Process Modeling and Optimization of Supercritical Carbon Dioxide-Enhanced Geothermal Systems in Poland

Abstract

This paper presents a comprehensive analysis of supercritical carbon dioxide (sCO₂)-enhanced geothermal systems (EGSs) in Poland, focusing on their energetic performance through process modeling and optimization. EGSs harness the potential of geothermal energy by utilizing supercritical carbon dioxide as the working fluid, offering promising avenues for sustainable power generation. This study investigates two distinct configurations of sCO₂-EGS: one dedicated to power generation via a binary system with an organic Rankine cycle and the other for combined power and heat production through a direct sCO₂ cycle. Through accurate process modeling and simulation, key parameters influencing system efficiency and performance are identified and optimized. The analysis integrates thermodynamic principles with geological and operational constraints specific to the Polish context. The results highlight the potential of sCO₂-EGSs to contribute to the country’s energy transition, offering insights into the optimal design and operation of such systems for maximizing both power and thermal output while ensuring economic viability and environmental sustainability.

1. Introduction

In recent years, global warming and energy-related concerns have emerged as a much-debated topic worldwide. Developing sustainable energy solutions is critical to address the global challenges of climate change and resource depletion. The main objective of meeting the energy and environmental requirements outlined in the 2020 Paris Agreement is to increase the quantity of power produced through renewable energy systems and to reduce dependence on fossil fuels. The European Commission is attempting to accelerate the EU’s shift to renewable energy sources (RESs) in accordance with long-term goals. This will enable the EU to considerably surpass its aim of reducing net greenhouse gas emissions by at least 55% by 2030 and achieving climate neutrality by 2050. One of the possibilities that relates to these concerns is geothermal energy, which is becoming more and more popular due to its clean energy production, reliability, and adaptability to be operated in a variety of settings and configurations.

1.1. Literature Review

Geothermal resources can be categorized into three types based on geological structure characteristics, heat flow transmission methods, temperature ranges, and development and usage methods: (i) shallow geothermal, (ii) hydrothermal geothermal (underground hot water), and (iii) deep geothermal. Among deep underground systems, hot dry rock (HDR) can be distinguished as a high-temperature heat source within rock formations. It refers to rocks that are either entirely fluid-free or contain minimal fluid content, typically exceed 150 degrees Celsius, and have the potential to be exploited through heat utilization under current technological and economic conditions [1].

1.1.1. Enhanced Geothermal Systems Overview

The power generation technologies used in conventional hydrothermal systems are mature and have been developed for approximately 100 years, although HDR allows access to more abundant heat through the fracturing process of hot rocks located at substantial depths. Through resource exploration, well drilling, and reservoir creation, fluid circulation is deployed for heat extraction and further power or heat generation (Figure 1). The units which operate in this way are called enhanced geothermal systems (EGSs). Their development is practically identical to traditional geothermal systems [2]. According to Lu S., there will be more than 70 GW_el installed from EGS units based on 85% of the estimated probability [2]. In many regions, such as the European Union, Japan, Australia, or the USA, research and initiatives, including innovative exploration, reservoir stimulation, and sustainable reservoir technologies, are being developed to accelerate the commercialization of EGS units [3].

Nevertheless, EGS plants must be placed at the location where a high-temperature reservoir is accessible. EGS operation is based on deep reservoirs whose high permeability is a result of hydraulic stimulation and fracturing. This process is necessary in order to enable sufficient working fluid flow [4]. Therefore, the wells are drilled and the bedrock is stimulated to structure the connected and open cracks to provide a stable network in which the fluid will be injected. Completion of the well drilling process is a crucial step in the development of a geothermal project. Drilling for geothermal fluids follows the same procedures as rotary drilling for gas and oil. However, rock in a geothermal well is typically harder, often metamorphic or igneous rather than sedimentary due to its properties. Furthermore, high temperatures affect the design of the drill string and casing, as well as the circulation system and cementing techniques [5].

