**3.2. Environmental and economic aspectsof EGS for power generation**

create a large vertical fracture, and ultimately to drill a second well to access that fracture at some distance above the first wellbore (Huenges, 2010; Stephens & Jiusto, 2010). It was the first attempt anywhere to make a deep, full-scale HDR reservoir. Significant research and production plants are under construction or operation to take advantage of this abundant renewable energy opportunity (Kalra et al., 2012). For example, power plants driven by EGS are currently being developed and tested in Australia, Germany, Japan, France, USA and Switzerland (DiPippo, 2008; Azim et al., 2010).The largest EGS project in the world is a 25 MWe demonstration plant currently being developed in Cooper Basin, Australia.There are several EGSprojects that are already or will soon produce power. Someof them are just for‐ research and development (R& D) and some are for commercial purpose. Examples of theEGS projects around the world with their location, capacity, welldepth, plant type, and

**EGS Project Type Country Capacity (MWe) Plant Type Depth (km)**

Cooper Basin Commercial Australia 250-500 Binary (Kalina) 4.3 The Geysers Demonstration USA (Unknown) Flash 3.5-3.8 Ogachi R&D Japan (Unknown) Flash 1.0-1.1

Commercial United Kingdom 10 Binary

An EGS project has several stages (Majorowicz& Grasby, 2010; Huenges, 2010; DiPippo, 2008); namely: (1) Identifying a potential site possessing the necessary characteristics through surface exploration, (2) Drilling an injection well to the depth required to reach the desired temperature, (3) Fracturing the rock in the subsurface by hydraulic stimulation (i.e. injecting a fluid at sufficient pressure to propagate fracture), (4) Creating and testing of the EGS reservoir storage capacity, (5) Drilling a production well for a doublet system or two‐ production wells for a triplet system (one injection & two production wells), (6) Creating fracture connectivity between the injection andthe production wells, (7) Extracting heat en‐

(ORC)

(ORC)

(ORC)

(ORC)

(ORC)

(ORC)

4.2

(Unknown)

3.3

4.1

4.5

3-4

Soultz (EU) R&D France 1.5 Binary

Desert Peak R&D USA 11-50 Binary

Landau Commercial Germany (EU) 3 Binary

Paralana (phase I) Commercial Australia 7-30 Binary

Eden Project Commercial United Kingdom 3 Binary

**Table 3.** EGS-based projects around the world (Azim et al., 2010).

the project type are summarized in Table 3.

316 New Developments in Renewable Energy

United Downs, Redruth

It was reported (Grasby et al., 2011) that impacts of geothermal development are relatively minor compared to many other energy developments, however there are still importantchal‐ lenges to be addressed. More particularly, it was reported (Azim et al., 2010) that the overall impact of EGS power generation plants on environment is remarkably lower than other con‐ ventional fossil fuel-fired and nuclear power plants.In addition, EGS plants may have lower impacts in comparison with other renewable energy sources such as solar, wind, hydroelec‐ tric, and biomass on an equivalent energy-output basis. This is primarily because a geother‐ mal energy source is contained subsurface and need not to be exposed in the atmosphere and the surface energy conversion system (e.g. ORC unit) is relatively compact, thus making the overall size of the entire system attractively small. EGS power plants also provide envi‐ ronmental benefits by having minimal GHGand other emissions (zero emissions for the case of using ORC technology). Distinct from the conventional fossil fuels, there are minimal dis‐ charges of CO2, nitrogen or sulphur oxides or particulate matter resulting from its use, and there is no need to dispose radioactive materials. However, still there are impacts that must be considered and managed if enhanced geothermal energy resource is to be developed as part of a more environmentally sound, sustainable energy source for the future. The major environmental challenges for EGS are associated with ground water use and contamination, with related concerns about induced seismicity or subsidence as a result of water injection and production. Induced seismicity is a phenomenon in which a change in fluid pressure within a stressed rock formation leads to movement of the fractured rocks; the energy re‐ leased is transmitted through the subsurface rock and may reach the surface with enough intensity to be heard or felt by people in the surrounding region (DiPippo, 2008; Majer et al., 2007). To mitigate risks related to induced seismicity, strategies and procedures are needed to set requirements for seismic monitoring and for prolonged EGS field operation. Technolo‐ gies for imaging fluid pathways induced/injected by hydraulic stimulation in EGS fields would constitute a key improvement of the EGS concept. Issues of noise, safety and land use associated with drilling and production operations are also important but can be fully man‐ ageable (Huenges, 2010; Majorowicz& Grasby, 2010; Azim et al., 2010).

