InTechOpen uses cookies to offer you the best online experience. By continuing to use our site, you agree to our Privacy Policy.

Engineering » Energy Engineering » "New Developments in Renewable Energy", book edited by Hasan Arman and Ibrahim Yuksel, ISBN 978-953-51-1040-8, Published: March 13, 2013 under CC BY 3.0 license. © The Author(s).

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

By Basel I. Ismail
DOI: 10.5772/52063

Article top

## Overview

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

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

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

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

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

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

Basel I. Ismail1

## 1. Introduction

Electrical power generation using innovative renewable and alternative geothermal energy technologies have shown merits and received renewed interest in recent years due to an increasing concern of greenhouse gas (GHG) emissions, being responsible for global warming & climate change, environmental pollution, and the limitations and conservation of natural energy resources. Organic Rankine Cycle (ORC) power generation using low-temperature geothermal resources is one of these innovative geothermal power generation technologies. The vast low-temperature geothermal resources found widely in most continental regions have not received much attention for electricity generation. Continuous development of ORC power generation and state-of-the-art drilling technologies and other factors make this renewable and nonconventional energy source one of the best future viable, alternate and available source to meet the required future electricity demand worldwide, significantly reducing GHG emissions and mitigating global warming effect. The first part of this chapter will introduce the ORC-based geothermal power generation technology. It will also present its fundamental concept for power generation and discusses its limitations, environmental & economic considerations, and energy conversion performance concept. Another novel “double-benefit” technology is enhanced (engineered) geothermal systems (EGS) using CO2 as the working fluid for combined renewable power generation and CO2 sequestration. CO2 is of interest as a geothermal working fluid mainly because it transfers geothermal heat more efficiently than water. While power can be produced more efficiently using this technology, there is an additional benefit for carbon capture and sequestration (CCS) for reducing GHG emissions. 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. The second part of this chapter will present and discuss the merits, limitations, environmental, economic and fundamental aspects of CO2-based EGS technology.

## 2. ORC-based geothermal power generation

### 2.1. Developments & utilization of low-temperature geothermal energy resources for power generation

The geothermal resources of the Earth are vast and abundant. For example, the part of geothermal energy stored at a depth of 3 km is estimated to be 43,000,000 EJ (equivalent to 1,194,444,444 TWh) which is much larger compared to all fossil fuel resources, whose energy equivalent is 36,373 EJ, combined (Chandrasekharam & Bundschuh, 2008). Conventional energy resources, such as oil, natural gas, coal, and uranium, being widely consumed in the world, originate from finite energy sources embedded in the crust of the Earth. Only one energy resource of the crust is renewable, namely geothermal energy. The word “geothermal” is originated from Greek words; “geo” meaning the Earth and “therme” meaning heat, so geothermal energy means the natural heat energy from the Earth. The source of geothermal energy is the continuous energy flux flowing from the interior of the Earth towards its surface. Unlike other conventional and renewable energy sources, geothermal energy has unique characteristics, namely it is abundantly available, stable at all times throughout the year, independent of weather conditions, and has an inherent storage capability (Hammons, 2004). Distinct from fossil-fuelled power generation, geothermal power generation is also considered to be a clean technology and environmentally friendly power source which could significantly contribute to the reduction of GHG emissions by replacing fossil fuels and other non-clean energy sourcesused for power generation (Chandrasekharam& Bundschuh, 2008).

