**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 geo‐ thermal 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 en‐ ergy 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 en‐ ergy 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 sur‐ face. 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& Bund‐ schuh, 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 o C to 350 o C. The high-temperature (high-enthalpy) geothermal resources (with temperature > 200 o C) are typically found in volcanic regions and island chains, whereas the moderate-temperature (150-200 o C) and low-temperature (low-enthalpy) geothermal resources (<150 o C) are usually found broadly in most continental regions and by far the most commonly available heat re‐ source (Chandrasekharam& Bundschuh, 2008; Gupta & Roy, 2007). The increase in tempera‐ ture 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 o C/100 m (Dickson& Fanelli, 2005). For example, at depth around 3 km below ground level, the tem‐ perature is about 90 o C. 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 electric‐ ity using one of the following methods depending on the compositional and thermal charac‐ teristics 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 lowtemperature geothermal resources (i.e. water-dominated resources) can be efficiently accom‐ plished using a Binary-cycle technique, such as, ORC (Ismail, 2011a; Chandrasekharam& Bundschuh, 2008; Dickson & Fanelli, 2005; DiPippo, 2008).

nomically viable,leading to more widespread utilization of geothermal energy. The second part of this chapter will present and discuss the merits, limitations, environmental, econom‐

**2.1. Developments & utilization of low-temperature geothermal energy resources for**

The geothermal resources of the Earth are vast and abundant. For example, the part of geo‐ thermal 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 en‐ ergy 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 en‐ ergy 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 sur‐ face. 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& Bund‐

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

typically found in volcanic regions and island chains, whereas the moderate-temperature

found broadly in most continental regions and by far the most commonly available heat re‐ source (Chandrasekharam& Bundschuh, 2008; Gupta & Roy, 2007). The increase in tempera‐ ture 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

(Dickson& Fanelli, 2005). For example, at depth around 3 km below ground level, the tem‐

from the average value (e.g. in some geothermal areas the gradient is ten times the average

C. There are, however, areas in which the geothermal gradient is far

C to 350 o

C) are usually

C.

C) are

C/100 m

based on their available temperature, thus enthalpy energy level, from about 50 o

The high-temperature (high-enthalpy) geothermal resources (with temperature > 200 o

C) and low-temperature (low-enthalpy) geothermal resources (<150 o

technology (e.g. over 10 km), the average geothermal gradient is about 2.5-3.0 o

ic and fundamental aspects of CO2-based EGS technology.

**2. ORC-based geothermal power generation**

**power generation**

304 New Developments in Renewable Energy

schuh, 2008).

(150-200 o

perature is about 90 o

Generating electricity from geothermal steam resources using an experimental 10 kW-elec‐ trical 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 ge‐ othermal energy, considering it to be economically competitive with other forms of energy. It was estimated (Dickson& Fanelli, 2005; Ruggero, 2007) that the worldwide installed geo‐ thermal-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% in‐ crease since 2007. It should be noted that projects under development grew the most in‐ tensely 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 in‐ stalled geothermal-electric power capacity was online globally, and is increasingly contribu‐ ting 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 (Chan‐ drasekharam& Bundschuh, 2008): (1) the sources should be large enough to sustain a longlasting 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 geo‐ graphic 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 pow‐ er 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).

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 technolo‐ gy to utilize renewable low-temperature geothermal energy sources for generating electrici‐ ty. 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& Bund‐

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

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 commis‐ sioned in 1987. Two geothermal production wells with geo-fluids temperature of 137 o

were used (Chandrasekharam& Bundschuh, 2008). In 1998, a third well with geo-fluid tem‐

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

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 geo‐ thermal production well which was drilled down toa depth of 1,300 m, and supplying geo-

In 1992, a binary ORC power generation unit which utilized a low-temperature geothermal

arderello, Italy. The geothermal power plant generated between 800 and 1,300 kWe of elec‐ tricity (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 approxi‐

plant was commissioned in Thailand in 1989, with an installed capacity of 300 kWe. The ac‐ tual production was reported to vary from 150 to 250 kWe and the geo-fluid temperature is

geroBertani, 2007) to be the lowest low-temperature geothermal energy resource world‐ wide) 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

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 (Rug‐

C 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

frequency stability and response to load changes (Rosca et al., 2010).

utilizes a low-temperature geothermal resource at a temperature of 74o

4 years (Holdmann, 2007). In Altheim, Austria, a geo-fluid of temperature 106 o

water resource with a temperature ranging from 90 to 115 o

mately 230 kWe using a geo-fluid temperature of 98 o

C was drilled to maintain the capacity of the plant at approximately 4 MWe.

