**2.2. Energy conversion and performance aspects of ORC-based low-temperature geothermal power generation**

The ORC is a thermodynamic Rankine cycle that uses an organic working fluid instead of steam (water). A schematic diagram showing a low-temperature geothermal ORC binaryfluid system used for electric power generation is shown in Figure 2. In this system, the first (primary) fluid being the geo-fluid (brine) is extracted from the low-temperature geothermal resource through the production well. The geo-fluid carries the heat from the liquid-domi‐ nated resource (thus called the geo-fluid heat carrier) and efficiently transfers this heat to the low-boiling point (BP) organic working fluid (the secondary fluid) using an effective heat exchanger; shell-and-tube heat exchangers arewidely used (Chandrasekharam& Bundschuh, 2008). In this binary-fluid system, the low-boiling point organic liquid absorbs the heat which is transferred by the geothermal fluid and boils at a relatively much lower tempera‐ ture (compared to water) and as a result develops significant vapor pressure sufficient to drive the axial flow or radial inflow turbine. The turbine is coupled to an electric generator which converts the turbinemechanical shaft power into electrical power. The organic work‐ ing fluid expands across theturbine and then is cooled and condensed in thecondenser be‐ fore it is pumped back as a liquid to the heat exchanger using a condensate

ORC-Based Geothermal Power Generation and CO2-Based EGS for Combined Green Power Generation and CO2 Sequestration http://dx.doi.org/10.5772/52063 309

**Figure 2.** A schematic diagram showing the basic concept of a low-temperature geothermal binary ORC system for electrical power generation.

**Figure 1.** Photograph of Chena ORC-based geothermal power plantat Chena, Alaska, USA (Holdmann, 2007).

**2.2. Energy conversion and performance aspects of ORC-based low-temperature**

fore it is pumped back as a liquid to the heat exchanger using a condensate

The ORC is a thermodynamic Rankine cycle that uses an organic working fluid instead of steam (water). A schematic diagram showing a low-temperature geothermal ORC binaryfluid system used for electric power generation is shown in Figure 2. In this system, the first (primary) fluid being the geo-fluid (brine) is extracted from the low-temperature geothermal resource through the production well. The geo-fluid carries the heat from the liquid-domi‐ nated resource (thus called the geo-fluid heat carrier) and efficiently transfers this heat to the low-boiling point (BP) organic working fluid (the secondary fluid) using an effective heat exchanger; shell-and-tube heat exchangers arewidely used (Chandrasekharam& Bundschuh, 2008). In this binary-fluid system, the low-boiling point organic liquid absorbs the heat which is transferred by the geothermal fluid and boils at a relatively much lower tempera‐ ture (compared to water) and as a result develops significant vapor pressure sufficient to drive the axial flow or radial inflow turbine. The turbine is coupled to an electric generator which converts the turbinemechanical shaft power into electrical power. The organic work‐ ing fluid expands across theturbine and then is cooled and condensed in thecondenser be‐

**geothermal power generation**

308 New Developments in Renewable Energy

pump to be re-evaporated, and the power cycle repeats itself. One of the most important performance criteria in low-temperature geothermal ORC power generation technology re‐ quires the optimal selection of the ORC organic working fluid. Organic fluids used in binary ORC technology have inherent feature (compared to water) and that is they have low boil‐ ing temperature and high vapor pressure at relatively low temperatures, compared with steam (water) (Dickson & Fanelli, 2005).

Typical ORC organic fluids may include pure hydrocarbons (e.g. pentane, butane, propane, etc), refrigerants (e.g. R134a, R218, R123, R113, R125, etc), or organic mixtures (Panea et al., 2010; Saleh et al., 2007; Hung, 2001; Wei et al., 2007). The optimal energy conversion per‐ formance of a low-temperature geothermal ORC power generation system depends mainly on the type of organic fluid being used in the system (Ismail, 2011a). The selection of the type of organic fluid is normally based on the following criteria (Hettiarachchi et al.,2007; Saleh et al., 2007; Chandrasekharam& Bundschuh, 2008; Ismail, 2011b):


It should be noted that many binary ORC fluids may not meet all these criteria (Chandrase‐ kharam& Bundschuh, 2008) but the selection of the organic fluid should be optimized, in terms of the above requirements, while meeting the demanded power generation. In gener‐ al, binary ORC systems exhibit great flexibility, high safety (installations are perfectly tight), and low maintenance (Wei et al., 2007). It was reported that the selection of suitable organic fluids for application in binary ORC systems for generating electricity still deserves exten‐ sive thermodynamic and technical studies (Maizza, V., & Maizza, A., 2001).

