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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 and/or residues i.e. biomass. Thus, source of energy is biomass. Use of bioenergy is classified mainly into two types, traditional/conventional and modern (Gupta et al. 2014, pp. 1–17). Conventional use is the direct utilization of source i.e. biomass (e.g. wood, agricultural waste, traditional charcoal etc.) combustion to obtain energy. Modern use is the indirect use i.e. conversion of biomass using different technologies into biofuels. Apart from biofuels, other recent bioenergy technologies includes bio‐refineries (a range of bio‐products are produced consuming biomass), biogas (obtained via anaerobic digestion of bio‐residues) and wood pellet heating systems. Bioenergy can be stored for a long period as opposed to other renewable energy sources. By 2015, its share in total final energy consumption was nearly 10%, and 1.4% in worldwide electricity production. Approximately, 80% of bioenergy is consumed traditionally in the developing countries i.e. for cooking, space heating, and lighting (Lund et al. 2016, pp. 36–38). In 2019, bioenergy power generation capacity was 124 GW with a 5% growth in the year. Its contribution is roughly 5% in the worldwide renewable generation capacity, and China contributed more than half (3.3 GW) of new capacity added (6 GW) in the same year (IRENA 2020b).

Large‐scale production of biofuels can be achieved when these are produced using agricultural crops, crop residues, agro‐industries wastes, forests residue etc. At present, production cost of biofuels is high, conversion efficiency is low and its cost‐effectiveness is also low compared with other energy sources. To mitigate these issues, scientific community is doing lot of research worldwide. Advances are taking place in technologies for pre‐treatment of lignocelluloses and cost‐effective production method for biofuels with no or minimum by‐products (Lund et al. 2016, pp. 36–38). Bioenergy can significantly contribute in energy sector in the twenty‐first century. It will be a key component for the development of rural areas, agricultural sector and forestry.

      2.3.5.1 Biopellets and Biogas

      Biopellets are the simplest form of bioenergy yet competitive to crude oil or natural gas for the household uses in Western countries. Research in pelletization process of leaves in order to use them as additives or substitute to wood provides an alternative for biopellets production (Verma et al. 2012). Transforming biomass to biogas is a modern use of bioenergy, anaerobic digestion of biomass using a digestor leads to formation of biogas. Different types of digestors used for the anaerobic digestion are fixed dome, floating drum and plug flow (Gupta et al. 2014, pp. 1–17). Chemical composition of biogas is a function of the type of feedstock, design of digestor used and processing conditions (Lund et al. 2016, pp. 36–38). Replacing wood and coal with biogas in rural areas will decrease the emission of GHGs (Pathak et al. 2009). Moreover, biogas and biomethane produced from medium and large‐scale plants can substitute the natural gas usage as fuel in transportation, leading to decarbonization (IRENA 2019b).

      2.3.5.2 Bioethanol and Biodiesel

      Most frequently used biofuel is bioethanol. It can be formed from three types of feed stocks, namely sugar‐based (most common), starchy and lignocellulosic (require pre‐treatment). Production of bioethanol from sugar‐based crops was the most common method till 2003 and contributed nearly 60% in the worldwide production (Gupta et al. 2014, pp. 1–17). After sugar‐based crops (sugarcane, sorghum etc.), starch feedstock (corn, wheat, barley etc.) is used for bioethanol production followed by lignocellulosic feedstock (wood, straw, corncob etc.). Chemically lignocelluloses are mainly composed of carbohydrates (cellulose and hemi‐cellulose) and lignin along with some other quantities of biomass. Owing to the varying composition of lignocelluloses, several pre‐treatment processes were explored to obtain fermentable sugars. СКАЧАТЬ