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СКАЧАТЬ China, Spain, Germany, Italy and India (REN21, Paris 2014). China leads the overall production of energy from renewable sources. However, the contribution of India is also noticeable among the Asian countries especially in the hydropower sector along with the major contribution from the solar industry. To change the scenario of energy contribution, numerous researches are carried out in the field of utilizing and generating electricity from the renewable energy sources such as solar, wind, biomass, geothermal and others as well. New methods and techniques are being developed to harness energy in an efficient and fruitful manner (Gong et al. 2019). Research based on design, size and new materials formation or invention is more renowned nowadays (Kannan and Vakeesan 2016).

      2.3.1 Solar Energy

      Solar energy is available in the form of heat and light. Sun emits solar energy at the rate of 3.8 × 10²³ kilowatt (kW), from this enormous amount of energy approximately 1.8 × 10¹⁴ kW is obstructed by the Earth (Panwar et al. 2011). Worldwide energy requirements can be achieved by utilizing solar energy due to its adequate and free availability. Further, this source of energy is inexhaustible in nature, and output efficiency is also becoming better constantly due to various researches done in the field of photovoltaics. Factors on which harnessing of solar energy depends are its distribution and intensity of radiations, and hence, the efficiency of the solar industry becomes dependent on geographical location of a country (Panwar et al. 2011). Considering the distribution of solar radiation worldwide, it can be clearly understood that Asian countries receive the maximum radiations as compared with the rest of the world. Further, IRENA report 2020 (IRENA 2020b) published that Asian countries continued to dominate the global solar capacity expansion with a 56 GW increase (about 60% of the global expansion in 2019).

      Exercising solar energy has dual results as it fulfils the energy demand and does not disturb the ecosystem contrary to the exploitation of fossil fuels. Further, applicability of solar energy is not different for rural and urban due to its easy installation and hence can be easily utilized with the same ease and equal efficiency.

      Solar–thermodynamic power plants or concentrating solar thermal power (CSP) and solar photovoltaic (PV) are the two main technologies that can be practically used to transform solar energy into electric power. Nowadays, solar heaters are also becoming very popular which consume heat from the sun to directly increase the temperature of a fluid.

      2.3.1.1 Solar Photovoltaic

This is the fastest growing technology with an average increase of 48% since 2002 (Kropp 2009). Six main types of solar PV which are used to transform solar energy directly into electricity are crystalline silicon, thin film solar cells, concentrated solar PV, organic/polymer cells, hybrid solar cells and dye‐sensitized solar cells (DSSCs) (Pandey et al. 2016). Apart from these main types, there are some other solar cells based on advanced technologies. Tuning of the band gap of solar cells using nanoscale composites revealed enhanced power conversion efficiency. These are often termed as third‐generation PV (tandem cells, impurity‐band and intermediate‐band devices, hot‐electron extraction and carrier multiplication) based on nanostructures. In the field of nanotechnology, carbon nanotubes, quantum dots and ‘hot‐carrier’ flat‐plate device based solar PV cells are produced (Razykov et al. 2011).

      Under the crystalline silicon solar cells which are one of the categories of solar PV, there are mono‐crystalline, poly‐crystalline and GaAs‐based solar cells. Mono‐crystalline is still popular among the manufactures due to high efficiency and easy availability; however, its cost is high for both manufactures and end users. So, other cost‐effective options are also evaluated to further decrease the cost, and ploy‐crystalline offers a good deal in terms of production cost. Another alternative under the category of crystalline silicon cell is GaAs‐based solar cells which provides high efficiency, and these are also low‐weight. However, again, its cost is high compared with other types of crystalline solar cells. These are resistant to high heat which makes them suitable for the concentrated PV (used in power generation), hybrid use and space applications (Deb 1998).

      Thin‐film solar cells are of three types, namely amorphous Si, CdS/CdTe and CIS/CIGS (copper indium gallium selenide). Amorphous Si‐based thin‐film solar cells are further classified into three types: single junction, double junction and triple junction (El Chaar et al. 2011). Thin‐film solar cells require less manufacturing materials which makes them cheaper compared with crystalline Si‐based cells. Amorphous Si‐based solar cells have higher absorption rate of light (40 times due to non‐crystalline and disordered structure) which makes them more popular than CdS/CdTe and CIS/CIGS among the same category owing to the higher efficiency of the former (Pandey et al. 2016). Let us consider a particular example of CdTe solar cell, where an experimental study (Soliman et al. 1996) to enhance the characteristics of CdTe showed that to produce better cells, chemical heat treatment is required. Another example in the same category is CIGS which has been popular because of its laboratory‐scale efficiency of about 20.3%. In the area of thin films, there is ongoing research to enhance the efficiency and lifetime of these cells (Pandey et al. 2016).

Concentrated solar PV (CPV) system is gaining popularity nowadays due to its high efficiency which is the major requirement to make it cost‐effective technology and also to make it feasible at user end. Different classification of concentrated solar irradiation based on a study (Looser et al. 2014) is shown in Figure 2.2. CPVs are used to generate electricity as well as heating of water to low or medium temperature by extracting heat using active cooling i.e. using heat transfer fluid. For the long‐term applications of CPV in different sectors, various studies are conducted worldwide. In a particular example, at the Institute of Nuclear Research in Taiwan, Kuo et al. 2009 worked on the design and development of the 100 kW high‐concentration photovoltaic (HCPV) with passive cooling system. This institute receives solar radiation of 850 W/m², with this solar radiation system module efficiency reported to be 26.1% with a concentration ratio of 476×.

Organic/polymer solar cells have efficiency between 8 and 10% (Dou et al. 2012). In addition to the low efficiency, these cells are used as an alternative material due to various properties such as low manufacturing cost, low weight and good mechanical flexibility. Globally many laboratories have developed high‐performance solar cells using P3HT (poly [3‐hexylthiophene]) as the donor and PCBM ([6, 6]‐phenyl C60 butyric acid methyl ester) as the acceptor and/or BHJ (bulk hetero‐junction) structures (Bagienski and Gupta 2011; Devi et al. 2011). Further, from an environmental point of view, these types of cells are the most desirable ones.

Figure 2.2 Classification of common technologies and system set‐up for concentrated solar irradiance conversion.

      Source: Based on ref. Looser et al. (2014).

      Hybrid solar cells offer a right blend of inorganic and organic materials. At present, this type of cells are gaining popularity due to cheap processing techniques of organic materials. Choice of organic and inorganic materials opens various options for the chemical synthesis and molecular design of hybrid solar cells (Pandey et al. 2016). Inorganic part of the cell possesses high charge‐carrier mobility while the organic part has strong optical absorption which makes them one of good options for energy fulfilment.

      DSSCs are simple to manufacture, similar to hybrid solar cells with low cost, low toxicity and ease of production. These cells have the potential in the solar industry in near future. At present, these cells cannot be used in commercialized PV systems owing to their poor efficiency (8–12%), a major concern for the solar cells in this category (Pandey et al. 2016). Recently, a new profitable platinum‐free СКАЧАТЬ