EGS technology has impacts on the global and local environment (Huenges, 2010; DiPippo, 2008). Therefore, it is important to identify and evaluate any impact which results from the implementation of an EGS plant at the beginning of a project. The goal must be to avoid or minimize adverse impacts on the environment during all stages of an EGS project (e.g., con‐ struction, operation, and deconstruction) and to meet the objectives and requirements of en‐ vironment protection and preservation of finite resources (Huenges, 2010; Stephens & Jiusto, 2010). Potential impactsneed to be addressedduring planning environmentally sound EGS plants. The relevance and extent of the addressed impacts can vary from location to location. Even if EGS plants are not related to (continuous) gaseous emissions during operation (due to the carrying of the geothermal fluid in a closed loop system on the surface), environmen‐ tal impacts such as airborne emissions or the consumptions of the finite energy resources (such as steel used for well completion or fuel for drilling rig operation) occur during other life cycle stages, therefore, all life cycle stages need to be considered in order to analyze the environmental performance of an EGS plant. In this regard, life cycle analysis (LCA) is a widely applied approach to evaluate and compare specific environmental impacts of differ‐ ent products or technologies (Huenges, 2010; Frick et al., 2010). The idea is to carry out a de‐ tailed analysis of the life cycle of the product or a duty emerged in response to increased environmental awareness of the public, the governments, and the industries. An LCA in‐ volves two main stages: the collection of a data, related to the product or duty and relevant for the environment, and interpretation of the collected information. For transparency and traceability of LCA results, standards, such as ISO 14040, ISO 14044 have been developed (Huenges, 2010; Frick et al., 2010). Based on this approach, aspects, which influence the envi‐ ronmental impact during the life cycle, and parameters, which need to be considered in the planning of the environmentally sound EGS plants, can be identified. According to given standards, the LCA is carried out in four steps: (1) Goal and scope definition to assess select‐ ed environmental effects (e.g., global warming potential, cumulated demand of finite energy resources, etc.) in the different life cycle stages and throughout the whole life cycle of EGS plants, (2) Inventory analysis, (3) Impact analysis to quantify the environmental effects (all inventoried material and energy flows are transformed to different impact indicators based on certain conversion factors), and (4) Interpretation of the results from the impact analysis.

**1.** Drilling operations: Drilling operations have a large impact on the surrounding envi‐ ronment and are associated withvarious risks. They are normally limited to the period of drilling operations (i.e., last only a couple of weeks or months). Environmental effects related to drilling operations may include: noise emissions, subsurface emissions, site preparations and alterations, airborne emissions, water usage (usually taken from near‐ by surface or groundwater bodies), waste disposal, and visual impact (due to night-

ORC-Based Geothermal Power Generation and CO2-Based EGS for Combined Green Power Generation and CO2

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**2.** Reservoir stimulation: Enhancing a geothermal reservoir for power generation includes injection of stimulation fluids under high pressure which could produce geo-mechani‐ cal changes in the subsurface (rocks may slip along pre-existing fractures. Depending on the surrounding rock formations and their magnitude, geo-mechanical alterations can be followed by build-up and dispersion of microseismic activities up to the surface. Induced seismicity, which can result from stimulation, helps to identify the extent of the fracture network in the reservoir. Despite the fact that geo-mechanical changes can lead to damage of buildings and even be hazardous for human beings and animals, the earth tremors caused by EGS reported so far can be categorized a sensible but not as adverse impacts on the environment.In almost all cases, these events in the deep reservoir are of such low magnitude that they are not felt at the surface. However, based on the present state of knowledge, larger impacts cannot be totally excluded since the knowledge about the stress situation and the development of larger microseismic events in the sub‐