EGS installations may be deployed with other working fluids besides conventional water. Especially, supercritical carbon dioxide (sCO₂) is gaining more and more attention due to the global crisis of CO₂ emissions [6] combined with favorable thermophysical properties for EGS. Rapid technological development in climate mitigation tools in the field of dynamic growth of carbon capture, utilization, and storage solutions has also affected EGS research and exploration that employs sCO₂. This approach may be perceived as a promising technology that matches near-zero carbon energy production with captured CO₂ usage and storage [4]. Many efforts have already been made to compare the performance of CO₂ and water as working fluids in EGS installations, including properties [7], operating parameters, heat recovery rates [8], as well as efficiency and power plant output [9]. One key difference compared to water-based systems is that sCO₂ systems require much higher circulated flow rates to achieve a similar rate of heat extraction due to sCO₂’s lower density and specific heat capacity. However, this may be considered an environmental benefit since the process in EGS involves capturing carbon dioxide, transporting it, and using it as a working medium in the cycle with its partial sequestration in rock formations so that it does not enter the atmosphere [10]. Wang C. et al. analyzed the water losses and the possibility of CO₂ sequestration in the reservoir, showing that it can achieve approximately 6–8% [11]. Comparing carbon dioxide and water, it can be concluded that CO₂ is a poor solvent for most rock minerals, while water can cause substances to break down and precipitate, which can result in reduced flow [12]. Reservoirs frequently extend over long distances and can be of considerable thickness, and interactions of CO₂ with methane present in the reservoir, brine, or minerals occur on a microscopic level. Moreover, carbon dioxide has a higher compressibility and expansiveness, which results in a lower power needed to pump this fluid [9]. In addition, CO₂ has a lower viscosity than water, which in turn has a better ability to behave under specific thermodynamic conditions. The lower viscosity results in higher CO₂ flow rates, which compensate for its lower heat capacity compared to water [10]. Nevertheless, in the case of the sCO₂-EGS system, an additional step is required during the establishment of the plant. After a period of stimulation, water is removed from the rock seam by decomposition in the flowing CO₂ stream along with the remaining reservoir fluids and subsequently, the water content decreases until the system is completely devoid of water [8]. Besides this additional stage, using supercritical carbon dioxide as a working fluid in EGS entails several benefits, such as mitigating greenhouse gas emissions by partial CO₂ sequestration, chemical stability, reduced water usage, or lower pumping power requirements.

The EGS deployment depends on many factors. Key aspects are related not only to their operational potential but also to cost-effectiveness and environmental impact. The latter was recently examined through life cycle assessment by the authors of [13]. The performed study focused on Polish and Norwegian cases with different system configurations. The results indicated that the construction phase has the highest environmental impact during the system’s lifetime. The analysis also showed that higher mass flow rates influence lower impact due to much higher production. The overall global warming potential of sCO₂ was calculated in the range of 12–54 kgCO_2eq/MWh_el. Based on the available literature, water-based EGS installations, due to high impacts in the construction phase, are characterized by similar values of global warming potential. Lacirignola et al. [14] obtained 36.7 kgCO_2eq/MWh_el for a water-based binary unit in Soultz, France. In [15], the authors analyzed various hypothetical water-based EGS units located in Germany and the resulting values are in the range of 42–62 kgCO_2eq/MWh_el for electricity generation. The climate-change impacts of EGS in the Upper Rhine Valley estimated in another EGS-LCA study [16] indicate values between 24.7 and 45.9 kgCO_2eq/MWh_el in terms of electricity production. From the economic point of view, well drilling accounts for the largest share of investment expenditures; thus, both water-based and CO₂-based systems are characterized by similar capital costs and consequently levelized cost of electricity (LCOE). The values vary from 90 [17] to 250 [18,19] EUR/MWh_el, depending on depth, temperature, and system configuration.

