**3.3.2 Heat exchanger**

490 Heat Exchangers – Basics Design Applications

However, GPK3 shows a high permeability while that of GPK4 is low (Table 5). Combining data of calcite content and permeability, one can infer that calcite may represent a serious threat to the EGS reservoir when the connectivity of the fractures is low while it does not impair the permeability when the connectivity is high. A solution can be brought by hydraulic fracturing that allows developing the extension of fractures. However, such process was employed in Basel (Switzerland) resulting in an earthquake of a 3.4 magnitude that scared the population in 2006. The EGS Basel project had to be stopped. At Soultz, an earthquake of 2.9 magnitude had been felt by local population during the stimulation of GPK3 in 2000 thus no further hydraulic stimulations were driven to prevent this problem. As a consequence, chemical stimulations had to be performed in order to improve the permeability and connectivity of the three deep wells. Particular efforts were put on GPK4. Figure 4 shows the results of chemical stimulations. The behaviour of the 3 deep wells has been largely improved. Given the good results of the circulation test conducted in 2005, and the improvement of the hydraulic performances of the three existing deep wells by stimulation, it was decided to build a geothermal power plant of Organic Rankine Cycle (ORC) type (using an organic working fluid). Thus, a first 1.5 MWe (electricity; equals 12 MW thermal) ORC unit was built and power production was achieved in June 2008 thanks to down-hole production pumps. The power plant was ordered to a European consortium made of Cryostar (France) and Turboden, Italy. A three year scientific and technical monitoring of the power plant has started on January 2009 focused on the reservoir

evolution and on the technologies used (pumps, exchanger; Genter et al., 2009).

reinjected in the rock reservoir through GPK3 and GPK1.

**Genter et al., 2010)** 

**3.3.1 Pumps** 

**3.3 Technical data about the heat exchanger and the EGS (after Genter et al., 2009 and** 

The geothermal fluid is produced from GPK2 and GPK4 thanks to two different kinds of pumps and, after electricity production (or only cooling if electricity is not produced), it is

It was necessary to install down-hole production pumps because the artesian production was not sufficient. Thus, two types of production pumps were deployed in the production wells: a Line Shaft Pump (LSP; in GPK2) and a Electro-Submersible Pump (ESP; in GPK4). The LSP itself is in the well while the motor is at surface. The connection is obtained through a line shaft. The main advantage is to avoid installing the motor in hot brine, but the possible installation depth is limited and the line shaft has to be perfectly aligned. The LSP was supplied by Icelandic Geothermal Engineering Ltd. The length of the shaft is 345 m. The shaft (40 mm diameter) is put in an enclosing tube (3" internal diameter) with bearings every 1.5 m. The enclosing tube is set by means of centralisers in the middle of the LSP production column (6" internal diameter) which is put into the 8" casing. The pump itself is from Floway (USA) and made of 17 different stages of 20 cm (3.4 m total length). The LSP flow rate can be modulated until 40l/s with a Variable Speed Drive. The maximum rotation speed is 3000 rpm at 50 Hz. The surface motor is vertical. Metallurgy is cast iron and injection of corrosion inhibitor can be done at the pump intake by mean of coiled tubing. Shaft lubrication is made with fresh water injected from surface in the enclosing tube. The pump has been installed at 350 m depth into GPK2 that presents good verticality and is the

A schematic view of the Soultz' binary power plant is given in Figure 5. As the purpose of the project was first to demonstrate the feasibility of power production, a binary system utilizing an organic working fluid called an Organic Rankine Cycle (ORC) technology was chosen. Due to the high salinity of the geothermal brine, the geothermal fluid cannot be vaporized directly into the turbine as occurs in classical "simple flash" power plants.

Then, a secondary circuit is used that involves a low boiling point organic working fluid (isobutane). As there is no easily accessible shallow aquifer around the geothermal site, an air-cooling system was required for the power plant, which also limits the impact on environment. It consists in a 9-fan system. The turbine is radial and operates around 13000 rpm. The generator is asynchronous and is running around 1500 rpm. The generator is able to deliver 11 kV and the produced power is to be injected into the 20 kV local power network.

The expected net efficiency of the ORC unit is 11.4%. Geothermal water may be cooled down to 80-90°C in the heat exchangers of the binary unit. After this cooling, the entire geothermal water flow rate is re-circulated in the reservoir. The system is built so that the production coming from one or two wells can easily be used to feed the power production loop. On surface, the pressure in the geothermal loop is maintained at 20 bars in order to

The Soultz-sous-Forêts' Enhanced Geothermal System:

**3.4 Review of 20 years of research at Soultz** 

propylene glycol.