**3.** Reservoir exploitation: The exploitation of an EGS reservoir can lead to different altera‐ tions in the reservoir and the surrounding subsurface. Impacts on the local subsurface environment such as hydraulic and thermal alterations as well as circulation losses need to be considered for sustainable reservoir management, but are not considered as

**4.** Installation and operation of surface facilities: The installation of the subsurface part of an EGS power plant, such as a binary ORC power unit is related to general environ‐ mental impacts which come with construction work such as noise emissions and dust

**5.** Dismantling of well infrastructure: In EGS plants, the decommissioning of the deep wells need be managed since abandoned wells are a potential source for material emissions to

For the development of EGS projects, different environmental regulations, standards, and permission procedures are binding depending on the country's legislations. A widely used tool in this context is the environmental impact assessment (EIA); first established in USA in the 1970s and used today in many countries to ensure that all possible environmental im‐ pacts of planned EGS projects are identified and assessed before a decision is made for get‐ ting permission for an EGS project for power generation (Huenges, 2010). The most

the subsurface. With proper closing and filling of the wells, this can be eradicated.

important steps of atypical EIA process are briefly outlined in Figure 4.

time lighting, etc., from drilling rig).

surface is still insufficient.

adverse environmental impacts.

creation.

EGS plants are related to different impacts on diverse parts of the local environment and dif‐ ferent characteristics regarding their duration (continuous or temporary), reversibility, and their degree of probability. Many of the impacts on the local environment are related to as‐ sessing the EGS reservoir. Most of these impacts are known and technologicallymanageable based on the oil and gas exploration experiences. Adverse effects due to reservoir exploita‐ tion can be avoided with proper reservoir management and monitoring. Environmental im‐ pacts from the construction and operating the surface facilities are comparatively low (Dickson & Fanelli, 2005; DiPippo, 2008). In an EGS project, some environmental impacts and risks, however,need to be considered and evaluatedin connection withthe following EGS phases and activities (Huenges, 2010; Majer et al., 2007; DiPippo, 2008; Stephens & Jius‐ to, 2010):

**1.** Drilling operations: Drilling operations have a large impact on the surrounding envi‐ ronment and are associated withvarious risks. They are normally limited to the period of drilling operations (i.e., last only a couple of weeks or months). Environmental effects related to drilling operations may include: noise emissions, subsurface emissions, site preparations and alterations, airborne emissions, water usage (usually taken from near‐ by surface or groundwater bodies), waste disposal, and visual impact (due to nighttime lighting, etc., from drilling rig).

EGS technology has impacts on the global and local environment (Huenges, 2010; DiPippo, 2008). Therefore, it is important to identify and evaluate any impact which results from the implementation of an EGS plant at the beginning of a project. The goal must be to avoid or minimize adverse impacts on the environment during all stages of an EGS project (e.g., con‐ struction, operation, and deconstruction) and to meet the objectives and requirements of en‐ vironment protection and preservation of finite resources (Huenges, 2010; Stephens & Jiusto, 2010). Potential impactsneed to be addressedduring planning environmentally sound EGS plants. The relevance and extent of the addressed impacts can vary from location to location. Even if EGS plants are not related to (continuous) gaseous emissions during operation (due to the carrying of the geothermal fluid in a closed loop system on the surface), environmen‐ tal impacts such as airborne emissions or the consumptions of the finite energy resources (such as steel used for well completion or fuel for drilling rig operation) occur during other life cycle stages, therefore, all life cycle stages need to be considered in order to analyze the environmental performance of an EGS plant. In this regard, life cycle analysis (LCA) is a widely applied approach to evaluate and compare specific environmental impacts of differ‐ ent products or technologies (Huenges, 2010; Frick et al., 2010). The idea is to carry out a de‐ tailed analysis of the life cycle of the product or a duty emerged in response to increased environmental awareness of the public, the governments, and the industries. An LCA in‐ volves two main stages: the collection of a data, related to the product or duty and relevant for the environment, and interpretation of the collected information. For transparency and traceability of LCA results, standards, such as ISO 14040, ISO 14044 have been developed (Huenges, 2010; Frick et al., 2010). Based on this approach, aspects, which influence the envi‐ ronmental impact during the life cycle, and parameters, which need to be considered in the planning of the environmentally sound EGS plants, can be identified. According to given standards, the LCA is carried out in four steps: (1) Goal and scope definition to assess select‐ ed environmental effects (e.g., global warming potential, cumulated demand of finite energy resources, etc.) in the different life cycle stages and throughout the whole life cycle of EGS plants, (2) Inventory analysis, (3) Impact analysis to quantify the environmental effects (all inventoried material and energy flows are transformed to different impact indicators based on certain conversion factors), and (4) Interpretation of the results from the impact analysis.

