*Article* **A Multicriteria Approach for Assessing the Impact of ICT on EU Sustainable Regional Policy**

## **Christiana Koliouska \* and Zacharoula Andreopoulou**

Laboratory of Forest Informatics, Faculty of Forestry and Natural Environment, Aristotle University of Thessaloniki, P.O. Box 247, 54124 Thessaloniki, Greece; randreop@for.auth.gr

**\*** Correspondence: ckolious@for.auth.gr; Tel.: +30-694-691-7724

Received: 26 March 2020; Accepted: 3 June 2020; Published: 15 June 2020

**Abstract:** As a global actor, the European Union (EU) plays a leading role in international efforts to promote sustainable development globally. All sustainable objectives and targets need Information and Communication Technologies (ICTs) as key catalysts, since ICTs constitute tools of unprecedented power which help people to face the growing challenges of rising population, poverty, epidemics and climate change. Policy makers in the EU are increasingly putting ICTs into relations with sustainable regional development. This paper aims to study and assess the impact of ICT on the EU regional policy in terms of sustainable development by applying the multicriteria approach, PROMETHEE II, using the software Visual PROMETHEE. The criteria that were used in this research are the criteria that both the European Commission and member states define to assess the ICT implications of new EU legislation since 2010. The results revealed that the impact of ICT on EU sustainable regional policy has gotten stronger in the last two decades.

**Keywords:** ICT; EU regional policy; sustainable development; multicriteria approach; policy assessment

## **1. Introduction**

Information and Communication Technology (ICT) advancements not only bring new opportunities, but also bring to light new risks for the achievement of sustainable development (SD) goals [1]. ICT proved to accelerate the worldwide socio-technological progress through knowledge transfer, marketing goods or services, network externalities and the development of cooperative relationships [2]. The contribution of ICT in addressing the major challenges of sustainable energy, climate change and sustainable development is highlighted by researchers, entrepreneurs, decision-makers and policy-makers [3]. The integration of ICT into social development and economic growth provides better opportunities for enhancing competitiveness and satisfying human needs [4,5].

The initiatives required to help ICT enable transformation to sustainable development and global competitiveness are as follows [6]:

	- ICT constitutes just a means of achieving sustainable development.
	- Active efforts are required to foster global inclusion.
	- Sustainable ICT should be economically feasible and create end-user value.
	- ICT for sustainable development research and practice should be participatory, collaborative and empowering for the solutions to be globally consistent and relevant.
	- Inspire and effectively engage all relevant stakeholders to have a shared vision to implement sustainable ICT.
	- Develop metrics to quantify the level of success and effectiveness and consider new academic rigor.
	- Focus on the challenges of modernization.
	- Plan innovative models for research and development.

ICT is supposed to trigger the co-evolutionary process that will meet sustainable development goals in the EU through the efficient use of natural resources [7]. National and regional agencies involved in activities related to the agenda about digital inclusion and web accessibility in rural and underserved areas should adopt a new technology, engage community participation and incorporate sustainable development as essential factors to link ICT with community development and prosperity [8]. ICT solutions constitute vehicles for sustainable development in a more effective and cost-efficient way, as they have great potential to assist poor people to improve the quality of their life [9].

Information and knowledge are essential to achieve regional development and economic growth, since they are performed as primal components of socio-economic activities for strategic management in developing countries [10]. According to OECD Report Greener and Smarter in 2010, ICT is a crucial component for green growth and green economy [11] as it promotes smart growth, "development that is economically sound, environmentally friendly and supportive of community livability–growth that enhances our quality of life" [12,13]. Realtime data streaming will support the process of effectively monitoring the impact of regulations regarding the environment and society [14]. The multimedia distributed systems such as ICT, communication networks and smart media are widely used for electronic data interchange (EDI) [15]. The international community has integrated many goals in public policy to ensure environmental sustainability since it constitutes part of global socio-economic well-being [16,17]. The sustainable management of natural resources plays a vital role in the achievement of global sustainability goals [18]. The aim of sustainable development is to "promote the human well-being, meet the basic needs of the poor and protect the welfare of future generations (intraand inter- generational justice), preserve environmental resources and global life-support systems (respecting limits), integrate economics and natural environment in decision-making, and encourage public involvement in development processes" [19,20]. However, the current framework for sustainable development is completely different from the framework developed over past years, as new factors (e.g., multinational corporate companies and civil society organizations) and modern ICT (computerized communication channels, such as Internet) influence the environmental and socio-economic aspects of the development [21].

According to the EU Green Paper on Innovation in 1995, the key factors to boosting innovation potential are the following: environmental policy, regional industrial policy, technology policy, education and training policy, research policy (RP), competition policy, Small and Medium Enterprises (SMEs) policy and taxation policy [22]. In today's digital era, the EU encourages the international cooperation in research and innovation as a key success factor in sustainability [23,24]. Sustainable regional and local development constitutes an integrated approach to the planning and development of our regions [25] but cooperation among different authorities is essential [26,27]. Regional Policy aims to create employment opportunities, support business agility and business competitiveness, while promoting sustainable economic growth to all European regions [28]. EU regional policy funding focuses on four categories [29]:


The main aim of the current policies and strategies for regional development is the flow of money from the rich nations to the poor nations, as well as the support to confront regional challenges through funding programs [30]. Aiming to create empowering partnerships among EU regions and EU member states in the context of these policies' implementation, the EU adopts some practices, such as monitoring and policy evaluation, in order to enhance the effectiveness of the policies [30]. The European Commission's involvement in regional development can be traced back to 1957 when the Treaty of Rome required the community to ensure "harmonious" development by reducing regional differences and the backwardness of less-favored regions [31].

According to the latest Eurobarometer regarding "Citizens' awareness and perceptions of EU regional policy" in 2017, almost 80% of the EU citizens were convinced that EU regional policy investments had a positive effect on their region or city, while almost half of the EU citizens supported the idea that EU regional investments should continue [32]. In order to be effective, a policy, such as regional policy, necessarily involves multiple partners operating at different spatial scales and different governance levels [33]. There are four structural funds: the European Regional Development Fund (ERDF), which is intended to finance large infrastructure projects and has the largest weight in the budget; the European Social Fund (ESF), which is the main financial instrument assisting the EU in realizing the strategy and the main objectives of its implementing policy; the European Agricultural Guidance and Guarantee Fund (EAGGF—Guidance Section), which accelerates the reformation of the agricultural structure; the Financial Instrument for Fisheries Guidance (FIFG), which is the specific fund for the reform of the structure of the fisheries sector [34]. The structural actions represent approximately one-third of whole the budget of the EU [34]. Infrastructure expenditures by are addressed to low-growth, low-employment and low-productivity regions [35].

It is widely accepted that EU regional policy has been an important contributing factor to the promotion of EU political regionalism and decentralization [36]. This is primarily as a result of the Funds of Partnership principle, taking into consideration that there are competent regional authorities and they get actively involved in the developing and implementation of regional aid [36]. Whereas a succession of income transfers may lead to a series of possible reductions in regional disparities, it should not be confused with the process of convergence on the regional level, which would be the successful implication of regional policies [37]. An emphasis on endogenous growth models has developed, and in connection with this, the intention to enhance the competitiveness among EU regions in contrast with simply getting involved in redistributive activities characterized by former regional policy interventions [38].

The European Regional Policy Research Consortium (EoRPA), originally launched in 1978, and was funded by government departments in Austria, Finland, France, Germany, Italy, the Netherlands, Norway, Poland, Sweden, Switzerland and the United Kingdom. It involves the monitoring and analysis of national regional policies in 30 European countries, and the study of the inter-relationships between EU Cohesion policy and EU Competition policy control of state aid [39].

The objective of this study is to investigate the impact of ICT on EU regional policy in the terms of sustainable development by applying the multicriteria approach, PROMETHEE II, using the software, Visual PROMETHEE. This approach was used for the assessment of the impact of ICT and for the ranking of the EU regional policies.

#### **2. Materials and Methods**

EU regional policies are retrieved from the official European Union website (www.europa.eu). The first step was to record the regulations, the directives, the decisions, the communications and other acts regarding the regional issues. EU sustainable regional policies form the alternatives. After the collection of the policies, a two-dimensional table was developed in order to find out the existence or lack of criteria that both the European Commission and member states have defined and proposed in order to assess the ICT implications of new EU legislation since 2010 [40]. These criteria constitute the variables X1, X2, ..., X12 (Table 1). Variable X1 refers to the requirement of the design of information rich

processes by the legislation, while variable X2 refers to the requirement of the design of new business processes. Variable X3 represents the requirement of large amounts of data gathering in these processes and variable X4 represents the requirement of collaboration between ICT systems of multiple DGs or institutions/organizations. Variable X5 is about the fact that the legislation concerns ICT systems or that ICT is a supporting function of the legislation. The first five criteria/variables describe the level of dependence on ICT solutions of the EU regional policies and the weight of each is 1. The rest of the criteria/variables describe the levels of complexity of the ICT solutions and the weight of each is 0.83. The weights of the criteria were defined according to the method used by the European Commission to assess the ICT implications of EU legislation [40], giving the same importance to the level of dependence on the ICT implications and the level of complexity of the ICT implications. Variable X6 refers to whether the legislation requires new ICT solutions or existing applications can fulfill the requirements, while variable X7 refers to the existence of legacy systems which might hamper the implementation. Variable X8 concerns the imposition of authentication requirements by the legislation and variable X9 concerns the requirement of large amounts of data exchange between member states and/or the Commission. Variable X10 is about the required lead-time of the implementation (urgency), variable X11 is about the requirement of new interoperability specifications and variable X12 is about the imposition of high security requirements on the ICT solution by the initiative. The total amount of criteria achieved by each EU regional policy was also studied.


**Table 1.** Criteria [41].

Furthermore, the impact of ICT was assessed, and the EU regional policies were ranked using the multicriteria analysis, PROMETHEE II. PROMETHEE constitutes a prescriptive methodology which enables the decision-maker to rank the actions regarding his preferences [42]. Two rankings are calculated: the partial ranking is calculated mainly by undisputable preferences (PROMETHEE I), while the complete ranking, which is probably weaker, is obtained according to the decision-maker's requirements, too (PROMETHEE II) [42]. The PROMETHEE IV method solves a choice problematic for an infinite set of actions. It uses the same outranking relation, but the flows are defined on a compact subset of R" [43].

The PROMETHEE method includes four processes [44]:


The method normalizes the weights of the criteria in order for their sum to be equal to 1.0 (100%) [44]. The PROMETHEE II method is described thoroughly in Brans and Mareschal (2005) [46] and in Andreopoulou et al. (2017) [47].

PROMETHEE provides the researcher with rankings of the alternatives and GAIA with a graphical representation of the decision problem [48]. The GAIA analysis is based on the uni-criterion net flows [42]. GAIA uses the principal components analysis (PCA), a well-known dimension-reduction technique for statistical data analysis [49].

PROMETHEE II methodology was selected in order to evaluate the impact of ICT on EU sustainable regional policies and to rank the policies because [50]:


#### **3. Results**

The research through the official European Union website (www.europa.eu) resulted in the retrieval of 50 regional policies, which are presented in the Appendix A. In total, 19 out of the 50 EU regional policies have been established since 2010 (when the European Commission began to assess the ICT implications of EU legislation). Figure 1 presents the partial rankings of the 19 EU regional policies based on the computation of the two preference flows (Phi+ and Phi-).

Figure 2 presents the complete rankings of the 19 EU regional policies, which are based on the total net preference flow (Phi). According to the results of PROMETHEE I and PROMETHEE II, COM(2012)19 "Waste Electrical and Electronic Equipment (WEEE)" is preferred to all other regional policies. COM(2012)19 has the highest score on Phi (0.5841), followed by Reg.2015/207 "The models for the progress report, submission of the information on a major project, the joint action plan, the implementation reports for the Investment for growth and jobs goal, the management declaration, the audit strategy, the audit opinion and the annual control report and the methodology for carrying out the cost-benefit analysis" and COM(2014)473 "Sixth report on economic, social and territorial cohesion: investment for jobs and growth", while COM(2015)118 "The Agreement on the European Economic Area" has the lowest score. The ranking of PROMETHEE I is confirmed by the ranking of PROMETHEE II.

PROMETHEE II.

two preference flows (Phi+ and Phi-).

**3. Results**

2) Calculate the preference index: this index is used to compare the alternatives in pairs,

3) Construct valued outranking graph: outgoing and incoming flows are determined by means

4) Rank alternatives according to the valued outranking graph: determination of the weights

The method normalizes the weights of the criteria in order for their sum to be equal to 1.0 (100%) [44]. The PROMETHEE II method is described thoroughly in Brans and Mareschal (2005) [46] and in

PROMETHEE provides the researcher with rankings of the alternatives and GAIA with a graphical representation of the decision problem [48]. The GAIA analysis is based on the uni-criterion net flows [42]. GAIA uses the principal components analysis (PCA), a well-known dimension-

PROMETHEE II methodology was selected in order to evaluate the impact of ICT on EU

 the use of the superiority relation is applied when the alternatives (sustainable regional policies) have to be ranked from the alternative with the highest score to the alternative

 the assessment and ranking process of complicated cases of sustainable regional policies is suitable for the application of PROMETHEE II methodology in the way that it seems

The research through the official European Union website (www.europa.eu) resulted in the retrieval of 50 regional policies. In total, 19 out of the 50 EU regional policies have been established since 2010 (when the European Commission began to assess the ICT implications of EU legislation).

there is now so much sensitivity of the estimated relation in small changes.

quantitatively taking into consideration all the defined criteria.

is an important step in most multi-criteria methods [45].

of relevant preference indices.

reduction technique for statistical data analysis [49].

with the lowest score.

to be closer to reality.

sustainable regional policies and to rank the policies because [50]:

the results can be easily interpreted and discussed.

Andreopoulou et al. (2017) [47].

**Figure 1. Figure 1.** Partial ranking of EU regional policies Partial ranking of EU regional policies. .

**Figure 2.** Complete ranking of EU regional policies. **Figure 2.** Complete ranking of EU regional policies.

Figure 3 shows the PROMETHEE diamond, which depicts in a better way the two rankings (Phi+ and Phi-), while the vertical dimension represents the total net flow (Phi) by the complete ranking process. It can be observed that all action cones are located on the left axis (Phi+), which means that the total net flow of the regional policies is less than 1. Figure 3 shows the PROMETHEE diamond, which depicts in a better way the two rankings (Phi+ and Phi-), while the vertical dimension represents the total net flow (Phi) by the complete ranking process. It can be observed that all action cones are located on the left axis (Phi+), which means that the total net flow of the regional policies is less than 1.

Table 2 shows the Phi scores of all the EU regional policies. The values calculated for the total net flows (Phi) present a large spectrum of values between +0.5841 and −0.4123, and that shows a great difference concerning "superiority" between the first and the last case in the ranking of EU regional policies according to the impact of ICT.

*Sustainability* **2020**, *12*, x FOR PEER REVIEW 7 of 13

**Figure 3.** PROMETHEE Diamond. **Figure 3.** PROMETHEE Diamond.


**Table 2.** Preference flows.

COM(2014)490 0.077 0.3094 −0.2324 COM(2011)146 0.077 0.3094 −0.2324 Reg.347/2013 0.0667 0.3865 −0.3198 Reg.240/2014 0.0547 0.4445 −0.3899 COM(2014)494 0.0547 0.4445 −0.3899 COM(2013)463 0.0547 0.4445 −0.3899 COM(2015)118 0.0564 0.4687 −0.4123 In Figure 4, the GAIA plane is displayed. The red axis is the decision axis, which indicates the In Figure 4, the GAIA plane is displayed. The red axis is the decision axis, which indicates the direction for the best solution according to the weight vectors on the GAIA plane. Because the direction of the decision axis is in the same direction as the variables X2 "The requirement of the design of new business processes" and X4 "The requirement of collaboration between ICT systems of multiple DGs or institutions/organizations", it can be expected that the PROMETHEE II ranked actions to be stronger on this variable and potentially weaker on variables X8 "The imposition of authentication requirements by the legislation" and X11 "The requirement of new interoperability specifications". COM(2010)553 "Regional Policy contributing to smart growth in Europe 2020" and Reg.1300/2013 "Cohesion Fund and repealing Council Regulation (EC) No 1084/2006" are very close to each other and

COM(2011)776 0.1752 0.1633 0.0119

direction for the best solution according to the weight vectors on the GAIA plane. Because the

they have similar actions, whereas the other policies are in the opposite direction. It can be concluded that they are different from other actions. to each other and they have similar actions, whereas the other policies are in the opposite direction. It can be concluded that they are different from other actions.

Reg.1300/2013 "Cohesion Fund and repealing Council Regulation (EC) No 1084/2006" are very close

*Sustainability* **2020**, *12*, x FOR PEER REVIEW 8 of 13

of multiple DGs or institutions/organizations", it can be expected that the PROMETHEE II ranked actions to be stronger on this variable and potentially weaker on variables X8 "The imposition of

**Figure 4.** PROMETHEE GAIA plane. **Figure 4.** PROMETHEE GAIA plane.

## **4. Discussion**

**4. Discussion**  Research into the official EU website retrieved 50 regional policies, while 19 out of them have been established since 2010 (when the European Commission began to assess the ICT implications of EU legislation). The fulfillment of these 12 criteria, used by the European Commission, was studied in order to depict the impact of ICT on EU sustainable regional policy. Multicriteria decision analysis Research into the official EU website retrieved 50 regional policies, while 19 out of them have been established since 2010 (when the European Commission began to assess the ICT implications of EU legislation). The fulfillment of these 12 criteria, used by the European Commission, was studied in order to depict the impact of ICT on EU sustainable regional policy. Multicriteria decision analysis and GAIA analysis, are presented to identify the impact of ICT on EU sustainable regional policies.

and GAIA analysis, are presented to identify the impact of ICT on EU sustainable regional policies. The results and conclusions are summarized as follows: The results and conclusions are summarized as follows:


• "The imposition of authentication requirements by the legislation" and "The requirement of new interoperability specifications" are found to be the weakest criteria.

As most EU sustainable regional policies present positive total net flows, this research confirms that the impact of ICT on EU sustainable regional policy has been getting stronger since 2010. The applied methodology constitutes an efficient planning tool at EU level for the policy makers for the assessment of sustainable regional policies, based on the impact of ICT. Furthermore, the findings of this research can be a supportive tool for the policy makers, as the superior EU regional policies can be used as benchmarks for future policies in terms of sustainable development. ICT adoption shows an enormous potential for accelerating the progress towards SDGs (Sustainable Development Goals), while at the same time it can improve the quality of life of people in fundamental ways [51]. However, it would be very interesting to apply different methods for multiple criteria decision-making because of some disadvantages of PROMETHEE method, such as the paradigm of the underlying method, the determination of the weights and the rank reversal problem [52].

**Author Contributions:** C.K. and Z.A. conceived of the presented idea. C.K. contributed to the design and the implementation of the research, to the analysis of the results and to the xriting of the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Appendix A**


**Table A1.** Sustainable regional policies.



#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **Efficiency Enhancement of Gas Turbine Systems with Air Injection Driven by Natural Gas Turboexpanders**

**Ali Rafiei Sefiddashti <sup>1</sup> , Reza Shirmohammadi 2,3,\* and Fontina Petrakopoulou <sup>3</sup>**


**Abstract:** The fuel source of many simple and combined-cycle power plants usually comes from a nearby natural gas transmission pipeline at a pressure from 50 to over 70 bar. The use of a turboexpander instead of throttling equipment offers a promising alternative to regulate the pressure of natural gas introduced to the power plant. Specifically, it helps recover part of the available energy of the compressed gas in the transmission pipeline, increase the power output and efficiency of the gas turbine system, and decrease the fuel use and harmful emissions. In this paper, the addition of such a turboexpander in a gas pressure-reduction station is studied. The recovered power is then used to drive the compression of extra air added to the combustion chamber of a heavy-duty gas turbine. The performance of this configuration is analyzed for a wide range of ambient temperatures using energy and exergy analyses. Fuel energy recovered in this way increases the output power and the efficiency of the gas turbine system by a minimum of 2.5 MW and 0.25%, respectively. The exergy efficiency of the gas turbine system increases by approximately 0.36% and the annual CO<sup>2</sup> emissions decrease by 1.3% per MW.

