**1. Introduction**

476 Heat Exchangers – Basics Design Applications

Sanner, B., Constantine Karytsasb, Dimitrios Mendrinosb & Rybachc, L. 2003. Current status

Seyboth, K., Beurskens, L., Langniss, O. & Sims, R. E. H. 2008. Recognising the potential for

renewable energy heating and cooling. *Energy Policy,* 36, 2460-2463. Wang, S. K. 2000. *Handbook of air conditioning and refrigeration,* New York, McGraw Hill.

*Geothermics,* 32, 579-588.

of ground source heat pumps and underground thermal energy storage in Europe.

The increasing need for energy, and electricity in particular, together with specific threats linked with the use of fossil fuels and nuclear power and the need to reduce CO2 emissions leads us to look for new energy resources. Among them, geothermics proves to be efficient and clean in that it converts the energy of the earth into heating (domestic, industrial or agricultural purposes) or electricity (Lund, 2007). Numerous geothermal programs are producing energy at present and some of them have been performing for several decades in the USA (Sanyal and Enedy, 2011), Iceland and Italy for example (Minissale, 1991; Romagnoli et al., 2010). From statistics presented in World Geothermal Congress 2010, the installed capacity of geothermal power generation reaches 10,715 MW in the world. It increased by nearly 20% in 5 years. Its average annual growth rate is around 4%. USA, Indonesia and Iceland increased by 530MW, 400MW and 373MW respectively. Many countries all around the world develop geothermal exploitation programs. As a consequence, scientists from the whole world meet each year at the Annual Stanford Workshop on Geothermal Reservoir Engineering to discuss new advances in geothermics.

Conventional geothermal programs use naturally heated groundwater reservoirs. In many sedimentary provinces, depths of a few hundreds of meters are enough to provide waters with a temperature around 90°C. Such resources give rise to low and very low enthalpy geothermics. Very low enthalpy geothermal resources are used through geothermal heat pumps for various purposes including hot water supply, swimming pools, space heating and cooling either in private houses or in public buildings, companies, hotels and for snowmelting on roads in Japan (Yasukawa and Takasugi, 2003). In 1999 the energy extracted from the ground with heat pumps in Switzerland reached 434 GWh. The same level of utilization in Japan would bring the Japanese gure to 8 TWh per year (Fridleifsson, 2000). Technically, heat pumps can be applied everywhere. It is the difference between surface (atmospheric) and underground temperatures at 20 m or deeper that provides the advantage of geothermal heat pumps over air-source heat pumps.

In volcanic zones (like in Iceland), geothermics depends on specific geological contexts that are rather rare on the earth even though quite numerous in specific zones e.g. in the vicinity

The Soultz-sous-Forêts' Enhanced Geothermal System:

Soultz-sous-Forêts

A Granitic Basement Used as a Heat Exchanger to Produce Electricity 479

Country Site Depth (m) Dates Production

(3 boreholes) 1985- 1.5 MWe

5000

MWe: MWelectricity (raw production minus consumption of electricty required for the production).

conventional geothermal power generation. It is expected that EGS will produce less dissolved gas (mostly carbon dioxide CO2 and hydrogen sulfide H2S) than conventional energy by recovering the heat through a heat exchanger and reinjecting the fluid without releasing any gas during operation. As concerns subsurface water contamination it is unlikely because all the produced fluid is reinjected. EGS is also characterized by a modest use of land since with directional-drilling techniques, multiple wells can be drilled from a single pad to minimize the total wellhead area (MIT, 2006). EGS requires no storage and the plant is built near the geothermal reservoir because long transmission lines degrade the pressure and temperature of the geofluid as well as the environment. As a consequence EGS power plants require about 200 m2/GWh while a nuclear plant needs 1200 m2/GWh and a solar photovoltaic plant 7500 m2/GWh (MIT, 2006). Most of EGS developments are likely to occur in granitic-type crystalline rocks, at great depth. Careful management of the water resource is unlikely to induce subsidence (lowering of the ground's level as in shallow mining activity; MIT, 2006). Seismic activity linked to engineering of EGS reservoirs during hydraulic stimulation (injection of water under pressure to create or open pre-existing joints) has conducted managers of these sites to prefer chemical stimulations (use of chemicals to dissolve minerals responsible for the sealing of joints) in or close to urban areas (Hébert et al., 2011; Ledésert et al., 2009) in order to avoid earthquakes. Another force of the EGS technology is the possible use of CO2 instead of water because of its favourable thermodynamic properties over water in EGS applications (Brown, 2000; Magliocco et al., 2011) thus leading to a possible sequestration of carbon dioxide produced by the use of fossil fuel. However EGS might have increased visual impact and noise levels compared to conventional geothermal power plants but no more than fossil fuel driven power plants (MIT, 2006). The highest noise levels are usually produced during the well drilling, stimulation, and testing phases (about 80 to 115 decibels). For comparison, congested urban areas typically have noise levels of about 70 to 85 decibels, and noise levels next to a major freeway are around 90 decibels. A jet plane just after takeoff produces noise levels of about 120 to 130 decibels (MIT, 2006). Finally, considering all these factors, EGS has a low overall

