**3. CO2 – based EGS for combined power generation& CO2 sequestration**

#### **3.1. Enhanced geothermal systems (EGS) – developments & utilization**

EGS, also known as engineered geothermal systems, are reservoirs that have been stimulat‐ ed(e.g. hydraulic stimulation) and engineered to extracteconomical amounts of heat from unproductive geothermal resources that lack heat-carrier fluid circulation, permeability and/or porosity. EGS is a new type of geothermal power technologyand has the potential to become a significant sustainable and renewable power source for the future (Grasby et al., 2011, Kalra et al., 2012). The EGS concept is currently the subject of several international re‐

#### ORC-Based Geothermal Power Generation and CO2-Based EGS for Combined Green Power Generation and CO2 Sequestration http://dx.doi.org/10.5772/52063 315

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

restrictive(Grasby et al., 2011). For small-scale geothermal power plants (<5 MWe) utilizing low-temperature resources, the subsurface cost typically accounts for approximately 30% of

**(US\$/year) Geothermal Resource Temperature (oC)**

**O&M Cost**

the total investment costs whereas the surface cost accounts for the remaining 70%.

**Capital Cost (US\$/net kWe)**

**100 120 140**

 2,786 2,429 2,215 21,010 2,572 2,242 2,044 27,115 2,357 2,055 1,874 33,446 2,143 1,868 1,704 48,400

**Table 2.** Unit cost of electricity generated from low-temperature based small power plants (Chandrasekharam&

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

**3. CO2 – based EGS for combined power generation& CO2 sequestration**

EGS, also known as engineered geothermal systems, are reservoirs that have been stimulat‐ ed(e.g. hydraulic stimulation) and engineered to extracteconomical amounts of heat from unproductive geothermal resources that lack heat-carrier fluid circulation, permeability and/or porosity. EGS is a new type of geothermal power technologyand has the potential to become a significant sustainable and renewable power source for the future (Grasby et al., 2011, Kalra et al., 2012). The EGS concept is currently the subject of several international re‐

**3.1. Enhanced geothermal systems (EGS) – developments & utilization**

**Net Power (kWe)**

long-term.

Bundschuh, 2008; DiPippo, 2008).

314 New Developments in Renewable Energy

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

The basis, on which today's EGS projects are developed were laid out in the early 1970s, when an EGS (hot dry rock (HDR)) development concept was implemented at Los Alamos National lab, involving drilling a well into hot crystalline rock, using pressurized water to create a large vertical fracture, and ultimately to drill a second well to access that fracture at some distance above the first wellbore (Huenges, 2010; Stephens & Jiusto, 2010). It was the first attempt anywhere to make a deep, full-scale HDR reservoir. Significant research and production plants are under construction or operation to take advantage of this abundant renewable energy opportunity (Kalra et al., 2012). For example, power plants driven by EGS are currently being developed and tested in Australia, Germany, Japan, France, USA and Switzerland (DiPippo, 2008; Azim et al., 2010).The largest EGS project in the world is a 25 MWe demonstration plant currently being developed in Cooper Basin, Australia.There are several EGSprojects that are already or will soon produce power. Someof them are just for‐ research and development (R& D) and some are for commercial purpose. Examples of theEGS projects around the world with their location, capacity, welldepth, plant type, and the project type are summarized in Table 3.

ergy from the rock by injecting cool fluid (typically water) through the injection well and producing hot fluid (in some cases steam)from the production wells, and (8) Operating the

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

Sequestration

317

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

EGS for power generation is still relatively novel technology and remains to be proved on a large scale. Engineers & researchers in several countries throughout the world have been working on advancing EGS technology for few decades, but the technology has received limited attention and minimal financial support from either the public or private sector, with the exception of Australia's significant market investments. The high cost of drilling, which is estimated to account for a third to a half of EGS projected costs, is a major challenge to the technology (Stephens & Jiusto, 2010). The risks and uncertainties associated with EGS

It was reported (Grasby et al., 2011) that impacts of geothermal development are relatively minor compared to many other energy developments, however there are still importantchal‐ lenges to be addressed. More particularly, it was reported (Azim et al., 2010) that the overall impact of EGS power generation plants on environment is remarkably lower than other con‐ ventional fossil fuel-fired and nuclear power plants.In addition, EGS plants may have lower impacts in comparison with other renewable energy sources such as solar, wind, hydroelec‐ tric, and biomass on an equivalent energy-output basis. This is primarily because a geother‐ mal energy source is contained subsurface and need not to be exposed in the atmosphere and the surface energy conversion system (e.g. ORC unit) is relatively compact, thus making the overall size of the entire system attractively small. EGS power plants also provide envi‐ ronmental benefits by having minimal GHGand other emissions (zero emissions for the case of using ORC technology). Distinct from the conventional fossil fuels, there are minimal dis‐ charges of CO2, nitrogen or sulphur oxides or particulate matter resulting from its use, and there is no need to dispose radioactive materials. However, still there are impacts that must be considered and managed if enhanced geothermal energy resource is to be developed as part of a more environmentally sound, sustainable energy source for the future. The major environmental challenges for EGS are associated with ground water use and contamination, with related concerns about induced seismicity or subsidence as a result of water injection and production. Induced seismicity is a phenomenon in which a change in fluid pressure within a stressed rock formation leads to movement of the fractured rocks; the energy re‐ leased is transmitted through the subsurface rock and may reach the surface with enough intensity to be heard or felt by people in the surrounding region (DiPippo, 2008; Majer et al., 2007). To mitigate risks related to induced seismicity, strategies and procedures are needed to set requirements for seismic monitoring and for prolonged EGS field operation. Technolo‐ gies for imaging fluid pathways induced/injected by hydraulic stimulation in EGS fields would constitute a key improvement of the EGS concept. Issues of noise, safety and land use associated with drilling and production operations are also important but can be fully man‐

**3.2. Environmental and economic aspectsof EGS for power generation**

ageable (Huenges, 2010; Majorowicz& Grasby, 2010; Azim et al., 2010).

ORC power plant and maintain the EGS reservoir.

technology are other barriers as well.


**Table 3.** EGS-based projects around the world (Azim et al., 2010).

An EGS project has several stages (Majorowicz& Grasby, 2010; Huenges, 2010; DiPippo, 2008); namely: (1) Identifying a potential site possessing the necessary characteristics through surface exploration, (2) Drilling an injection well to the depth required to reach the desired temperature, (3) Fracturing the rock in the subsurface by hydraulic stimulation (i.e. injecting a fluid at sufficient pressure to propagate fracture), (4) Creating and testing of the EGS reservoir storage capacity, (5) Drilling a production well for a doublet system or two‐ production wells for a triplet system (one injection & two production wells), (6) Creating fracture connectivity between the injection andthe production wells, (7) Extracting heat en‐ ergy from the rock by injecting cool fluid (typically water) through the injection well and producing hot fluid (in some cases steam)from the production wells, and (8) Operating the ORC power plant and maintain the EGS reservoir.

EGS for power generation is still relatively novel technology and remains to be proved on a large scale. Engineers & researchers in several countries throughout the world have been working on advancing EGS technology for few decades, but the technology has received limited attention and minimal financial support from either the public or private sector, with the exception of Australia's significant market investments. The high cost of drilling, which is estimated to account for a third to a half of EGS projected costs, is a major challenge to the technology (Stephens & Jiusto, 2010). The risks and uncertainties associated with EGS technology are other barriers as well.