Depending on the temperature and depth of the resource, the rock chemical composition and the abundance of ground water, geothermal heat energy resources vary widely from one location to another (Gupta & Roy, 2007). Geothermal heat sources are typically classified based on their available temperature, thus enthalpy energy level, from about 50 oC to 350 oC. The high-temperature (high-enthalpy) geothermal resources (with temperature > 200 oC) are typically found in volcanic regions and island chains, whereas the moderate-temperature (150-200 oC) and low-temperature (low-enthalpy) geothermal resources (<150 oC) are usually found broadly in most continental regions and by far the most commonly available heat resource (Chandrasekharam& Bundschuh, 2008; Gupta & Roy, 2007). The increase in temperature with depth in the Earth’s crust can be expressed in terms of what is known as the geothermal temperature gradient. Down to the depths accessible by drilling with modern technology (e.g. over 10 km), the average geothermal gradient is about 2.5-3.0 oC/100 m (Dickson& Fanelli, 2005). For example, at depth around 3 km below ground level, the temperature is about 90 oC. There are, however, areas in which the geothermal gradient is far from the average value (e.g. in some geothermal areas the gradient is ten times the average value) due to geothermal structure and composition of these areas (Dickson& Fanelli, 2005). The type of geothermal resource determines the type of system and method of its harvesting and utilization for electrical power generation. For example, high-temperature geothermal resources (vapour- and liquid-dominated) can be harvested and utilized to generate electricity using one of the following methods depending on the compositional and thermal characteristics of the resource: (1) single-flash steam power systems, (2) double-flash steam power systems, and (3) dry-steam power systems. Generating electricity from medium- and low-temperature geothermal resources (i.e. water-dominated resources) can be efficiently accomplished using a Binary-cycle technique, such as, ORC (Ismail, 2011a; Chandrasekharam& Bundschuh, 2008; Dickson & Fanelli, 2005; DiPippo, 2008).

Generating electricity from geothermal steam resources using an experimental 10 kW-electrical generator was made at Larderello of Italy in 1904 (Dickson& Fanelli, 2005; Panea et al., 2010). The commercial success of this attempt indicated the industrial value of geothermal energy and marked the beginning of a form of exploitation that was to develop significantly from the on. By 1942, the installed geothermal-electric capacity had reached approximately 128 MWe (Dickson& Fanelli, 2005). In the early 1950’s, many countries were attracted by geothermal energy, considering it to be economically competitive with other forms of energy. It was estimated (Dickson& Fanelli, 2005; Ruggero, 2007) that the worldwide installed geothermal-electric capacity reached 1.300 GWe (in 1975), 4.764 GWe (in 1985), 6.833 GWe (in 1995), 7.974 GWe (in 2000), 8.806 GWe (in 2004), 8.933 GWe (in 2005), 9.732 GWe (in 2007). In 2010, it was reported (Holm et al., 2010) that 10.715 GWe is online generating 67,246 GWh which represents a 20% increase in geothermal power online between 2005 and 2010. While power on-line grew 20% between 2005 and 2010, countries with projects under development grew at a much faster pace. In 2007, Geothermal Energy Association (GEA) reported that there were 46 countries considering geothermal power development. In 2010, this report identified 70 countries with projects under development or active consideration, a 52% increase since 2007. It should be noted that projects under development grew the most intensely in two regions of the world; namely, Europe and Africa (Holm et al., 2010).Very recently, it was reported (GEA, 2012) that as of May 2012, approximately 11.224 GWe of installed geothermal-electric power capacity was online globally, and is increasingly contributing to the electric power supply worldwide. It was estimated (Ruggero Bertani, 2007) that geothermal energy provides approximately 0.4% of the world global power generation, with a stable long term growth rate of 5%; the largest markets being in USA, Mexico, Indonesia, Philippines, Iceland, and Italy. Security for long-term electricity supply and GHG emission from fossil fuelled power plants is becoming a cause of concern for the entire world today. It was estimated (Chandrasekharam& Bundschuh, 2008) that the world net electricity demand is going to increase by approximately 85% from 2004 to 2030, rising from 16,424 TWh (in 2004) to 30,364 TWh in the year 2030. It was also reported (Chandrasekharam& Bundschuh, 2008; Dickson & Fanelli, 2005) that the emissions of GHG from geothermal power plants constitute less than 2% of the emission of these gases by fossil-fuelled power plants. To meet future energy demands renewable energy sources should meet the following criteria (Chandrasekharam& Bundschuh, 2008): (1) the sources should be large enough to sustain a long-lasting energy supply to generate the required electricity for the country, (2) the sources should be economically and technically accessible, (3) the sources should have a wide geographic distribution, and (4) the sources should be environmentally friendly and thus should be low GHG emitters in order to make significant contribution to global warming mitigation. Low-temperature (low-enthalpy) geothermal energy resources meet all the above criteria. It was reported in (Chandrasekharam& Bundschuh, 2008; Cui et al., 2009) that this huge low-temperature geothermal energy resource has already been used for power generation by typical countries, such as USA, Philippines, Mexico, Indonesia, Iceland, Germany, and Austria. The installations of several commercial low-temperature geothermal power systems in these countries have substantially proved the ability of low-temperature geothermal fluids to generate green electricity (Chandrasekharam& Bundschuh, 2008).