In terms of analyzing the potential of EGSs and their performance, numerical simulation and mathematical modeling are the main tools applied. The latter is usually used for the assessment of topside units [2]. Read et al. discussed the operational requirements of power generation equipment for EGS units. The authors compared different power plant systems in terms of their energetic outputs [20]. Rodríguez et al. compared the power output of the organic Rankine cycle (ORC) and Kalina cycle using heat extracted from HDR [21]. Heat and electricity production, including energy storage, was also investigated through numerical simulations in [22], showing the plant operation over a number of cycles and 30 years of operation. Nevertheless, these publications are mainly related to conventional water-based EGS units. Bonalumi D. examined CO₂-EGS installation from the ORC point of view with emphasis on the working fluid selection [23]. Gładysz et al. assessed the techno-economic potential of sCO₂-EGS units in Central Poland as part of a combined system with power plant integration and district heating supply [24], as well as their performance for different mass flows with an economic feasibility study [25]. Heat input into a topside EGS power generation system is determined by the production wellhead temperature, while the driving force required to maintain working fluid circulation within the geothermal reservoir is indicated by the production pressure difference. Consequently, the production wellhead temperature determines the expander’s output power, whereas the power consumption of the injection pump mainly influences the EGS’s net output power. The impact of different injection temperatures and fluid mass flows on system performance was investigated by the authors in [26]. Nevertheless, the conducted research was mainly focused on the general examination of the efficiency and outputs achieved from sCO₂-EGS installations. Moreover, the presented articles did not show the results in the context of the multiannual operation of the sCO₂-EGS installation. Thus, the novelty of this paper lies in its focus on reservoir degradation over time within sCO₂-EGS installations, considering the degradation rate as a function of operation—an essential aspect for sustained unit activity that has been overlooked in previous research. Additionally, this study presents results in the context of long-term operation, evaluating various system configurations, which was not fully addressed in existing literature.

1.1.2. sCO₂-EGS Worldwide Status

According to reports released after the last World Geothermal Congress 2023 and documents published by the International Geothermal Association, the installed capacity of geothermal energy worldwide reached 16,260 MW_e distributed in 197 geothermal fields in more than 30 countries. The installed capacity of geothermal heating and cooling worldwide will reach around 110 GW_th by 2022, which is a 50% increase from 2015 [27].

In terms of enhanced geothermal systems currently, around thirty EGS projects are either under development or in operation worldwide, with a total installed capacity of about 12 MW [1]. Of these, five are in routine operation, and fourteen are currently capable of producing power. Nevertheless, EGS projects have not yet reached large-scale commercial operation due to a number of obstacles that need to be overcome. In [1], the authors underlined aspects that may cause an impediment in rapid EGS deployment; some of those mentioned are as follows:

• Lack of adequate and efficient HDR artificial fracture management technology in EGS development may lead to isolated, disproportionately large artificial fractures, fluid circulation short circuits, early thermal breakthroughs, and consequently inefficient heat recovery.
• The processes of EGS formation and heat recovery are influenced by a number of variables, including water–rock interaction, seepage, heat transmission, medium deformation, and several others. It is yet unknown how multi-scale and multi-field coupling patterns and mechanisms influence geothermal reservoirs.
• Pressure drops during the lifting process in EGS producers can result in fluid flashing, which modifies the well’s flow and heat transfer properties and limits the extraction of hot fluid efficiently.

In spite of several problems, there are multiple EGS power, heat, or combined power and heat generation methods. HDR geothermal power generating has not yet been brought to broad commercial application. In Fenton Hill, the Los Alamos Laboratory carried out the first HDR project in the 1970s [28]. Since then, the UK, Japan, Germany, Australia, and France have also started HDR project development. Examples of pilot projects in commercial production with initial power generation capabilities of 1–5 MW include Soultz in France [29], Landau in Germany, Habanero in Australia, and Desert Peak and Geysers in the United States [2]. Most of the techniques and methods associated with well drilling and exploration come from the oil and gas industry, and it is presumed that based on the research and experience gained by oil and gas companies, the field of enhanced geothermal systems will continue to evolve. Nevertheless, EGS units are still under development and are not mature enough to be considered as competitive with other renewable energy sources. Future exploration of EGS may contribute to drilling cost reduction, which has the biggest share in capital expenditures related to EGS unit deployment and may cause major impediments [30].