318 New Developments in Renewable Energy

EGS plants are related to different impacts on diverse parts of the local environment and dif‐ ferent characteristics regarding their duration (continuous or temporary), reversibility, and their degree of probability. Many of the impacts on the local environment are related to as‐ sessing the EGS reservoir. Most of these impacts are known and technologicallymanageable based on the oil and gas exploration experiences. Adverse effects due to reservoir exploita‐ tion can be avoided with proper reservoir management and monitoring. Environmental im‐ pacts from the construction and operating the surface facilities are comparatively low (Dickson & Fanelli, 2005; DiPippo, 2008). In an EGS project, some environmental impacts and risks, however,need to be considered and evaluatedin connection withthe following EGS phases and activities (Huenges, 2010; Majer et al., 2007; DiPippo, 2008; Stephens & Jius‐

to, 2010):


For the development of EGS projects, different environmental regulations, standards, and permission procedures are binding depending on the country's legislations. A widely used tool in this context is the environmental impact assessment (EIA); first established in USA in the 1970s and used today in many countries to ensure that all possible environmental im‐ pacts of planned EGS projects are identified and assessed before a decision is made for get‐ ting permission for an EGS project for power generation (Huenges, 2010). The most important steps of atypical EIA process are briefly outlined in Figure 4.

estimation of the costs and revenues, which are related to a project. Since EGS projects are characterized by a long planning period, large initial investments and a long technical life‐ time, estimating prospective costs and revenues involves uncertainties and risks (Huenges, 2010). This is true because no reliable statements on markets development, detailed geologic site conditions, or technological problems can be made at the start of a project. In order to minimize existing risks, cost influences must be known and risks must be analyzed. The to‐ tal costs of an ORC-based EGS project are dominated by the investments at the start of the project (Huenges, 2010; Stephens & Jiusto, 2010; DiPippo, 2008). These investments mainly consist of costs for the subsurface components, including: (1) reservoir exploration, (2) well drilling and completion - the most significant cost factor in all geothermal operations is asso‐ ciated with drilling and well completion (Grasby et al., 2011), (3) reservoir engineering measures, (4) installation of the geothermal fluid loop, and (5) construction of the ORCbased power unit.Typical EGS drilling costs as a function of well depth are shown in Table 4. As mentioned before, drilling costs estimated to account for a third to a half of EGS pro‐ jected costs; a major challenge to the EGS technology (Stephens & Jiusto, 2010). As shown in Table 4, EGS well costs are not a linear function of depth, but additionally reflect, tempera‐ ture, extent of casing employed, difficulty in drilling, and lithologic characteristics(Grasby et al., 2011). In addition, surface related costs, such as operation & maintenance O& M costs (i.e., annual operating costs for personnel, material consumption, overhaul and mainte‐ nance) are considered. The operating and maintenance cost of an EGS power plant has two important components: (a) the O& M for the ORC power plant and (b) the well field mainte‐ nance cost. The ORC power plant O& M costs were estimated based on experience in similar power plant configurations and ORC installations. This is usually a percentage of the instal‐ led cost of the power plant on a yearly basis (Kalra et al., 2012). It was reported (Grasby et al., 2011) that the techniques and technologies related to EGS projects are evolving rapidly so as its estimated costs and that EGS technology may not be commercially viable at this time. However, it was suggested (Grasby et al., 2011) that renewable resources such as EGS

ORC-Based Geothermal Power Generation and CO2-Based EGS for Combined Green Power Generation and CO2

offer attractive power market contributions beyond power generation.