In most developing countries, low-temperature geothermal resources have not received much attention for electricity generation. The main reason for not utilizing these resources by most developing countries (and several industrialized countries) for commercial exploitation is that they are not considered as economically feasible for generating electricity (Chandrasekharam& Bundschuh, 2008). In contrast, in some industrialized countries, especially USA and in Europe, increasing energy demand and environmental awareness related to climate change have urged these countries to develop technologies which utilize low-temperature geothermal resources economically for power generation (Chandrasekharam& Bundschuh, 2008; Dickson & Fanelli, 2005). It was reported (Chandrasekharam& Bundschuh, 2008; Galanis et al., 2009) that developing countries, in general, need to benefit from these new and continually improving technologies for using low-temperature geothermal resources for generating electricity. It should be noted that for many developing countries, the use of low-temperature geothermal resources is not new. Many of developing countries have been using these resources for the past centuries for direct heating (but not power generation) applications (Chandrasekharam& Bundschuh, 2008). Recent increases in the cost and uncertainty of future conventional energy supplies for power generation are improving the attractiveness of low-temperature geothermal resources. Continuous development of innovative drilling and power generation technologies makes this nonconventional, renewable and clean energy source the best future viable, alternate and available source to meet the required future electricity demand worldwide, significantly reducing GHG emissions and mitigating global climate change (Chandrasekharam& Bundschuh, 2008).

As mentioned earlier, generating electricity from low-temperature geothermal resources (water-dominated resources) can be effectively achieved using a binary ORC technology. Low-temperature geothermal ORC technology has virtually no GHG emissions to the atmosphere (DiPippo, 2008; Hettiarachchi et al., 2007) and is an attractive energy-conversion technology due to its simplicity and its limited number of components, all of them being very common and commercially available. Nowadays, the ORC can be considered asthe only proved technology that is commonly used in ranges of afew kW up to 1 MW (Schuster et al., 2009). Despite the fact that ORC technology is currently associated with low conversion efficiencies,new applications of this technology are commonly examined and implementeddue to its possibility to utilize the low-grade heat from sources, such as low-temperature geothermal resources, for power generation (Ismail, 2011a). A number of successful & innovative ORC binary power plants were installed in different locations (e.g. remote and rural sites) worldwide which demonstrate the ability of this promising alternative technology to utilize renewable low-temperature geothermal energy sources for generating electricity. For example, two plants were installed in Nevada, USA in 1984 and 1987 with electric power generation capacity of 750 and 800 kWe, respectively (Chandrasekharam& Bundschuh, 2008). The production wells supply geo-fluid (water) temperature at 104 oC with a flow rate of 60 l/s to these plants. The ORC binary fluid used was initially R-114 but due to non-availability of this working fluid the plant switched to iso-pentane in 1998. In another location near Empire, Nevada, approximately four 1 MWe units were installed and commissioned in 1987. Two geothermal production wells with geo-fluids temperature of 137 oC were used (Chandrasekharam& Bundschuh, 2008). In 1998, a third well with geo-fluid temperature of 152 oC was drilled to maintain the capacity of the plant at approximately 4 MWe. The modular approach was used so that high plant availability factors of 98% and more were achievable (Hammons, 2004). In 1987, another plant was installed and commissioned in Taiwan with an electric power generation of 300 kWe. The plant draws geo-fluids from a 500 m deep well at a temperature of 130 oC. It was reported that the power generated from this facility was sold to the national power grid at 0.04 US$/kWh (Chandrasekharam& Bundschuh, 2008). In 1986, a low-temperature geothermal ORC unit (Mulka plant) with a power capacity of 15 kWe was commissioned in Australia. The unit was coupled to a geothermal production well which was drilled down toa depth of 1,300 m, and supplying geo-fluid at 86 oC. The unit was operated non-stop for about three and a half years, showing frequency stability and response to load changes (Rosca et al., 2010). In 1992, a binary ORC power generation unit which utilized a low-temperature geothermal water resource with a temperature ranging from 90 to 115 oC was tested at a location near arderello, Italy. The geothermal power plant generated between 800 and 1,300 kWe of electricity (Rosca et al., 2010). In Germany, the first low-temperature geothermal power plant using ORC technology was installed at Neustadt-Glewe, with a power capacity of approximately 230 kWe using a geo-fluid temperature of 98 oC (RuggeroBertani, 2007). Another plant was commissioned in Thailand in 1989, with an installed capacity of 300 kWe. The actual production was reported to vary from 150 to 250 kWe and the geo-fluid temperature is 116 oC with a flow rate of approximately 8 l/s (Chandrasekharam & Bundschuh, 2008). In Japan, binary ORC technology was experimentally operated for 5 years starting in 1993 by NEDO (Yamada & Oyama, 2004). More recently, in 2006, the first binary ORC plant which utilizes a low-temperature geothermal resource at a temperature of 74oC reported by (RuggeroBertani, 2007) to be the lowest low-temperature geothermal energy resource worldwide) was installed at Chena Hot Springs, Alaska, with a power generation capacity of 200 kWe. A photograph of Chena ORC-based geothermal power plant is shown in Figure 1. A second ORC unit was added, reaching the total installed capacity of 400 kWe net. The total project cost of this binary geothermal plant was$2.2 million with a simple payback period of 4 years (Holdmann, 2007). In Altheim, Austria, a geo-fluid of temperature 106 oC is utilized both for district heating and electric power generation using a binary plant technology. The net electric output of this plant is 500 kWe, selling to the electric grid 1.1 GWh in 2006 (RuggeroBertani, 2007).