**3.5 Future of the Soultz EGS** 

**4. Other EGS programs in the world** 

(SoultzNet, 2011).

project was conceived.

A Granitic Basement Used as a Heat Exchanger to Produce Electricity 493

Barriquand's exchangers to cool the brine to be reinjected. In that last case, the brine is injected in the 5 exchangers (Fig. 5). The fifth exchanger is used only when needed. The exchangers allow heat transfer between the brine and a fluid composed of water and mono

About 40 PhD theses have been written on the Soultz project in the last 20 years together with about 200 publications in international journals between 2001 and 2008. With this scientific background and the current production of electricity (1.5 MWe), the Soultz site is now a world reference for EGS. It appears that cooling 1 km³ of rock by only 20°C (initial temperature around 160-200°C) liberates as much energy as the combustion of 1 275 000

The total cost of the Soultz pilot operating now is 54 M€ (Soultznet, 2011). A prototype of 20- 30 MWe will follow the first pilot presently in production (1.5 MWe). On a longer term, industrial units will be constructed (Soultznet, 2011). Large-scale production units inspired from the Soultz EGS might transform the world of energy since it is clean and sustainable. It preserves fossil fuels and limits the emissions of GHG and allows a continuous production of electricity 8000 hours/year, at night as well as at day, whatever the climate conditions

The Soultz EGS is the only operating site at present. It benefited from the experience developed on other sites all around the world. The objective here is not to provide a complete review of these projects but to show the impact they had on the Soultz project. More details on these projects can be found in MIT (2006) and in the abundant literature easily available (e.g. Brown, 2000, 2009; Yanagisawa et al., 2011; Yasukawa and Takasugi,

The first attempt to extract the Earth's heat from rocks with no pre-existing high permeability was the Fenton Hill HDR experiment. It was initially totally funded by the U.S. government, but later involved active collaborations under an International Energy Agency agreement with Great Britain, France, Germany, and Japan. The Fenton Hill site is characterized by a high-temperature-gradient, a large volume of uniform, low-permeability, crystalline basement rock. It is located on the margin of a hydrothermal system in the Valles Caldera region of New Mexico, not far from the Los Alamos National Laboratory where the

The Fenton Hill experience demonstrated the technical feasibility of the HDR concept by 1980, but none of the testing carried out yielded all the performance characteristics required for a commercial-sized system (sufficient reservoir productivity, maintenance of flow rates with sufficiently low pumping pressures, high cost of drilling deep (> 3 km) wells in hard

rock becoming the dominant economic component in low-gradient EGS resources).

2003). An overview of HDR/EGS programs in the world is given in table 6.

**4.1 Fenton Hill (U.S.A.; after Brown, 2000, 2009; MIT, 2006)** 

tons of oil and thus saves as much non-renewable fossil fuel (SoultzNet, 2011).

Fig. 5. Schematic view of the Soultz' binary power plant after Genter et al. (2010). Each production well can be run separately thanks to the valves. The hot geothermal fluid (around 165°C) is filtered (150 µm) before entering the surface network. After the complete cycle, the fluid is reinjected in the natural rock exchanger thanks to one or two wells : with a reinjection pump in GPK3 and in addition if necessary by gravity in GPK1. The temperature, pressure and flow figures indicated are those obtained during the 8 months circulation test performed in 2008. If the Organic Rankine Cycle dedicated to electricity production is not activated (the last valve being closed), the geothermal fluid goes through 5 exchangers in the cooling cycle (lower part of the figure with the ORMAT aircooling system) in order to be reinjected after filtration (50 µm) at low temperature (around 50 to 67°C). The last of these five exchangers is used only when necessary.

avoid mineral precipitations. Locally, in the filtering system, some scaling was observed with barite, celestine, iron oxides, galena and calcite mainly. In order to investigate corrosion and scaling, an innovative corrosion pilot was set up on the surface geothermal loop and tested for the first time between September 2008 and February 2009. Different kinds of steel were investigated for corrosion in the geothermal conditions of re-injection (20bars, < 80°C).

The liquid hot brine is pumped from the rock reservoir and first filtered in the surface geothermal loop in a self-cleaning 150µm filter. Whether the ORC cycle is or not in function, either the geothermal fluid feeds the ORC exchanger to produce electricity or it feeds the Barriquand's exchangers to cool the brine to be reinjected. In that last case, the brine is injected in the 5 exchangers (Fig. 5). The fifth exchanger is used only when needed. The exchangers allow heat transfer between the brine and a fluid composed of water and mono propylene glycol.
