In this section, the methods of literature review and the materials used will be described.
2.1. Methods of Literature Review
In order to analyse the concept and all interpretations of SECs, a literature review was conducted by defining the main research trends, the current applications, the directives and the prospects. The Preferred Reporting Items for Systematic reviews and Meta-Analysis (PRISMA) methodology was used following a four-phase flow diagram, since systematic review methods help the researchers to keep up to date [
31]. The four steps followed are listed in the following bulleted list and further explored in
Figure 2:
Identification;
Screening;
Eligibility;
Inclusion.
The identification of investigated studies started by defining the databases, the keywords and the websites used for the sources search. Specifically, Scopus™, Science Direct™ and Web of Science™ were used as databases. The most used keywords were smart energy system, energy community, prosumer and renewable energy community. In the second phase, the identified records were screened, and all duplicates were deleted. Furthermore, the sources that appeared not focused on the topic were excluded after having analysed their title and abstract. In this mix of documents, specific criteria were adopted to define the eligibility of sources. The inclusion criteria are listed in the following bulleted list:
Full access to the source;
Article or conference paper;
Source belonging to the energy subject area;
Source intended to evaluate the energy, or the economic, or the environmental benefits arising from energy sharing;
Source dealing with the exploitation of at least one RES;
Source describing the results of a simulation or a case study;
Source developed by at least one energy expert;
Source referring to countries where the REDII directive has already been transposed into national law or with interest in energy sharing.
Instead, the exclusion criteria were:
Grey research;
Duplicate source;
Non-indexed source;
Non-English source;
Source antecedent to 2018 (year of publication of the EU REDII);
Source focused on sharing but without energy scopes;
Source containing not relevant keywords, such as those related to non-energy communities (e.g., microbial communities);
Conference paper not focused on the exploitation of RESs;
Journals with fewer than ten publications about the topic published after 2018.
The research carried out by assuming smart energy system as keywords produced 52,753, 162,104, and 61,181 results in Scopus™, Science Direct™ and Web of Science™, respectively. Next, the results were filtered to keep only references successive to 2018 and to focus on energy topic. Since this approach was not applicable in Web of Science™ database, it was excluded. To avoid the presence of duplicates and to ensure the access to the sources for all authors, only Scopus™ was used as database. The references coming from the websites were excluded as well, since they responded to the exclusion criteria. After the application of the inclusion and exclusion criteria, the title of 626 remaining documents had been evaluated by the authors. Subsequently, in order to avoid bias, the abstract of the remaining 245 documents had been analysed. The final set of references included 78 papers.
In
Table 1, the screenings for the number of documents published from 2018 to 2022, found in Scopus™ and Science Direct™ databases and belonging to the
energy subject area, are reported. In the last column of each row, the total number of papers published from 2018 to 2022 and referred to each keyword and database is specified. In the last row, the total number of papers found in each database using the Boolean function {“smart energy system” OR “energy community” OR “prosumer” AND “renewable energy community”} AND {“limit to the energy subject area”} AND {“year of publication”} as query is shown for each analysed year (from 2018 to 2022). The result is that in Scopus™ database, the
prosumer term realized the best increase in the number of citations. Moreover, with respect to 2018, it is possible to detect an increase between 17% and 36% of the number of papers about
energy community and
renewable energy community published in 2019 and 2020, thus following the publication of the REDII and the IEMD.
In carrying out the literature analysis, several topics were found to be able to summarize SECs’ common characteristics. They are listed in the following bulleted list:
Electric load sharing;
Thermal load sharing;
Photovoltaic (PV) systems adoption;
Exploitation of RESs different from solar;
High efficiency cogeneration (CHP) systems usage;
ESSs presence;
DSM programs;
Information and Communication Technologies (ICT) implementation.