1.1.3. sCO₂-EGS Potential in Poland

The geological conditions of Poland can be described as well recognized because research in this field has been carried out since the 1980s. In recent years, numerous initiatives have been taken to determine the conditions conducive to enhanced geothermal systems and, consequently, to assess the energy potential in areas where energy can be collected from hot, dry rocks. Poland is located at the meeting point of three European tectonic structures: (i) the Precambrian Eastern European Platform, (ii) the Paleozoic platform of Western and Central Europe, and (iii) the Alpine fold zone of Southern Europe [31].

Poland’s geothermal resources are included in four main hydrogeothermal provinces:

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Carpathian Foredeep;

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Outer Carpathians;

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Sudetes;

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Polish Lowlands.

The Polish Lowlands province is the largest one of those mentioned above. The geothermal resources of this region are related mainly to Mesozoic formations. High temperatures and capabilities were recorded, especially for the Lower Jurassic and Lower Cretaceous formations of this hydrogeothermal reservoir. These layers are based on sandstones with valuable reservoir parameters. Particularly favorable conditions in terms of EGS installations can be found in the region of Mogilno-Łódź basin, especially in the areas around Konin and Krośniewice as well as the area of Gorzów Block, which are characterized by the highest temperatures found in this region and indicate a high potential for energy development of the resources [31]. Another promising area for efficient hydrogeothermal energy management area is the Carpathian region, which is characterized by a diverse geological structure. The greatest use of geothermal resources is in the part of the Inner Carpathians—Podhale, where the first and largest geothermal heating plant in Poland was built. Favorable conditions in this area affect the high efficiency of the systems, amounting to approx. 960 m³/h and water temperatures in the range of 80–86 °C. Therefore, the main purpose of geothermal resources in Podhale remains heating, medical treatment, and balneotherapy [31].

Other geothermal conditions occur in the Sudetes area, which is located in the southwestern part of Poland; unlike the others, it is made of crystalline rocks locally covered with sedimentary rocks. The best parameters were recorded in the Karkonosze region, where the geothermal gradient temperature is approx. 4 °C/100 m. The rock temperature here reaches around 165 °C at a depth of 4000 m. The estimated gross electrical power possible for unit production for this depth is 12–13 kWh/(m³/h) [32].

Based on the geological and geothermal conditions in Poland, three main criteria were considered for further investigation. They include (i) reservoir characteristics—thermal parameters, (ii) reservoir characteristics—permeability and porosity, as well as (iii) reliability of the system. These criteria are schematically presented in Figure 2.

Regarding the potential for CO₂ sequestration in Poland, the best structures are sedimentation basins, which are usually located in tectonically stable areas. Studies already conducted on the potential of sCO₂-EGS in Poland were based on the Krośniewice-Kutno area [24,25]. Thus, taking into account high geothermal potential in the western part of Poland and due to the depth and expected favorable thermal conditions as well as petrophysical parameters, the Gorzów Block was chosen as a structure for further investigation within this study. In this area, the temperature reaches 145 °C in the Osno-IG1 well (4950 m depth). The fractured zone permeability is 425.57 mD and porosity 0.03. This region was also selected based on regional structural modeling, including the distribution of the main petrophysical parameters of the rocks (density, porosity, permeability, etc.).

2. Materials and Methods

The geological parameters and results obtained from the Gorzów Block geological model, including the sCO₂ flows, indicate the potential for heat utilization in the EGS unit in this area. Thus, the main goal of the study was to assess the sCO₂-EGS performance based on Gorzów Block conditions in the Polish Lowlands area by evaluating the energy key performance indicators of the investigated system. The analyzed productivity and efficiency of such a system will point out the possible energy outputs of the facility as well as underline the technological gaps or obstacles which should be addressed in the future research in the field of enhanced geothermal systems in Poland.

2.1. Case Studies

Within the study, two different sCO₂-EGS configurations are investigated. Variants were selected on the basis of different scenarios which aimed to maximize net total power, based on following formula:

$$W_{net}=W_{{CO}_2 turbine}+W_{ORC turbine}-\sum W_{pump,i}$$

(1)

where W is total power and ‘CO₂ turbine’, ‘ORC turbine’, ‘pump’ refer to particular equipment parts.

The following units were proposed for assessment:

• Case 1: direct sCO₂ cogeneration cycle; unit with direct sCO₂ expansion in the turbine for combined power and heat generation;
• Case 2: hybrid cycle for power generation only; unit with direct sCO₂ turbine and organic Rankine cycle (ORC) only for power generation.