The quality of heat energy which can be supplied by any heat source depends on its temper‐ ature level. For ORC-based geothermal power system, this is the temperature of the pro‐ duced geo-fluid from the geothermal production well. The theoretical overall performance of low-temperature geothermal binary systems can be evaluated using the thermal efficien‐ cy of a heat engine, given by (Cengel& Boles, 2008)

$$
\eta\_{th} = \frac{\dot{W}\_{cut}}{\overline{Q}\_{gs}} \tag{1}
$$

*ηth* =0.000648 *Tgeo*,*in* - 0.036 (3)

C. The thermal efficiency as a function of the ge‐

) (4)

)(*Tgeo*,*in* - *Tsink* ) (5)

C and geo-fluid flow rate > 900

C and ambient temper‐

Sequestration

311

http://dx.doi.org/10.5772/52063

C. The estimated net

For example, using Eq. (3) it can be estimated that a thermal efficiency of approximately 4.8% could be achieved for power generation with a geo-fluid extracted from a low-temper‐

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

othermal heat resource temperature, *Tgeo*,*in* (in K), and ambient temperature, *To* (in K) is

*T geo*,*in* + *To*

power output produced by the geothermal power system can also be determined using (Di‐

In Eq. (5), *m*˙ *geo* is the geo-fluid mass flow rate; and *Tsink* is the heat sink temperature. It should be noted that the above correlations given by Eqs. (2) through (5) provide quick esti‐ mate of the thermal efficiency and net power output. However, for more accurate system performance predictions, a detailed energy analysis should be performed to predict the net power, the available geothermal heat, and overall thermal efficiency using Eq. (1). Since the geothermal energy is produced at low enthalpy levels,ORC-based low-temperature geother‐ mal power generation plants tend to have low thermal efficiencies: 10-13% reported by (Di‐ Pippo, 2007), 2.8-5.5% reported by (Gupta & Roy, 2007), and 5-9% reported by (Hettiarachchi et al., 2007). Maximizing generating power capacity is normally sought from these power plants by maximizing the geo-fluid flow rate (depending on the capability of the production well) with a limited geo-fluid temperature available from the geothermal re‐ source. It was reported (Chandrasekharam& Bundschuh, 2008) that low-temperature geo‐

l/min could generate electric power ranging from 50 to 700 kWe. When appropriate, multiple production wells could be installed using the same low-temperature geothermal energy res‐ ervoir so that a number of ORC power generation units could be cascaded to obtain larger power production rates from the plant (Gupta & Roy, 2007). Limited by the second-law of thermodynamics, the ideal (absolute maximum) efficiency of a thermoelectric power cycle, such as the low-temperature geothermal ORC power cycle, operating as a reversible heat en‐ gine between a heat source at a temperature *T <sup>H</sup>* and a heat sink at a temperature *T <sup>L</sup>* is Car‐

( *<sup>T</sup> geo*,*in* - *To T geo*,*in* + *To*

C, the thermal efficiency is estimated to be 8.7%, using Eq. (4). It should be noted

*<sup>η</sup>th* <sup>≅</sup>0.58 ( *<sup>T</sup> geo*,*in* - *To*

So for example, with a geothermal heat resource temperature of 130o

that Eq. (4) is valid for resource temperatures between 100 and 140 o

*<sup>W</sup>*˙ *out* <sup>≅</sup>2.47 *<sup>m</sup>*˙ *geo*

thermal production wells with geo-fluid temperature < 150 o

not efficiency, given as (Cengel& Boles, 2008)

ature geothermal resource available at 130 o

given by (DiPippo, 2007)

ature of 25o

Pippo, 2007)