It results that none of the 2019 reviewed papers considers SECs which implement thermal load sharing, while the electric load sharing topic tends to be quite equally distributed within the analysed years. As a matter of fact, the former was discussed for 70.9% in 2018 papers, for 17.7% in 2020 papers and for 11.4% in 2021 papers. The latter was discussed for 24.8% in 2018 papers, for 38.8% in 2019 papers, for 14.0% in 2020 papers and for 22.4% in 2021 papers. This outcome is indicative of the great attention paid by the REDII to the sharing of electric energy, which is easier to apply than the thermal energy sharing since electric grids are public and already extended and branched. The topic about the exploitation of RESs other than solar was argued only by papers of 2020 and 2021 for 61.0 and 39.0%, respectively. On the contrary, the topic about the adoption of PV systems appears to be quite equally distributed within the analysed papers, since it was discussed for 26.8% in 2018 papers, for 18.6% in 2019 papers, for 26.8% in 2020 papers and for 27.7% in 2021 papers. These outcomes reflect the continued attempt of the literature to find profitable ways to boost the exploitation of RESs. Nowadays, PV systems are affordable and easy to install, thus representing an optimum tool for the promotion of renewable electricity generation also at the community level. However, the exploitation of other types of RESs is gaining more interest, thanks to the opportunity to improve SECs’ energy self-sufficiency (SS) and to counteract RESs’ uncertainty. For this to happen, it is also useful to design SECs equipped with ESSs and/or CHP units. This assumption explains the quite equal distribution of these topics among analysed papers. The former has been analysed for 33.9% by 2018 papers, for 23.5% by 2019 papers, for 15.1% by 2020 papers and for 27.5% by 2021 papers. The latter has been analysed for 7.1% in 2018 papers, for 44.4% in 2019 papers, for 44.4% in 2020 papers and for 4.0% in 2021 papers.
A similar analysis has been carried out about SECs’ principal purposes, which, alongside with better local RESs exploitation and improvement in users’ SS and self-consumption (SC), turn out to be as follows:
Primary energy saving;
Operating energy costs reduction;
GHG emissions saving;
Electric peak shaving;
Power quality supply adjustment;
Social outcomes (local economy improvement, energy democratization, gaps in energy access resolution).
The most equal distributions in terms of year coverage are found for the topics regarding the reduction in operating energy costs and GHG emissions. In fact, each publication year covers this topic between 20.2% and 35.0%, as evidence that both goals have been recognized to be essential key drivers of energy transition and, according to the REDII, the creation of SECs is expected to enhance their achievement. In the case of the other aims, the variability of the percentages is much more pronounced. For example, in relation to power quality enhancement, the year coverage varies between 1.4% and 41.0%. In relation to social outcomes, the year coverage varies between 2.7% and 66.8%, and in relation to energy consumption reduction, the year coverage varies between 3.1% and 77.5%. In the stacked graph of
Figure 3, the percentage distribution of each topic among the analysed years is reported to assess the shift in the interest in the literature. It results that the social outcomes expected by SECs have progressively gained importance after 2018, as they cover 6.7% of analysed papers from 2018, 33.3% of 2019 papers and 58.3% of 2020 papers. This trend can in part be attributed to the publication of the REDII, and specifically to the role given to SECs about ensuring social and environmental benefits rather than financial profits. As further evidence of this, from the values obtained, it results that the environmental aims had always been recognized essential goals of SECs. In fact, the reduction in GHG emissions results to be quite equally distributed among the analysed papers in all the years considered, since the percentage values vary between 8 and 27%. The same values are found for the reduction in the operating energy costs, while the variability is much more pronounced for the other goals. However, this outcome could be attributed to the specific field of engineering which the selected papers belong to.
The most diffused topic among all reviewed years is the adoption of PV systems, which has been discussed by a total of 18 papers, followed by the presence of ESSs, included in 15 papers. Thereby, it is further evident that the practical possibility of sharing electric energy among different users regulated and incentivised in Europe by the REDII, together with the affordability and the easiness to install of solar panels, boosted the focus of the literature about the exploitation of solar energy and the adoption of electric ESSs. Indeed, the exploitation of RESs other than solar emerges to be the less diffused topic, with only 2 references dealing with it dating to 2020 and 2021, respectively. The interest paid by the RED II to the renewable electricity supply is also reflected in the objectives of SECs. In fact, as already stated, both the reduction of GHG emissions and of operating energy costs expected by SECs recur frequently within the literature, and both are described in 12 papers among the analysed ones. Yet, social outcomes result to be the most popular aims of SECs, included in a total of 13 papers. These outcomes furtherly account for the main benefits expected from SECs’ according to the REDII and demonstrate the potential to simultaneously reap the benefits from the energy, environmental, social, and economic point of view through their implementation. Therefore, it may be concluded that after the launch of the REDII in 2018, the interest of the literature has been shifted towards issues more in line with the regulatory framework.