**Table 4.** Typical EGS drilling costs as a function of well depth (Azim et al., 2010).

**Well Depth (km) EGS Well Category Estimated EGS Drilling Cost (\$**

**1.5** Shallow well 0.90 **2.5** 1.81 **3.0** 2.55 **4.0** Mid range well 5.10 **5.0** 6.45 **6.0** Deep well 8.92 **7.5** 13.83

**millions)**

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**Figure 4.** A schematic diagram showing the important steps in an EIAprocessfor an EGS project (Huenges, 2010).

Costs for geothermal plants have been dropping and are becoming more competitive (Pruess, 2006). EGS-based ORC power technology has the potential to replace other more costly and environmentally destructive technologies.It also has the potential to provide greenpower generation at affordable prices, thereby improving the standard of living andso‐ cio-economic potential through creation of jobs in many regions (Grasby et al., 2011). The‐ planning of EGS projects and especially the decision to realize a project is based on the estimation of the costs and revenues, which are related to a project. Since EGS projects are characterized by a long planning period, large initial investments and a long technical life‐ time, estimating prospective costs and revenues involves uncertainties and risks (Huenges, 2010). This is true because no reliable statements on markets development, detailed geologic site conditions, or technological problems can be made at the start of a project. In order to minimize existing risks, cost influences must be known and risks must be analyzed. The to‐ tal costs of an ORC-based EGS project are dominated by the investments at the start of the project (Huenges, 2010; Stephens & Jiusto, 2010; DiPippo, 2008). These investments mainly consist of costs for the subsurface components, including: (1) reservoir exploration, (2) well drilling and completion - the most significant cost factor in all geothermal operations is asso‐ ciated with drilling and well completion (Grasby et al., 2011), (3) reservoir engineering measures, (4) installation of the geothermal fluid loop, and (5) construction of the ORCbased power unit.Typical EGS drilling costs as a function of well depth are shown in Table 4. As mentioned before, drilling costs estimated to account for a third to a half of EGS pro‐ jected costs; a major challenge to the EGS technology (Stephens & Jiusto, 2010). As shown in Table 4, EGS well costs are not a linear function of depth, but additionally reflect, tempera‐ ture, extent of casing employed, difficulty in drilling, and lithologic characteristics(Grasby et al., 2011). In addition, surface related costs, such as operation & maintenance O& M costs (i.e., annual operating costs for personnel, material consumption, overhaul and mainte‐ nance) are considered. The operating and maintenance cost of an EGS power plant has two important components: (a) the O& M for the ORC power plant and (b) the well field mainte‐ nance cost. The ORC power plant O& M costs were estimated based on experience in similar power plant configurations and ORC installations. This is usually a percentage of the instal‐ led cost of the power plant on a yearly basis (Kalra et al., 2012). It was reported (Grasby et al., 2011) that the techniques and technologies related to EGS projects are evolving rapidly so as its estimated costs and that EGS technology may not be commercially viable at this time. However, it was suggested (Grasby et al., 2011) that renewable resources such as EGS offer attractive power market contributions beyond power generation.