### 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 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 binary-fluid 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-dominated 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 temperature (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 working fluid expands across theturbine and then is cooled and condensed in thecondenser before it is pumped back as a liquid to the heat exchanger using a condensate

### Figure 2.

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

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 requires 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 boiling 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 performance 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):

• The ORC organic fluid should be environmentally friendly; less in ozone depletion potential (ODP) and global warming potential (GWP).

• It should result in high thermal efficiency by allowing maximum utilization of the available low-temperature geothermal heat source.

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

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

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

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

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

• It should have appropriate low critical temperature and pressure.

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

• It should result in low maintenance.

It should be noted that many binary ORC fluids may not meet all these criteria (Chandrasekharam& 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 general, 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 extensive 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 temperature level. For ORC-based geothermal power system, this is the temperature of the produced geo-fluid from the geothermal production well. The theoretical overall performance of low-temperature geothermal binary systems can be evaluated using the thermal efficiency of a heat engine, given by (Cengel& Boles, 2008)

 ηth=W˙outQ˙geo (1)

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 resource (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 available thermal power from the geo-fluid flowand inlet temperature of the geo-fluid, given by

 W˙out=0.0036 Q˙geo0.18 Tgeo,in-10 (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 oC) available at the production well, is given by

 ηth=0.000648 Tgeo,in-0.036 (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-temperature geothermal resource available at 130 oC. The thermal efficiency as a function of the geothermal heat resource temperature, Tgeo,in (in K), and ambient temperature, To (in K) is given by (DiPippo, 2007)

 ηth≅0.58 Tgeo,in-ToTgeo,in+To (4)

So for example, with a geothermal heat resource temperature of 130oC and ambient temperature of 25oC, 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 oC. The estimated net power output produced by the geothermal power system can also be determined using (DiPippo, 2007)

 W˙out≅2.47 m˙geoTgeo,in-ToTgeo,in+ToTgeo,in-Tsink (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 estimate 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 geothermal power generation plants tend to have low thermal efficiencies: 10-13% reported by (DiPippo, 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 resource. It was reported (Chandrasekharam& Bundschuh, 2008) that low-temperature geothermal production wells with geo-fluid temperature < 150 oC 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 reservoir 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 engine between a heat source at a temperature T H and a heat sink at a temperature T L is Carnot efficiency, given as (Cengel& Boles, 2008)

 ηideal =ηCarnot=1-TLTH (6)

For example, for an ORC power system using a geo-fluid extracted from a low-temperature geothermal heat source at 130 oC (403 K) and a heat sink (condenser) at 40 oC (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 temperature 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

 ηex=W˙outEx˙geo (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 dead-state. This involves determining the rate of exergy carried by the geo-fluid to the ORC power 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 technology for power generation (Dickson & Fanelli, 2005) and it tends to have the largest technological potential compared to other renewable energy sources used for power generation (Hammons, 2004). Once up and running, GHG emissions are typically zero when low-temperature 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 usage per installed megawatt (DiPippo, 2008). As far as physical environmental effects, geothermal 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 geothermal installations are in remote areas where the natural level of noise is low and any additional 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 systems, 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 insignificant (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.