**Keywords:** gas turbine; air injection; turboexpander; performance enhancement; emission reduction

#### **1. Introduction**

A prominent technology today for the energy conversion of fossil fuels, such as natural gas (NG) and oil, are the gas turbine systems. These machines reach high energy conversion efficiencies due to technological progress and advanced materials in their design and construction. Nevertheless, the associated environmental impact of these machines plays a key role in climate change, highlighting the necessity of energy efficiency improvement policy in power plants and energy policies overall [1]. Currently, such policies motivate governments to improve the efficiency of gas turbines by further recuperating thermal energy from the exhaust gasses to produce steam and drive a steam turbine [2]. However, this kind of relatively high-investment cost of solutions force private companies to seek cheaper solutions [3,4].

In a simple gas turbine system, the temperature and pressure of the ambient air increases by passing through the compressor. After mixing with the fuel and the ignition, the high-pressure combustion products reach the highest operating temperature. The hot combustion product (gases) is expanded in the turbine, moving the rotating blades, and consequently rotating the turbine shaft to provide power for rotating the compressor and the generator [5]. The amount of required power for the compressor depends on the inlet volumetric flow of the air; more power is required to compress the same mass flow of air of lower density to a given outlet pressure.

**Citation:** Sefiddashti, A.R.; Shirmohammadi, R.; Petrakopoulou, F. Efficiency Enhancement of Gas Turbine Systems with Air Injection Driven by Natural Gas Turboexpanders. *Sustainability* **2021**, *13*, 10994. https://doi.org/ 10.3390/su131910994

Academic Editors: Georgios Tsantopoulos and Evangelia Karasmanaki

Received: 27 August 2021 Accepted: 27 September 2021 Published: 3 October 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

A means to decrease the inlet air temperature and boost the turbine output recommended by most gas turbine manufacturers is the use of cooling equipment. Cooling equipment includes evaporative coolers, fogging, and chillers that significantly increase the capital cost of the plant. Although cooling systems improve operation, their efficacy is highly dependent on ambient temperature [6] and humidity. Steam injection into the combustion chamber for power enhancement is another method, but it requires large quantities of demineralized water, and is linked to combustion and other operational challenges.

Another measure to increase the generated power of gas turbine systems is the compressed air injection (CAI), i.e., the injection of additional pressurized air into the combustion chamber or at the compressor outlet. This additional air flow requires then more fuel to maintain the inlet temperature of the expander. Nevertheless, in such applications, the fuel increase pales in comparison to the gas turbine power increase. The significant power increase is due to the higher mass flow in the turbine, and consequently, the increased work generated in comparison to the compressor's required work. This leads to an overall enhancement of the thermal efficiency of the gas turbine. Nakhamkin et al. [7] proposed injecting compressed air in a highly efficient electrically driven compressor upstream of the combustion chamber. The air can be injected through the ports of steam injection that are already available in some commercial gas turbines. CAI also helps to increase the lifetime of the gas turbine by reducing the inlet temperature of the turbine without a reduction in the power generation. Akita et al. showed that the reduction of firing temperature with air injection by approximately 110 ◦C increases the maintenance intervals and reduces the maintenance costs by a factor of two in both cases [8]. Typically, up to 10% of a gas turbine's airflow at ISO conditions (temperature = 15 ◦C, relative humidity = 60%, and pressure = 101.3 kPa) can be used for injection purposes. However, avoiding compressor surge and the torque limit of the shaft restrict the maximum retrieved air at any given ambient temperature. An electrical motor or an efficient reciprocating engine may drive an intercooled compressor that compresses ambient air and adds it to the compressor outlet [9]. Internal combustion engines are less sensitive to temperature and humidity, maintaining their nominal power output and efficiency over a broader range of ambient conditions. Hence, some companies designed a series of standardized building block modules which can be connected together to operate at high injection air flows [10]. Combined diesel-engine gas turbine systems enable distributed power generation plants to attain high thermal efficiencies while enjoying the operational advantages of both diesel engines and gas turbines [11]. Abudu et al. evaluated the implication of the steady-state injection of compressed air into two multi-spool gas turbines for power enhancement. The steadystate analysis demonstrated that with an 8% flow injection, a power increase of at least 16% is obtained [12]. Gas turbines also play a key role in synchronous power generation and back-up systems for intermittent renewable systems. Igie et al. [13] considered the extraction of compressed air from a single-shaft gas turbine to store energy when surplus power is available and then the reinjection of the pressurized air at peak demand. CAI can constitute thus an alternative solution for energy storage, required by most renewable power sources.

Although a wide range of fuels can be used in gas turbines, compressed natural gas is the most common fuel used. Natural gas is transported through pipelines over long distances. The pressure of the natural gas must be significantly decreased before it is supplied to the combustion chamber of the gas turbine system. The pressure reduction of the natural gas that usually occurs in throttling valves is accompanied by substantial energy and exergy losses [14].

Today, many researchers study energy recovery devices for the decompression of high-pressure natural gas. The amount of energy that can be recovered depends on various parameters including both operating conditions (pressure difference, temperature, and mass flow) and design parameters (efficiency, capacity, performance map, etc.) [15–19]. Furthermore, the quality of NG (in terms of hydrate formation) is also crucial [14]. Many authors, such as Morgese et al. [20], propose an optimization design procedure of a tur-

boexpander by considering fluid dynamic and technical requirements. Recovery of waste energy of the gas stations can also be used for both producing power and freshwater with a potentially substantial effect on the reduction of greenhouse gases and air emissions [21]. Golchoobian et al. [22] investigated the feasibility of using a turboexpander coupled with a refrigeration cycle to decrease the inlet temperature of air and increase the generated power. Although many studies evaluate waste energy recovery from pressure-reducing stations and air injection into the combustion chamber separately, the combined use of waste energy to inject air into gas turbine combustion chambers is still missing. This paper aims to address this research gap with energy and exergy analyses of a hybrid system of a gas turbine including a natural gas turboexpander and air injection for performance enhancement. Lastly, since the capacity and operating conditions of pressure-reducing stations in power plants vary moderately, important parameters and their effects are studied in this work as well. waste energy of the gas stations can also be used for both producing power and freshwater with a potentially substantial effect on the reduction of greenhouse gases and air emissions [21]. Golchoobian et al. [22] investigated the feasibility of using a turboexpander coupled with a refrigeration cycle to decrease the inlet temperature of air and increase the generated power. Although many studies evaluate waste energy recovery from pressurereducing stations and air injection into the combustion chamber separately, the combined use of waste energy to inject air into gas turbine combustion chambers is still missing. This paper aims to address this research gap with energy and exergy analyses of a hybrid system of a gas turbine including a natural gas turboexpander and air injection for performance enhancement. Lastly, since the capacity and operating conditions of pressure-reducing stations in power plants vary moderately, important parameters and their effects are studied in this work as well.

19]. Furthermore, the quality of NG (in terms of hydrate formation) is also crucial [14]. Many authors, such as Morgese et al. [20], propose an optimization design procedure of a turboexpander by considering fluid dynamic and technical requirements. Recovery of

*Sustainability* **2021**, *13*, x FOR PEER REVIEW 3 of 17

#### **2. Process Description** The pressure-reducing station is the endpoint of the natural gas transmission system.

**2. Process Description** 

The pressure-reducing station is the endpoint of the natural gas transmission system. There, the pressure of the delivered gas is decreased to the final domestic or industrial consumer [23]. These stations have usually two or three parallel pressure regulator lines to provide redundancy in case of changing filters and for safety purposes. There are several pieces of equipment on each uniform line but their arrangement or configuration in each station may change based on ambient and operating conditions. The common elements of all stations, and probably the most important, are the control or reduction valves that maintain the pressure downstream of the station constant. In some stations due to the ambient conditions or the high-pressure reduction ratio, a heating element, such as a bath heater, is provided to reduce the risk of hydrate formation from the Joule–Thomson effect. There, the pressure of the delivered gas is decreased to the final domestic or industrial consumer [23]. These stations have usually two or three parallel pressure regulator lines to provide redundancy in case of changing filters and for safety purposes. There are several pieces of equipment on each uniform line but their arrangement or configuration in each station may change based on ambient and operating conditions. The common elements of all stations, and probably the most important, are the control or reduction valves that maintain the pressure downstream of the station constant. In some stations due to the ambient conditions or the high-pressure reduction ratio, a heating element, such as a bath heater, is provided to reduce the risk of hydrate formation from the Joule–Thomson effect. Currently, a commercially available alternative technology to throttling is axial or

Currently, a commercially available alternative technology to throttling is axial or radial turbines (also called turboexpanders) coupled with a generator to convert mechanical energy into electrical energy. Figure 1a shows the schematic placement of a turboexpander unit in the bypass line which can be isolated by two shut-off valves. In some arrangements, it is necessary to preheat the high-pressure gas because of the throttling process before the entrance of the turbine. radial turbines (also called turboexpanders) coupled with a generator to convert mechanical energy into electrical energy. Figure 1a shows the schematic placement of a turboexpander unit in the bypass line which can be isolated by two shut-off valves. In some arrangements, it is necessary to preheat the high-pressure gas because of the throttling process before the entrance of the turbine.

(**a**) (**b**)

In this work, a novel cycle for the arrangement of a turboexpander in a gas station of simple and combined-cycle power plants is proposed as illustrated in Figure 1b. The turboexpander is connected to a compressor that compresses ambient air. The air that is led In this work, a novel cycle for the arrangement of a turboexpander in a gas station of simple and combined-cycle power plants is proposed as illustrated in Figure 1b. The turboexpander is connected to a compressor that compresses ambient air. The air that is led to the combustion chamber of the gas turbine system after it is passed through a heat exchanger to preheat the high-pressure gas. This reduces the risk of hydrate formation in the turboexpander. Injecting extra air to the combustion chamber increases the

mass flow of the GT and produces more power. Moreover, using this arrangement eliminates the need for a natural gas bath heater and generator, increases the plant efficiency, and decreases the plant's capital investment in comparison to individual turboexpander energy-recovery systems.

#### **3. Methodology**

To determine the effects of the air-injection system on the performance of the chosen gas turbine, a computer code was developed in the engineering equation solver (EES). The calculation procedure of the EES code is summarized in Figure 2. This code calculates the thermodynamic properties and off-design performance of the gas turbine with and without the high-pressure injection system. Another model was simulated using the Thermoflex software to validate the in-house code results. Operational compatibility between the turbine and the compressor of the gas turbine (matching calculations) depends on mass flow compatibility, pressure ratio (work), and rotational speed [24]. The characteristic curves of mass flow, pressure ratio, and efficiency with rotational speed of the compressor, turbine, and combustion chamber were obtained for the gas turbine model V94.2. It should be noted that the demonstrated flow chart has been developed on the assumption that the turbine inlet temperature (TIT) remains constant. This assumption depends on the control system mode of the gas turbine, and it can be adjusted for other GT control modes. Considering constant TIT and compatibility of speed and flow for a single-shaft machine, the pressure ratio and other performance characteristics of the gas turbine were determined. *Sustainability* **2021**, *13*, x FOR PEER REVIEW 5 of 17

**Figure 2.** Procedure of component matching of the gas turbine system. **Figure 2.** Procedure of component matching of the gas turbine system.

<sup>×</sup> ඥTଵ ඥTଷ

The pressure ratio of the gas turbine is a function of the compressor pressure ratio

where Pଶ and Pଷ are the pressures at the inlet and outlet of the combustor, respectively. The mass flow that passes through the turbine is equal to the outlet mass flow of the com-

The turbine and compressor shafts were coupled together to assure compatible rota-

In most gas turbines, the TIT is constant during the operation due to metallurgical limitations. Although there are various definitions and positions to measure the TIT (T03), in this study it was considered constant so that for given ambient conditions, the square root of the temperature ratio was constant as well. Moreover, the non-dimensional flow term expresses the compatibility of the flow between the compressor and the turbine as

+ mሶ ୧୬୨. (2)

(3)

pressor plus the fuel flow and the additional compressed air:

ሶ + mሶ

<sup>=</sup> <sup>N</sup> ඥTଵ

and the pressure drop within the combustor.

mଷ ሶ = mଵ

N ඥTଷ

tional speed.

follows:

In this study, the effect of the proposed air-injection system was studied on the heavy-duty gas turbine of Siemens V94.2, a model widely used in power plants. V94.2 is a single-shaft gas turbine with a rated power of 162 MW. This turbine incorporates a 16-stage compressor, two large silo-type combustion chambers, and a four-stage turbine. Performance data (including the compressor and turbine data) for the simulation were found in various references and official original equipment manufacturer (OEM) websites of Siemens and Alstom [25–28]. The design performance characteristics of the gas turbine are presented in Table 1. Calculated performance parameters with the EES code, including power and efficiency at various ambient temperatures, agree with published OEM data with an accuracy of more than 98%.

**Table 1.** Design performance characteristics of the simulated GT.


The pressure ratio of the gas turbine is a function of the compressor pressure ratio and the pressure drop within the combustor.

$$\mathbf{P}\_{03}/\mathbf{P}\_{02} = 1 - \Delta \mathbf{P}\_{\mathbf{CC}}/\mathbf{P}\_{02} \tag{1}$$

where P<sup>02</sup> and P<sup>03</sup> are the pressures at the inlet and outlet of the combustor, respectively. The mass flow that passes through the turbine is equal to the outlet mass flow of the compressor plus the fuel flow and the additional compressed air:

$$
\dot{\mathbf{m}}\_3 = \dot{\mathbf{m}}\_1 + \dot{\mathbf{m}}\_F + \dot{\mathbf{m}}\_{\text{in}\dot{\mathbf{q}}}.\tag{2}
$$

The turbine and compressor shafts were coupled together to assure compatible rotational speed.

$$\frac{\mathbf{N}}{\sqrt{\mathbf{T}\_{03}}} = \frac{\mathbf{N}}{\sqrt{\mathbf{T}\_{01}}} \times \frac{\sqrt{\mathbf{T}\_{01}}}{\sqrt{\mathbf{T}\_{03}}}.\tag{3}$$

In most gas turbines, the TIT is constant during the operation due to metallurgical limitations. Although there are various definitions and positions to measure the TIT (T03), in this study it was considered constant so that for given ambient conditions, the square root of the temperature ratio was constant as well. Moreover, the non-dimensional flow term expresses the compatibility of the flow between the compressor and the turbine as follows: . . .

$$\frac{\dot{\mathbf{m}}\_3 \sqrt{\mathbf{T}\_{03}}}{\mathbf{P}\_{03}} = \frac{\dot{\mathbf{m}}\_1 \sqrt{\mathbf{T}\_{01}}}{\mathbf{P}\_{01}} \times \frac{\mathbf{P}\_{01}}{\mathbf{P}\_{02}} \times \frac{\mathbf{P}\_{02}}{\mathbf{P}\_{03}} \times \frac{\sqrt{\mathbf{T}\_{03}}}{\sqrt{\mathbf{T}\_{01}}} \times \frac{\dot{\mathbf{m}}\_3}{\mathbf{m}\_1} \tag{4}$$

where . <sup>m</sup><sup>1</sup> is the inlet mass flow of the compressor, . m<sup>3</sup> is the inlet mass flow of the turbine, T<sup>01</sup> is the ambient temperature at the inlet of the compressor, P<sup>01</sup> is the pressures at the inlet of the compressor. The adiabatic work of the compressor can be calculated with Equation (5): .

$$\mathcal{W}\_{\text{compressor}} = \dot{\mathbf{m}}\_1 \times \mathbf{C} \mathbf{p}\_A \times (\mathbf{T}\_{02} - \mathbf{T}\_{01}) \tag{5}$$

where Cp<sup>A</sup> is the specific heat capacity of the air at constant pressure and T<sup>02</sup> is the outlet temperature of the compressor.

The actual compressor outlet temperature (T02), considering its isentropic efficiency (η<sup>c</sup> ), can be estimated with the following equation:

$$\mathbf{T\_{02}} = \mathbf{T\_{01}} + \frac{\mathbf{T\_{01}}}{\eta\_{\mathbf{c}}} \left( \left( \frac{\mathbf{P\_{02}}}{\mathbf{P\_{01}}} \right)^{\frac{\gamma - 1}{\gamma}} - 1 \right) \tag{6}$$

where η<sup>c</sup> is the compressor efficiency and γ the air-specific heat ratio.

The outlet pressure of the combustion chamber (P03) is also calculated from the compressor's delivery pressure (P02) and the pressure drop of the air in the combustor (∆PCC). For most available combustors it is in the range of 0.03–0.05 of the inlet pressure [29].

$$\mathbf{P\_{03}} = \mathbf{P\_{02}} - \Delta \mathbf{P\_{CC}} \tag{7}$$

With constant blade dimensions and negligible changes in efficiency, higher inlet mass flow will lead to an off-design operation of the turbine. A similar equation to the compression process is used for the calculation of the turbine's expansion work (WT) by considering the total mass flow of the gas calculated with Equation (2):

$$\mathbf{W\_{T}} = \dot{\mathbf{m}}\_{3} \times \mathbf{C} \mathbf{p\_{G}} \times \left(\mathbf{T\_{04}} - \mathbf{T\_{03}}\right) \tag{8}$$

where Cp<sup>G</sup> . is the specific heat of the exhaust gas and T<sup>04</sup> is the temperature at the outlet of the turbine.

The actual turbine outlet temperature (T04). can be estimated with Equation (9), considering the isentropic efficiency of the turbine (ηT):

$$\mathbf{T\_{04}} = \mathbf{T\_{03}} - \eta\_{\mathbf{T}} \times \mathbf{T\_{03}} \left( 1 - \left( \frac{\mathbf{P\_{04}}}{\mathbf{P\_{03}}} \right)^{\frac{\gamma - 1}{\gamma}} \right) \tag{9}$$

The net or useful work of the gas turbine can be obtained by subtracting the consumed work of the compressor from the produced work of the turbine.

$$\mathbf{W\_{Net}} = \boldsymbol{\eta}\_{\rm m} \times \mathbf{W\_{T}} - \mathbf{W\_{c}}.\tag{10}$$

where η<sup>m</sup> is the mechanical efficiency of the gas turbine.

Similar equations can be used to determine the mass flow of the additional compressed air in the turboexpander at different conditions.

$$
\eta\_{\rm lm} \times \mathbf{W}\_{\rm TE} = \mathbf{W}\_{\rm AIC} \tag{11}
$$

where ηm. is the combined mechanical efficiency, WTE. the produced work of the turboexpander, and WAIC the shaft power of the air-injection compressor.

In this study, the effect of various parameters on the mass flow of injected air were investigated. The mass flow of the fuel that expands in the turboexpander plays a key role on the mass flow of the injected air. Based on OEM data of several industrial gas turbine models, up to 5% of the main gas turbine inlet flow can be injected into the combustion chamber safely [30]. Table 2 presents selected parameters used to model the proposed system.


**Table 2.** Considered assumptions for model calculation.

To evaluate the environmental performance of the proposed system and compare it to that of a conventional system, the amount of generated CO2, CO, and NOx have been calculated. The emitted CO<sup>2</sup> was calculated using the combustion and equilibrium reactions. Empirical relations proposed in [31] are used to determine the emission of CO and NOx, using adiabatic flame temperature in the primary zone of the combustion chamber as follows [32]:

$$\begin{cases} \begin{aligned} \mathbf{T}\_{\rm ad} &= \mathbf{A}\sigma^{\rm \alpha} \exp\left(\mathfrak{\boldsymbol{\beta}} \left(\sigma + \lambda\right)^{2}\right) \pi^{\rm x} \mathfrak{\boldsymbol{\sigma}}^{\rm y} \boldsymbol{\Psi}^{\rm z} \\ &\quad \mathbf{x} = \mathbf{a}\_{1} + \mathbf{b}\_{1}\sigma + \mathbf{c}\_{1}\sigma^{2} \\ &\quad \mathbf{y} = \mathbf{a}\_{2} + \mathbf{b}\_{2}\sigma + \mathbf{c}\_{2}\sigma^{2} \\ &\quad \mathbf{y} = \mathbf{a}\_{3} + \mathbf{b}\_{3}\sigma + \mathbf{c}\_{3}\sigma^{2} \end{aligned} \end{cases} \tag{12}$$

where θ is a dimensionless temperature, π is a dimensionless pressure, σ is the fuel to air equivalent ratio, and ψ is the H/C atomic ratio. Parameters A, α, β, λ, a<sup>i</sup> , b<sup>i</sup> , and c<sup>i</sup> are constants, depending on σ. and θ, available in [33]. Accordingly, by using adiabatic flame temperature, the produced CO and NOx can be estimated based on the following empirical equations in grams per kilogram of fuel flow:

$$\dot{m}\_{\rm NOx} = \frac{1.5 \times 10^{15} \tau^{0.5} \exp(-7110/\text{T}\_{\rm ad})}{\text{P}\_{2}^{0.05} (\Delta \text{P}\_{\rm cc}/\text{P}\_{2})^{0.5}} \tag{13}$$

$$\dot{\mathbf{m}}\_{\rm CO} = \frac{0.179 \times 10^9 \exp(7800/\mathcal{T}\_{\rm ad})}{\mathcal{P}\_2^2 \pi (\Delta \mathcal{P}\_{\rm cc}/\mathcal{P}\_2)^{0.5}}.\tag{14}$$

where P<sup>2</sup> is the pressure at the inlet of the combustor, ∆Pcc is the dimensionless pressure loss in the combustion chamber, and τ is the residence time in the combustion zone (considered constant at 0.02 s).