France Le Mayet 1975-1989

Germany Bad Urach 4500 and 2600 1976- Falkenberg 300 1975-1985 UK Rosemanowes 2700 1975-1991 Australia Cooper Basin 4300 2001- USA Fenton Hill 5000 1973-2000 Coso 2001- Desert Peak 5420 2001- Japan Hijori 2300 1987-

Data after MIT (2006), Davatzes and Hickmann (2009) and Genter et al. (2009) Table 1. Overview of some HDR or EGS programs in the world.

of electricity

Since June 2008

of subduction zones like around the Pacific Ocean as in Japan (Tamanyu et al., 1998) or in zones where the earth's crust is expanding like in Iceland (Cott et al., 2011). Such geothermics is called high enthalpy. It allows the production of electricity like in the Uenotai geothermal power plant in Japan which started operation in 1994 as a 27.5 MW electric power generation facility (Tamanyu et al., 1998). Electricity production from geothermal resources began in 1904 in Italy, at Larderello (Lund, 2004; Massachusetts Institute of Technology [MIT], 2006). Since that time, other hydrothermal developments led to an installed world electrical generating capacity of nearly 10,000 MWe and a direct-use, nonelectric capacity of more than 100,000 MWt (thermal megawatts of power) at the beginning of the 21st century from the steam field at The Geysers (California, USA), the hotwater systems at Wairakei (New Zealand), Cerro Prieto (Mexico), Reykjavik (Iceland), Indonesia and the Philippines (MIT, 2006).

Complementary to conventional geothermics, Enhanced Geothermal Systems (EGS; also called Engineered Geothermal Systems) aim to develop reservoirs in rocks where little (or no) water is available (Redden et al., 2010). This concept was invented, patented and developed in the early 1970s at Los Alamos National Laboratory and was first called Hot Dry Rock (HDR) geothermal energy. As defined by these early researchers, the practical HDR resource is the heat contained in those vast regions of the earth's crust that contain no fluids in place—the situation characterizing by far the largest part of the earth's drillingaccessible geothermal resource (Brown, 2009).

This concept was developed for electricity production in any kind of area at the surface of the earth even though the geodynamical context is not in favour of geothermics (Redden et al., 2010 and references therein).

However, because of the general low thermal gradient in the earth (30°C/km in sedimentary basins), reaching a temperature around 150-200°C needed for the production of electricity make things more difficult than first considered. Technical and economical problems linked to such deep drillings (Culver, 1998; Rafferty, 1998) have restricted EGS to zones where the thermal gradient is high enough to reduce the depth of the exchanger. Now, EGS include all geothermal resources that are currently not in commercial production and require stimulation or enhancement (MIT, 2006).

Table 1 gives an overview of HDR/EGS programs in the world.

In such difficult technical conditions, one can wonder whether the geothermal energy resource and electricity production process are sustainable. According to Clarke (2009) and authors cited therein, the management and use of the geothermal resource (Rybach and Mongillo, 2006) and the environmental impacts during geothermal energy production (Bloomfield et al., 2003; Reed and Renner, 1995) were the first concern about sustainability of geothermal energy. These studies have shown that there is less impact on land use, air emissions including greenhouse gases, and water consumption from geothermal electricity generation than from fossil-fuel–based electricity generators. However, the environmental impacts from the construction of geothermal energy production facilities being less well understood, especially for enhanced geothermal systems (EGS) subsequent studies were conducted. The life-cycle analysis of the EGS technology (including pre-production process such as drilling, construction, production and transportation) had to be discussed, especially when the potential for large-scale development exists. Because of increased depth and decreased water availability, environmental impacts may be different from those of