In Eq. (1), *W*˙ *out* is the net power output produced by the geothermal power system (in kWe); and *Q*˙ *geo* is the thermal heat supplied by the geo-fluid from the available geothermal re‐ source (in kWt ). A correlation is proposed (Dickson & Fanelli, 2005) to calculate the actual net power output (used for a quick estimate with rough accuracy) as a function of the availa‐ ble thermal power from the geo-fluid flowand inlet temperature of the geo-fluid, given by

$$
\dot{W}\_{\text{out}} = 0.0036 \,\text{\AA} \,\text{\bar{Q}}\_{\text{geo}} \{ 0.18 \,\, T\_{\text{geo},in} \text{ - 10} \} \tag{2}
$$

Substituting Eq. (2) in Eq. (1), the estimated thermal efficiency of the low-temperature based geothermal power generation system, as a function of geo-fluid inlet temperature (in o C) available at the production well, is given by

ORC-Based Geothermal Power Generation and CO2-Based EGS for Combined Green Power Generation and CO2 Sequestration http://dx.doi.org/10.5772/52063 311

$$
\eta\_{th} = 0.000648 \text{ } T\_{\text{gee,in}} \text{ - 0.036} \tag{3}
$$

For example, using Eq. (3) it can be estimated that a thermal efficiency of approximately 4.8% could be achieved for power generation with a geo-fluid extracted from a low-temper‐ ature geothermal resource available at 130 o C. The thermal efficiency as a function of the ge‐ othermal heat resource temperature, *Tgeo*,*in* (in K), and ambient temperature, *To* (in K) is given by (DiPippo, 2007)

**•** The ORC organic fluid should be environmentally friendly; less in ozone depletion poten‐

**•** It should result in high thermal efficiency by allowing maximum utilization of the availa‐

**•** It should have a low-boiling temperature and should evaporate at atmospheric pressure.

**•** It should not react or disassociate at the pressures and temperatures at which it is used.

It should be noted that many binary ORC fluids may not meet all these criteria (Chandrase‐ kharam& Bundschuh, 2008) but the selection of the organic fluid should be optimized, in terms of the above requirements, while meeting the demanded power generation. In gener‐ al, binary ORC systems exhibit great flexibility, high safety (installations are perfectly tight), and low maintenance (Wei et al., 2007). It was reported that the selection of suitable organic fluids for application in binary ORC systems for generating electricity still deserves exten‐

The quality of heat energy which can be supplied by any heat source depends on its temper‐ ature level. For ORC-based geothermal power system, this is the temperature of the pro‐ duced geo-fluid from the geothermal production well. The theoretical overall performance of low-temperature geothermal binary systems can be evaluated using the thermal efficien‐

In Eq. (1), *W*˙ *out* is the net power output produced by the geothermal power system (in kWe); and *Q*˙ *geo* is the thermal heat supplied by the geo-fluid from the available geothermal re‐

net power output (used for a quick estimate with rough accuracy) as a function of the availa‐ ble thermal power from the geo-fluid flowand inlet temperature of the geo-fluid, given by

Substituting Eq. (2) in Eq. (1), the estimated thermal efficiency of the low-temperature based geothermal power generation system, as a function of geo-fluid inlet temperature (in o

). A correlation is proposed (Dickson & Fanelli, 2005) to calculate the actual

*<sup>W</sup>*˙ *out* =0.0036 *<sup>Q</sup>*˙ *geo*(0.18 *Tgeo*,*in* - 10) (2)

(1)

C)

*<sup>η</sup>th* <sup>=</sup> *<sup>W</sup>*˙ *out Q*˙ *geo*

tial (ODP) and global warming potential (GWP).

**•** It should be safe (non-flammable and no-toxic) and non-corrosive.

**•** It should lead to optimum design and cost effectiveness of the ORC system.

**•** It should have suitable thermal stability and high thermal conductivity.

**•** It should have small specific volume, low viscosity and surface tension.

sive thermodynamic and technical studies (Maizza, V., & Maizza, A., 2001).