As mentioned, the validation of the energy model of the gas turbine with injection was carried out with the Thermoflow software—commercially available thermal engineering software for analyzing the performance of thermodynamic cycles. In this validation process, the total pressure loss of intake and exhaust were assumed to be 10 and 5 mbar, respectively. The air compressor was fed with power from the natural gas turboexpander. A schematic of the Thermoflow model is shown in Figure 3.

**Figure 3.** Thermoflow model for validating the EES code. **Figure 3.** Thermoflow model for validating the EES code.

**Table 3.** Validation of the EES energy code. **Parameter EES Code Thermoflow Error**  Power w/o CAI 161.4 MW 161.8 MW 0.25% The validation results are reported in Table 3, where the gas turbine power and efficiency were calculated with and without CAI. It is seen that there is generally good agreement between the EES code and the Thermoflow results, with acceptable errors for both power and efficiency.

Power with CAI 163.6 MW 164.1 MW 0.30% **Table 3.** Validation of the EES energy code.

.


where Exሶ ,୩ and Exሶ ,୩ are the fuel and product exergy of each component, respectively, and Exሶ ୈ,୩ is the exergy destruction within component k. Exergy loss is not defined at the By applying the laws of thermodynamics within component k, exergy destruction is obtained, which is a relation between the fuel and product exergy as follows [34]:

$$
\dot{\mathbf{E}}\mathbf{x}\_{\rm D,k} = \dot{\mathbf{E}}\mathbf{x}\_{\rm F,k} - \dot{\mathbf{E}}\mathbf{x}\_{\rm P,k} \tag{15}
$$

ሶ = ሶ ு + ሶ ு (16) The exergetic efficiency of each thermodynamic component is calculated as: where ExF,k and ExP,k . are the fuel and product exergy of each component, respectively, and . ExD,k is the exergy destruction within component k. Exergy loss is not defined at the component level, as it is only relevant for the overall process [35–37]. All exergy calculations of streams are based on the sum of chemical and physical exergies as follows [38]:

tions of streams are based on the sum of chemical and physical exergies as follows [38]:

$$
\dot{\mathbf{E}} = \dot{\mathbf{E}} \mathbf{x}^{\text{PH}} + \dot{\mathbf{E}} \mathbf{x}^{\text{CH}} \tag{16}
$$

(17)

୍େ Exሶ ,୍େ = Exሶ − Exሶ

All components are analyzed based on their exergy destruction and exergy efficiency, The exergetic efficiency of each thermodynamic component is calculated as:

$$
\varepsilon\_{\rm GT} = \frac{\dot{\rm Ex}\_{\rm P,k}}{\dot{\rm Ex}\_{\rm F,k}} = 1 - \frac{\dot{\rm Ex}\_{\rm D,k}}{\dot{\rm Ex}\_{\rm F,k}} \tag{17}
$$

**Table 4.** Definitions of the fuel and product exergy for each component. **Components Fuel Exergy,** ሶ , **Product Exergy,** ሶ , Gas Turbine Exሶ ,ୋ = Exሶ <sup>ଵ</sup> + Exሶ <sup>ଷ</sup> + Exሶ ଼ − Exሶ <sup>ଶ</sup> Exሶ ,ୋ = Wሶ ୋ All components are analyzed based on their exergy destruction and exergy efficiency, determined by the definition of exergy of the fuel and exergy of the product of each component, as shown in Table 4.

Turbo Expander Exሶ , = Exሶ <sup>ସ</sup> − Exሶ <sup>ଷ</sup> Exሶ , = Wሶ

Heat Exchanger Exሶ ,ୌଡ଼ = Exሶ ଼ − Exሶ Exሶ ,ୌଡ଼ = Exሶ <sup>ସ</sup> − Exሶ <sup>ଷ</sup>

comparison, these values are shown for both systems with and without the turboexpander

The calculated values of the thermodynamic parameters and the total rate of exergy

air-injection system.

.

Air Injection Compressor Exሶ ,୍େ = Wሶ

εୋ <sup>=</sup> Exሶ ,୩ Exሶ ,୩


**Table 4.** Definitions of the fuel and product exergy for each component.

The calculated values of the thermodynamic parameters and the total rate of exergy at various points of the system are shown in Figure 3 and listed in Table 5. To facilitate comparison, these values are shown for both systems with and without the turboexpander air-injection system.


**Table 5.** Thermodynamic data of the streams.

#### **4. Results**

In a single-shaft machine, the air injection does not affect rotational speed of the engine or the compressor airflow. With the inlet temperature and the fuel input of the turbine fixed, the cycle pressure ratio of the system must increase. Figure 4a depicts a simplistic interpretation of the effect of air injection on the T-s diagram of the Brayton cycle. This figure, based on a semi-perfect gas model, shows that one of the main effects of air injection is the increase of the pressure ratio. With fixed turbine blade design, higher flow rates through the combustor result in a higher turbine pressure ratio and for a fixed compressor inlet pressure, the compressor pressure ratio increases. Subsequently, with fixed turbine inlet temperature, the turbine work output and exhaust temperature increase. It should be mentioned that the power output and the efficiency of the gas turbine decreases with higher ambient temperature, due to the lower density and, subsequently, the lower compressor mass flow. Power and, to some extent, efficiency can be restored through the injection of compressed air because the work required by the turbocompressor is covered with the turboexpander. At higher ambient temperatures, the power output of the turbine decreases at a rate of 1 MW per degree of centigrade (Figure 4b). Injecting approximately 5% of the turbine's exhaust mass flow at ISO conditions (or merely 25 kg/s) results in rapidly increasing the power output by around 11% (16 MW).

**4. Results** 

idly increasing the power output by around 11% (16 MW).

**Table 5.** Thermodynamic data of the streams. **Stream M (kg/s) P (bar) T (C) h (kJ/kg) s (kJ/kgC)** ሶ **(kW)**  Without turboexpander air injection 1 512.7 1.013 15 −10.13 0.1435 1584.7 2 522 1.013 544.4 568.4 1.338 145426 3 9.39 60 25 50047 −2.266 492640 With turboexpander air injection 1 512.7 1.013 15 -10.13 0.1435 1584.7 2 525.8 1.013 543.6 567.6 1.337 146236 3 9.45 17 16.62 50029 −1.56 493698 4 9.45 58.8 88.5 50193 −1.747 495667 5 9.45 60 25 50047 −2.266 495560 6 3.65 1.013 15 −10.13 0.1435 11.34 7 3.65 17.1 417.3 407.7 0.232 1447 8 3.65 17 55 27.5 −0.5415 898

**Figure 4.** (**a**) Effect of air injection on a single-shaft gas turbine cycle; (**b**) air injection impact on the output power of the V94.2 turbine. **Figure 4.** (**a**) Effect of air injection on a single-shaft gas turbine cycle; (**b**) air injection impact on the output power of the V94.2 turbine. *Sustainability* **2021**, *13*, x FOR PEER REVIEW 10 of 17

Performance enhancement of the gas turbine leads to overall fuel savings. Although the used fuel increases for a range of ambient temperatures due to the increased air mass flow, the performance of the gas turbine improves considerably. The latter has a strong impact on the overall consumption of fuel and, consequently, on the generated emission, as also shown in Figure 5a. As seen, the proposed system results in approximately 200 kg/h of fuel savings, while the CO<sup>2</sup> emissions reduce by 500–600 kg/h (or 4000–4800 tons/year) for a wide range of ambient temperatures. Performance enhancement of the gas turbine leads to overall fuel savings. Although the used fuel increases for a range of ambient temperatures due to the increased air mass flow, the performance of the gas turbine improves considerably. The latter has a strong impact on the overall consumption of fuel and, consequently, on the generated emission, as also shown in Figure 5a. As seen, the proposed system results in approximately 200 kg/h of fuel savings, while the CO2 emissions reduce by 500–600 kg/h (or 4000–4800 tons/year) for a wide range of ambient temperatures.

In a single-shaft machine, the air injection does not affect rotational speed of the engine or the compressor airflow. With the inlet temperature and the fuel input of the turbine fixed, the cycle pressure ratio of the system must increase. Figure 4a depicts a simplistic interpretation of the effect of air injection on the T-s diagram of the Brayton cycle. This figure, based on a semi-perfect gas model, shows that one of the main effects of air injection is the increase of the pressure ratio. With fixed turbine blade design, higher flow rates through the combustor result in a higher turbine pressure ratio and for a fixed compressor inlet pressure, the compressor pressure ratio increases. Subsequently, with fixed turbine inlet temperature, the turbine work output and exhaust temperature increase. It should be mentioned that the power output and the efficiency of the gas turbine decreases with higher ambient temperature, due to the lower density and, subsequently, the lower compressor mass flow. Power and, to some extent, efficiency can be restored through the injection of compressed air because the work required by the turbocompressor is covered with the turboexpander. At higher ambient temperatures, the power output of the turbine decreases at a rate of 1 MW per degree of centigrade (Figure 4b). Injecting approximately 5% of the turbine's exhaust mass flow at ISO conditions (or merely 25 kg/s) results in rap-

**Figure 5.** (**a**) Fuel savings and CO2 emission reduction; (**b**) normalized emission values of NOx and CO at various ambient temperatures. **Figure 5.** (**a**) Fuel savings and CO<sup>2</sup> emission reduction; (**b**) normalized emission values of NOx and CO at various ambient temperatures.

The generation of the air pollutants CO and NOx per megawatt have been calculated using Equations (13) and (14). Two factors play an important role in the generated emissions: the combustor inlet pressure and the relative fuel savings (per megawatt of produced power). Figure 5b demonstrates the variation of CO and NOx emissions relative to the conventional gas turbine system. As it is seen, the ratio of pollutants per megawatt are lower than those of the conventional system for most ambient temperatures studied. At lower ambient temperatures, the proposed system results in a marginal increase of the CO emissions due to the decrease of the inlet temperature of the turbine (constant maximum power of GT and lower combustor inlet temperature). However, the NOx and CO emissions reduce at higher temperatures by about 1% and 2%. Considering 8000 h of GT operating per year and average ambient temperature of 25 °C, the overall fossil fuel savings The generation of the air pollutants CO and NOx per megawatt have been calculated using Equations (13) and (14). Two factors play an important role in the generated emissions: the combustor inlet pressure and the relative fuel savings (per megawatt of produced power). Figure 5b demonstrates the variation of CO and NOx emissions relative to the conventional gas turbine system. As it is seen, the ratio of pollutants per megawatt are lower than those of the conventional system for most ambient temperatures studied. At lower ambient temperatures, the proposed system results in a marginal increase of the CO emissions due to the decrease of the inlet temperature of the turbine (constant maximum power of GT and lower combustor inlet temperature). However, the NOx and CO emissions reduce at higher temperatures by about 1% and 2%. Considering 8000 h of GT operating per year and average ambient temperature of 25 ◦C, the overall fossil

As mentioned before, the required fuel of the gas turbine determines the recovered energy and the mass flow of injected air. Figure 6a illustrates the variation of fuel flow in the V94.2 gas turbine versus the ambient temperature. The pressure and temperature of the gas transmission pipelines are assumed to be 60 bar and 25 °C, respectively. The required fuel mass flow decreases as the ambient temperature increases due to the control system of the GT that maintains the inlet temperature of the GT constant. Injecting highpressure air into the combustion chamber increases the mass flow of the exhaust and, subsequently, the required fuel. As shown in Figure 6a, the proposed system does not improve the performance at lower ambient temperatures, due to mechanical limitations of the GT. However, at lower ambient temperatures, the constant power of the GT and the increasing exhaust mass flow result in a decrease in the fuel mass. In other words, the GT control system decreases the TIT to maintain the power constant at lower temperatures that results in higher efficiencies. The high pressure of roughly around 9 kg/s of fuel can be recovered and used to inject about 3–4 kg/s of air into the combustion chamber. This amount of air is less than 0.8% of the air flow of the GT, and hence, has no drawback on

tively.

the stability of the gas turbine.

fuel savings and CO<sup>2</sup> emission reductions are estimated at about 1600 and 4800 tons per year, respectively.

As mentioned before, the required fuel of the gas turbine determines the recovered energy and the mass flow of injected air. Figure 6a illustrates the variation of fuel flow in the V94.2 gas turbine versus the ambient temperature. The pressure and temperature of the gas transmission pipelines are assumed to be 60 bar and 25 ◦C, respectively. The required fuel mass flow decreases as the ambient temperature increases due to the control system of the GT that maintains the inlet temperature of the GT constant. Injecting high-pressure air into the combustion chamber increases the mass flow of the exhaust and, subsequently, the required fuel. As shown in Figure 6a, the proposed system does not improve the performance at lower ambient temperatures, due to mechanical limitations of the GT. However, at lower ambient temperatures, the constant power of the GT and the increasing exhaust mass flow result in a decrease in the fuel mass. In other words, the GT control system decreases the TIT to maintain the power constant at lower temperatures that results in higher efficiencies. The high pressure of roughly around 9 kg/s of fuel can be recovered and used to inject about 3–4 kg/s of air into the combustion chamber. This amount of air is less than 0.8% of the air flow of the GT, and hence, has no drawback on the stability of the gas turbine. *Sustainability* **2021**, *13*, x FOR PEER REVIEW 11 of 17

**Figure 6.** Variation of the (**a**) consumed fuel and the (**b**) power output of the gas turbine with and without air injection at various ambient temperatures. **Figure 6.** Variation of the (**a**) consumed fuel and the (**b**) power output of the gas turbine with and without air injection at various ambient temperatures.

Injecting high-pressure air into the combustion chamber can enhance the performance of the gas turbine system. As shown in Figure 6b, recovering the available fuel energy in the studied V94.2 gas turbine increases the output power by approximately 2.5 MW for a wide range of ambient temperatures, and similarly, the efficiency can increase by about 0.25%. At lower ambient temperatures (about 5 °C), air injection has no major impact on the gas turbine power due to GT mechanical and maximum power limitations, but it still improves the efficiency by somewhat decreasing the required fuel flow. Although here, one gas reducing station is included in the analysis, more than one station usually exists in real power plants. Therefore, in most real cases, more high-pressure air can be generated for injection into the combustion chamber. The potential energy recovery from gas can thus provide the required energy to compress 3% to 5% more air into the Injecting high-pressure air into the combustion chamber can enhance the performance of the gas turbine system. As shown in Figure 6b, recovering the available fuel energy in the studied V94.2 gas turbine increases the output power by approximately 2.5 MW for a wide range of ambient temperatures, and similarly, the efficiency can increase by about 0.25%. At lower ambient temperatures (about 5 ◦C), air injection has no major impact on the gas turbine power due to GT mechanical and maximum power limitations, but it still improves the efficiency by somewhat decreasing the required fuel flow. Although here, one gas reducing station is included in the analysis, more than one station usually exists in real power plants. Therefore, in most real cases, more high-pressure air can be generatedfor injection into the combustion chamber. The potential energy recovery from gas can thusprovide the required energy to compress 3% to 5% more air into the combustion chamber.

combustion chamber. It is estimated that in conventional pressure-reducing stations, roughly up to 40% of the energy of the consumed fuel can be recovered to supply high-pressure air. Since air injection can result in a decline of the surge margin, OEM recommends air injection with a mass flow lower than 3% of the compressor's inlet flow [39]. The impact on power and efficiency of the amount of injected air into the V94.2 gas turbine is shown in Figure 7. It is seen that adding 1% more air into the combustion chamber can increase the power and It is estimated that in conventional pressure-reducing stations, roughly up to 40% of the energy of the consumed fuel can be recovered to supply high-pressure air. Since airinjection can result in a decline of the surge margin, OEM recommends air injection with a mass flow lower than 3% of the compressor's inlet flow [39]. The impact on power and efficiency of the amount of injected air into the V94.2 gas turbine is shown in Figure 7. It is seen that adding 1% more air into the combustion chamber can increase the power and the efficiency by about 2% and 0.75%, respectively. The addition of compressed air into

the GT leads to a slightly higher compressor pressure ratio (Figure 8a). A 3% air-injection ratio increases the compressor pressure ratio by about 3%. Although the temperature of the inlet fuel of the turboexpander affects the outlet pressure, it plays a minor role and can

**Figure 7.** Performance variation of the V94.2 gas turbine with air-injection ratio.

0.5 1 2 3

Ratio of air injection to compressor inlet flow (%)

be considered negligible.

Power Efficiency

Power & Efficiency

enhancement (%)

various ambient temperatures.

0 5 10 15 20 25 30

Fuel flow with injected Air Fuel flow w/o injected air

Ambient Temperature (C)

8.6 8.8 9 9.2 9.4 9.6 9.8 10

Fuel mass flow (kg/s)

the GT leads to a slightly higher compressor pressure ratio (Figure 8a). A 3% air-injection ratio increases the compressor pressure ratio by about 3%. Although the temperature of the inlet fuel of the turboexpander affects the outlet pressure, it plays a minor role and can be considered negligible. the efficiency by about 2% and 0.75%, respectively. The addition of compressed air into the GT leads to a slightly higher compressor pressure ratio (Figure 8a). A 3% air-injection ratio increases the compressor pressure ratio by about 3%. Although the temperature of the inlet fuel of the turboexpander affects the outlet pressure, it plays a minor role and can be considered negligible.

Injecting high-pressure air into the combustion chamber can enhance the performance of the gas turbine system. As shown in Figure 6b, recovering the available fuel energy in the studied V94.2 gas turbine increases the output power by approximately 2.5 MW for a wide range of ambient temperatures, and similarly, the efficiency can increase by about 0.25%. At lower ambient temperatures (about 5 °C), air injection has no major impact on the gas turbine power due to GT mechanical and maximum power limitations, but it still improves the efficiency by somewhat decreasing the required fuel flow. Although here, one gas reducing station is included in the analysis, more than one station usually exists in real power plants. Therefore, in most real cases, more high-pressure air can be generated for injection into the combustion chamber. The potential energy recovery from gas can thus provide the required energy to compress 3% to 5% more air into the

Power

0 5 10 15 20 25 30 35 40 45

Enhanced GT Base GT

Efficiency

Ambient Temperature (C)

32 32.5 33 33.5 34 34.5 35 35.5

GT Efficiency (%)

It is estimated that in conventional pressure-reducing stations, roughly up to 40% of the energy of the consumed fuel can be recovered to supply high-pressure air. Since air injection can result in a decline of the surge margin, OEM recommends air injection with a mass flow lower than 3% of the compressor's inlet flow [39]. The impact on power and efficiency of the amount of injected air into the V94.2 gas turbine is shown in Figure 7. It is seen that adding 1% more air into the combustion chamber can increase the power and

*Sustainability* **2021**, *13*, x FOR PEER REVIEW 12 of 17

combustion chamber.

*Sustainability* **2021**, *13*, x FOR PEER REVIEW 11 of 17

 (**a**) (**b**) **Figure 6.** Variation of the (**a**) consumed fuel and the (**b**) power output of the gas turbine with and without air injection at

GT Power (MW)

**Figure 7.** Performance variation of the V94.2 gas turbine with air-injection ratio. **Figure 7.** Performance variation of the V94.2 gas turbine with air-injection ratio.

**Figure 8.** Variation of the (**a**) GT pressure ratio and the (**b**) inlet temperature of the turbine versus the air-injection ratio with constant power output. **Figure 8.** Variation of the (**a**) GT pressure ratio and the (**b**) inlet temperature of the turbine versus the air-injection ratio with constant power output.