Both cases are presented in Figure 3. This work evaluates facilities in terms of their performance mainly from an energetic standpoint in two distinct configurations, including a cogeneration plant and an electricity-only generation plant. In the first one, heated sCO₂ is brought to the surface and transfers the heat to the district heating system (DHS). Afterward, it is expanded in an sCO₂ turbine for electricity generation. Subsequently, it is mixed with the make-up stream, compressed (if needed), and cooled before entering the injection well. The cogeneration of heat and electricity increases the system’s energy utilization efficiency, which was also proved by the authors in [24]. In the second system, the primary working fluid exiting the sCO₂ turbine feeds into the ORC system, which utilizes heat only for electricity production in the ORC turbine.

2.2. Process Synthesis and Design

Based on the geothermal model of Gorzów Block, the simulations of the two cases were performed in IPSEpro 8.0 software. This software allows the building of simulation models of power systems through a graphical interface. Building a model involves assembling a system from components for individual devices and determining the relationships between them. The model of each component is based on algebraic equations (in particular, the equations of mass and energy balances), mathematically describing the processes occurring in these devices. IPSEpro software allows for the following:

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Process modeling of different energy systems;

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Conducting thermodynamic analyses;

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Simulation and analysis of real thermal systems;

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Optimization of thermal systems [33].

In addition, IPSEpro, as a process modeling software, has its limitations related to the level of detail for complex models that can be efficiently simulated or referred to integration with other tools, although there are additional modules that can also provide the ability to create their own models from scratch.

This study aims to provide insights into the thermodynamic behavior and performance optimization of EGSs through advanced process simulation. Thus, two different system models were developed, reflecting differences in the layout and performance of included components. The methodology applied in this work involves integrating geological data, thermodynamic principles, and operational parameters to develop accurate representations of EGS behavior under various conditions. Through systematic analysis and optimization, key performance metrics such as energy efficiency, power output, and system reliability are evaluated. Each plant simulation included three different working fluid mass flows: 50, 100, and 150 kg/s. Further modeling assumptions are presented in

Table 1. Summary of assumptions for process modeling.

Parameter	Value
Isentropic Efficiency:
sCO2 Turbine	85%
sCO2 Compressor	94%
ORC Turbine	85%
ORC Pump	94%
Mechanical efficiency:
Turbine	98%
Generator	98%
Motors (for pumps and compressors)	98%
Generator electrical efficiency	96%
Pressure drops in heat exchangers	0.5 bar
Minimum temperature difference for heat exchangers	10 K
Injection temperatures (50/100/150 kg/s)	38/43/44.5 °C
Cooling water parameters:
Temperatures	20/25 °C
Pressure	1 bar
District heating system parameters	50/var °C

.

Those key working fluid parameters were assumed to maintain the supercritical state of carbon dioxide as well as to provide stable unit operation along with assuring buoyant drive to reduce the electricity requirements for circulating pumps. The flow rates have been adjusted to the reservoir parameters based on the Gorzów Block geothermal model to ensure maximum energy production and stable, long-term operation of the installation. Different flow rate values were also considered to demonstrate the variability in unit performance.

Other process modeling assumptions regarding Gorzów Block were taken into account, as follows:

• The circulating medium is pure CO₂, free from pollutants and other particles;
• Plant operating hours: 7884 h (90% of the year);
• Geothermal doublet—1 injection and 1 production well.

2.3. Assessment Methods

Process modeling allows for the assessment of the overall energy efficiency of the power plant by analyzing the conversion efficiency of geothermal heat into electricity or heat generation. It indicates how effectively the system utilizes the available thermal energy at the surface level to generate power. Thus, key performance indicators used to evaluate the simulation of the sCO₂-EGS unit include the following:

• Power output—gross and net electricity generation under various operating conditions. This parameter is crucial for assessing the capacity of the plant to meet energy demand and for optimizing the system’s power generation capabilities.
• Heat output—quantified the amount of heat transferred to the district heating system under particular conditions.
• Exergy output—quantified the amount of exergy transferred to the district heating system under particular conditions associated with the thermal energy generated by the plant, calculated based on the average mean temperature difference.