**Table 4.** Typical EGS drilling costs as a function of well depth (Azim et al., 2010).

**Figure 4.** A schematic diagram showing the important steps in an EIAprocessfor an EGS project (Huenges, 2010).

320 New Developments in Renewable Energy

Costs for geothermal plants have been dropping and are becoming more competitive (Pruess, 2006). EGS-based ORC power technology has the potential to replace other more costly and environmentally destructive technologies.It also has the potential to provide greenpower generation at affordable prices, thereby improving the standard of living andso‐ cio-economic potential through creation of jobs in many regions (Grasby et al., 2011). The‐ planning of EGS projects and especially the decision to realize a project is based on the

#### **3.3. EGS using CO2 as the working fluid for green power generation and simultaneous carbon sequestration**

mal installation situated close to a coal-fired power plant), the captured CO2 from the plant can be run through the geothermal reservoir first and then sequestered in a geologic seques‐

ORC-Based Geothermal Power Generation and CO2-Based EGS for Combined Green Power Generation and CO2

back to the EGS reservoir. For example, in a study by (Gurgenic et al., 2008), it was reported that there is a significant potential to use supercritical CO2 as working fluid in the power loop as illustrated (Gurgenic et al., 2008) in Figure **5**. Significantly higher energy conversion efficiencies were predicted using a single-loop system with the CO2 being both the heat exchange and the power cycle working fluid. It was reported (Gurgenic et al., 2008; Atrens et al., 2011) that the loops in either of the two cycles (i.e. subsurface loop and surface power loop) do not have to be closed. For example, if there is ready access to CO2 (e.g., at a geothermal installation situated close to a coal-fired power plant), the captured CO2 from the plant can be run through the geothermal reservoir first and then sequestered in a

CO2-based EGS has been examined in (Atrens et al., 2011) from a reservoir oriented perspec‐ tive, and as a result thermodynamic performance was investigated. It was reported (Atrens et al., 2011) that economics of the system are still not well understood, however. In their study, the economics of the CO2–based EGS technology was explored for an optimized pow‐ er plant design and best-available cost estimation data. It was demonstrated in (Atrens et al., 2011) that near-optimum turbine exhaust pressure can be estimated from surface tempera‐ ture. It was found that achievable cooling temperature is an important economic site consid‐ eration alongside EGS resource temperature. The role of sequestration as part of CO2–based EGS was also examined in (Atrens et al., 2011), and it was concluded that if fluid losses oc‐ cur, the economic viability of the concept depends strongly on the price associated with CO2 (Atrens et al., 2011). Potential barriers to implementation of CO2–based EGS technology in‐ clude access to CO2 at an acceptable cost, proximity of the EGS to the electricity grid, and access to cooling water. Similar issues related to long-term responsibility for the resultant reservoir, including the liability for future CO2 leakage from the geologic sequestration site.In another study by (Randolph & Saar, 2011), it was suggested that using CO2 as the working fluid in geothermal power systems may permit utilization of lower temperature geologic formations than those that are currently deemed economically viable,l eading to more widespread utilization of geothermal energy. However, additional exploration of eco‐ nomics regarding the opportunities and issues for CO2–based EGS technology for combined

CO2-based EGS has been examined in (Atrens et al., 2011) from a reservoir oriented perspective, and as a result thermodynamic performance was investigated. It was reported (Atrens et al., 2011) that economics of the system are still not well understood, however. In their study, the economics of the CO2–based EGS technology was explored for an optimized power plant design and best-available cost estimation data. It was demonstrated in (Atrens et al., 2011) that near-optimum turbine exhaust pressure can be estimated from surface temperature. It was found that achievable cooling temperature is an important economic site consideration alongside EGS resource temperature. The role of sequestration as part of CO2– based EGS was also examined in (Atrens et al., 2011), and it was concluded that if fluid losses occur, the economic viability of the concept depends strongly on the price associated with CO2 (Atrens et al., 2011). Potential barriers to implementation of CO2–based EGS technology include access to CO2 at an acceptable cost, proximity of the EGS to the electricity grid, and access to cooling water. Similar issues related to long-term responsibility for the resultant reservoir, including the liability for future CO2 leakage from the geologic sequestration site. In another study by (Randolph & Saar, 2011), it was suggested that using CO2 as the working fluid in geothermal power systems may permit utilization of lower temperature geologic formations than those that are currently deemed economically viable, leading to more widespread utilization of geothermal energy. However, additional exploration of economics regarding the opportunities and issues for CO2–based EGS technology for combined carbon sequestration and power generation is