As seen in Figure 8b, air injection may be used to reduce the inlet temperature of the turbine as well. Turbine inlet temperature reduction has a great impact on extending the lifetime of gas turbines and increases the maintenance intervals and the overall GT life cycle costs. Approximately, adding 1% extra air into the combustion chamber may result in a 12 °C reduction of the TIT keeping the power output constant. As seen in Figure 8b, air injection may be used to reduce the inlet temperature of the turbine as well. Turbine inlet temperature reduction has a great impact on extending the lifetime of gas turbines and increases the maintenance intervals and the overall GT life cycle costs. Approximately, adding 1% extra air into the combustion chamber may result in a 12 ◦C reduction of the TIT keeping the power output constant.

As mentioned, the amount of energy that can be recovered by the turboexpander depends on various parameters including the expander pressure ratio and the temperature of the fuel at the inlet of the expander. Figure 9a shows the power produced with the turboexpander based on the expander's operating parameters. As mentioned, the amount of energy that can be recovered by the turboexpander depends on various parameters including the expander pressure ratio and the temperature of the fuel at the inlet of the expander. Figure 9a shows the power produced with the turboexpander based on the expander's operating parameters.

25 °C 0.2 0.25 0.3 0.35 0.4 0.45 Air Injection mass flow per kg fuel 25 °C 50 °C The amount of compressed air that can be supplied to the gas turbine can be estimated by considering the power output of the turboexpander in conjunction to the air compressor. As seen in Figure 9b, the mass flow of compressed air is directly related to both the working pressure ratio and the inlet temperature of the turboexpander. To compare the two systems with and without the air-injection unit, a component-level exergy analysis was performed, and the results are presented in Table 6. As seen, with the proposed modification of the gas turbine, exergy efficiency increases by approximately 0.36%. In addition, the exergy destruction of the gas turbine with the turboexpander system is approximately 2 MW lower than that of the gas turbine without the turboexpander.

The amount of compressed air that can be supplied to the gas turbine can be estimated by considering the power output of the turboexpander in conjunction to the air compressor. As seen in Figure 9b, the mass flow of compressed air is directly related to both the working pressure ratio and the inlet temperature of the turboexpander. To compare the two systems with and without the air-injection unit, a component-level exergy analysis was performed, and the results are presented in Table 6. As seen, with the proposed modification of the gas turbine, exergy efficiency increases by approximately 0.36%. In addition, the exergy destruction of the gas turbine with the turboexpander system is approximately 2 MW lower than that of the gas turbine without the turboexpander.

1.5 2 2.5 3 3.5 4

Turbocompressor pressure ratio

75 °C 100 °C

0.1 0.15

50 °C 75 °C 100 °C

 (**a**) (**b**) **Figure 9.** Effect of the turbocompressor pressure ratio and the fuel temperature on (**a**) the recoverable work and (**b**) the

amount of compressed air per kg of fuel.

1.5 2 2.5 3 3.5 4

Turbocompressor pressure ratio

Recoverable work per kg fuel (kw)

with constant power output.

0.5 1 1.5 2 2.5 3

Ratio of air injection to compressor inlet flow (%) −40

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50

Compressor outlet pressure

increase (%)

 (**a**) (**b**) **Figure 8.** Variation of the (**a**) GT pressure ratio and the (**b**) inlet temperature of the turbine versus the air-injection ratio

**Turbine Inlet Temperature** 

**Reduction (°C)**

−35 −30 −25 −20 −15 −10 −5 0

in a 12 °C reduction of the TIT keeping the power output constant.

turboexpander based on the expander's operating parameters.

As seen in Figure 8b, air injection may be used to reduce the inlet temperature of the turbine as well. Turbine inlet temperature reduction has a great impact on extending the lifetime of gas turbines and increases the maintenance intervals and the overall GT life cycle costs. Approximately, adding 1% extra air into the combustion chamber may result

0.0 0.5 1.0 1.5 2.0 2.5 3.0

**Ratio of air injection to compressor inlet flow (%)**

As mentioned, the amount of energy that can be recovered by the turboexpander depends on various parameters including the expander pressure ratio and the temperature of the fuel at the inlet of the expander. Figure 9a shows the power produced with the

**Figure 9.** Effect of the turbocompressor pressure ratio and the fuel temperature on (**a**) the recoverable work and (**b**) the amount of compressed air per kg of fuel. **Figure 9.** Effect of the turbocompressor pressure ratio and the fuel temperature on (**a**) the recoverable work and (**b**) the amount of compressed air per kg of fuel.


The amount of compressed air that can be supplied to the gas turbine can be estimated by considering the power output of the turboexpander in conjunction to the air **Table 6.** Exergy efficiency and destruction of each component. **Table 6.** Exergy efficiency and destruction of each component.

The Sankey diagram of exergy flows can provide important information of the operation of an energy system. The Sankey diagram showing the distribution of exergy flows of the proposed system is presented in Figure 10. In this diagram, the exergy destruction flows are shown in red. As seen, the exergy destruction of the GT systems accounts for about one third of the total exergy input to the turbine, mainly associated with irreversibilities within the combustion process. Moreover, the total exergy destruction of other components (including AIC, HX, and TE) is less than 1 MW. The Sankey diagram of exergy flows can provide important information of the operation of an energy system. The Sankey diagram showing the distribution of exergy flows of the proposed system is presented in Figure 10. In this diagram, the exergy destruction flows are shown in red. As seen, the exergy destruction of the GT systems accounts for about one third of the total exergy input to the turbine, mainly associated with irreversibilities within the combustion process. Moreover, the total exergy destruction of other components (including AIC, HX, and TE) is less than 1 MW.

vary largely with the fuel transmission pressure. Specifically, it is found that the sum of exergy destruction of these three components increases from 678 to 1266 kW for fuel pressures from 40 to 90 bar. The bar diagram in Figure 11a presents the ratio of exergy destruction of these components of the proposed system with the fuel feed pressure. Figure 11a shows that the exergy destruction of the turboexpander increases with increasing fuel pressure, while the exergy destruction of the heat exchanger presents the opposite trend.

**Figure 10.** Exergy flow Sankey diagram of the turboexpander air-injection system. **Figure 10.** Exergy flow Sankey diagram of the turboexpander air-injection system.

The total exergy destruction of the air compressor, heat exchanger, and expander vary largely with the fuel transmission pressure. Specifically, it is found that the sum of exergy destruction of these three components increases from 678 to 1266 kW for fuel pressures from 40 to 90 bar. The bar diagram in Figure 11a presents the ratio of exergy destruction of these components of the proposed system with the fuel feed pressure. Figure 11a shows that the exergy destruction of the turboexpander increases with increasing fuel pressure, while the exergy destruction of the heat exchanger presents the opposite trend. *Sustainability* **2021**, *13*, x FOR PEER REVIEW 14 of 17

**Figure 11.** (**a**) Variation of the exergy destruction ratio of components with the inlet pressure of the fuel; (**b**) variation of the exergy efficiency of the GT with ambient temperature, with and without air injection. **Figure 11.** (**a**) Variation of the exergy destruction ratio of components with the inlet pressure of the fuel; (**b**) variation of the exergy efficiency of the GT with ambient temperature, with and without air injection.

The exergy efficiency of the system with and without the turboexpander increases directly with the ambient temperature (Figure 11b). However, the efficiency enhancement of the proposed system is higher at elevated temperatures and varies from about 0.3% to 0.5% with increasing ambient temperature from 0 to 45 °C. Hence, at elevated ambient temperatures, this system shows a higher efficiency than at lower ambient temperatures. The exergy efficiency of the system with and without the turboexpander increases directly with the ambient temperature (Figure 11b). However, the efficiency enhancement of the proposed system is higher at elevated temperatures and varies from about 0.3% to 0.5% with increasing ambient temperature from 0 to 45 ◦C. Hence, at elevated ambient temperatures, this system shows a higher efficiency than at lower ambient temperatures.

#### **5. Conclusions 5. Conclusions**

In this article, a turboexpander was introduced in a conventional high-pressure natural gas pressure-reduction station. The power recovered from the expansion of the natural gas was used to compress and introduce extra air into the combustion chamber of a heavy-duty gas turbine V94.2 of Siemens for performance enhancement. In this article, a turboexpander was introduced in a conventional high-pressure natural gas pressure-reduction station. The power recovered from the expansion of the natural gas was used to compress and introduce extra air into the combustion chamber of a heavy-duty gas turbine V94.2 of Siemens for performance enhancement.

The exergy analysis revealed that the exergy destruction of the gas turbine with the new turboexpander system is approximately 2 MW lower than that of the conventional system without a turboexpander. In other words, the proposed system results in an increase in the overall exergy efficiency of the gas turbine of approximately 0.36%. The recovery of the potential energy of the fuel led to an increase of the power output and efficiency of the gas turbine by 2.5 MW and 0.25%, respectively. In addition, the proposed system led to considerable fuel savings and reduced generated pollutants. Considering 8000 h of operating per year, annual fuel savings of at least 2 million cubic meters and an annual CO2 reduction of 4000–4800 tons (depending on site conditions) are estimated. Finally, the NOx and CO emissions of the system decrease by about 1% and 2%, respec-The exergy analysis revealed that the exergy destruction of the gas turbine with the new turboexpander system is approximately 2 MW lower than that of the conventional system without a turboexpander. In other words, the proposed system results in an increase in the overall exergy efficiency of the gas turbine of approximately 0.36%. The recovery of the potential energy of the fuel led to an increase of the power output and efficiency of the gas turbine by 2.5 MW and 0.25%, respectively. In addition, the proposed system led to considerable fuel savings and reduced generated pollutants. Considering 8000 h of operating per year, annual fuel savings of at least 2 million cubic meters and an annual CO<sup>2</sup> reduction of 4000–4800 tons (depending on site conditions) are estimated. Finally, the NOx and CO emissions of the system decrease by about 1% and 2%, respectively.

tively. Overall, it was shown that a single-shaft gas turbine can benefit from this hybridization not only as a strategy to increase the output power and efficiency of the gas turbine but also as an innovative way to recover energy and reduce the required fuel and emissions. It is noteworthy that this hybrid system results in better performance at higher ambient temperatures, when compared to the conventional gas turbine. The amount of recoverable work depends on the fuel feeding line and pressure ratio of the turboexpander. It is estimated that in conventional pressure-reducing stations, roughly up to 40% of the energy of the consumed fuel can be recovered. This power can be used to supply high-Overall, it was shown that a single-shaft gas turbine can benefit from this hybridization not only as a strategy to increase the output power and efficiency of the gas turbine but also as an innovative way to recover energy and reduce the required fuel and emissions. It is noteworthy that this hybrid system results in better performance at higher ambient temperatures, when compared to the conventional gas turbine. The amount of recoverable work depends on the fuel feeding line and pressure ratio of the turboexpander. It is estimated that in conventional pressure-reducing stations, roughly up to 40% of the energy of the consumed fuel can be recovered. This power can be used to supply high-pressure air. However, consulting with the gas turbine manufacturer is recommended for injecting air

injecting air with a flow rate higher than 3% the compressor's inlet flow. Another important point is that the proposed system can be used in gas turbines to lower the inlet temperature of the turbine by at least 10 degrees to extend the lifetime of gas turbine parts

when more power is not required.

with a flow rate higher than 3% the compressor's inlet flow. Another important point is that the proposed system can be used in gas turbines to lower the inlet temperature of the turbine by at least 10 degrees to extend the lifetime of gas turbine parts when more power is not required.

**Author Contributions:** Investigation, A.R.S.; resources, A.R.S.; data curation, A.R.S.; writing original draft preparation, A.R.S.; software, A.R.S.; visualization, A.R.S.; validation, A.R.S.; formal analysis, A.R.S.; conceptualization, A.R.S., R.S.; methodology, A.R.S., R.S., F.P.; writing—review and editing, A.R.S., R.S., F.P.; supervision, R.S., F.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Reza Shirmohammadi would like to acknowledge the Erasmus + International Credit Mobility (KA107-2020 project), Alianza 4 Universidades, and International Affairs at University of Tehran and Carlos III University of Madrid. Fontina Petrakopoulou would like to thank the Spanish Ministry of Science, Innovation and Universities and the Universidad Carlos III de Madrid (Ramón y Cajal Programme, RYC-2016-20971).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Abbreviations**



#### **References**


## *Article* **Assessment of the Greek National Plan of Energy and Climate Change—Critical Remarks**

**Efthimios Zervas 1,\* , Leonidas Vatikiotis <sup>1</sup> , Zoe Gareiou <sup>1</sup> , Stella Manika <sup>1</sup> and Ruth Herrero-Martin <sup>2</sup>**


**\*** Correspondence: zervas@eap.gr; Tel.: +30-2610-367-566

**Abstract:** The Greek National Energy and Climate Plan was validated by the Greek Governmental Committee of Economic Policy on 23 December 2019. The decisions included in this plan will have a significant impact on the Greek energy mix as the production of electricity from lignite combustion ceases in 2028, when lignite will be replaced by natural gas (NG) and renewable energy sources (RES). This work presents an assessment of the Greek National Energy and Climate Plan by analyzing its pros and cons. The main critiques made are focused on the absence of risk analysis and alternative scenarios, the proposed energy mix, the absence of other alternatives on the energy mix and energy storage, the low attention given to energy savings (transport, buildings), the future energy prices, and the economic and social impacts. This analysis shows that delaying this transition for some years, to better prepare it by taking into consideration the most sustainable paths for that transition, such as using more alternatives, is the best available option today.

**Keywords:** climate change; energy and climate plan; energy price; energy transition; Greece; lignite; natural gas; RES

#### **1. Introduction**

Climate change is one of the main current environmental problems [1] and its mitigation requires a great effort from scientists to find adapted solutions, from policy makers to find adapted policy measures, and from different stakeholders to apply them. One of these measures is the transition from the production of electricity via coal combustion to more efficient or renewable energy sources.

Coal was and is still today one of the major sources for electricity production in Europe, as it accounts for 22.9% of the total final energy production in EE27 in 2017 [2]. However, the target set by the European Green Deal is to decrease greenhouse gas emissions by 40% in EE27 in 2030, compared to the 1990 emissions [3], and to reach climate neutrality (80–95%) in the EE countries in 2050 [4]. In that sense, coal's participation in the EE energy mix has to decrease to 12% by 2050, with the complete elimination of oil as a power-generating source [5]. To achieve these goals, several countries set up measures to decrease coal's participation in their energy mix. Greece is one of them, as electrical energy production from lignite was 29.3% in 2019 [6].

All EU countries recently released National Energy and Climate Plans [7]. The Greek plan [8] sets the goal of greenhouse gas emissions in Greece for 2030 and the main actions proposed to achieve this goal. It is an important milestone for the current national policy on energy and climate, as it sets out climate goals at the heart of development policy in Greece and actions to protect the environment. This plan sets several very ambitious goals. However, the Greek NECP is one of the most critical for several reasons: Greece was heavily impacted by the recent economic crisis, and for this reason both economic growth and available funds are limited; also, Greece is very dependent on local lignite

**Citation:** Zervas, E.; Vatikiotis, L.; Gareiou, Z.; Manika, S.; Herrero-Martin, R. Assessment of the Greek National Plan of Energy and Climate Change—Critical Remarks. *Sustainability* **2021**, *13*, 13143. https:// doi.org/10.3390/su132313143

Academic Editors: Georgios Tsantopoulos and Evangelia Karasmanaki

Received: 30 September 2021 Accepted: 22 November 2021 Published: 27 November 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

production and imported oil; thus, the radical change of the energy mix in just a few years is very challenging.

The aim of this work is to address the points of this plan that could pose some issues for the future, and to serve as a guideline for other European NECPs in the cases of similar issues. This work follows the general structure of several similar policy works on energy regulations, policies, or research agendas that can be found in the literature [9–13]. It should be noted that this work assesses some major points in a high-level critique of this plan and that this work is clearly of an application nature. For each one of these points, a specific analysis will be conducted to precisely quantify the economic, social, and environmental pros and cons. These detailed analyses will be presented in future dedicated works.

#### **2. Presentation of the Mains Points of the Greek National Energy and Climate Change Plan**

The purpose of the NECP is described in detail in the introduction of the plan (published on the website for the Greek Ministry of Environment and Energy [14]). More specifically, it is stated that the National Plan for Energy and Climate is, for the Greek government, a strategic plan for the issues of climate and energy. A detailed roadmap for the achievement of specific energy and climate objectives by 2030 is given. The NECP presents and analyzes policy priorities and measures in a wide range of development and economic activities for the benefit of Greek society, making this text a reference for the next decade.

It is noted that the NECP is a tool for national policy in the field of energy and for the mitigation of climate change, which highlights the priorities of, and development opportunities in, Greece. The aim of the NECP is to be the main tool for the establishment of national economic, energy, and climate policies over the next decade, taking into account the recommendations of the European Commission and the UN's Sustainable Development Goals. The strategic goals of the Greek government in the field of energy and climate are set until 2030, and they aim to contribute decisively to the necessary energy transition in the most competitive way for the national economy, achieving a drastic reduction in greenhouse gas emissions. In this way, Greece can emerge as one of the member states with the most ambitious climate and energy goals through a comprehensive and coherent program of measures and policies for both 2030 and 2050.

The NECP sets specific targets for greenhouse gas emissions from Greece in 2030 and determines the Greek energy mix until that year by increasing the participation of natural gas and renewable energy sources [8], as well as by increasing energy efficiency. At the same time, the NECP identifies the policies and measures that are necessary for achieving those goals, analyzes the evolution of the Greek energy system until 2030, and reports the investment needs and the different impacts on society, the economy, and the environment. In summary, the objectives of this plan [8] are the following:


#### **3. Methodology**

The Greek National Energy and Climate Plan is analyzed from two points of view: first, the general concept of this plan is analyzed; then, a specific analysis is conducted for each one of its parts. For every one of these points, the general methodological approaches are analyzed and the specific problematic points are revealed. Concrete proposals are

formulated for these approaches and points. Then, an analysis of the specific parts of the plan is performed; as previously, the general methodological approaches are analyzed and the specific problematic points are revealed, followed by concrete proposals. In both cases, alternatives are suggested for several points, following a high level of argumentation.

More specifically, the critiques analyzed in this article concern the application of a single scenario and the absence of updating and alternatives, the de-lignification of energy production and the proposed future energy mix (natural gas and renewable energy sources), the absence of other alternatives for the energy mix/production (biomass, gasification of lignite, carbon storage, and energy storage), vehicle electrification and public transport, the energy savings in buildings, the local production of products and dietary habits, the cost of energy and its economic and social consequences, the impact on labor issues, employees education and training, the employees and citizen information, and the regional and sectoral plans.

Several proposals, based on the general needs of the Greek economy, the protection of the environment, and the mitigation of climate change are presented for the above issues. Their general feasibility is also shown. These proposals aim to address the economic development of Greece, the decrease of energy poverty, with the parallel respect of the environment and the mitigation of climate change. As mentioned above, only a high-level critique is addressed in this article; a specific and detailed study is necessary for each one of the proposals presented here, and these studies will be presented in the near future.

#### **4. Results—Assessment of the Plan**

#### *4.1. Implementation of a Single Scenario and the Absence of Alternative Scenarios*

The submitted National Energy and Climate Plan is, with a few exceptions, a linear implementation of a single scenario. However, it should be noted that the energy and climate sectors are highly unpredictable. Taking this high variability into consideration, the National Plan should include a risk assessment and multiple alternative scenarios. However, the possible cases of deviation from, or even the failure of, this linear implementation are not analyzed, and no alternative scenarios are given.

Obviously, scientific progress leads to better materials, more efficient technologies, and finally to a decrease in the cost of energy production. In the field of energy, many technological achievements have been developed in recent years, such as the extraction of marine shale gas [15] or new photovoltaic materials, such as the recent progress in chlorinated organic photovoltaic materials, the discovery of two-dimensional photovoltaic materials accelerated by machine learning, or their new applications, such as in highways for signal systems, for agricultural and livestock purposes due to the need for water during periods of intense sunshine, for charging car and boat batteries, etc. [16] However, while technology has a positive effect on energy production, either in terms of efficiency or price, geopolitical events can affect the price and availability of energy in the opposite direction. The price of natural gas, which will be one of the major components of the future Greek energy mix, can be very significantly affected by these events in the future. The relationship between the geopolitical situation and energy is interdependent, as geopolitical changes can affect energy markets and, conversely, energy trends can disrupt geopolitical dynamics [17].

The 1973 oil crisis is the easiest example. Geopolitically, the Middle East, an important oil-producing region, is a politically unstable region with two ongoing conflicts, one in Iraq and one in Syria. Additionally, the political and economic developments in China have a global impact. Libya is another hotbed of instability in the close-to-Greece region; moreover, the relationship between Greece and Turkey is often very tense. Other global tensions, however, especially those concerning oil-producing countries (such as Iran or Venezuela), have a significant impact on energy prices.