**•** It should have appropriate low critical temperature and pressure.

ble low-temperature geothermal heat source.

310 New Developments in Renewable Energy

**•** It should result in low maintenance.

cy of a heat engine, given by (Cengel& Boles, 2008)

available at the production well, is given by

source (in kWt

$$\eta\_{\rm flt} \cong 0.58 \left( \frac{T\_{g\alpha,in} \cdot T\_o}{T\_{g\alpha,in} \cdot T\_o} \right) \tag{4}$$

So for example, with a geothermal heat resource temperature of 130o C and ambient temper‐ ature of 25o C, the thermal efficiency is estimated to be 8.7%, using Eq. (4). It should be noted that Eq. (4) is valid for resource temperatures between 100 and 140 o C. The estimated net power output produced by the geothermal power system can also be determined using (Di‐ Pippo, 2007)

$$\dot{W}\_{\text{out}} \cong 2.47 \text{ } \dot{m}\_{\text{geo}} \text{(} \frac{T\_{\text{geo},in} \cdot T\_o}{T\_{\text{geo},in} \cdot T\_o} \text{)} \text{(} T\_{\text{geo},in} \text{ - } T\_{\text{sink}}\text{)}\tag{5}$$

In Eq. (5), *m*˙ *geo* is the geo-fluid mass flow rate; and *Tsink* is the heat sink temperature. It should be noted that the above correlations given by Eqs. (2) through (5) provide quick esti‐ mate of the thermal efficiency and net power output. However, for more accurate system performance predictions, a detailed energy analysis should be performed to predict the net power, the available geothermal heat, and overall thermal efficiency using Eq. (1). Since the geothermal energy is produced at low enthalpy levels,ORC-based low-temperature geother‐ mal power generation plants tend to have low thermal efficiencies: 10-13% reported by (Di‐ Pippo, 2007), 2.8-5.5% reported by (Gupta & Roy, 2007), and 5-9% reported by (Hettiarachchi et al., 2007). Maximizing generating power capacity is normally sought from these power plants by maximizing the geo-fluid flow rate (depending on the capability of the production well) with a limited geo-fluid temperature available from the geothermal re‐ source. It was reported (Chandrasekharam& Bundschuh, 2008) that low-temperature geo‐ thermal production wells with geo-fluid temperature < 150 o C and geo-fluid flow rate > 900 l/min could generate electric power ranging from 50 to 700 kWe. When appropriate, multiple production wells could be installed using the same low-temperature geothermal energy res‐ ervoir so that a number of ORC power generation units could be cascaded to obtain larger power production rates from the plant (Gupta & Roy, 2007). Limited by the second-law of thermodynamics, the ideal (absolute maximum) efficiency of a thermoelectric power cycle, such as the low-temperature geothermal ORC power cycle, operating as a reversible heat en‐ gine between a heat source at a temperature *T <sup>H</sup>* and a heat sink at a temperature *T <sup>L</sup>* is Car‐ not efficiency, given as (Cengel& Boles, 2008)

$$
\eta\_{\text{ideal}} = \eta\_{\text{Carnot}} = 1 - \frac{\text{T}\_{\text{L}}}{\text{T}\_{\text{H}}} \tag{6}
$$

tems, the thermal impact is much reduced by disposing of waste geothermal water using deep re-injection approach so that the thermal impact of the waste heat becomes insignifi‐ cant (Dickson & Fanelli, 2005). Appropriate measures should be applied to prevent leakage of the binary working fluid from ORC power generation units to the environment (Yamada & Oyama, 2004); normally the installations of these units are made perfectly tight to meet

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

In theory, geothermal energy potential is present below the entire surface of the Earth. In practice however, special geologic settings are required for geothermal energy to be eco‐ nomically exploited (Grasby et al., 2011). Generating electricity using ORC-based geother‐ mal technology is very cost-effective and reliable (Chandrasekharam& Bundschuh, 2008; Dickson & Fanelli, 2005). Table 1 compares electrical energy costs produced by various re‐ newable energy technologies.The cost of geothermal energy for generating electricity is fa‐ vourable compared to other energy sources. The reported costs of low-temperature based small geothermal power plants vary from 0.05 to 0.07 US\$/kWh for units generating < 5

> **Turnkey Investment Cost (US\$/kWe)**

**Potential Future Energy Cost (US cents/kWh)**

Sequestration

313

http://dx.doi.org/10.5772/52063

high safety standards.