Fig. 5. A conceptual model showing a single-loop system with CO2 used for combined heat

**Figure 5.** A conceptual model showing a single-loop system with CO2 used for combined heat exchange and power

An increasing concern of environmental issues of emissions & pollution, in particular global warmingand the constraints on consuming conventional energy sources has recently result‐ ed in extensive research into innovative renewable and green technologies of generating

Subsurface

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tration site of choice.

needed.

cycle (Gurgenic et al., 2008).

**4. Conclusion**

Heat exchange and power single-loop

carbon sequestration and power generation is needed.

exchange and power cycle (Gurgenic et al., 2008).

geologic sequestration site of choice.

It was reported (Pruess, 2006) that previous attempts to develop EGS in Japan, USA, Europe and Australia have all employedwater as a heat transmission fluid. Although, water has many properties that make it a favorable medium for this purpose, it also has serious short‐ comings. An unfavorable property of water is that it is astrong solvent for many rock miner‐ als, especially at elevated temperatures. In this case, injecting water at high pressure intohot rock fractures, as part of an EGS resource operation & utilization, results in strong dissolu‐ tion and precipitation effects that change fracture permeabilityand make it very difficult to operate an EGS reservoir in a stable manner. In 2000, Brown, D. (Pruess, 2006) proposed a novel EGS concept that would utilize supercritical CO2 instead of water as heat exchange (carrier) fluid, and would simultaneously achieve CO2 geologic sequestration as an addition‐ al benefit. There are only very few investigations that characterized the performance of CO2 as working fluid in EGS applications. For example, Pruess (Pruess, 2006) performed numeri‐ cal simulations and evaluated thermophysical properties in order to explore the heat trans‐ fer and fluid dynamics characteristics in an EGS reservoir that would be operated with CO2. It was found that CO2 is superior to water in its ability to exchange heat from hot fractured rock. Carbon dioxide also offers certain advantages with respect to wellbore hydraulics, in that its larger compressibility and expansivity as compared to water would increase buoyan‐ cy forces and would decrease the parasitic power consumption (thus reduce pumping cost) of the EGS fluid circulation system. This is because the larger expansivity of CO2 would gen‐ erate large density differences between the cold CO2 in the injection well and the hot CO2 in the production well, and therefore provide buoyancy force that would reduce the power consumption of the fluid circulation system. Another interesting feature of CO2 is that its lower viscosity, tend to yield larger flow velocities for a given pressure gradient. In addi‐ tion, CO2 would be much less effective as a solvent for rock minerals, which would reduce or eliminate scaling problems, such as silica dissolution and precipitation in water-based systems (Pruess, 2006). It was also reported (Pruess, 2006) that while the thermal and hy‐ draulic aspects of aCO2-based EGS system look promising, major uncertainties remain with regard to geochemical interactions betweenfluids and rocks. It was concluded in (Pruess, 2006) that an EGS system running on CO2 has sufficiently attractive features to warrant fur‐ therinvestigation. It was suggested that an EGS using CO2 as heat transport and exchange fluid could have favorable geochemical properties, as CO2 uptake and sequestration by rock minerals would be quite rapid.

Supercritical CO2 can also be used as the working fluid of the power cycle before it is sent back to the EGS reservoir. For example, ina study by (Gurgenic et al., 2008), it was reported that there is a significant potential to use supercritical CO2 as working fluid in the power loop as illustrated (Gurgenic et al., 2008) in Figure 5. Significantly higher energy conversion efficiencies were predicted using a single-loop system with the CO2 being both the heat ex‐ change and the power cycle working fluid. It was reported (Gurgenic et al., 2008; Atrens et al., 2011) that the loops in either of the two cycles (i.e. subsurface loop and surface power loop) do not have to be closed. For example, if there is ready access to CO2 (e.g., at a geother‐ mal installation situated close to a coal-fired power plant), the captured CO2 from the plant can be run through the geothermal reservoir first and then sequestered in a geologic seques‐ tration site of choice. back to the EGS reservoir. For example, in a study by (Gurgenic et al., 2008), it was reported that there is a significant potential to use supercritical CO2 as working fluid in the power loop as illustrated (Gurgenic et al., 2008) in Figure **5**. Significantly higher energy conversion