Figure 1 shows the change in the price of crude oil for the period of 1946–2021. This figure shows a course of sharp changes and alternating increases and decreases in crude oil price; this price shows a range of variations, from less than USD 20 to near USD 180 during

*Sustainability* **2021**, *13*, x FOR PEER REVIEW 4 of 18

*Sustainability* **2021**, *13*, x FOR PEER REVIEW 4 of 18

Iran or Venezuela), have a significant impact on energy prices.

Iran or Venezuela), have a significant impact on energy prices.

this time period. This sharply changing picture is also reflected in Figure 2, which shows the changes in the price of crude oil during only the last two years (January 2020–October 2021). shows the changes in the price of crude oil during only the last two years (January 2020– October 2021). October 2021).

region; moreover, the relationship between Greece and Turkey is often very tense. Other global tensions, however, especially those concerning oil-producing countries (such as

figure shows a course of sharp changes and alternating increases and decreases in crude oil price; this price shows a range of variations, from less than USD 20 to near USD 180 during this time period. This sharply changing picture is also reflected in Figure 2, which

Figure 1 shows the change in the price of crude oil for the period of 1946–2021. This

region; moreover, the relationship between Greece and Turkey is often very tense. Other global tensions, however, especially those concerning oil-producing countries (such as

figure shows a course of sharp changes and alternating increases and decreases in crude oil price; this price shows a range of variations, from less than USD 20 to near USD 180 during this time period. This sharply changing picture is also reflected in Figure 2, which shows the changes in the price of crude oil during only the last two years (January 2020–

Figure 1 shows the change in the price of crude oil for the period of 1946–2021. This

**Figure 1.** Change in the price of crude oil from 2010 to today (source: https://www.macrotrends.net/1369/crude-oil-price-history-chart, (accessed on 5 September 2021)). **Figure 1.** Change in the price of crude oil from 2010 to today (source: https://www.macrotrends. net/1369/crude-oil-price-history-chart, (accessed on 5 September 2021)). **Figure 1.** Change in the price of crude oil from 2010 to today (source: https://www.macrotrends.net/1369/crude-oil-price-history-chart, (accessed on 5 September 2021)).

data: https://www.macrotrends.net/1369/crude-oil-price-history-chart, (accessed on 5 September 2021). **Figure 2.** Change in the price of crude oil in the period January2020–October 2021 (Source of the data: https://www.macrotrends.net/1369/crude-oil-price-history-chart, (accessed on 5 September **Figure 2.** Change in the price of crude oil in the period January2020–October 2021 (Source of the data: https://www.macrotrends.net/1369/crude-oil-price-history-chart, (accessed on 5 September 2021).

While unexpected health crises, such as the current crisis of COVID-19, along with new technological advances and geopolitical developments play an important role in the global supply chain and the energy markets [18], it is a fact that the situation in the international market becomes even more unpredictable given the uncharted course of the coronavirus, the announcements about mutations, the preventive measures taken, as well as 2021). While unexpected health crises, such as the current crisis of COVID-19, along with new technological advances and geopolitical developments play an important role in the global supply chain and the energy markets [18], it is a fact that the situation in the inter-While unexpected health crises, such as the current crisis of COVID-19, along with new technological advances and geopolitical developments play an important role in the global supply chain and the energy markets [18], it is a fact that the situation in the international market becomes even more unpredictable given the uncharted course of the coronavirus, the announcements about mutations, the preventive measures taken, as well as the course

navirus, the announcements about mutations, the preventive measures taken, as well as

fossil fuels.

of vaccinations. These conditions directly determine consumption, which, in turn, affects the supply–demand balance and the course of prices. the course of vaccinations. These conditions directly determine consumption, which, in turn, affects the supply–demand balance and the course of prices. pact of these events on the price of crude oil.

the course of vaccinations. These conditions directly determine consumption, which, in

years, indicating some major global political events [[19]. This figure shows the great im-

Figure 3 shows a well-known figure of the evolution of crude oil prices the last 20

*Sustainability* **2021**, *13*, x FOR PEER REVIEW 5 of 18

turn, affects the supply–demand balance and the course of prices.

Figure 3 shows a well-known figure of the evolution of crude oil prices the last 20 years, indicating some major global political events [19]. This figure shows the great impact of these events on the price of crude oil. Figure 3 shows a well-known figure of the evolution of crude oil prices the last 20 years, indicating some major global political events [[19]. This figure shows the great impact of these events on the price of crude oil.

**Figure 3.** Evolution of crude oil price during the last 20 years. The major political events are indicated (source: [https://doi.org/10.1016/j.ribaf.2020.101357 ], (accessed on 24 November 2021)). **Figure 3.** Evolution of crude oil price during the last 20 years. The major political events are indicated (source: [https: //doi.org/10.1016/j.ribaf.2020.101357 ], (accessed on 24 November 2021)). cated (source: [https://doi.org/10.1016/j.ribaf.2020.101357 ], (accessed on 24 November 2021)).

Recent research has shown the correlation between daily new cases or total number of deaths due to communicable diseases and adverse stock returns in the Chinese equity market [20] and presented the negative, direct, and indirect effects of COVID-19 on global markets [21]. Meanwhile, even the way that COVID-19 is covered by the media, or that panic is caused by them, seems to affect stock market volatility [22]. Recent research has shown the correlation between daily new cases or total number of deaths due to communicable diseases and adverse stock returns in the Chinese equity market [20] and presented the negative, direct, and indirect effects of COVID-19 on global markets [21]. Meanwhile, even the way that COVID-19 is covered by the media, or that panic is caused by them, seems to affect stock market volatility [22]. of deaths due to communicable diseases and adverse stock returns in the Chinese equity market [20] and presented the negative, direct, and indirect effects of COVID-19 on global markets [21]. Meanwhile, even the way that COVID-19 is covered by the media, or that panic is caused by them, seems to affect stock market volatility [22].

Recent research has shown the correlation between daily new cases or total number

In addition to the previous figures, Figure 4 shows the index of prices for the three In addition to the previous figures, Figure 4 shows the index of prices for the three fossil fuels. In addition to the previous figures, Figure 4 shows the index of prices for the three fossil fuels.

The above figures show the instability and uncertainty of the price of fuels; however, the linear application of only one scenario cannot take into consideration these changes. **Figure 4.** Price index for the three fossil fuels (source of data: https://ourworldindata.org/fossil-fuels, (accessed on 05/09/2021)). **Figure 4.** Price index for the three fossil fuels (source of data: https://ourworldindata.org/fossilfuels, (accessed on 5 September 2021)).

the linear application of only one scenario cannot take into consideration these changes.

alities, such as:

The above figures show the instability and uncertainty of the price of fuels; however, the linear application of only one scenario cannot take into consideration these changes. At the same time, the Intergovernmental Panel on Climate Change (IPCC) has recognized that many parameters affect the emissions of greenhouse gas and, therefore, there

At the same time, the Intergovernmental Panel on Climate Change (IPCC) has recognized that many parameters affect the emissions of greenhouse gas and, therefore, there is considerable uncertainty for the prediction of future emissions. For this reason, six groups of greenhouse gas emission scenarios have been proposed (A1T, A1B, A1F1, A2, B1, and B2) with a total of 40 scenarios, the implementation of which give different future temperatures in the Earth's atmosphere (Figure 5) [23]. is considerable uncertainty for the prediction of future emissions. For this reason, six groups of greenhouse gas emission scenarios have been proposed (A1T, A1B, A1F1, A2, B1, and B2) with a total of 40 scenarios, the implementation of which give different future temperatures in the Earth's atmosphere (Figure 5) [23].

*Sustainability* **2021**, *13*, x FOR PEER REVIEW 6 of 18

**Figure 5.** Index of carbon dioxide emissions produced by energy and industry for the various IPCC scenarios (1990 = 1) (source: [23]). **Figure 5.** Index of carbon dioxide emissions produced by energy and industry for the various IPCC scenarios (1990 = 1) (source: [23]).

Based on the previous analysis, two additional works must be conducted to complement the NECP. The first one concerns the inclusion of alternative scenarios to cover the probability that unpredictable factors inhibit the implementation of the initial planning. Based on the previous analysis, two additional works must be conducted to complement the NECP. The first one concerns the inclusion of alternative scenarios to cover the probability that unpredictable factors inhibit the implementation of the initial planning. The alternative scenarios should take into account both the positive and negative eventualities, such as:


demic or a major natural disaster in Greece, e.g., an earthquake), changes in institutional, economic, or social parameters, etc.; • Geopolitical issues that may affect the availability of an uninterrupted gas supply in Greece, or a great change in energy prices, etc. The second necessary work is to have a specific provision for the regular reporting of the progress of the plan's actions and, based on this reporting, the establishment of an annual update of the objectives of the plan, of the policies to be set, and of the actions to be implemented. It should be noted that this regular reporting and the regular update of the objectives of the plan are not mentioned in the current plan.

#### *4.2. De-Lignification and Energy Mix*

*4.2. De-Lignification and Energy Mix*

The second necessary work is to have a specific provision for the regular reporting of the progress of the plan's actions and, based on this reporting, the establishment of an annual update of the objectives of the plan, of the policies to be set, and of the actions to be implemented. It should be noted that this regular reporting and the regular update of The plan proposes the complete de-lignification of the country in 2028 and the increase of the participation of natural gas and RES in Greece's energy mix. This option has some positive and some negative points. The main positive point is the reduction of greenhouse gas production. However, this advantage is not so obvious. Natural gas produces 201.96 kg

The plan proposes the complete de-lignification of the country in 2028 and the in-

crease of the participation of natural gas and RES in Greece's energy mix. This option has some positive and some negative points. The main positive point is the reduction of greenhouse gas production. However, this advantage is not so obvious. Natural gas produces 201.96 kg of CO2/MWh, compared to 363.6 kg of lignite [24]; or, put differently, natural gas produces 55% of the GHG produced by lignite. However, methane has a 100-year global warming potential, 25 times that of CO2, or 72 times for a 20-year period [25], and leaks of methane from the production, transport, and consumption of NG are significant [26]. For the above reasons, the gain in GHG emissions will be much smaller than initially

of CO2/MWh, compared to 363.6 kg of lignite [24]; or, put differently, natural gas produces 55% of the GHG produced by lignite. However, methane has a 100-year global warming potential, 25 times that of CO2, or 72 times for a 20-year period [25], and leaks of methane from the production, transport, and consumption of NG are significant [26]. For the above reasons, the gain in GHG emissions will be much smaller than initially estimated, as already reported [27,28]. The additional risk of accidents in the natural gas circuit also decreases this difference. In conclusion, leaks of 5% of methane can completely cancel this difference.

Aside from the previous statement, there are also some other issues about the proposed energy mix. The first one is the deterioration of the country's trade balance, both from the import of raw materials for the installation of RES and natural gas installations and also from imports of natural gas to replace a domestic product, lignite. The other negative point is the energy dependence of Greece on an imported fuel instead of a locally produced one, leading to higher risks of national energy autonomy.

It is clear that the deterioration of the country's trade balance will affect the entire Greek economy, and heavy actions should be proposed to mitigate this. However, the analysis of these actions and/or of alternative scenarios is not performed in this plan.

It should be also noted that countries that use natural gas to a large extent in their energy mix, such as the Netherlands, have decided to become independent of it in the coming years [29]. Taking this into consideration, natural gas can be considered only as a transition solution, and the cost of replacing lignite with natural gas, which will be also replaced in two or three decades, is not examined in the NECP.

The proposed energy mix also has several uncertain elements, such as the final price of the energy, which can be much higher than the current one, or the uncertainty of finding domestic hydrocarbon reservoirs, as the NECP estimates that the domestic fuel production will increase from 281ktoe in 2020 to 536ktoe in 2030. However, this last estimation is not consolidated, and no alternatives are examined in case of failure.

Even if a close economic analysis of the substitution of lignite by natural gas is out of the scope of this work, some elements are given here. The current cost of electricity production from lignite is EUR 105/MWh (EUR 35 is the direct cost and EUR 70 is the emission price) [30], while the corresponding cost for natural gas is EUR 75/MWh, and for RES is EUR 135.6/MWh [31].

However, these values, and the great instability of prices shown in Figures 1–4, indicate that the complete abandonment of domestic energy sources (lignite) and the use of only imported fossil fuels (oil and natural gas) can have a very high cost for the energy in Greece and, as a consequence, for the entire Greek economy.

Therefore, a more detailed examination of the possibility of continuing to exploit lignite, an available domestic fuel, for a longer time, instead of completely substituting it for imported natural gas (at least until the cost of energy produced from RES decreases significantly) is proposed. This can be done by using modern, more environmentally efficient technologies, by combining lignite combustion with biomass combustion or gasification technology, and using synthetic gas and, in addition, carbon storage technology, as will be exposed here.

#### *4.3. Biomass Combustion*

The development of more renewable energy sources started in a more systematic manner after the oil crisis of 1973. During this period, scientists adopted a systematic approach to energy and coined the term biomass [32]. Biomass is a renewable energy source because the CO<sup>2</sup> released from its combustion is bound to the plants for their development. Therefore, its use as a fuel can have a positive impact on the overall GHG balance. Due to this positive impact, the use of biomass as fuel has increased during the last years. In addition, biomass is abundant, which is why biomass energy has become the world's fourth largest energy source today, following coal, oil, and natural gas, indicating its significant economic, societal, and environmental potential [33].

In several countries affected by the economic crisis after 2008, or even in the case of citizens with economic hardship in economically developed countries, the shift in the use of biomass as a heating fuel is mainly due to its lower price, or to the ability to burn materials that had not previously found a suitable route of exploitation through domestic combustion. Several citizens of low economic status have used pruning or even organic waste as heating fuel during the last years [34,35].

However, the combustion of biomass has a significant negative impact: the high emission of pollutants, mainly of particulate matter (PM) [36]. The increased use of biomass for domestic heating in recent years has led to a very poor air quality in Greek and many European cities, especially in winter. This poor air quality has serious consequences for both quality of life and human health, and these consequences will strongly appear in the next years. The European Respiratory Society has already highlighted the serious effects of biomass-burning on human health in cities in developed countries, and recommends limiting its use [37]. However, the poor air quality comes from the domestic combustion of biomass, where no pollution control system exists. Central power plants using biomass and equipped with pollution-control systems are widely available. Biomass co-firing has already received wide acceptance in many European countries, mostly in the northern and central parts of Europe, such as the United Kingdom, Germany, and the Nordic countries [38]. From this point of view, the use of biomass combustion to produce electrical energy for central power plants can be a very serious alternative to lignite combustion. This alternative is not proposed in this plan. Very roughly, the following data can show the feasibility of this alternative.

The consumption of lignite in Greece is 4.5 million tons of oil equivalent [39] or 46 million tons of lignite [40]. The typical thermal power of wood is quite similar to that of lignite, of course depending on the wood type [41]. The total timber production in Greece was almost 1.1 million m<sup>3</sup> in 2013 [42]; considering an average density of wood of 600 kg/m<sup>3</sup> [41], almost 0.7 million tons of wood was produced in Greece in 2013. However, in other neighboring or European countries with an equivalent or smaller surface area than Greece, the production is several times higher: 6.1 million m<sup>3</sup> in Bulgaria, 5.5 in Croatia, 15.3 in the Czech Republic, 7.6 in Estonia, 7.0 in Lithuania, 6.0 in Hungary, 17.4 in Austria, 8.0 in Slovakia, etc. [42]. Moreover, the forest cover of Greece is about 3.9 million hectares, with 3.5 million available for wood production [43]. In addition to the previous data, 52 thousand acres of forests were burned in Greece in 2018. Considering a wood density of about 10 m3/acre (although, depending on the tree species, it can reach up to 40 m3/acre), the total volume of burned forests corresponds to 0.5–4 times the annual timber production in Greece [44,45]. The data for the forest fires of 2021 are even worse, as 1.55 million acres of the total forest area was burned in Greece [46], which corresponds to 4.2% of the total forest area (of 36.8 million acres) [47].

It is important to mention that funds allocated in 2021 for fire protection was only EUR 1,700,000, which corresponds to only 10% of the costs requested by the relevant institutions [48]. This indicates that, with a very small increase of this fund, a significantly decreased amount of forest fires will occur in the future, allowing for the better exploitation of forests for timber to be used as fuel, rather than being devastated by fires.

The above data and calculations are approximate and, of course, a more detailed analysis is necessary. However, the above data show that a ten-times increase in the total timber production in Greece in the coming years could be an achievable goal. This production could be specifically focused on the mountainous areas of Western Macedonia, where the majority of the lignite mines are found today, but also on the many mountainous/semimountainous areas of Greece that are currently bare of forests and could accommodate special fast-growing tree plantations. This amount could replace about 15% of the current lignite consumption and will have several advantages, such as:

• Zero contribution to the emission of CO2, because the CO<sup>2</sup> produced from biomass combustion is absorbed by the plants for their development;


The decentralized electricity production, i.e., the creation of plants in many areas, e.g., 1–2 per county, will be more efficient due to the shorter transportation distance of biomass. This will also have a positive impact on the control of particles emissions, as the pollution will not be emitted in the same area.

The above (approximate) analysis shows that this route, albeit complementary to the import of natural gas, should be better exploited. In this case, the future use of lignite, combined with carbon capture technologies, may be more advantageous for the Greek economy than the transition to natural gas. A comprehensive study is necessary to take into account all the pros and cons of this alternative. Of course, a comprehensive technical and economic study should be carried out, including the external costs of this alternative, ensuring that there are no major environmental nuisances or degradations [49].

The above analysis shows that the further exploitation of biomass, a domestic product, can replace lignite to a certain extent with zero-equivalent CO<sup>2</sup> emissions, and it is therefore proposed that this is taken into consideration.

#### *4.4. Gasification of Lignite*

The gasification of lignite and the production of fuel gas is another alternative; however, this alternative is not considered by this plan. This technology, used in several parts of the world in the past, is found in recession after the mid-20th century due to the high competitiveness of oil and gas prices, but is again on the rise because of the necessity to reduce greenhouse gas emissions [50].

It is therefore proposed that the use of this alternative technology is explored in more technological, economic, and environmental detail. The gaseous fuel could be used for electricity generation or in large central plants (industry, hot water production, etc.), or even be considered as an addition to the domestic natural gas network.

The above process could continue the use of lignite, in combination with the use of biomass and carbon capture technologies, and continue to produce energy with domestic raw materials, lower costs for the Greek economy, and lower CO<sup>2</sup> emissions. A comprehensive technical, economic, and environmental study is again necessary to take into consideration all the pros and cons of this alternative.

#### *4.5. CO<sup>2</sup> Storage*

The use of CO<sup>2</sup> storage technology is not mentioned in this plan and is proposed only at one point, concerning future research actions.

It is true that this technology has, so far, been used worldwide in a limited number of facilities [51]. However, many countries, such as Canada, or the Netherlands in the port of Amsterdam [52], invest significantly in this technology. The plan proposes a research action for this technology, but only after the end of lignite production. In this case, the implementation of carbon storage technology will be of very limited value.

An immediate examination of the technological and economic uses of this technology, in combination with the continued use of lignite and in biomass combustion, is proposed here. This alternative can possibly have lower CO<sup>2</sup> emissions than the use of natural gas combustion [53]. In addition, a combination of an existing domestic source (lignite) with a new one (biomass), and a combination of a mature technology and infrastructure (lignite combustion) with a new one (biomass combustion and carbon storage) will be used.

#### *4.6. Energy Storage*

It is well known that the production of electricity from RES does not necessarily go hand-in-hand with consumption. The highest production of energy from photovoltaics occurs during the sunshine hours of the day, falling to zero during evening hours, when a peak in consumption occurs. The energy produced by wind turbines is quite unstable, as it depends on the windy hours. Moreover, the distribution of winds in the year and in space is also of high variability. In contrast to that, energy production from thermal plants, using fossil fuels or biomass combustion, and from hydropower plants, can be adapted to energy consumption.

Therefore, in order to efficiently use the energy produced from certain RES, such as wind turbines and photovoltaics, it is necessary to store the energy produced during the low consumption hours in order to use it during the high consumption hours. The main available energy storage techniques are pumped storage hydropower (using the pumping of water from a reservoir of low elevation to one of high elevation during low consumption hours and then allowing the flow of water from the high to low reservoir for the production of electric energy during peak hours), batteries, and hydrogen [54].