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 economically exploited (Grasby et al., 2011). Generating electricity using ORC-based geothermal technology is very cost-effective and reliable (Chandrasekharam& Bundschuh, 2008; Dickson & Fanelli, 2005). Table 1 compares electrical energy costs produced by various renewable energy technologies.The cost of geothermal energy for generating electricity is favourable compared to other energy sources. The reported costs of low-temperature based small geothermal power plants vary from 0.05 to 0.07 US$/kWh for units generating < 5 MWe (Chandrasekharam& Bundschuh, 2008).  Renewable Energy Source Current Energy Cost(US cents/kWh) Turnkey Investment Cost(US$/kWe) Potential Future 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 (Hammons, 2004; Dickson & Fanelli, 2005).

### Table 4.

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

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

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 shortcomings. An unfavorable property of water is that it is astrong solvent for many rock minerals, 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 dissolution 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 additional 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 numerical simulations and evaluated thermophysical properties in order to explore the heat transfer 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 buoyancy 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 generate 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 addition, 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 hydraulic 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 furtherinvestigation. 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 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,l eading 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 needed.

### Figure 5.

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

## 4. Conclusion

An increasing concern of environmental issues of emissions & pollution, in particular global warmingand the constraints on consuming conventional energy sources has recently resulted in extensive research into innovative renewable and green technologies of generating electrical power. One of these innovative emerging technologies includes renewable low-temperature (low-enthalpy) geothermal energy source for clean electrical power generation. This promising technology offers potential applications in generation of electric power which can be produced using the vast renewable low-temperature geothermal energy resources available worldwide.In this chapter, the concept of ORC binary technologyfor power generation using low-temperature geothermal heat source was introduced and its potential applications and limitations for small-scale geothermal power generation and its relevant environmental and economic considerations were presented and discussed. Also, recent developments of ORC-based low-temperature geothermal power generation with their significant and relevant applications were presented and discussed. A number of successful ORC binary plants were installed in different locations (e.g. remote and rural sites) worldwide which demonstrated the ability of this promising alternative and green technology to utilize renewable low-temperature geothermal energy sources for generating electricity. Also, several patents were reported on the application of this innovative technology. 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. Currently, ORC power cycles exhibit great flexibility, high safety (installations are perfectly tight), and low maintenance when coupled with low-enthalpy geothermal heat sources. The future use of low-temperature geothermal energy resources for generating electricity would very much depend on further overcoming technical barriers both in utilization and production, and its economic viability compared to other conventional and renewable energy sources used for power production. Another emerging “dual-benefit” technology is EGS using CO2 as the working fluid for combined clean power generation and geologic CO2 sequestration. CO2 is of interest as a geothermal working fluid mainly because it transfers geothermal heat more efficiently than water. While power can be produced more efficiently using this technology, there is an additional benefit CCS for reducing GHG emissions. The second part of the chapter presented the merits and fundamental aspects of CO2-based EGS technology.In 2000, Brown, D. (Pruess, 2006) proposed a novel EGS concept that would utilize supercritical CO2 instead of water as a more efficient heat exchange (carrier) fluid (due to its favorable properties over water), and would simultaneously achieve CO2 geologic sequestration as an additional benefit.It was found that CO2 is superior to water in its ability to exchange heat from EGS hot fractured rock and reduce hydraulic power consumption for fluid injection and circulation in the EGS reservoir. It was concluded that an EGS system running on CO2 has sufficiently attractive features to warrant furtherinvestigation.It was also concluded that EGS for power generation is still relatively a novel technology and remains to be proved on a large scale and that further research is needed for additional exploration oftechnological and economic aspects regarding the opportunities and challenges for CO2–based EGS technology for combined carbon sequestration and power generation.