The selected papers for this research involve many researchers from different countries. In
Figure 4, the thematic map about the number of papers considered for each country is reported. These values are furtherly detailed in
Table 2. Each value has been calculated by assigning to each paper the country of its corresponding author. In the case of papers with more than one corresponding author coming from different countries, the score of each involved country has been evaluated accordingly as a fraction of one. It results that the countries counting more than five papers are Austria, Italy, Portugal and the United Kingdom.
2.3. Aims of Smart Energy Communities
The sharing approach had already been proved to be environmentally, economically, and socially sound before REDII launch. In [
33], it emerged that the adequate selection of users suitable for sharing their electrical and thermal loads leads to satisfactory performance from an energetic point of view thanks to the reduction in primary energy demand by 18.8% in Benevento (Italy) and 17.2% in Milan (Italy). In addition, the proposed system involved also environmental advantages, thanks to the 26.9% and 25.2% reduction in CO
2 emissions in Benevento and Milan, respectively. In [
34], the authors showed the load-sharing approach to increase energy SC, thus reducing grid perturbation and providing economic benefits. In addition, the increased SC reflected in the saving of primary energy and CO
2 emissions. The primary energy saving was equal to 6.2% in Naples (Italy) and to 8.8% in Turin (Italy), while CO
2 emissions were reduced by 6.7% and 8.3% in Naples and Turin, respectively. These results anticipate SECs’ primary target according to the REDII, which is to offer their members environmental, economic, and social benefits rather than financial profits [
52]. Starting from this assumption, in this section an overview of the principal purposes of SECs will be defined. The discussion will be developed by defining three categories, listed as follows:
Each one of these categories will be detailed in the following subsections, according to the recent literature review and in the best of authors’ knowledges.
Section 2.3.1 aims to detail the energy, technical and environmental objectives of SECs, while
Section 2.3.2 defines the economic aims and
Section 2.3.3 defines the social ones. In the last subsection,
Section 2.3.4, the analysed aims of SECs will be briefly discussed.
2.3.1. Energy, Technical and Environmental Aims
Some of the principal purposes of SECs from the energy and technical point of view are listed in the following bulleted list:
Decrease in global energy consumption due to the adoption of DESs [
62];
Massive use of RESs-based energy conversion systems [
63];
Improvement of energy grid stability [
64];
Widespread exploitation of RESs in energy networks [
65];
Development of hybrid energy systems that will increase the steadiness of RES-based energy conversion systems in supplying the energy requests [
66].
According with the first bulleted aim, the support of DESs leads to positive environmental outcomes [
67]. Franzoi et al. [
43] argue that the broader diffusion of on-site energy generation units arising from SECs’ spreading is one of the key drivers in buildings’ carbon footprint reduction. According to [
49], with respect to the baseline scenarios where RESs exploitation is neglected, “zero-energy” scenarios involve strong carbon emissions and costs reduction. In detail, the former ranges between 71.2% and 90.9%. On the other hand, the improvement of energy grid stability is a consequence of the energy sharing and the interaction among SEC participants. In the absence of the SEC configuration, the surplus of decentralized energy “generation” is sold to power grid, thus worsening its instability [
68]. Instead, within the SEC, the use of the surplus energy by other consumers/prosumers or in CESs is allowed. Therefore, users’ SC and SS increase, as proved in [
36]. In fact, thanks to the reduction in PV electricity exports allowed by the energy sharing, the amount of energy consumed on-site rises between 60.7 and 74.4%, and the amount of electric load covered by PV electricity grows to values between 77.8 and 81.3%. Both results are due to greater exploitation of surplus electricity, which is distributed within the community. Additional advantages in terms of grid stability enhancement are found with ESSs. Starting from the assumption that ESSs are essential for the proper integration of renewable energy generation, both imports and exports, and thus the interaction with the grid, can be further reduced when ESSs are implemented as CESs. In [
44], it is demonstrated that the adoption of CESs reduces monthly imports by 91.0 MWh, whereas the reduction is limited to 59.0 MWh with the adoption of household batteries. In parallel, CESs reduce communities’ monthly exports by 102.2 MWh, whereas the reduction allowed by household batteries is limited to 80.5 MWh. Nevertheless, CESs total storage size is equal to 8.5 MWh in the community scenario, thus lower than in the household scenario, where it is equal to 13.0 MWh. The reason for this is the community aggregating affect, by which 39.0% of households do not need any batteries. More generally speaking, the integration of polygeneration systems and different ESSs in multi-energy districts can provide the following:
Given technical and economic constraints which could make power grid extension unviable, RESs are recognised as having a fundamental role in rural electrification plans [
71]. In this respect, community-based energy systems allow a greater energy access than centralized ones, even in areas with no access to the national grid [
72]. As an example, solar-driven SECs may represent an interesting alternative in meeting off-grid rural communities’ electricity demand with sustainable electricity, thus boosting the decarbonization of energy islands, i.e., areas with poor or inexistent connections to the grid [
70], where the electricity generation relies on diesel-fuelled systems [
73]. In [
74], a SEC is designed to increase energy SS, stability and flexibility of the Italian island of Ischia. The SEC is based on an organic Rankine cycle plant interacting with a geothermal source at medium temperature. In contrast to the traditional system, where the electricity demand would have been met by the national power grid, the considered system provides electric energy for both pure electric and thermal load by the exploitation of a “programmable” RES, often insufficiently exploited within the Italian territory. The results confirm the great environmental benefit expected: the proposed system leads to a large CO
2 emission saving, equal to 29.9 tCO
2/y. The accomplishment of this result is facilitated by the fact that small island electricity networks are characterised by lower complexity, hence representing ideal places for improving pilot projects of SECs [
75].