Moreover, based on mathematical modeling of reservoir characteristics, it was possible to estimate the CO₂ loss ratio and, consequently, the total amount of CO₂ sequestrated during the system operation.

3. Results

Since pressure difference and production temperature play key roles in the plant performance evaluation, the parameters were assessed at the beginning of system operation (year 0) and at the end (year 30) for three different mass flows. The wellhead pressure difference corresponds to the net resulting thermosiphon effect, which reduces, or even removes completely, the need for sCO₂ compression before entering the injection well. With the increase in mass flow rate, both the working fluid production temperature and the wellhead pressure difference decrease. At higher mass flows, the thermosiphon effect is increasingly challenged by larger frictional pressure losses combined with less initial driving force from density differences in the two wells. The end result of these effects can be observed in Figure 4.

3.1. Energy Assessment

Results obtained during energy assessment conducted based on mathematical modeling and plant simulation show the outputs of two analyzed power plant cases. The main outcomes from process modeling and comparison between the two cases are presented in

Table 2. Key performance indicators of analyzed sCO₂-EGS cases (working fluid mass flow of 100 kg/s).

Parameter	Case 1 Direct sCO2 Cycle (Power and Heat Generation)	Case 2 Indirect sCO2 Cycle with ORC (Only Power Generation)
Gross power (at time = 0), MWe	0.8	2.1
Net power (at time = 0), MWe	0.5	1.8
Total gross electricity production, MWhe	175,884	451,058
Total electricity own consumption, MWhe	55,609	81,818
Total net electricity production, MWhe	120,235	369,240
Total DHS heat supply, MWhth	2,219,119	n/a
Total DHS exergy supply, MWhth	396,693	n/a
Total amount of CO2 sequestrated, ton	10,764,096

. The values correspond to a nominal mass flow of 100 kg/s. Total production, consumption, or supply refers to values accumulated over 30 years.

As can be noted in Figure 5, the stream determines the power output of the system; thus, the cases were also evaluated in order to investigate their performance under different working fluid mass flows. The change in both electricity and heat production for three levels of mass flow was assessed over the system’s entire lifetime of 30 years. The highest mass flow reaches the highest energy outputs for up to 18 years. Its rapid decrease is also visible on the graphs, which stems from the faster drop in temperature and pressure in the system.

3.1.1. Case 1 Direct sCO₂ Cycle (Power and Heat Generation)

In the following Figure 6 and Figure 7, the characteristics of electrical as well as heat output from the simulations of the first case with direct sCO₂ cycle are presented. The graphs reflect the change in production over 30 years of operation, including gross and net electricity generation and heat and exergy supply to the DHS.

The simulations were performed for different mass flows, among which the highest mass flow was characterized by the highest energy performance but also experienced the most dynamic drop. The significant output from heat production is noticeable for all analyzed variables. What is worth mentioning is that more than 95% of auxiliaries’ power consumption is associated with the CO₂ make-up booster. The overall results for Case 1 are summarized in

Table 3. Main operational results from energy assessment for Case 1.

Parameter	50 kg/s	100 kg/s	150 kg/s
Net electricity production, MWhe	24,732.3	120,235.1	163,161.7
DHS heat supply, MWhth	926,901.3	2,219,119.2	2,754,275.2

.

3.1.2. Case 2 Indirect sCO₂ Cycle with ORC (Only Power Generation)

Figure 8 presents the annual electricity production from the second analyzed configuration that includes ORC. Due to additional power generated from the ORC turbine and lack of heat supply, the electrical output is three to five times higher than in Case 1. Additionally, more than 60% of auxiliaries’ power consumption is associated with the CO₂ make-up booster in this configuration. Case 2 reflects approximately 50/50% production between the ORC and sCO₂ cycle. The overall results for Case 2 are summarized in

Table 4. Main operational results from energy assessment for Case 2.

Parameter	50 kg/s	100 kg/s	150 kg/s
Gross electricity production, MWhe	179,823.7	451,058.1	566,014.5
Net electricity production, MWhe	117,991.7	369,240.1	473,362.9

.