## Acknowledgements

The author of this chapter would like to acknowledge the funding contribution by Goldcorp Canada Ltd.-Musselwhite Gold Mine that mainly supported the collaborative geothermal energy & heat pump (GHP) technology research project (author was the PI of the project) at their site in Northern Ontario; a contracted research project with Lakehead University (2007-09). Acknowledgement also goes to Natural Sciences and Engineering Research Council of Canada (NSERC) for the Discovery Grant (Individual) funding that was provided to the author’s research in the area of clean energy technologies related to CO2 membrane gas separation from industrial flue gases for GHG emissions reduction.

## References

1 - A. D. Atrens, H. Gurgenic, V. Rudolph, (2011, 2011 Economic optimization of a CO2-based EGS plant. Energy & Fuels, 25 3765 3775 ACS Publications.
2 - M. R. Azim, M. S. Amin, A. Shoeb, 2010 Prospects of enhanced geothermal system in baseload power generation. IEEE, 10 176 180 0000-9781
3 - Y. A. Cengel, M. A. Boles, 2008 Thermodynamics: an engineering approach (6th ed.), McGraw-Hill press, 978-0-07352-921-4 New York.
4 - D. Chandrasekharam, J. Bundschuh, 2008 Low-enthalpy geothermal resources for power generation CRC Press Taylor & Francis Group, 978-0-41540-168-5 New York. 10.1201/9780203894552
5 - J. Cui, J. Zhao, C. Dai, B. Yang, 2009 Exergetic performance investigation of medium-low enthalpy geothermal power generation. IEEE Computer Society, 636 639 0000-9780
6 - M. H. Dickson, M. Fanelli, (2005, 2005 Geothermal energy: utilization and technology, Earthscan, an imprint of James & James (Science Publishers) Ltd. in Association with the International Institute for Environment and Development, 1-84407-184-7
7 - R. Di Pippo, 2008 Geothermal power plants: principles, applications, case studies and environmental impact 2nded, Elsevier, 978-0-75068-620-4 New York.
8 - R. Di Pippo, 2007 Ideal thermal efficiency for geothermal binary plants. Geothermics, 36 276 285 0375-6505
9 - Energyinformative. How enhanced geothermal systems (EGS) work. Available from: http://energyinformative.org
10 - S. Frick, M. Kaltschmitt, G. Schröder, (2010, 2010 Life cycle assessment of geothermal binary power plants using enhanced low-temperature reservoirs Energy 35 2281 2294 0360-5442
11 - N. Galanis, E. Cayer, P. Roy, E. S. Denis, M. Desilets, 2009 Electricity generation from low temperature sources. Journal of Applied Fluid Mechanics, 2 2 55 67 1735-3645
12 - Geothermal Energy Association, GEA, 2012 Geothermal: International market overview report. Available from: http://www.geo-energy.org
13 - S. E. Grasby, D. M. Allen, S. Bell, Z. Chen, G. Ferguson, A. Jessop, M. Kelman, M. Ko, J. Majorowicz, M. Moore, J. Raymond, R. Therrien, 2011 Geothermal energy resource potential of Canada Geological Survey of Canada, Open File 6914. Available from: http://geoscan.ess.nrcan.gc.ca 10.4095/288745
14 - H. Gupta, S. Roy, 2007 Geothermal energy: an alternative resource for the 21st century, Elsevier, 978-0-44452-875-9 New York.
15 - H. Gurgenic, V. Rudolph, T. Saha, M. Lu, 2008 Challenges for geothermal energy utilisation. Proceedings, 33rdWorkshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California,
16 - T. J. Hammons, 2004 Geothermal power generation worldwide: global perspective, technology, field experience, and research and development. Electric Power Components and Systems, 32 529 553 1532-5008
17 - H. D. M. Hettiarachchi, M. Golubovic, W. M. Worek, Y. Ikegami, 2007 Optimum design criteria for an organic Rankine cycle using low-temperature geothermal heat sources Energy, 32 1698 1706 0360-5442
