(IRENA 2019b) and ocean energy (Wilberforce et al. 2019). Technologies in the solar and wind energy sector are growing at a faster rate compared with other alternative sources of energy. One of the paradigm shifts in solar energy usage is conversion of CO2 into fuels and highly valued chemicals (He and Janáky 2020). Such advances will have dual benefits i.e., generating fuels using CO2 which is a threat to environment and utilizing the renewable solar energy. Further, solar energy is also exploited to resolve the freshwater crises via advanced evaporation systems (Xu et al. 2020). Harnessing onshore and offshore wind energy can become competitive to power generation from the non‐renewable energy sources due to evolution of new and innovative technologies in the wind energy sector (IRENA 2019a).
2.2 Need of Novel Research in Alternative Sources of Energy
The world population is expected to be around 9.8 billion by 2050 as projected by the UN's World Population Prospects 2019. This increase in population despite a continuous decline in birth rate globally demands a huge volume of energy resources to accomplish their daily requirements. If we look at the industrial needs of energy resources then also, we can understand the ever‐increasing demand due to intense industrialization in the past and will continue to rise in future as well. Reserves of coal, natural gas and crude oil which are available worldwide are predicted to be exhausted by 2300 if consumed at the same rate (Kougias et al. 2019). Geographical location of fossil fuels is not uniformly spread across the world. This uneven distribution of fossil fuels leads to high and unstable prices of petroleum‐based fuels. To accomplish the energy demand of the world population, use of renewable resources should also be increased consistently and in an exponential mode so that by 2300 the world can rely only on renewable resources. Further, usage of non‐renewable energy resources also increases the carbon emission. Total carbon emission from the consumption of energy resources is around 33,000 million metric tons in 2012 as reported by Global Status Report, 2014 (REN21, Paris 2014). This massive emission of CO2 leads to global warming and change in the earth atmosphere.
To maintain a balance between carbon emission and usage of energy resources, there is a need for such energy resources which have minimum or zero carbon emission. In the planetary emergency of climate change, there is a need for valuable lessons from the past. Remedy of this problem lies in the usage of renewable sources of energy which are in use from the past few decades but in small percentage. In order to decrease the dependency on fossil fuels, renewable energy sources consumption has to be promoted. At present, if one carefully looks at the various alternative sources of energy, namely solar, wind, biomass, hydropower, ocean and geothermal energy, then it can be realized that fossil fuels cannot be completely replaced by these energy sources. Alternative sources of energy are complementary to non‐renewable energy resources so this is the need of the hour to harness as much energy as possible via these sources. For the uniform and sustainable economic growth of a nation, use of renewable energy sources should be enhanced by the policymakers of a country (Bhattacharya et al. 2016). Also, some subsidies must be provided to the industry and end users to encourage the usage of alternative sources of energy. Recently, very promising research is performed in the process of harnessing energy through renewable energy sources in an efficient manner, explained in detail in the following sections of the chapter.
2.3 Recent Advances in Renewable Sources of Energy
Till 2017, global production of electricity was 74% from fossil fuels and nuclear sources and 26% from all the renewable energy sources as reported by the International Energy Agency (www.iea.org). If one looks at the contribution from the different renewable sources, then it was majorly from hydro power (16%) followed by wind (4%), bio power (2%), solar (2%) and others (2%). Figure 2.1 shows the contribution of all the renewable energy technologies and their sub‐technologies in electricity generation worldwide from 2010 to 2018.
Further, 2019 data shows that the capacity of power generation by renewable energy sources was 2537 gigawatts (GW) worldwide. In the same year, renewable power generation capacity growth was 7.4% with an addition of 176 GW, slightly lower than that in 2018 (7.9% with an addition of 171 GW). Contribution of hydropower was maximum i.e. 1190 GW. Capacity of wind and solar energy was 623 GW and 586 GW, respectively. Bioenergy share was also noticeable i.e. 124 GW. Geothermal energy and marine energy accounted for 14 GW and 500 MW, respectively. Among the various renewable energy sources, solar energy growth dominated with a growth rate of 20% followed by wind (10%), hydropower (1%) and bioenergy (5%) as per International Renewable Energy Agency (IRENA) report, 2020 (IRENA 2020b).
Figure 2.1 Worldwide electricity generation (GWh) between 2010 and 2018 using all the renewable energy technologies.
Source: www.irena.org © IRENA.
Majorly, six countries around the globe are harnessing energy from renewable sources, and these countries are the United States, 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