**Renewable Energy Source**

(Hammons, 2004; Dickson & Fanelli, 2005).

MWe (Chandrasekharam& Bundschuh, 2008).

**Current Energy Cost (US cents/kWh)**

Geothermal 2-10 800 – 3,000 1-8 Wind 5-13 1,100 – 1,700 3-10 Solar photovoltaic 25-125 5,000 – 10,000 5-25 Solar thermal 12-18 3,000 – 4,000 4-10 Biomass 5-15 900 – 3,000 4-10 Tidal 8-15 1,700 – 2,500 8-15 Hydro 2-10 1,000 – 3,000 NA

**Table 1.** Energy and investment costs for electric power production from different renewable energy sources

The unit cost of electricity generated from low-temperature geothermal based small power plants is compared in Table 2. Moreover, the unit cost of electricity from small-scale geother‐ mal plants (<5 MWe) is much lower than the average cost of 0.25 US\$/kWh supplied through diesel generators (Chandrasekharam& Bundschuh, 2008). The total investment for a geo‐ thermal power plant mainly includes the following types of costs: (1) cost of exploitation, (2) cost of drilling, (3) cost of power plant (capital cost of design and construction), and (4) op‐ erating & maintenance costs (Chandrasekharam& Bundschuh, 2008). The first two types are referred to as subsurface costs whereas the other two are referred to surface costs. The high initial investments incurred through the exploration, drilling and development of wells and the production field is an important constraint on future geothermal power development. Despite low maintenance and operational costs, high initial investments are often a strong

For example, for an ORC power system using a geo-fluid extracted from a low-temperature geothermal heat source at 130 o C (403 K) and a heat sink (condenser) at 40 o C (313 K), the maximum ideal Carnot efficiency can be calculated using Eq. (6) to be approximately 22.3%. For an actual (irreversible) ORC-based geothermalsystem operating between the same tem‐ perature limits would have lower efficiency. Another measure of the performance of the low-temperature geothermal ORC power plant can be obtained using the Second-Law of thermodynamics in the form of exergetic efficiency, *ηex* , given as

$$
\eta\_{ex} = \frac{\dot{W}\_{out}}{\mathbb{E}\mathbf{x}\_{syn}} \tag{7}
$$

The exergetic efficiency in Eq. (7) is defined as the ratio of the actual net power output from the power generation system to the maximum theoretical power that could be extracted from the geo-fluid at the geothermal resource state relative to the thermodynamic deadstate. This involves determining the rate of exergy carried by the geo-fluid to the ORC pow‐ er system. Typically, the design and operation of geothermal binary power generation systems should be optimized in order to increase their thermal and exergetic efficiencies guided by the Carnot efficiency (Ismail, 2011b).

#### **2.3. ORC-based low-temperature geothermal power generation: Environmental & economic aspects**

Geothermal power generation is relatively pollution-free and considered to be a clean tech‐ nology for power generation (Dickson & Fanelli, 2005) and it tends to have the largest tech‐ nological potential compared to other renewable energy sources used for power generation (Hammons, 2004). Once up and running, GHG emissions are typically zero when low-tem‐ perature geothermal energy reservoirs are utilized using ORC power systems, since all of the produced geo-fluid is injected back into the reservoir (Hammons, 2004). In this case, one of the effective ways of getting rid of hazardous chemical constituents of geothermal water (e.g. trace metals) is re-injection. ORC-based low-temperature geothermal power generation systems are far less environmentally intrusive than alternative power generation systems in several respects, e.g. they are essentially zero-GHG emission systems and have low land us‐ age per installed megawatt (DiPippo, 2008). As far as physical environmental effects, geo‐ thermal projects may cause some kind of disruption activities as other same size and complexity of civil engineering projects. Also, the locations of excavations and sitting of boreholes and roads will have to be taken into account, soil and vegetation erosion, which may cause changes in ecosystems, has to be watched. It should be noted that many geother‐ mal installations are in remote areas where the natural level of noise is low and any addi‐ tional noise is very noticeable (Dickson & Fanelli, 2005). There is a relatively larger production of waste-heat energy in geothermal systems, and this needs to be dissipated in an environmentally acceptable way. In ORC-based low-temperature geothermal power sys‐ tems, the thermal impact is much reduced by disposing of waste geothermal water using deep re-injection approach so that the thermal impact of the waste heat becomes insignifi‐ cant (Dickson & Fanelli, 2005). Appropriate measures should be applied to prevent leakage of the binary working fluid from ORC power generation units to the environment (Yamada & Oyama, 2004); normally the installations of these units are made perfectly tight to meet high safety standards.