2.3.2. Economic Aims
From the economic point of view, SECs lead to significative reduction in both systems costs, thanks to profitable economy of scale [
76], and electricity costs, because of load aggregation and shifting [
77]. The load-sharing approach is proved to be very cost effective in [
39], where the economic saving is equal to 71.0% as a consequence of a 28.0% increase in energy SC and the reduction in the energy demand in certain hours of the day. In fact, the increment of community SC makes the imports reduction prevail on the income loss deriving from the reduced energy exports [
68]. Similarly, in [
35], it results that by allowing DESs to sell electricity to the grid and to interact with each other it is possible to minimize the total operating energy cost and the net energy cost. In fact, the energy sharing maximizes the usage of the electricity “produced” by the CHPs. Moreover, SECs may play a significative role in making the decarbonization of energy islands affordable, as proved in [
78]. This study evidences that the design of a multi-vector energy system to supply the energy demand of a community in Neom (Saudi Arabia) allows the surplus of electricity to be used to charge a battery and/or to produce hydrogen that can be used during deficit periods. It results that the cost-effective and reliable design of the proposed system leads to a reduction in costs by around 67.3%. SECs also allow the reduction in system costs for ESSs [
76]. In turn, the adoption of ESSs makes the incentives gained for improved SC by SEC’s participants increase, especially when ESSs act as CES [
44]. The adoption of decentralized EVs as decentralized ESSs is analysed in [
61] from the economic standpoint. It results that the recharge of the EV daily costs EUR 1.58 when the consumer acts individually, whereas the cost is largely lower and equal to EUR 0.03 when the consumer operates at the community level. Therefore, the presence of EVs leads to significative economic savings, which can be shared within all members of the SEC, according to their agreements. This cost-saving solution, together with the enhancement of local RESs exploitation [
73], demonstrates that SECs may improve local economy. In addition, SECs have the potential of unlocking private investments and financing renewable energy generation units, thus increasing their pervasiveness and social acceptability [
79].
2.3.3. Social Aims
SECs allow the shifting of responsibilities from centralized to decentralized entities [
50], and thus the sharing of benefits and governing powers among their members by aligning the concepts of “energy community” and “energy democracy” [
72]. In fact, SECs have a great potential in transforming current socio-technical centralized regimes into distributed and decentralized organizations [
80], whereby energy democratization is a principal consequence [
81]. Prosumers’ main concern is to guarantee sustainable energy provisioning and to contribute towards innovation and local value creation [
82]. In this regard, driving the implementation of DESs, prosumerism is ever more considered as a fundamental solution to energy systems’ challenges [
83]. Therefore, new business models have been defined to deliver value and reduce the costs of “merit goods” [
62,
84]. Their principal characteristics are the following:
The concept of “community” includes the sharing of investments, infrastructures and expertise for the creation of local value [
85] by guaranteeing the energy access and mitigating the energy poverty in vulnerable contexts [
52]. The improvement of SS is indeed one of the main reasons for users to participate into a SEC [
86]. In [
87], it is demonstrated the fundamental role which can be played by SECs in electrifying poor rural communities, characterised by dispersed settlement patterns and significative poverty levels which challenge rural citizens grid connections affordability. In [
72], Ambole et al. analyse the role which SECs can play in counteracting strong energy access gaps in Sub-Saharan Africa, which in turn worsen poverty and health issues. In this context, where the access to electric energy is missing for about 600 million people and about 890 million still use unsafe traditional fuel-based devices for energy provision, SECs are gradually emerging as an instrument to enhance households’ sustainability and resilience, by providing them cheaper electricity and globally extending energy access. By entrusting SECs’ members with ownership and control, the community is given the possibility of improving local economy and entrepreneurship. Moreover, the ownership of RESs-based generation units and infrastructure projects increases their value in the eyes of community, thus facilitating their commitment [
88].