C. The unit was operated non-stop for about three and a half years, showing

C. It was reported that the power generated from

C was tested at a location near

C (RuggeroBertani, 2007). Another

C reported by (Rug‐

C is utilized

C with a

Sequestration

307

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

C

schuh, 2008). The production wells supply geo-fluid (water) temperature at 104 o

perature of 152 o

fluid at 86 o

116 o

geroBertani, 2007).

500 m deep well at a temperature of 130 o

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 exploi‐ tation is that they are not considered as economically feasible for generating electricity (Chandrasekharam& Bundschuh, 2008). In contrast, in some industrialized countries, espe‐ cially USA and in Europe, increasing energy demand and environmental awareness related to climate change have urged these countries to develop technologies which utilize low-tem‐ perature geothermal resources economically for power generation (Chandrasekharam& Bundschuh, 2008; Dickson & Fanelli, 2005). It was reported (Chandrasekharam& Bund‐ schuh, 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 gen‐ eration) 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 in‐ novative drilling and power generation technologies makes this nonconventional, renewa‐ ble 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 at‐ mosphere (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 on‐ ly 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 implemented‐ due to its possibility to utilize the low-grade heat from sources, such as low-temperature ge‐ othermal 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 technolo‐ gy to utilize renewable low-temperature geothermal energy sources for generating electrici‐ ty. 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& Bund‐ schuh, 2008). The production wells supply geo-fluid (water) temperature at 104 o C 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 commis‐ sioned in 1987. Two geothermal production wells with geo-fluids temperature of 137 o C were used (Chandrasekharam& Bundschuh, 2008). In 1998, a third well with geo-fluid tem‐ perature of 152 o C 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 o C. 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 geo‐ thermal production well which was drilled down toa depth of 1,300 m, and supplying geofluid at 86 o C. 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).

should be economically and technically accessible, (3) the sources should have a wide geo‐ graphic 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 pow‐ er 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).

306 New Developments in Renewable Energy

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 exploi‐ tation is that they are not considered as economically feasible for generating electricity (Chandrasekharam& Bundschuh, 2008). In contrast, in some industrialized countries, espe‐ cially USA and in Europe, increasing energy demand and environmental awareness related to climate change have urged these countries to develop technologies which utilize low-tem‐ perature geothermal resources economically for power generation (Chandrasekharam& Bundschuh, 2008; Dickson & Fanelli, 2005). It was reported (Chandrasekharam& Bund‐ schuh, 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 gen‐ eration) 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 in‐ novative drilling and power generation technologies makes this nonconventional, renewa‐ ble 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 at‐ mosphere (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 on‐ ly 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 implemented‐ due to its possibility to utilize the low-grade heat from sources, such as low-temperature ge‐ othermal resources, for power generation (Ismail, 2011a). A number of successful & In 1992, a binary ORC power generation unit which utilized a low-temperature geothermal water resource with a temperature ranging from 90 to 115 o C was tested at a location near arderello, Italy. The geothermal power plant generated between 800 and 1,300 kWe of elec‐ tricity (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 approxi‐ mately 230 kWe using a geo-fluid temperature of 98 o C (RuggeroBertani, 2007). Another plant was commissioned in Thailand in 1989, with an installed capacity of 300 kWe. The ac‐ tual production was reported to vary from 150 to 250 kWe and the geo-fluid temperature is 116 o C 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 74o C reported by (Rug‐ geroBertani, 2007) to be the lowest low-temperature geothermal energy resource world‐ wide) 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 o C 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 (Rug‐ geroBertani, 2007).

Electric Generator

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

**Condenser**

Electric Power Output

Condensate Pump

Subsurface

S b f

Surface

Re-injection well

Cooling Tower

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

Sequestration

309

**Turbine** 

Heat Exchanger (Evaporator)

ORC BINARY POWER CYCLE

Low-temperature geothermal liquid-dominated resource

Saleh et al., 2007; Chandrasekharam& Bundschuh, 2008; Ismail, 2011b):

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

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

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;

Production well

steam (water) (Dickson & Fanelli, 2005).

electrical power generation.

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