Energy. Группа авторов
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Название: Energy
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Физика
isbn: 9781119741558
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In the high head hydropower plants, fishways find some restrictions, so in order to overcome these restrictions, some advances are taking place in the turbine designs. For safe passage of fishes through hydraulic turbines, a technology termed as fish‐friendly turbine has emerged, owing to the developments in the area of advances in turbines. Recently, two different turbines with better designs have been introduced, the Alden turbine and the Minimum Gap Runner turbine. Usage of these turbines leads to more power generation, reduction in fish mortality rate and injury along with better downstream water quality (Hogan et al. 2014).
2.3.4 Geothermal Energy
Geothermal energy is generated within the sub‐surface of earth in the form of heat. Direct use of this energy is for heating purposes (Lund and Boyd 2016), and/or it can also be harnessed for electricity generation using a geothermal power plant (Bertani 2016). Compared with the intermittent sources of energy (solar and wind), geothermal energy is constant around the year and it is available worldwide. Electricity generation from the geothermal energy comes with many benefits as compared with some other renewable energy sources such as higher capacity factors (>90%), flexibility in power production, ability of efficiently providing baseload electricity, low cost of electricity generation due to lower cost of operation, lower impact on environment along with less CO2 emissions and land usage is also small (Geirdal et al. 2015). Electricity production from geothermal has lower life‐cycle greenhouse gas (GHG) emissions as compared with fossil fuels (https://archive.ipcc.ch/pdf/special‐reports).
Despite many advantages of geothermal energy, its share in worldwide energy production is very small owing to many challenges such as substantial initial capital cost, longer time period before a geothermal plant becomes operational and high risk at early stage (pre‐survey, exploration and test drilling) (IRENA 2017). As mentioned earlier in Section 3, contribution of geothermal energy was 14 GW by the end of 2019 which is a relatively small percentage (<0.6%) of the total generation capacity of all the renewable sources of energy.
Geothermal energy has tremendous potential for development in near future owing to the huge amount of energy present within 10 km of the earth's surface which contains 50000 times more energy than all the fossil fuel reserves existing globally (Shere 2013, pp. 201). Depending upon the amount of heat present in a geothermal reservoir, different technologies are used for power generation. Medium or high‐temperature resources are required for power production which are generally located in the vicinity of tectonically active regions. To transfer the heat present within the earth's surface, water or steam is used. At present, four different technologies are used for power production from geothermal energy, namely dry steam plants, flash plants (single, double and triple), binary plants, and combined‐cycle or hybrid plants (Long et al. 2003). These technologies are based on exploitation of conventional geothermal resources.
Figure 2.3 Comparison of geothermal power capacity (MW) by different countries in 2016 and projected values in 2025 and beyond 2025.
Source: Based on data in Ref. (IRENA 2017).
Recently, due to more developments in enhanced geothermal systems (EGS), which are the deep geothermal resources, power production from these types of plants will add a new dimension in the geothermal energy sector. Power generation will increase by many folds and can reach to 70 GWe (installed capacity) by 2050 (Lu 2018). Figure 2.3 shows the geothermal power capacity of some countries in 2016 along with projected values in 2025 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 СКАЧАТЬ