2.3.4. Discussion about SECs Aims
Local investments and income generation are recognized to be two of SECs’ most important drivers, together with environmental and ethical commitment, by SECs’ participants interviewed through an online survey whose results are discussed in [
89]. SECs’ members were also asked which were, according to their opinion, the most important factors characterizing the community. Emerging from this is their focus on the positive environmental impacts following from the SEC constitution, which reflects the fact that no financial benefit is perceived from the SEC. The same approach has been adopted in [
90], which is a documentary study based on an online survey conducted in nine EU countries to highlight the current state of EU energy cooperatives. Regarding SECs’ key drivers, over 60% of respondents answered that their main purpose was to tackle the climate change problem. In fact, environmental benefits, together with increased comfort, energy independence, greater participation into the electricity market, reduced energy bills and increased energy supply reliability can enhance consumers’ participation to SECs [
82].
To sum up, according to the case studies reviewed, energy, environmental and technical issues (energy demand reduction, increase in energy SS and SC, local RESs valorisation, GHG emissions reduction, power peak shaving and power quality supply enhancement), in addition to the reduction in the energy costs, the improvement of local economy, the transition to energy democracy and the resolution of existing gaps in energy access emerge to be some of the main purposes of SECs. These results are summarised in
Table 4, where some selected references are linked to the previously listed goals of SECs. The synthesis is carried out by following the categorization adopted to identify the subsections previously detailed. The check mark states the presence of a specific topic in the selected paper, the cross mark its absence. Each reference is reported in the first column of the table, while each goal is listed in each column of the second row. The second column is about the energy demand reduction, and out of a total of twenty-four rows, only six are filled with the check mark. In fact, only references [
35,
39,
43,
49,
68,
74] deal with this topic. The number of rows filled with the check mark increases to thirteen in the case of the third column, which refers to the increase in energy SC and SS. This topic has been analysed in references [
13,
36,
37,
38,
39,
43,
49,
61,
68,
70,
74,
78,
86]. The fourth column refers to local RESs valorisation, and accounts for eight check marks corresponding to references [
36,
49,
69,
70,
72,
73,
74,
78]. No more than ten check marks are found in all remaining columns. Specifically, the fifth one refers to power peak shaving and power quality supply enhancement, and is filled with five check marks corresponding to references [
36,
64,
69,
77,
87]; the sixth one refers to GHG emissions reduction, and includes six check marks corresponding to references [
70,
73,
74,
78,
89,
90]; the seventh one is about the reduction in energy costs, which has been discussed in references [
13,
35,
38,
39,
49,
61,
68,
76,
77,
78]; the eighth one is about the local economy improvement, the penultimate refers to the transition to energy democracy and the last one refers to the resolution of gaps in energy access. The minimum number of check marks is found in the antepenultimate and penultimate column, where it is equal to two and one, respectively, corresponding to references [
72,
89]. Instead, seven references describe the SECs’ goal of solving gap in energy access, namely [
70,
71,
72,
73,
74,
78,
87].