This plan mentions storage in batteries or in gas production (e.g., hydrogen), without giving specific data, but pumped storage hydropower is not included. However, pump storage could be an efficient way of storing energy. In addition, the creation of new water reserves could be very beneficial for agricultural purposes. The storage of energy in batteries on the level of an entire country can be quite problematic, as the cost of these batteries may be too high. Moreover, the environmental consequences of this very high amount of necessary batteries are not negligible [55,56].

Therefore, it is proposed that the alternative of pumped storage hydropower, instead of the battery storage that is proposed in this plan, is developed.

#### *4.7. Vehicle Electrification*

One of the actions of the NECP to reduce the use of fossil fuels is to increase the electrification of vehicles. The plan presents an estimation for the development of electric mobility in Greece until 2030. However, the estimated numbers are rather high.

The total market for new passenger cars is projected to increase from 103,431 units in 2018 (reference year) to 275,133 units in 2030, which corresponds to an annual increase of 8–11%, which is rather high. It should be noted that sales of 280–320,000 units/year took place in Greece during the period of 2000–2006.

However, the economic development of that past period cannot be compared to the current economic situation of Greek households. In addition, the plan estimates that Greece's GDP will be approximately the same as in 2008, when the economic crisis started, only by 2030. It should be noted that the rest of the European countries will have recovered much earlier from this economic crisis and will be at much higher corresponding levels of GDP in 2030. Having gone through a severe economic crisis, with declining incomes, high unemployment, and a large exodus abroad, especially of young scientists, it is probably very difficult to have such a large increase in the market for new passenger cars. Additional components that support this argument are:


Moreover, the estimated rate of electric vehicle penetration (24–30% by 2030) may be overestimated. Electric vehicles, from almost non-existent today, are projected to have a quite-high penetration in 2030. Given the current available technology for electric cars (such as the number of kilometers that an electric vehicle can travel, sufficient for urban travel but not always for long-distance, or battery life, etc.), the necessary infrastructure to be created to recharge a car's battery, especially in public places, and the higher prices of electric cars compared to conventional vehicles, the above objective for the penetration of these cars may not be met so early. Several researchers already expressed their reservations about the announced rapid introduction of electric cars to the market [58]. It should also be noted that, as an additional difficulty, the battery-charging infrastructure of electric cars in public places is quite problematic in Greek cities, due to the general insufficient width of sidewalks and, moreover, to the high lack of parking availability in all Greek cities.

Due to the higher price of electric vehicles compared to conventional ones, high financial incentives for their purchase will probably be required, and this will be another additional charge for the Greek national budget and the Greek National Balance, as all these vehicles are imported.

For the above reasons, the existence of several scenarios with alternatives is more than necessary.

#### *4.8. Public Transportation*

Although there is a specific chapter in this plan on the electrification of passenger cars, the increase of public transportation in Greece is not taken into consideration. It is well known that the use of public transportation emits lower CO<sup>2</sup> emissions than the use of passenger cars [59], and this difference is even higher in a complete product life-cycle analysis, with all the external costs taken into consideration, since the total impact of policies or measures in the long term are unclear [60].

Greece has one of the lowest percentages of train-passenger kilometers, and the second lowest percentage of railways for the transportation of goods in the EU [61,62]. Moreover, Greece showed a very high decrease in the share of public transport in total passenger traffic, from 28% in 2000 to 18% in 2018 [62].

The shift to public transport is therefore of paramount importance, and this action should be immediately taken into consideration in this plan.

#### *4.9. Energy Savings*

The remarks concerning the energy savings are analyzed as a function of the type of the building: public administration buildings or residential buildings.

#### 4.9.1. Energy Savings in Public Administration Buildings

The plan provides an annual energy upgrade of 3% of the total surface of the buildings of the central public administration. Some facts should be mentioned here. The first is that the ages of Greek public administration buildings are quite high [63]. In addition, many of them are listed; therefore, they require a specific process for their restoration and the targeted energy results cannot be achieved easily. Moreover, the procedures for such upgrades in the public sector are very time-consuming. These facts show that the target of the energy upgrade of 3% of the total surface each year is very probably unattainable, at least during the first years. On the other hand, there is an urgent need to upgrade much more than one-third of the total buildings' area by 2030.

The above shows that it is initially necessary to radically review and accelerate the current procedures for the energy upgrade of public buildings, of course with the necessary protection of listed buildings, and set a more ambitious target. The energy-saving measures must be first implemented, as saving energy is one of the most efficient measures to decrease CO<sup>2</sup> emissions.

#### 4.9.2. Energy Savings in Residential Buildings

In Greece, there were about 3 million households and more than 6 million residences in 2019 [63]. More than 55% of these residences were built before 1980, i.e., they have very poor energy performance [63]. The plan proposes the energy upgrade of 60,000 residences per year, a number that corresponds to less than 10% of all residences by 2030. This percentage is obviously very small.

If the estimated economic growth is taken into account (the country's GDP will be by 2030 equal to that of 2008), it seems that the disposable income of citizens for energy upgrades will not be very high. This statement indicates that either there will be financial difficulty in upgrading many buildings, or that large public funds will be required for subsidizing this upgrade. For an estimated cost of EUR 10,000–15,000 for a residence of 80–100m<sup>2</sup> (depending on the climatic zone, the age of the residence, the exposure, etc.), the total cost will be more than EUR 60 billion. For comparison, the public revenues, spending, and the Program of Public Investment of the Greek state was, in 2019, EUR 53.02, 57.79, and 6.75 billion, respectively, and the GDP of Greece was EUR 192.75 billion in 2020 [64].

The plan also proposes an increase of the use of domestic natural gas. Taking into consideration the current coverage of Greece in the use of natural gas (only 5.4% of the residences used natural gas from 2011–2012 [65]) and the large and time-consuming projects required to increase the natural gas network, there are high reservations for the rapid penetration of the use of natural gas. Therefore, it is necessary to change the priorities and practices followed so far in order to achieve this goal. At this point, we can again express the previous comment concerning the choice to use domestic natural gas while other countries choose to abandon it.

#### *4.10. Local Production of Products, Dietary Habits*

There are several additional measures to decrease the emission of greenhouse gases. One of them is local/global production/consumption. The large penetration of globalization led to the high increase of the transportation of products on a global scale. However, a very effective action to decrease CO<sup>2</sup> emissions is to enhance the local production/consumption of products, as their transportation is significantly decreased. This policy has proven to be one of the most effective policies/actions to reduce greenhouse gas emissions from product transport (e.g., "food-kilometers") in the case of food transport [66].

However, this action is completely missing from this plan. In addition, this policy can strengthen the Greek economy and Greek businesses, and help with the creation of new jobs, especially in small cities or suburban areas. The dynamic integration of this policy is proposed in this plan. Specific policies and actions must be implemented immediately, as this policy will bring only positive results.

The food sector is responsible for a large proportion of greenhouse gas emissions, stemming from the production of primary food products, their process, transport, etc. [67]. Greece has an average consumption of 3353 kcal/cap/day calories in 2017, against the 2000 calories a day for women and 2500 for men that is recommended by the WHO [68]. Meat consumption in Greece is 76.7 Kg/cap/year [69], compared to 63.12 Kg/cap/year in

Europe in 2013 [70]. The decrease in extra calories and meat consumption can be two very efficient methods for the decrease in the greenhouse gas emissions of a country [67], with a very low cost and, moreover, with several other significant health advantages. However, there is no mention in this plan about these, or similar, alternative and low-cost measures for the decrease of greenhouse gases emissions.

#### *4.11. Energy Price, Social Impact, Energy Poverty*

The causes of energy poverty are low incomes, high fuel prices, and the poor thermal conditions of houses [71,72]. A total of 35% of Greeks have debts to energy bills (first place among the member countries of the European Union) and the percentage of Greeks who cannot keep their home warm is very high, ranking Greece in third place among the member states of the European Union [73].

It is, therefore, absolutely necessary that the actions of the plan should focus on mitigating energy poverty. However, there are some reservations about the effectiveness of the proposed actions. Reducing energy poverty requires either a high increase in income or a high decrease in the price of energy. However, the plan does not foresee either of those two options. Based on projected GDP growth in Greece, the Greek GDP will reach that of 2008, i.e., before the economic crisis, only in 2030.

The plan estimates a decrease in the price of RES in the coming years without providing more information about the final prices or about how this decrease will take place. It should be noted that it is very difficult to obtain the real cost of energy production of RES from official sources and, thus, this point cannot be verified.

It should be noted that the complete de-lignification and change of the energy mix of Greece in such a short period of time carries the risk of a significant increase in the final price of electricity. In recent years, from 2006 to 2017, there has been an increase of 28% in the price of electricity for medium-sized industries and 177% for households [74]. The current worldwide increases in fuel and energy prices are another example. Therefore, several reservations can be expressed about the announced decrease of energy prices in the coming years. In addition, the plan does not present alternative scenarios if the projected final energy price does not occur, nor possible actions or legal shields in case of speculative trends from the liberalized energy market.

Therefore, the decrease of energy poverty mentioned in the plan seems to be very difficult to achieve. It is proposed here that more generous, but also more specific measures to deal with this phenomenon are adopted. The control of household energy prices is the first of those measures.

Is should be noted that the environmental and social impacts from the energy transition to the main lignite area of Greece, that of Western Macedonia, are analyzed in another work [75].

#### *4.12. Education, Public Awareness*

The change of the energy mix of Greece with the shift to RES and natural gas, the gradual change of the fleet of vehicles to electric cars, and the actions of energy-upgrading buildings, etc., will bring significant changes in many technical professions directly related to the above issues.

It is obvious that not only these professions must be protected from any downgrading, but also that a substantial improvement of their role as well as their working conditions should take place. To upgrade these professions, the role of education and training should be enhanced, in addition to institutional and legislative upgrading. Education and training, both conventional and lifelong, and both in presence and distance, need to be upgraded to issues related to energy, the environment, and climate change. These issues need to be more strongly integrated at all levels of education, starting from the lowest, even that of primary school. However, in Greece, during the last years, there is a severe lack of technical workers to face the necessary technical works of the energy transition, and this is not taken into consideration in this plan.

Additionally, it is already known that well-informed and sensitized citizens can implement environmental protection actions much better than those who are not properly or fully informed [76,77]. Several points of the plan refer to citizens/consumers' actions on energy and climate change; however, these points are quite unclear. A specification of the increase of public information and awareness on issues of the safe and proper use of energy, energy savings, environmental protection, and climate change is proposed.

#### *4.13. Regional and Sectoral Plans*

The plan proposes individual regional plans to better implement the objectives of the project in the regions of Greece. According to the National Statistical Authority of Greece, the regions of Greece have an imbalanced contribution to the country's GDP [78]. Special mention should be given to regions with a small contribution, so as not to exclude these regions or to have them find themselves at second speed from the actions to be developed.

However, in addition to regional targeting, a sectoral dimension of this plan is missing. The establishment of a sectoral plan is probably more important than those of regional plans and should be established very soon.

#### **5. Discussion**

The main pillars of a National Energy and Climate Plan should first take into consideration the future energy mix of the country to mitigate climate change. However, energy is one of the main pillars of society and economy. For this reason, available energy, for example, without power interruptions or blackouts, and energy at an affordable price must be ensured so that the economy can function efficiently, but also to protect the disposable income of citizens and small enterprises. The high price of energy is the main reason for energy poverty, a very significant social problem in Greece, but also in many other countries in Europe or worldwide.

As a first step, a policy maker should guarantee that his proposal is efficient. If, for some reason, this proposal cannot be implemented, efficient alternatives should be applied. It is very strange that the Greek NECP lacks a risk analysis and proposes no alternatives to cover the probable cases of failure. This is a major shortcoming of this plan and should be restored as soon as possible, especially taking the very high increase of energy prices during 2020 into consideration. Greece, especially, is characterized as the most expensive wholesale electricity market in Europe, with the wholesale price at EUR 157 per MWh, a 70% increase since the beginning of the year [79].

Energy mixes and alternative technologies are the other critical points of this plan. The very rapid de-lignification of the country and the transition to natural gas can lead to a very problematic situation. The first point is that the decrease in greenhouse gas emissions will not be as high as is expected, mainly due to leaks of natural gas and its high global warming potential [26–28]. Additionally, the deterioration of the country's trade balance from the transition to natural gas is not taken into consideration in this plan. However, this is a major point, as the trade balance of Greece is, generally, highly negative. For example, the trade balance of Greece reached its highest point in 2015 (EUR -1.8 billion), during the debt crisis due to the collapse of aggregate demand, while it reached its lowest point on 2020 (EUR -12.52 billion) because of the collapse of tourism. Moreover, the trade balance of Greece becomes more negative when the GDP increases [80].

The energy dependence of Greece is also not taken into consideration, as a local product, lignite, is substitute with an imported one. A very specific analysis, using several scenarios of energy prices, should have been conducted to prove the advantages of this transition. However, this analysis is not shown in this plan. It is also clear that natural gas will be used only for some decades, and this statement adds a supplementary cost for the replacement of the infrastructure created only for a limited period of time.

More sustainable alternatives, such as the combustion of biomass, the gasification of lignite, or carbon storage, are not considered in this plan. However, the precise economic, environmental, and social evaluation of these alternatives, using different scenarios, should

have been completed first. The same is valid in the case of energy storage, which is necessary due to the high future penetration of RES, where no comparative analysis, with different scenarios, is provided. Pumped storage hydropower, one of the established techniques for energy storage, is not compared with the other techniques to prove that storage in batteries has a lower cost, is more efficient, and more environmentally friendly.

The plan proposes a gradual shift to electric passenger cars. However, the targets set are overestimated and cannot be reached very easily. Moreover, the increase in the use of public transportation, one of the most efficient methods for decreasing CO<sup>2</sup> emissions from the transport sector, is not taken into consideration in this plan.

Energy saving is one of the most efficient ways to decrease greenhouse gas emissions. However, the proposed energy savings in public buildings and private residencies are too low and, moreover, will not be achieved very easily. Energy-saving measures must be first implemented before examining a radical change to an energy mix, which will probably not be adequate for the new consumption of energy.

Other alternatives for the mitigation of greenhouse gas emissions, such as the enhancement of local production/consumption, or the very low action of changing dietary habits, which also have several other advantages for human health, are not taken into consideration in this plan. In addition, the establishment of a circular economy is not strong enough in this plan.

The social impacts of this energy transition are weakly taken into consideration in this plan. Some examples are the provision for technical workers, and their education/training, or the social awareness and information of citizens.

The absence of sectoral plans is another shortcoming of this plan; these plans should be established as soon as possible.

Finishing the list of main shortcomings, the future low energy price is not guaranteed in this plan. It is mentioned that future energy prices will be lower than the current ones without providing, however, a precise analysis. Even if there are some actions to tackle energy poverty, achieving this with a low economic growth, high energy prices, and a low percentage of dwellings renovated each year is impossible.

From the previous analysis, a more detailed examination of the possibility of continuing to exploit lignite, an available domestic product, for a longer time, instead of its complete substitution with imported natural gas, (at least until the cost of energy produced from RES decreases significantly) is therefore proposed. This can be done by using modern, more environmentally efficient technologies, by combining lignite combustion with biomass combustion or gasification technology, and by using synthetic gas and, in addition, carbon storage technology, as is exposed here.

There is also a very strong necessity to ensure the availability of all forms of energy at an affordable price. It is proposed that special care is taken to verify the projected reduction in the prices of energy and that actions, institutional initiatives, and other arrangements are taken so that the final price of energy does not increase and, additionally, so that any speculative trends are avoided.

#### **6. Conclusions**

The Greek National Energy and Climate Plan set the priorities of the Greece in the terms of Energy and Climate. A complete de-lignification by 2028 is proposed. The future energy mix will be mainly composed of natural gas and RES.

This plan has several shortcomings: there is no risk analysis and no alternative scenarios are proposed; the participation of other energy sources, such as biomass or the gasification of lignite, is not considered; energy storage is mainly focused on batteries and hydrogen, while pump energy storage is not considered; the targets set for the electrification of the passenger car fleet are too difficult to achieve, and at the same time, there is no provision for the enhancement of sustainable mobility by increasing the participation of public transportation; energy savings in buildings are not so ambitious; the tackling of energy poverty is almost impossible; and the same goes for the control of energy prices.

It is suggested that this transition be delayed for some years, taking into consideration the most sustainable paths for transitioning, such as using more alternatives.

**Author Contributions:** Conceptualization, E.Z.; methodology, E.Z. and L.V.; investigation, L.V., Z.G. and S.M.; resources, L.V., Z.G., S.M. and R.H.-M.; data curation, Z.G. and S.M.; writing—original draft preparation, Z.G. and S.M.; writing—review and editing, E.Z., L.V. and R.H.-M.; supervision, E.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Hyung-Seok Jeong , Ju-Hee Kim and Seung-Hoon Yoo \***

Department of Energy Policy, Graduate School of Convergence Science, Seoul National University of Science & Technology, Seoul 01811, Korea; azar76@seoultech.ac.kr (H.-S.J.); jhkim0508@seoultech.ac.kr (J.-H.K.)

**\*** Correspondence: shyoo@seoultech.ac.kr; Tel.: +82-2-970-6802

**Abstract:** South Korea has set up a plan to convert 24 coal-fired power plants into natural gas-fired ones by 2034 in order to reduce carbon dioxide (CO<sup>2</sup> ) emissions. This fuel transition can succeed only if it receives the public support. This article seeks to investigate the public acceptance of the fuel transition. For this purpose, data on South Koreans' acceptance of the fuel transition were gathered on a nine-point scale from a survey of 1000 people using face-to-face individual interviews with skilled interviewers visiting households. The factors affecting acceptance were identified and examined using an ordered probit model. Of all the interviewees, 73.6 percent agreed with and 12.2 percent opposed the fuel transition, respectively, agreement being about six times greater than opposition. The model secured statistical significance and various findings emerged. For example, people living in the Seoul Metropolitan area, people who use electricity for heating, people with a low education level, young people, and high-income people were more receptive of the fuel transition than others. Moreover, several implications arose from the survey in terms of enhancing acceptance.

**Keywords:** coal; natural gas; CO<sup>2</sup> emissions; public acceptance; ordered probit model

#### **1. Introduction**

Since coal has a higher carbon content than other fossil fuels such as oil and natural gas (NG), it emits more carbon dioxide (CO2), a greenhouse gas, during the combustion process. As a result, countries around the world are making various efforts to reduce coal power generation to prevent climate change [1,2]. In other words, energy transition is being pushed around the world to change from coal, which is high-carbon energy, to low-carbon or zero-carbon energy [3–5].

For example, the United States will shut down one-fifth of all coal-fired power plants by 2025 [6,7]. Germany plans to abolish all coal-fired power plants by 2038 by enacting the so-called 'de-coal law' and establishing a three-stage de-coal schedule [8,9]. As of December 2019, the plan is to reduce the capacity of 43.9 GW coal-fired power plants in Germany to 30 GW by 2022, 17.8 GW by 2030, and zero by 2038. In China, NG-fired power plants, replacing coal-fired power plants, are expected to grow at an average annual rate of 10% from 2025 to 2030, with a capacity of 235.7 GW by 2030 [10]. Japan also has a plan to abolish 100 of its 140 coal-fired power plants by 2030 [11].

South Korea is no exception to this trend [12]. The Ninth Basic Plan for Electricity Demand and Supply (2020–2034), which was finalized at the end of December 2020, proposed a goal to reduce the proportion of coal-fired power generation from 40.4 percent in 2019 to 29.9 percent in 2034 [13]. This is quite challenging as it will abolish 30 coal-fired power plants, half of the total of 60 coal-fired power plants in operation, by 2034. Of the 30 coal-fired power plants that will be abolished, six will be shut down completely, while the remaining 24 will be converted to NG-fired power plants. In producing the same amount of electricity, an NG-fired power plant emits less than half of the greenhouse gases emitted by a coal-fired power plant. In addition, if this trend continues, the remaining

**Citation:** Jeong, H.-S.; Kim, J.-H.; Yoo, S.-H. South Korean Public Acceptance of the Fuel Transition from Coal to Natural Gas in Power Generation. *Sustainability* **2021**, *13*, 10787. https://doi.org/10.3390/ su131910787

Academic Editors: Georgios Tsantopoulos and Evangelia Karasmanaki

Received: 17 August 2021 Accepted: 23 September 2021 Published: 28 September 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

30 coal-fired power plants that are supposed to operate without being abolished by 2034 are expected to be converted into NG-fired power plants or abolished as early as possible.