CO2-based EGS has been examined in (Atrens et al., 2011) from a reservoir oriented perspec‐ tive, and as a result thermodynamic performance was investigated. It was reported (Atrens et al., 2011) that economics of the system are still not well understood, however. In their study, the economics of the CO2–based EGS technology was explored for an optimized pow‐ er plant design and best-available cost estimation data. It was demonstrated in (Atrens et al., 2011) that near-optimum turbine exhaust pressure can be estimated from surface tempera‐ ture. It was found that achievable cooling temperature is an important economic site consid‐ eration alongside EGS resource temperature. The role of sequestration as part of CO2–based EGS was also examined in (Atrens et al., 2011), and it was concluded that if fluid losses oc‐ cur, the economic viability of the concept depends strongly on the price associated with CO2 (Atrens et al., 2011). Potential barriers to implementation of CO2–based EGS technology in‐ clude access to CO2 at an acceptable cost, proximity of the EGS to the electricity grid, and access to cooling water. Similar issues related to long-term responsibility for the resultant reservoir, including the liability for future CO2 leakage from the geologic sequestration site.In another study by (Randolph & Saar, 2011), it was suggested that using CO2 as the working fluid in geothermal power systems may permit utilization of lower temperature geologic formations than those that are currently deemed economically viable,l eading to more widespread utilization of geothermal energy. However, additional exploration of eco‐ nomics regarding the opportunities and issues for CO2–based EGS technology for combined carbon sequestration and power generation is needed. efficiencies were predicted using a single-loop system with the CO2 being both the heat exchange and the power cycle working fluid. It was reported (Gurgenic et al., 2008; Atrens et al., 2011) that the loops in either of the two cycles (i.e. subsurface loop and surface power loop) do not have to be closed. For example, if there is ready access to CO2 (e.g., at a geothermal installation situated close to a coal-fired power plant), the captured CO2 from the plant can be run through the geothermal reservoir first and then sequestered in a geologic sequestration site of choice. CO2-based EGS has been examined in (Atrens et al., 2011) from a reservoir oriented perspective, and as a result thermodynamic performance was investigated. It was reported (Atrens et al., 2011) that economics of the system are still not well understood, however. In their study, the economics of the CO2–based EGS technology was explored for an optimized power plant design and best-available cost estimation data. It was demonstrated in (Atrens et al., 2011) that near-optimum turbine exhaust pressure can be estimated from surface temperature. It was found that achievable cooling temperature is an important economic site consideration alongside EGS resource temperature. The role of sequestration as part of CO2– based EGS was also examined in (Atrens et al., 2011), and it was concluded that if fluid losses occur, the economic viability of the concept depends strongly on the price associated with CO2 (Atrens et al., 2011). Potential barriers to implementation of CO2–based EGS technology include access to CO2 at an acceptable cost, proximity of the EGS to the electricity grid, and access to cooling water. Similar issues related to long-term responsibility for the resultant reservoir, including the liability for future CO2 leakage from the geologic sequestration site. In another study by (Randolph & Saar, 2011), it was suggested that using CO2 as the working fluid in geothermal power systems may permit utilization of lower temperature geologic formations than those that are currently deemed economically viable, leading to more widespread utilization of geothermal energy. However, additional exploration of economics regarding the opportunities and issues for

CO2–based EGS technology for combined carbon sequestration and power generation is

Fig. 5. A conceptual model showing a single-loop system with CO2 used for combined heat exchange and power cycle (Gurgenic et al., 2008). **Figure 5.** A conceptual model showing a single-loop system with CO2 used for combined heat exchange and power cycle (Gurgenic et al., 2008).