4. Discussion

Many technical, economic, and environmental factors must be considered when choosing a working fluid. Quoilin et al. [21] have developed a detailed guideline that may be useful in the selection of ORC units. In practice, only a few fluids may be deployed when creating a geothermal binary plant. Among the most frequently used are isobutane, n-pentane, and R245fa, whose performance in EGS is similar; thus, the most eco-friendly and economically justified one should be chosen [23]. Supercritical CO_2, compared to other fluids, represents an interesting alternative due to the possibility of its direct use and partial deposition in rock formations, which can be considered as part of the carbon capture, utilization, and storage chain. Injected CO₂ will be permanently stored in the reservoir through different mechanisms, and the system must be regularly supplied with CO₂ make-up.

The fluid loss rate depends on site-specific parameters such as permeability, porosity, water chemistry, and mineralogy. The circulation tests conducted at the first EGS site at Fenton Hill, New Mexico, indicated water losses in the range of 7–12% of injected rates [8], although the newest literature underlines that in sCO₂-EGS units, the sequestration rate should not be calculated based on water flow in traditional systems. Therefore, for such studies, a detailed geothermal model should be developed [11]. sCO₂-EGS units have a similar environmental impact to water-based systems, just as the costs of electricity generation range over similar values from around 90 to 250 EUR/MWh_e [17,18,19]. The costs are site-specific and depend mainly on the wells’ depth, temperature, and applied configuration. Nevertheless, compared to conventional shallow geothermal units (LCOE around 60 EUR/MWh_e [25]) the costs still remain high and may decrease with future technology development. However, in terms of economics, the operational costs of CO₂ sequestration may be offset via coupling with a geothermal operation [34]. Deposited CO₂ may potentially be eligible for carbon credits based on existing legislation, although the eligibility for carbon credits is determined by specific regulatory frameworks and carbon trading schemes that set the criteria for permanent CO₂ sequestration. Carbon credits for EGSs as units that provide electricity or/and heat production, as well as CO₂ sequestration, must be addressed in the future.

The performance of enhanced geothermal systems is dependent on the rock formation parameters; thus, the ability of the geothermal reservoir to maintain its heat exchange capacity over extended periods is crucial. Continuous monitoring and modeling are necessary to predict and manage the depletion rates and ensure the longevity of the geothermal source. Moreover, to ensure the safety of its operation, detailed risk assessment along with seismic activity monitoring and system management have to be taken into account.

5. Conclusions

The goal of this paper was to present the energy performance of an enhanced geothermal system with supercritical carbon dioxide as a working fluid assessed for the specific conditions of the Gorzów Block in Poland. The main conclusions stemming from this study are as follows:

• The geological properties of the reservoir play a crucial role in determining the feasibility and effectiveness of EGS units. Key factors include the permeability of the rock formation, its porosity, temperature gradient, and depth. High permeability is essential for fluid circulation and heat extraction, while adequate porosity allows for fluid storage. A reservoir with a substantial temperature gradient and sufficient depth ensures access to the desired heat source for efficient power production or power and heat generation.
• Both analyzed cases utilized a thermosiphon effect, which enabled the system to operate without an additional CO₂ compressor installed before the injection well, which stems from the pressure difference between the wellheads and reduces the consumption of electricity for the auxiliaries.
• At nominal flows (100 kg/s), sCO₂-EGS can produce:

  -

  Case no 1: 0.4 MW_e and 9 MW_th for up to 18 years;

  -

  Case no 2: 1.7 MW_e for up to 20 years.

  After this period, fairly stable conditions can be observed for lower mass flows, or a gradual decline, the dynamics of which are dependent on the flow rate.
• The proposed simulation may have a valuable impact on further techno-economic assessments, taking into account additional income related to disposing of CO₂ through its sequestration, which may change the revenue calculation significantly.

The study findings give valuable insights into the design, operation, and optimization of EGSs for maximizing energy extraction. This research contributes to the advancement of geothermal energy technologies and underlines the significance of process modeling in optimizing the performance of renewable energy systems.