South Korea's total CO<sup>2</sup> emissions in 2018 stood at 728 million tons, ranking eighth in the world; its per capita CO<sup>2</sup> emissions ranked higher, at sixth in the world. More specifically, the electricity and heat sector, industry sector, transportation sector, and commercial and other sector accounted for 287 million tons (39.4%), 243 million tons (33.4%), 98 million tons (13.5%), and 100 million tons (13.7%), respectively. Total CO<sup>2</sup> emissions in 2018 reached 149 percent of the amount in 1990, but CO<sup>2</sup> emissions in the electricity and heat sector reached 480 percent of the amount in 1990. That is to say, since the increase in CO<sup>2</sup> emissions in the electricity and heat sector is due to the increase in coal-fired power generation, reducing coal-fired power generation has emerged as the most basic challenge to reducing CO<sup>2</sup> emissions [14,15].

Under the Paris Agreement signed in 2015, South Korea submitted a nationally determined contribution (NDC) with a goal of reducing its 2018 greenhouse gas emissions by 26.3% by 2030 to the United Nations Framework Convention on Climate at the end of December last year. Currently, raising the reduction target on the NDC is discussed in preparation for the 26th Conference of the Parties to be held in Glasgow, United Kingdom in November 2021. For the successful implementation of the NDC, the power generation sector should abate its 2018 greenhouse gas emissions (269.6 million tons of CO2e) to 192.7 million tons of CO2e by 2030. To this end, the South Korean government intends to change the mix of the power generation sources in two directions [13].

First, South Korea will increase the share of renewable energy (RE) generation from 6.5% in 2019 to 20.8% by 2030. In particular, current global trends suggest that RE has become more reliable and cost-effective [16]. In South Korea, RE-related costs have been gradually decreasing. However, not a small amount of subsidies are still required for RE generation projects. This is because the price of RE facilities continues to fall, but the costs of compensation for local residents are increasing significantly as the acceptance of the residents in areas where RE facilities are being installed is getting worse. Therefore, the implementation of the NDC is almost impossible with the expansion of RE alone.

Second, replacing some of the coal-fired power plants with NG-fired ones will be promoted. Some argue that a significant portion of the coal-fired power plants being abolished should be substituted with RE. However, there are many limitations in terms of intermittency and variability to significantly expanding RE immediately. Moreover, unfortunately, South Korea's power grid is not linked to that of neighboring countries, and its technological capabilities and market systems are not fully equipped to cope with the variability of RE. Thus, most of the coal-fired power plants that are abolished will be replaced by NG-fired ones for the time being. Of course, since RE will expand significantly in the long term, NG-fired power plants are expected to serve as bridges in the era of energy transition [17–19].

Three major problems have been raised in this regard, causing some controversy. First, South Korea is one of the highest-cost countries in the world to consume NG, as it must liquefy, transport, vaporize, and use NG produced overseas. In other words, converting power generation fuel from coal to NG could increase costs and eventually lead to higher electricity bills [20], which could increase the burden on the people and weaken industrial competitiveness. Second, almost all of the NG consumed in the country depends on imports, and due to its high dependence on the Middle East, such as Qatar and Oman, NG supply and price stability are weak depending on international political circumstances [21]. Therefore, it is pointed out that it may be not desirable to expand the use of NG in terms of improving energy security [22]. Third, comparing the number of people employed at coal-fired power plants with those employed at NG-fired power plants of equal capacity, the fuel transition is expected to reduce the number of jobs as the latter is about half of the former.

Because the above three issues have become disputable problems and no one is willing to take responsibility, public consensus on fuel transition is insufficient due to a lack of public debate and discussion [23]. In some cases, the fuel transition may not be carried out and may only have a declarative meaning, or it could run aground in the face of public opposition. Thus, it is necessary to draw clear implications after determining the public's acceptance of the fuel transition from coal to NG at this point in time [24,25]. This is because it is the people who bear the social costs of fuel transition, and energy policymakers desperately need information about their acceptance.

With countries around the world struggling to reduce greenhouse gas emissions, the policy of converting power generation fuels from coal to NG is a global trend [26]. This fuel transition can be seen from two perspectives. First, both coal and NG are fossil fuels, but the fuel transition is inevitable because in generating the same amount of electricity combustion of NG emits much less both CO<sup>2</sup> and particulate matters than that of coal. For example, de Gouw et al. [27] reported that emissions of greenhouse gases and air pollutants decreased by 56% and 40–44%, respectively, when converting fuel from coal to NG. Eventually, in order to reduce emissions of air pollutants as well as greenhouse gases, the transition of power generation fuel from coal to NG will accelerate [14,28]. As mentioned earlier, the representative countries that are pursuing this fuel transition are the United States, Germany, China, and Japan [26].

Second, because NG also emits CO<sup>2</sup> during combustion, NG plays a role as a bridge energy that maintains an intermediate stage to a complete RE society [18,19,29–31]. Of course, since NG is also a fossil fuel, the expansion of its use may be limited at some point in the future. However, it is expected that its role as a bridge energy will expand [32,33]. Regardless of which of the two perspectives is more important, the fuel transition will continue for the time being.

However, so far as the authors know, there is no literature that analyzes the public acceptance of the fuel transition and the factors affecting it. Since the fuel transition leads to an increase in power generation costs, the public acceptance of the transition needs to be clearly examined. This study aims to meet this need. In other words, this study intends to add this analysis using the specific case of South Korea to the literature at a time when coal as a power-generation fuel is being replaced by NG, but investigation of the public acceptance of the replacement and the factors influencing it remains scarce.

The public acceptance covered in this paper is not about technology or the fuel itself. The fuel transition can have several effects, but the most important effect is an increase in electricity bills. Therefore, the main target of the public acceptance dealt with in this study can be said to be the increase in electricity rates caused by the fuel transition. This is because abolishing coal-fired power plants and converting them into NG-fired power plants will reduce greenhouse gas emissions while increasing electricity rates. In the South Korean situation, NG is more expensive than coal.

After simply examining the public acceptance of the transition of power generation fuel from coal to NG, this study attempts to analyze the factors that affect its acceptance. To this end, a survey of 1000 people nationwide was conducted and the results reported. To the best of the authors' knowledge, since this attempt is the first in South Korea and there are few cases internationally, this research is believed to be able to contribute significantly to the related literature. The remainder of this paper consists of four sections. The next section reports a brief literature review, and the history and the present situation of coal-fired power generation in South Korea. Section 3 presents a description of the materials and methods adopted in this work. Section 4 explains and discusses the results. Section 5 deals with conclusions.

#### **2. Brief Literature Review and History of Coal-Fired Power Generation in South Korea**

#### *2.1. Literature Review*

Public acceptance, as well as expert opinion, is an important consideration in the establishment and implementation of energy policies [34]. Therefore, a number of studies have been conducted to analyze the public acceptance of various RE sources and energy policies. For example, the public acceptance of hydrogen technology [35], hydrogen charging stations [36,37], solar energy [38], wind energy [38–41], geothermal energy [42], hydroelectric energy [43], energy infrastructure [44,45], various energy sources [46], RE cooperatives [47], RE policies [48], and energy transition policy [4] have been analyzed in the literature.

Some of these studies identified factors influencing public acceptance and then examined them as independent variables. For instance, Huijts et al. [37] used variables on individual perceptions such as individual behavior, perception of the effectiveness of hydrogen charging stations, subjective norms, personal norms, trust in industries, trust in local governments, awareness of environmental issues, and awareness of fairness in hydrogen charging station deployment. Mistur [46] employed health level, political orientation, ideology, gender, education, age, race, religion, and residential area as variables. Tabi and Westenhagen [43] adopted gender, age, income, education, political views, residential areas, and membership of environmental organizations as variables.

Kim et al. [40] utilized gender, education, age, income, residence in the metropolitan area, home solar power retention, average monthly electricity consumption per household, perception of the proportion of NG-fired power generation in the nation's total energy generation, and political tendencies as variables. Fischer et al. [47] included risk-taking perception, patience, political orientation, environmental perception, age, gender, education, income, rural area residence, and West Germany as variables. Kim et al. [4] deployed gender, age, education, income, residence in the metropolitan area, electric heating, cooktop use, environmental awareness, and prior knowledge of the renewable energy 100% campaign as variables. Furthermore, Venkatesh et al. [49] formulated the unified theory of acceptance and use of technology, and analyzed user acceptance of information technology. They found that the socio-demographic factors and individual experience affect individual acceptance of information technology.

Looking closely at these factors, they are largely divided into four categories. The first category is the socioeconomic variables of the respondent. Most of the studies that analyzed the public acceptance considered variables such as gender, age, education, and income of respondents. The second category is related to respondents' perception and includes respondents' perception of energy policy, RE-related technology, or prior knowledge of the object to be investigated. The third category is the variables concerning the respondent's living environment, the location of the respondent's residence, and the characteristics of the respondent's house. The fourth category relates to the characteristics of respondents. For example, whether respondents engage in environmental group activities or use eco-friendly products. The variables used in previous studies are summarized in Table 1.


**Table 1.** Factors affecting public acceptance used in previous studies.

#### *2.2. History of Coal-Fired Power Generation in South Korea*

Coal-fired power plants, along with nuclear power plants, have contributed significantly to enhancing the industrial competitiveness of the export-driven South Korean

economy by serving as a source of low electricity bills over the past 30 years. With the country's successful localization of coal-fired power plants with a capacity of 500 MW, coal-fired power plants have not only played a role in offering a cheap and stable power supply, but also helped create jobs. South Korea suffered a nationwide rolling power outage on 15 September 2011 due to a lack of power supply facilities. A stable power supply emerged as an important task, and the Sixth Basic Plan for Electricity Demand and Supply (2013–2027), announced in February 2013, reflected the new construction of 10.5 GW capacity of coal-fired power plants [50].

In the course of establishing the Seventh Basic Plan for Electricity Demand and Supply (2015–2029), announced in July 2015, reducing the proportion of coal-fired power plants was discussed for the first time to reduce emissions of CO<sup>2</sup> and particulate matter [51]. However, the trend of expanding coal-fired power plants remained, as a stable electricity supply was considered more important than the environment. Until April 2017, a policy of continuously expanding coal-fired power generation had been implemented since the cost of coal-fired power generation was lower than that of NG-fired power generation or that of RE generation. In particular, the Ministry of Trade, Industry and Energy—a South Korean government department in charge of electric power policy—judged that maintaining cheap electricity bills through coal-fired power plants was more important than supplying eco-friendly power.

However, with the launch of a new government advocating energy transition in May 2017, policies began to be implemented to control the pace of the increase in coal-fired power plants. The coal-fired power plants under construction would be completed to stabilize the supply and demand of electricity, but all of the planned new coal-fired power plants would be scrapped, and older coal-fired power plants that reach 30 years of operation would be abolished [52]. The coal-fired power plants which were abolished were old and had a small capacity of 500 MW, but the coal-fired power plants under construction were the latest model, with a capacity of more than 1000 MW. Therefore, the speed of increase in the number of coal-fired power plants was reduced, but it was not planned to reduce the overall capacity of the coal-fired plants significantly.

Since then, severe particulate matter problems occurred in March and April 2019. People's anxiety about particulate matter overwhelmed other social problems. In April 2019, the National Climate and Environment Council was launched as a state agency to take responsibility for and deal with particulate matter under the President's direct control. Ban Ki-Moon, who served as the Secretary-General of the United Nations, was appointed chairman of the Council, to take charge of international cooperation to reduce particulate matter. The Korea Ministry of Environment announced that about 15 percent of particulate matter generated in the country was emitted from coal-fired power plants, and the issue of how to deal with coal-fired power plants was seriously discussed.

As a first step, the Council proposed the introduction of a particulate matter seasonal management system that would stop or minimize the operation of coal-fired power plants for four months from December 2019 to March 2020. The Korea Ministry of Trade, Industry and Energy opposed the introduction of the system, citing a surge in electricity demand for heating during winter, but the system was introduced and implemented in the name of reducing particulate matter. Under the system, coal-fired power plants should not operate as much as possible. If coal-fired power plants are inevitably operated to meet the increased demand for electricity, their power generation should be lowered to 80% of normal operation. This is because it is impossible for South Korea's coal-fired power plants to reduce their power generation to less than 80% due to their design. Therefore, unlike other countries that aimed to reduce CO<sup>2</sup> emissions, South Korea has begun to push for reducing coal-fired power generation to abate serious particulate matter emissions.

In November 2020, the year after the Council was launched, it proposed to remove all coal-fired power plants by 2045. Considering Germany's push to eliminate coal-fired power generation in 2038, a vote was taken among selected people in the nation with three proposed dates to cease coal-fired power generation: 2040, 2045 and 2050; the majority chose 2045. These people, called national representatives, were 500 people selected from all over the country by the Council in terms of age, income, gender, and region. In the meantime, China declared its intention to reach carbon neutrality by 2060 in September 2020, and Japan declared its intention to reach carbon neutrality by 2050 in October 2020. South Korea, geopolitically located between the two countries, also declared carbon neutrality by 2050 in October 2020. The specific carbon neutrality route and implementation means will be determined through further discussions, which are currently active.

The Ninth Basic Plan for Electricity Demand and Supply (2020–2034), which began in January 2019, was finalized and announced in December 2020, about two years later [13]. The reason it took such a long time was to reflect the greatly strengthened goal to reduce CO<sup>2</sup> emissions compared to the Eighth Basic Plan for Electricity Demand and Supply (2017–2031). The Ninth Basic Plan for Electricity Demand and Supply (2020–2034) calls for an additional reduction in CO<sup>2</sup> emissions in the power generation sector of 34.1 million tons by 2030 compared to the Eighth Basic Plan for Electricity Demand and Supply (2017–2031).

Due to time constraints, the Ninth Plan did not reflect carbon neutrality by 2050, but the Tenth Plan, which is scheduled to be finalized at the end of 2022, decided to reflect it. The Ninth Plan decided to abolish coal-fired power plants when they reached the age of 30 and to introduce a price-bidding mechanism on coal-fired power plants from April 2022 by setting an upper limit on coal-fired power generation [13]. The amount of coal-fired power generation decided annually will decrease year by year, and South Korea's coal-fired power plants will be shut down in the near future.

#### **3. Materials and Methods**

#### *3.1. How to Investigate Public Acceptance*

Public acceptance of a particular policy pursued by the government means the quantity of people who agree with the implementation of the policy. The implementation could gain momentum if many people vote for it. However, it is difficult to secure momentum for the policy if many people disagree with its implementation. Thus, as mentioned in Section 2, there are quite a number of studies analyzing public acceptance of a newly introduced policy or technology in the field of energy. Of course, people's approval is not the only consideration in pursuing a particular policy, but it must be one of the most important considerations [53].

The first thing to do here is to determine the methodology for analyzing public acceptance. This study aims to collect data by asking about public acceptance through a survey of a large number of people selected at random. Surveys are an effective means of collecting people's opinions directly [4,34,40]. The study will investigate whether, in order to reduce CO<sup>2</sup> emissions, they are in favor of or against the policy of abolishing coal-fired power generation early and replacing the corresponding capacity with NG-fired generation. Not only are the pros and cons examined, but they are also tallied. In fact, comprehensively aggregating pros and cons is a very simple task. A more complex and meaningful task is to derive the implications by analyzing the determinants of those pros and cons, which is also performed in this work.

First, various issues related to survey data collection, such as the survey target, sampling method, sample size, and survey method, are reviewed below, given that a survey is used for this study. Next, how to construct specific questions to identify public acceptance is explained. Finally, an econometric model is presented that can derive implications by analyzing the determinants while considering the nature of the data collected on public acceptance.

#### *3.2. How to Gather the Data through a Survey*

Regarding the collection of data, the survey method, survey target, sampling method, sample size, and survey area are determined. First, the survey method used in this study is a costly person-to-person individual interview with households. Of course, other low-cost survey methods were also available, such as postal interviews, telephone interviews, and

internet interviews; however, person-to-person individual interviews were essential to fully explain the background to the transition of power generation fuel from coal to NG. In the case of postal interviews, there is no guarantee that people will properly look at the enclosed data, and the collection rate of questionnaires is extremely low in South Korea. In the case of telephone interviews, it is difficult to fully explain background information to respondents. While internet interviews have the advantage of being the lowest cost of the survey methods, they can cause problems that make random sampling difficult, leading to an increased probability of sample selection bias.

Second, the survey target in this study was selected as adults aged between 20 and 65 years. Of course, the opinions of people under 20 or over 65 may be important, but to obtain a responsible answer, the survey target was limited to those who could engage in economic activities. In South Korea, one graduates from high school at the age of 19 and becomes a true adult from the age of 20, becoming a university student or getting a job, and engaging in economic activities. In addition, people aged 65 usually retire from economic activity. Meanwhile, the proportions of men and women in the survey were the same.

Third, the sample size in this study was 1000. If a relatively large sample size is used, it is desirable in that it reflects many people's opinions, but it also creates disadvantages that increase the cost of the survey. In the end, it was necessary to determine the sample size that could collect people's opinions with some representation and avoid the problem of a sharp increase in survey costs. In this regard, Arrow et al. [54] pointed out that a sample size of 1000 may be appropriate to collect people's opinions. The Korea Development Institute [55] also proposed that the sample size be 1000 when conducting a survey for public sector decision-making. The sample size of 1000 used in this study is consistent with the suggestions made in these studies.

Fourth, the interviews were conducted by experienced interviewers belonging to a professional survey company. The authors first fully discussed the content of the questionnaire with the company's supervisors. Next, the supervisors trained the interviewers. In the course of the training, interviewers practiced questioning each other with the questionnaire. Interviewers who completed the training visited households and had them complete the questionnaire. The interviewers checked that there was no response to the main questions in the questionnaire and asked respondents to correct and supplement the questionnaire if necessary. Respondents who completed the questionnaire received simple household items such as a shopping bag, toothpaste, or a portable sewing kit as gifts.

Fifth, out of a total of 17 provinces in South Korea, 16 provinces were targeted, excluding Jeju Island. This was because Jeju-do, an island far from the mainland, had the lowest population among the 17 provinces, while the unit price of the survey was the highest. For these reasons, Jeju-do is usually excluded when conducting person-to-person individual interview surveys in South Korea. The Korea Development Institute [55] also suggests excluding Jeju-do when conducting a national opinion survey.

#### *3.3. How to Prepare the Questionnaire*

The final version of the questionnaire used in this study consisted of three main parts. After explaining the purpose of the survey, the first part asked about basic perceptions of several things. The second part asked about acceptance of the policy of converting 24 currently operating coal-fired plants into equal-capacity NG-fired power plants by 2034. The third part contained questions about the general characteristics of respondents. Answers to these questions are considered as candidates for factors affecting acceptance. The questions were about residential area, heating system, household income, personal income, education level, age, gender, etc.

For the second part, which is a key part of the questionnaire, how to ask questions about public acceptance had to be decided. For example, Ono and Tsunemi [56] used four views: "absolutely disagree," "slightly disagree," "slightly agree," and "absolutely agree." However, as a result of requesting a preliminary survey of 30 people from a professional survey company, two points were raised. First, those who participated in the preliminary

survey asked to add a "neutral" view. In practice, the most widely used Likert scale adopts five levels. For example, it would be appropriate to use the five options "strongly disagree," "disagree," "neutral," "agree," and "strongly agree."

Second, participants required more granularity in the five views. For example, instead of two views, "strongly disagree" and "disagree," it was proposed to use more granular views. Thus, these two were divided into four: "absolutely disagree," strongly disagree," "disagree," and "slightly disagree." The same was true of "strongly agree" and "agree." In fact, the analytic hierarchy process, developed by Saaty [57] and widely used in decision analysis, suggests the use of a nine-point scale rather than a five-point scale in value judgment. Consequently, this research finally used a total of nine views, from opposition to affirmation, as measures of acceptance. In other words, 1 to 9 correspond to "absolutely disagree," "strongly disagree," "disagree," "slightly disagree," "neutral," "slightly agree," "agree," "strongly agree," and "absolutely agree."

#### *3.4. How to Identify the Factors Affecting Public Acceptance*

Public acceptance will be affected by a variety of characteristics of respondents. This research considers a total of 11 variables in relation to these characteristics. The names and definitions of these are shown in Table 2. In addition, basic statistics such as average and standard deviation are included in the table. In determining these 11 variables, the previous studies that investigated some factors affecting the public acceptance presented in Table 1 were referred to as important. As shown in Table 1, the main variables used in previous studies were income, residential area, gender, age, education, personal life characteristics, health and faith, and personal perception. These variables are generally composed of three categories: the characteristics of respondent households, the individual characteristics of respondents, and the perception and judgment of respondents. Therefore, the 11 variables presented in Table 2 are similarly divided into three categories.