2.4. Indexes for Smart Energy Communities Analysis
A share of literature review was conducted by analysing the indexes and evaluation parameters used in scientific studies to compare and quantify the benefits of SECs, also with respect to other energy user configurations. Zhou et al. [
91] define three parameters for SECs optimization. The first one evaluates the maximization of energy load of electric appliances in residential SEC, the second one measures the efficiency of electric storages in the SEC and the third one, the user credit index, calculates the product between the unit price of subsidy for participants and the rating energy of participants’ appliances. In Khanna et al. [
92], the demand response methodology is applied to a small smart electric grid and a minimization of net cost of electricity index is proposed. For calculating this index, the energy export earnings are subtracted to the consumed electricity costs, thus evaluating the economic advantage of energy sell. Karunathilake et al. [
93] propose the evaluation of the Levelized Cost Of Energy parameter for describing the cost of energy production during the entire lifetime of the analysed facility. In particular, the index is calculated as the ratio of the present value of life cycle costs to that of lifetime energy generation. The evaluation of this parameter for different energy appliances makes it possible to compare them. The grid parity factor is set equal to 1.2 for guaranteeing the new renewable energy system Levelized Cost Of Energy not exceed the cost of grid electricity by more than 20%. Such a type of index is introduced also in [
70], where it is defined the Levelized Cost of Energy Consumed (LCOE
consum) indicator, and it is used together with Energy Potency (E
pot), Energy Self-Sufficiency (E
SS) and CO
2 intensity (CO
2consm) indicators in assessing the sustainability and SS of multi-energy vectors used in energy islands. Specifically, the E
pot index evaluates the efficiency of variable RESs integration in multi-vectors energy communities, E
SS is introduced to complement E
pot by evaluating the curtailment of solar and wind energy production and the LCOE
consum index quantifies the costs due to the use of energy, including capital, fuel, operation, maintenance and imported energy costs. Lastly, CO
2consm evaluates the average CO
2 emissions in each of the multi-vector energy community contexts. In Kahwash et al. [
94], a renewable penetration indicator for electrical system only is defined as the ratio between the renewable power production and the difference between both the total and the unmet electric load. In a similar way, the thermal penetration index is defined. In addition, the excess ratio is evaluated for quantifying the surplus power coming from renewable energy generation plants. The SS and the SC indexes are defined by Mutani et al. [
95]. The first index quantifies the amount of total energy consumption consisting in the share of locally self-consumed energy. The second one quantifies the amount of total RES energy generation shared for local energy SC. The same authors propose the definition of the energy exposure indicator for evaluating the user’s dependence on the national electric grid. Ceglia et al. propose the evaluation of
s and
d indexes [
36]. The former expresses the amount of electricity demand of SEC covered by the renewable energy production; the latter is the ratio between the renewable electricity self-consumed with respect to the total amount of renewable electricity produced. Maturo et al. [
96] define other two indexes: the self-sustaining services ratio, which quantifies in percentage terms the self-consumed energy used for providing heating and cooling services and the SC ratio, which calculates the amount of self-consumed energy over the global energy production, thus resulting in related local energy production.
Wang et al. propose energetic, environmental, and economic SEC indexes [
97]. From energy point of view, they define two indexes described as follows:
Promotion on Energy Balance index, which quantifies energy exchanges with the distribution network both in form of imports and exports;
Peak Reduction Index, which proves the maximum absolute power reduction achieved in the community scenario with respect to the reference case.
From an environmental point of view, they define the Emission Reduction Rate Index, which computes the waste emissions reduction rate with respect to the baseline case. From an economic point of view, they define two indexes as following description:
Total Cost Reduction Index, which compares the community overall cost reduction rate with the reference values;
Participation Intention Index, which measures in percentage terms the number of members who gain larger profits thanks to their participation to the new energy trading schemes with respect to benchmark conditions, thus representing the overall willingness to join the alliance.
Luthander et al. [
98] and Viti et al. [
68] calculate an index that expresses the ratio between SS and SS as defined in [
95]. In Braeuer et al. [
99], the electrical SC rate, the degree of electrical self-sufficiency and the degree of electrical autonomy are employed. Furthermore, the Grid Interaction Index and the normalised Grid Interaction Index are calculated for assessing the self-generated energy usage in energy communities composed by multi-family buildings in Germany.
In Minuto et al. [
100], two key performance indices (KPI) were defined to assess the economic and environmental performance of SEC in Italian condominium context. Dimensionless KPIs are defined as a weighted sum of economic and environmental parameters for the economic and environmental analysis, respectively. Specifically, for each scenario, the economic KPI is evaluated considering its Net Present Value, Internal Rate of Return and Pay-Back Time, whereas for the environmental KPI evaluation CO
2 emissions reduction, renewable electricity generation and energy consumption reduction due to increased energy efficiency are considered too.
In
Table 5, all cited documents related to SEC indices are grouped in three classes of index’s subject. Only Herenĉić et al. [
70], Wang et al. [
97] and Minuto et al. [
100] define indexes that include all economic, environmental and energy aspects.