ηideal =ηCarnot

thermodynamics in the form of exergetic efficiency, *ηex* , given as

guided by the Carnot efficiency (Ismail, 2011b).

**economic aspects**

geothermal heat source at 130 o

312 New Developments in Renewable Energy

=1 - TL TH

C (403 K) and a heat sink (condenser) at 40 o

For example, for an ORC power system using a geo-fluid extracted from a low-temperature

maximum ideal Carnot efficiency can be calculated using Eq. (6) to be approximately 22.3%. For an actual (irreversible) ORC-based geothermalsystem operating between the same tem‐ perature limits would have lower efficiency. Another measure of the performance of the low-temperature geothermal ORC power plant can be obtained using the Second-Law of

> *<sup>η</sup>ex* <sup>=</sup> *<sup>W</sup>*˙ *out E*˙*x geo*

**2.3. ORC-based low-temperature geothermal power generation: Environmental &**

Geothermal power generation is relatively pollution-free and considered to be a clean tech‐ nology for power generation (Dickson & Fanelli, 2005) and it tends to have the largest tech‐ nological potential compared to other renewable energy sources used for power generation (Hammons, 2004). Once up and running, GHG emissions are typically zero when low-tem‐ perature geothermal energy reservoirs are utilized using ORC power systems, since all of the produced geo-fluid is injected back into the reservoir (Hammons, 2004). In this case, one of the effective ways of getting rid of hazardous chemical constituents of geothermal water (e.g. trace metals) is re-injection. ORC-based low-temperature geothermal power generation systems are far less environmentally intrusive than alternative power generation systems in several respects, e.g. they are essentially zero-GHG emission systems and have low land us‐ age per installed megawatt (DiPippo, 2008). As far as physical environmental effects, geo‐ thermal projects may cause some kind of disruption activities as other same size and complexity of civil engineering projects. Also, the locations of excavations and sitting of boreholes and roads will have to be taken into account, soil and vegetation erosion, which may cause changes in ecosystems, has to be watched. It should be noted that many geother‐ mal installations are in remote areas where the natural level of noise is low and any addi‐ tional noise is very noticeable (Dickson & Fanelli, 2005). There is a relatively larger production of waste-heat energy in geothermal systems, and this needs to be dissipated in an environmentally acceptable way. In ORC-based low-temperature geothermal power sys‐

The exergetic efficiency in Eq. (7) is defined as the ratio of the actual net power output from the power generation system to the maximum theoretical power that could be extracted from the geo-fluid at the geothermal resource state relative to the thermodynamic deadstate. This involves determining the rate of exergy carried by the geo-fluid to the ORC pow‐ er system. Typically, the design and operation of geothermal binary power generation systems should be optimized in order to increase their thermal and exergetic efficiencies

(6)

(7)

C (313 K), the

In theory, geothermal energy potential is present below the entire surface of the Earth. In practice however, special geologic settings are required for geothermal energy to be eco‐ nomically exploited (Grasby et al., 2011). Generating electricity using ORC-based geother‐ mal technology is very cost-effective and reliable (Chandrasekharam& Bundschuh, 2008; Dickson & Fanelli, 2005). Table 1 compares electrical energy costs produced by various re‐ newable energy technologies.The cost of geothermal energy for generating electricity is fa‐ vourable compared to other energy sources. The reported costs of low-temperature based small geothermal power plants vary from 0.05 to 0.07 US\$/kWh for units generating < 5 MWe (Chandrasekharam& Bundschuh, 2008).