**Table 2.** Information on variables in the model.

The first category is the characteristics of respondent households. This category contains three variables: Metro, Heating, and Income. The Metro, Heating, and Income variables are a dummy for the interviewee's living in the Seoul Metropolitan area (0 = no; 1 = yes), a dummy for the interviewee households' using electricity for heating

(0 = no; 1 = yes), and a dummy for the interviewee household's monthly income being larger than KRW 4.88 million (USD 5.75 thousand) (0 = no; 1 = yes), respectively. The Income variable is defined as a dummy that identifies whether the interviewee household income is greater or less than the average value of the sample. That is, considering that the average monthly household income in the sample is KRW 4.88 million, the Income variable has a value of one if the interviewee household's income is greater than KRW 4.88 million and zero otherwise.

In the second category, two variables, Education and Age, were used as individual characteristics. The Education and Age variables are dummies for interviewees having more than twelve years' education (0 = no; 1 = yes) and interviewees' age in years, respectively. Other personal characteristic variables, such as the gender of the respondents and whether they are household owners, were also considered candidates for reflection, but were eventually excluded from Table 2 as the analysis indicated that they had little effect on acceptance.

Six variables related to respondents' recognition and judgment were utilized as the third category. Know1, Know2, Environment, Forest, Fsolar, and H2-car are dummies for interviewees knowing about the energy transition policy well before the survey (0 = no; 1 = yes), a dummy for interviewees knowing about hydrogen vehicles well before the survey (0 = no; 1 = yes), interviewees' subjective judgment on which is more important between jobs and the environment (0 = jobs; 1 = environment), a dummy for interviewees being in favor of the utilization of unused forest biomass (0 = no; 1 = yes), a dummy for interviewees being in favor of expanding floating solar power facilities (0 = no; 1 = yes), and a dummy for interviewees being in favor of expanding hydrogen vehicles (0 = no; 1 = yes), respectively.

#### *3.5. How to Model the Data*

Two important points should be taken into account in establishing a model in which acceptance is a dependent variable and the factors affecting this acceptance are independent variables. First, the observed dependent variable has a range of only 1 to 9—that is, the minimum is 1 and the maximum is 9. Therefore, the range is not the whole real number, but a natural number between 1 and 9. Second, the acceptance values are ordinal, not cardinal. Looking at the score, the measure of acceptance, it is clear that the larger the number, the greater the acceptance. However, this value is not cardinal. Performing classical regression without reflecting these two points can produce misleading analysis results.

To simplify the analysis, Ono and Tsunemi's [56] work transformed the collected data on a four-point scale into a two-point scale for pros and cons, and then applied a discrete logit model to the transformed data. However, adopting this approach results in the loss of important information from data collected on a nine-point scale. Therefore, it is necessary to apply a model that fully utilizes the collected data. The ordered probit model is useful in dealing with data on acceptance evaluated on the Likert scale as in this study [58,59]. The model defines a latent variable distributed over the whole real number instead of an observed variable on a nine-point scale and sets it as a dependent variable. In addition, the likelihood function reflects the relationship between the observed and latent variables.

The ordered probit model used in this research can be formulated as follows. For respondent *i* (*i* = 1, · · · , *I*), the latent variable variable, *A* ∗ *i* , and the observed variable, *Ai* , are:

$$\begin{cases} \begin{array}{c} A\_i^\* = y\_i^\prime \beta + \omega\_i \\ A\_i = J \text{ if } \sigma\_{I-2} < A\_i^\* \le \sigma\_{I-1} \text{ for } I = 1, \cdots, 9 \end{array} \end{cases} \tag{1}$$

where *y<sup>i</sup>* is a vector of a constant term and the variables given in Table 2, *y<sup>i</sup>* 0 means the transpose matrix of *y<sup>i</sup>* , *β* is a vector of the parameters matching *y<sup>i</sup>* , *ω<sup>i</sup>* is the disturbance term, and *σ* is a threshold value that is not known and should be estimated.

However, following the usual practice in the literature, *σ*−<sup>1</sup> = −∞, *σ*<sup>0</sup> = 0, and *σ*<sup>8</sup> = ∞ are assumed. Furthermore, the disturbance term is assumed to be distributed as normal with a standard deviation of one. Therefore, the probability that the observed variable has one value of 1 to 9 can be induced as:

$$\text{Prob}(y\_i = f) = F\left(\frac{\sigma\_{I-1} - y\_i^\prime \beta}{1}\right) - F\left(\frac{\sigma\_{I-2} - y\_i^\prime \beta}{1}\right) \tag{2}$$

where *F*(·) indicates the standard normal cumulative distribution function. The finally derived likelihood function is:

$$L = \prod\_{i=1}^{I} \prod\_{J=1}^{9} D\_t^s \text{Prob}(y\_i = J) \tag{3}$$

where *D<sup>s</sup> <sup>t</sup>* = 1(*y<sup>i</sup>* = *J*) for *J* = 1, 2, · · · , 9 where 1(·) is a function that returns one if the argument is true and zero otherwise. The maximum likelihood estimates can be obtained by finding the parameter values that maximize Equation (3).

#### **4. Results and Discussion**

*4.1. Data*

The authors sought to focus on three points in conducting field surveys. First, scientific sampling should reflect the characteristics of the population. Second, experienced professional interviewers should obtain reliable responses from respondents. Third, the person-to-person interviews should be carried out maintaining the distancing rules in the pandemic situation caused by COVID-19. To meet these three points, a professional survey company took charge of the entire process of the survey. The survey was conducted for one month from mid-March to mid-April 2021. Judging from the comments of the interviewers belonging to the company, respondents responded to the survey without any difficulties. In particular, people were actively involved in the survey because of the controversy over coal-fired power generation due to issues such as particulate matter and CO<sup>2</sup> emissions.

The results from interviewees selecting one of the nine views are presented in Table 3. Four views—"absolutely disagree," "strongly disagree," "disagree," and "slightly disagree"—can be aggregated as "disagree with implementing the fuel transition;" while four views—"slightly agree," "agree," "strongly agree," and "absolutely agree"—can be aggregated as "agree with implementing the fuel transition". Of the 1000 respondents, 122 opposed the fuel transition and 736 supported the fuel transition, the latter (73.6%) being about six times the former (12.2%). Overall, therefore, the approval rate was higher than the disapproval rate. A total of 142 respondents said "neutral." It was interesting that 14.2 percent of people were neutral or indifferent to fuel transition.


**Table 3.** Summary of responses regarding acceptance of replacing coal-fired power plants with natural gas-fired ones.

#### *4.2. Estimation Results of the Model*

The estimation results of the ordered probit model are given in Table 4. *R* 2 , which indicates goodness of fit, was 0.165. In the analysis using cross-sectional data, the value of *R* 2 is usually low [60,61]. In particular, Gans [62] pointed out that it is a kind of norm for *R* 2 to be between 0.1 and 0.2 when analyzing data obtained from the survey. Thus, it can be seen that the value of 0.165 for *R* <sup>2</sup> obtained in this study is not particularly low. A likelihood ratio test can be applied for the specification test of the model. In this case, the null hypothesis is that all estimated coefficients except the constant term are zero—that is, the model is mis-specified. The computed likelihood ratio test statistic was 175.62, which corresponds to a *p*-value of 0.000. Thus the statistical significance of this model, determined at a significance level of 1 percent, is ascertained.

**Table 4.** Estimation results of the ordered probit model.


<sup>a</sup> The variables are described in Table 2. \* and # indicate statistical significance at the 5% and 10% levels, respectively. *σ* 's are parameters to be estimated in the model.

All of the *σ* values used to define the observed variable, *A<sup>i</sup>* , were estimated to be positive, having statistical significance at the 1 percent level. Moreover, the estimation results of the coefficients corresponding to all the variables defined in Table 4 had statistical significance at the 10 percent level. Thus, the estimation results of the ordered probit model are significant. Interestingly, the model provides reasonably good performance. The application of the ordered probit model in this study was an appropriate strategy.

The estimated coefficients for the Metro, Heating, Income, Know1, Know2, Environment, Forest, Fsolar, and H2-car variables had a positive sign. The positive sign of each estimated coefficient implies that the greater the value of each variable, the greater the acceptance of the fuel transition. For example, those who lived in the Seoul Metropolitan area, those who used electricity for heating, those whose household income was high, those who knew about the energy transition policy before the survey, those who knew about hydrogen vehicles before the survey, those who considered the environment more important than jobs, those who were in favor of utilizing unused forest biomass, those who were in favor of expanding floating solar power facilities, and those who were in favor of expanding hydrogen vehicles were more prone to the fuel transition than others.

On the other hand, the sign of the estimated coefficients for the Education and Age variables was negative. A negative sign indicates that the greater the value of each variable, the lower the acceptance of the fuel transition. Respondents with more than twelve years' education and those who were older than 48 years were less receptive to the fuel transition than others.

#### *4.3. Discussion of the Results*

After data on acceptance of the fuel transition were collected on a nine-point scale, the determinants of acceptance were identified and analyzed. The results derived from this work can contribute to the literature, having various implications in terms of both research and policy. First of all, the usefulness of applying an ordered probit model to ordinal data collected on a nine-point scale was ascertained. The specification test confirmed the statistical significance of the model, and both the threshold values appearing in the model and the estimated coefficients for the eleven variables of interest were statistically significant. Thus, it is possible to derive various implications from the results.

The fact that the approval rate for the fuel transition (73.6%) exceeded the disapproval rate (12.2%) by about six times was a positive finding in promoting the fuel transition in South Korea. In order to achieve the goal of reducing CO<sup>2</sup> emissions declared to the international community, the country should carry out the fuel transition continuously and robustly while maintaining a stable power supply [13,14,28]. This finding can be recognized as an encouraging sign for the country. Without public support, the fuel transition cannot succeed. It is also worth noting that 14.2 percent of the respondents considered themselves neutral or indifferent.

Interestingly, three interviewee household characteristic variables had a significant impact on acceptance. First, those living in the Seoul Metropolitan area were more receptive to the fuel transition than those who were not. The Seoul Metropolitan area is home to about half of the population, an important area that determines public opinion in South Korea. Therefore, it is quite difficult to implement policies that residents in the area oppose. The finding that the acceptance of residents in the area of the fuel transition is secure is quite encouraging in promoting the fuel transition.

Second, those who use electricity for heating were more receptive than those who do not. The main fuel for residential heating in South Korea is city NG since electricity for heating is more expensive than city NG. Nevertheless, some people use electricity instead of city NG for heating because they prefer electricity to other fuels. This is because electricity is partially made from RE, and even if it is produced using fossil fuels, fossil fuel-fired power plants greatly reduce particulate matter through air pollutant reduction facilities, while city NG boilers are not equipped with these facilities. In other words, since people who use electricity for heating have a high interest in the environment, they seem to make more positive judgments on the fuel transition.

Third, the household income of the respondent was positively correlated with acceptance. The transition of power generation fuel from coal to NG will inevitably lead to higher electricity bills. From an individual household's point of view, income is limited, and an increase in electricity costs means spending on other goods should be reduced. In fact, the high-income group is more likely to accept an increase in electricity bills than others. In particular, low-income people may be opposed to an increase in electricity bills caused by the fuel transition rather than to the fuel transition itself. Therefore, even if the fuel transition is made, measures will need to be in place to alleviate the burden by continuously applying the electricity rate discount system for low-income people.

Moreover, two individual characteristics of interviewees had a significant impact on acceptance. The education level of the respondent had a negative impact on acceptance of the fuel transition, while older interviewees were less receptive to the fuel transition than younger interviewees. In fact, in South Korea, the higher the level of education and the older the person is, the more they tend to settle for the present situation. This is because fuel transition can not only increase electricity bills, but also reduce jobs and cause problems in the stability of electricity generation fuel supply. To help increase the acceptance of fuel transition among people with a high education level or older age, the fuel transition should be promoted in more persuasive and appealing ways.

Three issues concerning the fuel transition were addressed in the introduction: rising electricity bills due to increased power generation costs, reduced fuel supply stability due to increased NG use, and job losses [20–23]. Interviewees responded after hearing full explanations of the issues during the survey, and it was found that the approval rate for the fuel transition was six times higher than the opposition rate. However, if these three issues become a reality, the public acceptance of the fuel transition may drop significantly, which could place South Korea in a difficult situation. It may happen that coal-fired power generation facilities are reduced but NG-fired power facilities are not increased in time, which could give rise to a crisis in the supply and demand of electricity. Therefore, the three issues need to be discussed here in conjunction with qualitative implications obtained from respondents in the process of the survey.

First, since the unit cost of NG-fired power generation is higher than that of coal-fired power generation, the fuel transition will bring about an increase in electricity bills, which could negatively affect acceptance of the fuel transition. As of March 2021, South Korea's unit cost of NG-fired power generation was 99.72 KRW per kWh, about 10 percent higher than that of coal-fired power generation, which is 90.19 KRW per kWh. However, because NG prices are linked to oil prices, rising international oil prices could have a significant impact on NG prices, which could widen the gap. Usually, NG prices soar during periods of high oil prices and plunge or stabilize during periods of low oil prices. Recently, as oil prices have been rising, voices opposing the fuel transition have already begun to appear.

In addition, since NG will serve as a bridge in an era of energy transition, NG prices are likely to rise further in the future as demand for NG increases around the world. As coal-fired power generation decreases and NG-fired power generation continues to increase, the cost burden for electricity distribution companies is expected to increase. This could eventually lead to higher electricity bills, which could place a burden on consumers. Therefore, it is necessary to persuade the public by fully informing them that fuel transition is inevitable and that they must endure an increase in electricity bills to implement the fuel transition. Fuel transition will gain momentum only when there is a social consensus that electricity produced from NG-fired power plants is inevitably more expensive than that from coal-fired ones, just as organic products are more expensive than regular products.

Second, given the geopolitical situation of South Korea, NG is more vulnerable to supply instability than coal. Almost all of the NG consumed in the country is imported from abroad. In particular, the country relies heavily on the Middle East, including Qatar and Oman, as NG suppliers. Therefore, if political instability occurs in the Middle East, South Korea's NG-fired power plants may have to be shut down. In addition, liquefied NG produced in the Middle East is transported to South Korea, and the country's NG procurement costs are the highest in the world due to excessive liquefaction, transportation, and vaporization costs.

On the other hand, as coal is imported from all over the world without the need for liquefaction, it has higher supply stability than NG and its procurement cost is also low. Thus, it is quite important to secure a stable supply of NG in order for fuel transition to succeed. In Europe, NG is supplied stably mainly in the form of pipeline NG, but South Korea is introducing NG through liquefied NG carriers only. Currently, efforts are being made to import NG produced from other regions besides the Middle East. For example, South Korea is trying to increase NG imports from Australia, the United States, Indonesia, and Mozambique. These efforts must yield significant results. If South Korea fails to secure stability in NG supply, it will be difficult for the fuel transition to succeed and receive public support.

Third, if jobs are lost due to the fuel transition, this can lead to serious social conflict. Currently, a 500 MW coal-fired power plant in South Korea has a total of 168 to 200 employees in the operating and maintenance sectors, while the number of workers in an NG-fired power plant of the same capacity is between 90 and 110. In other words, the latter is about half the former. Consequently, converting all 24 (12.6 GW) coal-fired power plants to NG-fired power plants could result in the loss of between 2000 and 2400 jobs. Since all 24 coal-fired power plant operators are public companies, not private ones, responding to the fuel transition by reducing working hours and increasing the retraining of staff may not actually reduce jobs; however, this approach will be neither long-term nor stable.

The decline in the number of jobs will not only increase unemployment, making workers' lives difficult, but also cause considerable damage to the local economies where power plants are located. The 24 coal-fired power plants are located on the shores of rural areas, not urban areas, because they are a form of "not in my backyard" facility and need to use seawater as cooling water. Since most of them are located in underdeveloped areas, they contribute greatly to the local economy through local taxes, donations, public utility expenditures, and procurement of resources within the region. However, because it is advantageous for NG-fired power plants to be located in urban areas or industrial complexes adjacent to the demand for electricity, they are likely to change their location during the transition. Consequently, the areas where coal-fired power plants used to be located could face economic difficulties due to the fuel transition.

Due to these problems, during a recent visit to Chungnam Province, where the largest number of coal-fired power plants are located, President Moon Jae-In stressed that "The energy transition, which replaces existing coal-fired power plants with NG-fired ones, should be done in a just way so that no-one loses a job and the local economy is not damaged." Therefore, the just and fair transition of fuel is an important issue that South Korea will face in the future. There should be in-depth consideration of this and immediate preparation of concrete measures to implement this in practice. The measures currently proposed include the following.


#### **5. Conclusions**

In Introduction section, the authors looked at previous studies dealing with the transition of power generation fuel from coal to NG. An important implication was that this transition is inevitable to reduce CO<sup>2</sup> and air pollutant emissions, and is a strategy adopted by a number of countries. South Korea is no exception. The previous policy to expand coal-fired power generation has been changed to a reduction policy since May 2017. It was clearly pointed out that the use of NG will be expanded as an intermediate stage because it is difficult to immediately engage in a significant expansion of RE. It was also mentioned that the speed of fuel transition will be faster for the implementation of carbon neutrality by 2050. In the end, concerning coal-fired power generation, the current status and prospects of South Korea are consistent with those of other countries presented in several previous studies.

In Section 2, previous studies that analyzed energy-related public acceptance were examined. It was found that, to the best of the authors' knowledge, this study was the first to analyze the public acceptance of converting power generation fuel from coal to NG. In particular, it is an important discovery, differentiated from previous research, that the public acceptance of the fuel transition policy is secured to some extent and that many people support the transition even though there is an increase in cost. In addition, this study not only identified various factors affecting the public acceptance of the fuel transition, but also derived various implications by proposing and applying a framework for analyzing the impacts of the factors on the acceptance. The results of this study could be important

information for South Korean policymakers. Although the main findings of this study are unique to South Korea, the structure of this study can be extended to other countries as much as possible. The main qualitative findings of this study are not so different from those of the research that has analyzed the public acceptance of RE expansion policy [63], largescale offshore wind power generation construction [40], and energy transition policy [4] in South Korea.

South Korea's CO<sup>2</sup> emissions in 2018 totaled about 728 million tons. However, the 2030 CO<sup>2</sup> emissions target submitted to the United Nations Framework Convention on Climate Change is 536 million tons. Ultimately, South Korea needs to reduce its CO<sup>2</sup> emissions drastically, and the power generation sector should play an important role in this as it is very difficult to reduce CO<sup>2</sup> emissions in other sectors such as transportation, industry, building, and agriculture. To that end, South Korea has set out a plan to convert 24 coal-fired power plants into NG-fired power plants by 2034. This study sought to ascertain and analyze public acceptance of this through a survey of 1000 people nationwide. The results were statistically significant and reveal a number of useful implications.

The finding that the approval rate for the fuel transition was six times the opposition rate suggests that the government-led fuel transition should be carried out continuously. However, since respondents were concerned about three challenges that could arise in the process of fuel transition, the authors have attempted to discuss them above. These were higher electricity bills due to rising power generation costs, reduced fuel supply stability due to the increased use of NG, and job losses and a consequent negative impact on the local economy. Ultimately, dealing with these three challenges effectively will determine whether or not the fuel transition succeeds.

Of course, further challenges remain to be addressed. For example, is it socially desirable to tear down a coal-fired power plant that has only been in operation for 30 years? Given that NG-fired power plants are also fossil-fuel-utilizing facilities and there is strong opposition from residents to new construction of these, how will we facilitate fuel transition? It is clear that NG is the bridge energy to a complete RE society, but for how long will it play that role? In other words, if NG-fired power plants become stranded assets in a not-too-distant future in which carbon neutrality is realized, is it reasonable to invest in constructing NG-fired power plants now? Subsequent studies should be able to answer these questions.

**Author Contributions:** Conceptualization, H.-S.J. and J.-H.K.; methodology, S.-H.Y.; software, J.- H.K.; validation, H.-S.J.; formal analysis, J.-H.K.; data curation, H.-S.J.; writing—original draft preparation, H.-S.J.; writing—review and editing, S.-H.Y.; visualization, J.-H.K.; supervision, S.-H.Y.; project administration, J.-H.K.; funding acquisition, S.-H.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by the Research Program funded by the SeoulTech (Seoul National University of Science and Technology) (grant number: 2021-0787).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available because they were purchased for a fee.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