of subduction zones like around the Pacific Ocean as in Japan (Tamanyu et al., 1998) or in zones where the earth's crust is expanding like in Iceland (Cott et al., 2011). Such geothermics is called high enthalpy. It allows the production of electricity like in the Uenotai geothermal power plant in Japan which started operation in 1994 as a 27.5 MW electric power generation facility (Tamanyu et al., 1998). Electricity production from geothermal resources began in 1904 in Italy, at Larderello (Lund, 2004; Massachusetts Institute of Technology [MIT], 2006). Since that time, other hydrothermal developments led to an installed world electrical generating capacity of nearly 10,000 MWe and a direct-use, nonelectric capacity of more than 100,000 MWt (thermal megawatts of power) at the beginning of the 21st century from the steam field at The Geysers (California, USA), the hotwater systems at Wairakei (New Zealand), Cerro Prieto (Mexico), Reykjavik (Iceland),

Complementary to conventional geothermics, Enhanced Geothermal Systems (EGS; also called Engineered Geothermal Systems) aim to develop reservoirs in rocks where little (or no) water is available (Redden et al., 2010). This concept was invented, patented and developed in the early 1970s at Los Alamos National Laboratory and was first called Hot Dry Rock (HDR) geothermal energy. As defined by these early researchers, the practical HDR resource is the heat contained in those vast regions of the earth's crust that contain no fluids in place—the situation characterizing by far the largest part of the earth's drilling-

This concept was developed for electricity production in any kind of area at the surface of the earth even though the geodynamical context is not in favour of geothermics (Redden et

However, because of the general low thermal gradient in the earth (30°C/km in sedimentary basins), reaching a temperature around 150-200°C needed for the production of electricity make things more difficult than first considered. Technical and economical problems linked to such deep drillings (Culver, 1998; Rafferty, 1998) have restricted EGS to zones where the thermal gradient is high enough to reduce the depth of the exchanger. Now, EGS include all geothermal resources that are currently not in commercial production

In such difficult technical conditions, one can wonder whether the geothermal energy resource and electricity production process are sustainable. According to Clarke (2009) and authors cited therein, the management and use of the geothermal resource (Rybach and Mongillo, 2006) and the environmental impacts during geothermal energy production (Bloomfield et al., 2003; Reed and Renner, 1995) were the first concern about sustainability of geothermal energy. These studies have shown that there is less impact on land use, air emissions including greenhouse gases, and water consumption from geothermal electricity generation than from fossil-fuel–based electricity generators. However, the environmental impacts from the construction of geothermal energy production facilities being less well understood, especially for enhanced geothermal systems (EGS) subsequent studies were conducted. The life-cycle analysis of the EGS technology (including pre-production process such as drilling, construction, production and transportation) had to be discussed, especially when the potential for large-scale development exists. Because of increased depth and decreased water availability, environmental impacts may be different from those of

Indonesia and the Philippines (MIT, 2006).

accessible geothermal resource (Brown, 2009).

and require stimulation or enhancement (MIT, 2006).

Table 1 gives an overview of HDR/EGS programs in the world.

al., 2010 and references therein).


MWe: MWelectricity (raw production minus consumption of electricty required for the production). Data after MIT (2006), Davatzes and Hickmann (2009) and Genter et al. (2009)

Table 1. Overview of some HDR or EGS programs in the world.

conventional geothermal power generation. It is expected that EGS will produce less dissolved gas (mostly carbon dioxide CO2 and hydrogen sulfide H2S) than conventional energy by recovering the heat through a heat exchanger and reinjecting the fluid without releasing any gas during operation. As concerns subsurface water contamination it is unlikely because all the produced fluid is reinjected. EGS is also characterized by a modest use of land since with directional-drilling techniques, multiple wells can be drilled from a single pad to minimize the total wellhead area (MIT, 2006). EGS requires no storage and the plant is built near the geothermal reservoir because long transmission lines degrade the pressure and temperature of the geofluid as well as the environment. As a consequence EGS power plants require about 200 m2/GWh while a nuclear plant needs 1200 m2/GWh and a solar photovoltaic plant 7500 m2/GWh (MIT, 2006). Most of EGS developments are likely to occur in granitic-type crystalline rocks, at great depth. Careful management of the water resource is unlikely to induce subsidence (lowering of the ground's level as in shallow mining activity; MIT, 2006). Seismic activity linked to engineering of EGS reservoirs during hydraulic stimulation (injection of water under pressure to create or open pre-existing joints) has conducted managers of these sites to prefer chemical stimulations (use of chemicals to dissolve minerals responsible for the sealing of joints) in or close to urban areas (Hébert et al., 2011; Ledésert et al., 2009) in order to avoid earthquakes. Another force of the EGS technology is the possible use of CO2 instead of water because of its favourable thermodynamic properties over water in EGS applications (Brown, 2000; Magliocco et al., 2011) thus leading to a possible sequestration of carbon dioxide produced by the use of fossil fuel. However EGS might have increased visual impact and noise levels compared to conventional geothermal power plants but no more than fossil fuel driven power plants (MIT, 2006). The highest noise levels are usually produced during the well drilling, stimulation, and testing phases (about 80 to 115 decibels). For comparison, congested urban areas typically have noise levels of about 70 to 85 decibels, and noise levels next to a major freeway are around 90 decibels. A jet plane just after takeoff produces noise levels of about 120 to 130 decibels (MIT, 2006). Finally, considering all these factors, EGS has a low overall

