and ahead of 2025 (IRENA 2017).
2.3.4.1 Direct Dry Steam Plants
Direct dry steam geothermal power plant uses steam which is at 150 °C or higher, produced from the production well, to generate electricity with the help of geothermal steam turbines. Owing to design, these turbines effectively use the low‐pressure and high‐volume fluid produced in the steam field for conversion. Usually, these plants make use of condensing turbines, and thus produced condensate is re‐injected (closed cycle) or evaporated in wet cooling towers (https://iea‐etsap.org). The power generation capacity of these plants lies between 8 and 140 MW (www.platts.com).
2.3.4.2 Flash Power Plants
At present, most of the operational geothermal power plants are flash power plants. These plants work similar to dry steam plants; the only difference is steam production step. Steam is produced from flashing, a separation process for the two‐phase fluid, and then passed into the turbine. Flash plants are of three types, single, double and triple depending upon the capacity of the plant. In a single flash plant, condensate resulting from the turbine is re‐injected into the well while in a double‐flash plant, the condensate is directed for further separation at lower pressure to generate more steam. In a triple‐flash plant, the process of flashing is repeated one more time. Flash power plants are suitable for those reservoirs which possess well temperature >180 °C (IRENA 2017). Triple‐flash plants have the maximum power capacity (60–150 MW) followed by double (2–110 MW) and then single having the least capacity between 0.2–80 MW (www.platts.com).
2.3.4.3 Binary Plants
These plants are established for the reservoirs which possess well temperature between 100 and 170 °C. These plants use a process fluid which obtains heat from the geothermal fluid through heat exchangers in a closed loop. Depending upon the well‐matched boiling and condensation points of the geothermal fluid, different process fluids can be used such as ammonia/water mixtures used in Kalina cycles or hydrocarbons in organic Rankine cycles (https://iea‐etsap.org). The capacity of these plants varies between <1 MW and 50 MW (www.platts.com).
2.3.4.4 Combined‐Cycle or Hybrid Plants
In these plants, an additional Rankine cycle is employed to harness more electricity from the heat generated from the binary cycle, thus increasing the efficiency of the plant. These plants have power capacity up to 10 MWe (DiPippo 2015). On the other hand, hybrid plants make use of an additional source of heat such as CSP to enhance the efficiency of the plant by increasing the temperature of geothermal fluid. For example, a hybrid plant in Italy uses biomass energy to increase the temperature of brine and the Stillwater project in the United States which has combined CSP and solar photovoltaics with a binary plant (IRENA 2017).
2.3.4.5 Enhanced Geothermal Systems (EGS)
Conventional geothermal plants use the heat reservoirs which are present at intermediate (0.1–4 km) or shallow (up to 0.1 km) depths (Olasolo et al. 2016). Heat content of these reservoirs is limited so to access more heat from deep geothermal reservoirs (4–5 km or more), it has been proposed that reservoirs possessing huge amount of heat can be created artificially. This technique of creating large heat reservoirs using advanced drilling technology to make artificial fractures in the deep rocks having low permeability is known as EGS (Johnston et al. 2011). Figure 2.4 depicts the basic outline of a geothermal power plant using EGS technology. EGS decreases the dependency on conventional hydrothermal reservoir along with the generation of large quantities of electricity. Hydraulic or chemical stimulation is used to create artificial fractures to make the flow of water easier through the rocks (less permeable) which are at greater depths. In the process of stimulation, water and a small amount of chemicals are injected to create or reopen fissures in the rocks that exist in greater depths. When the injection pressure decreases, there is a probability that artificial fractures in the rocks are blocked so proppants are added to avoid the closing of fractures. Binary power plants are used in the EGS technology to generate electricity using the hot brine. Since there is no natural flow of water in the deep hard rock so to exploit the heat from these hot rocks, water is used as the working fluid. To maintain the constant pressure and production, brine solution has to be re‐injected into the well, thus avoiding any release of air during the lifespan of the power plant. By the end of 2018, there were 18 important EGS sites around the globe which are harnessing electricity using the advanced drilling techniques (Lu 2018). Recently, due to advances in the area of EGS systems, some techniques are used in EGS plants to make their performance better such as use of dense fluids for hydraulic stimulation and CO2 as the working fluid.
Figure 2.4 Outline of an EGS power plant.
Source: IRENA (2017).
By making use of hydraulic stimulation, artificial fractures are created in deep and hot rock. These fractures increase permeability of the rock via increasing the natural pathways (Olasolo et al. 2016). While creating fractures in a rock, there is a high probability that this hydraulic overpressure may cause micro‐seism in the vicinity. So, use of dense fluid for hydraulic stimulation is one of the options to control overpressure at the surface. Further, use of dense fluid has many advantages such as increased and improved underground circulation pathways; less permeability of these pathways; and better mass flow ratio and heat extraction ratio (Olasolo et al. 2016).
Use of CO2 as a working fluid is more efficient at medium and high pressures while at low pressure, water is a better working fluid. Due to its high heat extraction ratio (50% higher) than water along with its better performance at low temperatures, its use as working fluid in the EGS has increased (Olasolo et al. 2016).
Many developments are taking place with respect to software packages which are used for estimating and simulating costs of the EGS plants. GEOPHIRES and EURONAT are the most suitable software packages (Olasolo et al. 2016). EURONAT is a European and GEOPHIRES is the US software package. These packages have their own pros and cons; however, GEOPHIRES has the unique feature of being capable to simulate cost for electricity generation as well as for direct use heating and combined heat and power.
Worldwide, installed capacity of geothermal can be enhanced by using some advances which increase the efficiency of operational plants. Low‐temperature bottoming cycle is one such way which increases the efficiency of power generation by making use of a binary cycle in a conventional flash power plant. Another way is co‐generation, this technique uses the heat of condensate to raise the temperature of different water source before re‐injecting into the well. To cut down cost of electricity production from the geothermal plants, use of co‐produced resources i.e. by‐product of some industrial methods as a geothermal fluid offers a viable option (IRENA 2017).
Supercritical geothermal systems are the geothermal reservoirs where fluid is present in its supercritical state i.e. at very high temperature and pressure. Use of these supercritical geothermal systems as the wells for injection can increase the efficiency of current power plant (Friðleifsson et al. 2015). Another option is establishing a plant which directly uses the supercritical systems as the heat reservoirs. These plants will have better economic performance than the traditional plant due to high‐temperature wells (IRENA 2017).
2.3.5 Bioenergy
Bioenergy is one of the alternative forms of energy derived from biological sources