**Table 1.** Energy and investment costs for electric power production from different renewable energy sources (Hammons, 2004; Dickson & Fanelli, 2005).

The unit cost of electricity generated from low-temperature geothermal based small power plants is compared in Table 2. Moreover, the unit cost of electricity from small-scale geother‐ mal plants (<5 MWe) is much lower than the average cost of 0.25 US\$/kWh supplied through diesel generators (Chandrasekharam& Bundschuh, 2008). The total investment for a geo‐ thermal power plant mainly includes the following types of costs: (1) cost of exploitation, (2) cost of drilling, (3) cost of power plant (capital cost of design and construction), and (4) op‐ erating & maintenance costs (Chandrasekharam& Bundschuh, 2008). The first two types are referred to as subsurface costs whereas the other two are referred to surface costs. The high initial investments incurred through the exploration, drilling and development of wells and the production field is an important constraint on future geothermal power development. Despite low maintenance and operational costs, high initial investments are often a strong restrictive(Grasby et al., 2011). For small-scale geothermal power plants (<5 MWe) utilizing low-temperature resources, the subsurface cost typically accounts for approximately 30% of the total investment costs whereas the surface cost accounts for the remaining 70%.

search investigations and once brought to successful production, will significantly expand the regions where geothermal powergeneration is feasible. In an EGS, a fluid (typically cold water or brine) is injected and fractures are induced to form subsurface heat exchange sys‐ tems. The heated fluid is then produced from a parallel well where heat can be used at sur‐ face to generate electricity (Huenges, 2010; Majorowicz& Grasby, 2010). Additional production-injection wells are drilled to extract more heat from large volumes of rock mass to meet power generation requirement (Azim et al., 2010). This technology does not require conventional natural convective hydrothermal resources located at depth, nor an initial high permeability of the reservoir, for power generation(once linked with ORC power technolo‐ gy). A schematic diagram showing a typical EGS concept is shown in Figure 3. EGS has the potential for accessing the Earth's vast resources of heat located at depth to help meet future increasing energy demands. The EGS concept has driven increased interest in widelydevel‐ opment of this geothermal energy potential by orders of magnitude (Huenges, 2010; Majoro‐ wicz& Grasby, 2010; Azim et al., 2010). It was reported (Chandrasekharam& Bundschuh, 2008) that developing countries can access all low-temperature geothermal and EGS sources

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

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for green electricity generation immediately.

**Figure 3.** A conceptual model showing how EGS works (Source: http://energyinformative.org)

The basis, on which today's EGS projects are developed were laid out in the early 1970s, when an EGS (hot dry rock (HDR)) development concept was implemented at Los Alamos National lab, involving drilling a well into hot crystalline rock, using pressurized water to


**Table 2.** Unit cost of electricity generated from low-temperature based small power plants (Chandrasekharam& Bundschuh, 2008; DiPippo, 2008).

Generating electricity using low-temperature geothermal ORC technology is very reliable due to its advanced technological aspects. However, the maintenance costs and shutdowns could be reduced when the technical complexity of the plant is on a level that is accessible to local technical personnel or to experts who are readily available (Dickson & Fanelli, 2005). As mentioned before, geothermal ORC power generation plants are normally constructed and installed in small modular power generation units. These units can then be linked up to create power plants with larger power production rates. Their cost depends on a number of factors, but mainly on the temperature of the geothermal fluid produced, which influences the size of the ORC turbine, heat exchangers and cooling system. It was reported (Dickson & Fanelli, 2005) that the total size of the plant has little effect on the specific cost, as a series of standard modular units is linked together to obtain larger power capacities. It was also re‐ ported (Panea et al., 2010) that the modular units have a satisfying economic efficiency, be‐ cause modular construction reduces installation time and costs. Ultimately, the economic viability of the geothermal power plant depends on its ability to generate revenue in the long-term.