18 - G. Holdmann, 2007 ORC technology for waste heat applications. Presented at the Diesel Heat Recovery and Efficiency Workshop, Chena Power, Chena, Alaska, December 2007. Available from: http://www.akenergyauthority.org
19 - A. Holm, L. Blodgett, D. Jennejohn, K. Gawell, Energy. Geothermal, G. E. A. Association, 2010 Geothermal energy: International market update. Available from: http://www.geo-energy.org
20 - E. Huenges, (2010, 2010 Geothermal energy systems, Wiley-VCH Verlag GmbH & Co. 978-3-52740-831-3 KGaA, Weinheim, Germany. 10.1002/9783527630479
21 - Hung T.-C. 2001 Waste heat recovery of organic Rankine cycle using dry fluids Energy Conversion and Management 42 539 553 0196-8904
22 - B. I. Ismail, 2011a Advanced electrical power generation technology using renewable & clean low-enthalpy geothermal energy sources. Recent Patents on Mechanical Engineering, 4 2 168 179 0187-4477 X
23 - B. I. Ismail, 2011b Power generation using nonconventional renewable geothermal and alternative clean energy technologies, Chap. 18, in: Planet earth 2011 Global warming challenge and opportunities for policy and practice, Edited by Carayannis, E. G., INTECH Open Access Publisher, 978-9-53307-733-8 Croatia.
24 - C. Kalra, G. Becquin, J. Jackson, A. L. Laursen, H. Chen, A. Hardy, H. Klockow, J. Zia, 2012 High-potential working fluids and cycle concepts for next generation binary Organic Rankine Cycle for enhanced geothermal systems. Proceedings, 37th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California,
25 - V. Maizza, A. Maizza, 2001 Unconventional working fluids in organic Rankine-cycles for waste energy recovery systems Applied Thermal Engineering 21 381 390 1359-4311
26 - E. Majer, R. Baria, M. Stark, S. Oates, J. Bommer, B. Smith, H. Asanuma, (2007, 2007 Induced seismicity associated with Enhanced Geothermal Systems. Geothermics, 36 185 222 0375-6505
27 - J. Majorowicz, S. E. Grasby, 2010 Heat flow, depth-temperature variations and stored thermal energy for enhanced geothermal systems in Canada. J. Geophys. Eng., 7 232 241 1742-2132
28 - C. Panea, G. M. Rosca, C. A. Blaga, 2010 Power generation from low-enthalpy geothermal resources. Journal of Sustainable Energy, 1 2 1 5 2067-5538
29 - K. Pruess, 2006 Enhanced geothermal systems (EGS) using CO2 as working fluid- A novel approach for generating renewable energy with simultaneous sequestration of carbon. Geothermics, 35 351 367 0375-6505
30 - J. B. Randolph, M. O. Saar, 2011 Coupling carbon dioxide sequestration with geothermal energy capture in naturally permeable, porous geologic formations: Implications for CO2 sequestration. Energy Procedia, 4 2206 2213
31 - M. G. Rosca, K. Karytsas, D. Mendrinos, 2010 Low enthalpy geothermal power generation in Romania. Proceedings World Geothermal Congress, Bali, Indonesia, April 2010.
32 - E. Ruggero-Bertani, 2007 World geothermal generation in 2007, GHC Bulletin, 1 12
33 - B. Saleh, G. Koglbauer, M. Wendland, J. Fischer, 2007 Working fluids for low-temperature organic Rankine cycles Energy, 32 1210 1221 0360-5442
34 - A. Schuster, S. Karellas, E. Kakaras, H. Spliethoff, 2009 Energetic and economic investigation of Organic Rankine Cycle applications. Applied Thermal Engineering, 29 1809 1817 1359-4311
35 - J. C. Stephens, . Jiusto, S. , 2010 Assessing innovation in emerging energy technologies: Socio-technical dynamics of carbon capture and storage (CCS) and enhanced geothermal systems (EGS) in the USA. Energy Policy, 38 2020 2031 0301-4215
36 - D. Wei, X. Lu, Z. Lu, J. Gu, 2007 Performance analysis and optimization of organic Rankine cycle (ORC) for waste heat recovery Energy Conversion and Management 48 1113 1119 0196-8904
37 - S. Yamada, H. Oyama, 2004 Small capacity geothermal binary power generation system. Fuji Electric Review, 51 86 89