The Soultz-sous-Forêts' Enhanced Geothermal System:

limited.

the literature.

electricity

correct management

A Granitic Basement Used as a Heat Exchanger to Produce Electricity 481

thanks to thorough characterization of the fracture network (Hébert et al., 2010, 2011; Ledésert et al., 2009). When the rock heat exchanger is finally operated, careful reservoir engineering and monitoring has to be performed to ensure the viability of the EGS (Satman, 2011). The produced hot fluid is continuously replaced by cooled injected water. After the thermal breakthrough time the temperature of the produced fluid decreases. However if after a time the field is shut-in the natural energy flow will slowly replenish the geothermal system and it will again be available for production. Therefore when operated on a periodic basis, with production followed by recovery, doublets are renewable and sustainable (Satman, 2011). Triplets (one injection and two production boreholes) are now considered as being the best configuration (MIT, 2006; Genter et al., 2009). However, it must be taken into consideration that when an EGS reservoir is developed through hydraulic fracturing, the size of the reservoir might extend too much and attendant high water losses might occur compromising the sustainability of the project as at Fenton Hill (MIT, 2006; Brown, 2009). When shearing occurs through reopening of pre-existing sealed fractures during hydraulic stimulation (e.g. at Fenton Hill; Brown, 2009) or when the mineral deposits are dissolved through chemical stimulation (e.g. at Soultz; Nami et al., 2008; Portier et al., 2009; Genter et al., 2009) the size of the exchanger is better constrained and fluid losses are

Other risks such as technology (reliable supply of produced geofluids with adequate flow rates and heat content), finances (cost of construction, drilling, delays), scheduling, politics, etc…have to be estimated in order to make an EGS project viable. They are presented in MIT (2006) for the different stages of a project. In addition, seismic risk has to be fully taken into account where EGS programs are to be developed in urban areas in order to produce both electricity and central heating (Giardini, 2009): the Basel (Switzerland) experience (see section 4.4) had to be stopped because of a 3.4 magnitude earthquake generated by stimulation in a naturally seismic area. As a consequence to these numerous constraints, no financially viable EGS program is operating at present but the production of electricity that

As a conclusion, Table 2 shows some forces and difficulties of EGS programs inferred from

Production of electricity Deep drilling (technical and financial

Low to no GHG emissions Engineering of the reservoir to increase

Available on continents worldwide No financially viable program operating at present

Table 2. Forces and difficulties of EGS programs inferred form the abundant literature on

difficulties)

fracturing

permeability

Risk of water loss in case of pure hydraulic

Adequate flow rate and temperature

began at Soultz-sous-Forêts (France) in June 2008 is highly promising.

Forces Difficulties

Sustainability of the resource provided its

Low global environmental impact compared to fossil-fuel and nuclear

the subject (see reference list). GHG: greenhouse gases.

environmental impact when the production of electricity is considered compared to fossil or nuclear generation (MIT, 2006). As concerns the demand, supply and economic point of view, MIT (2006) provides a rather detailed analysis.

Geothermal energy and EGS in particular is studied world-wide and the annual Workshop on Geothermal Reservoir Engineering (Stanford, California, USA) allows scientists and industrials to compare data and improve renewable energy production. Proceedings of the workshop can be downloaded very easily and for free on the Internet. As a consequence, the latest advances in geothermal technology are available to the scientific community.
