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СКАЧАТЬ (Ahmad et al. 2014). Graphene nanoplatelets (GNPs) or multi‐wall carbon nanotubes (MWCNTs), or various weight % of hybrid GNPs and MWCNTs mixtures were used to make counter electrodes. A marginal increase in conversion efficiency was reported in the study. Using Ru (II) dyes, the efficiency of current DSSCs was reported to reach 12% (Sharma et al. 2018), which is still less as compared with the efficiency of the first‐ and second‐generation solar cells (20–30%).

Several researches are conducted in the techniques for improving the efficiency of PV panels apart from the advancement of cell material itself. Because of fluctuating solar flux, PV systems are not efficient to capture all available energy. So, to capture all the available solar energy, solar tracking (one axis and two axis) is performed. To increase the solar radiation collection, a tracker keeps PV photo thermal panels in a particular position perpendicular to sun rays during the day (Roth et al. 2005). One of the studies concluded that use of two‐axis tracking surfaces increases the total daily collection of solar radiation by approximately 41.34% compared with fixed one (Mamlook et al. 2006). A CSP system consisting of parabolic, trough‐shaped mirrors focuses the sunlight on the tubes which contain a heat transfer fluid. Temperature is raised to 734 °F by repeated exchange of heat. Heated fluid is used to generate the super‐heated steam which powers turbine generators to produce electricity (Devabhaktuni et al. 2013).

      A different technique in which solar energy is not concentrated is becoming popular in a short span of time. In this technique, flat plates and evacuated tubes are used as solar energy collectors for heating and cooling purposes. This technique is cost‐effective with good efficiency and can be used in low‐intensity solar areas. Insulated copper tubes consisting of water or air are used to absorb solar energy. Water or air present in the tubes is heated up before returning to the storage system (Kannan and Vakeesan 2016). In a modification evacuated tube collector is used where heating pipes are shielded by vacuum. This modification is 20–45% more efficient than flat‐plate collectors (Mangal et al. 2010).

      2.3.1.2 Solar Power Generation

      For electrical and mechanical connections, a solar power generation plant has many parts, namely arrays and modules of solar cells and means of controlling systems. Immense research is done in the area of power generation using solar energy at the practical level to evaluate the efficiency, lifespan, cost and durability of such power plants or even small‐scale grid systems. For the fulfilment of energy demand of people, use of hybrid power systems (Parida et al. 2011) is suggested where PV panels cannot generate regular electricity for consumption. In hybrid systems, PV systems are combined with hydro or wind turbines and sometimes with diesel or petrol‐driven generators for uninterrupted power supply. This hybrid system reduces the usage of fossil fuels.

      For power generation using solar energy, different combinations are evaluated from time to time such as hybrid wind/PV or fuel cell power generation systems, wind/PV/battery system and wind/PV/fuel cell electrolyser system. In one of the studies researchers have developed a system by combining PV, wind and fuel cells to transfer maximum power to a fixed direct current voltage bus (El‐Shatter et al. 2006). In this study, fuzzy logic control was used to obtain maximum power tracking of PV and wind energy. Recently, this fuzzy scheme was used by different researchers to track the wind and PV energy so that maximum available solar and wind energy can be extracted.

      2.3.1.3 Photovoltaic/Thermal (PV/T) Collectors

      It is a hybrid system of PV cells and thermal collectors which are flat and tube‐shaped material for absorbing heat from sun. PV/T has dual benefits i.e. while generating electricity using PV cells, thermal collectors utilize heat energy from solar radiations which are not consumed by PV cells as well as waste energy from PV cell (Kannan and Vakeesan 2016). Recently, PV/T systems are becoming popular owing to higher efficiency. Numerous researches are conducted from the different aspects to evaluate the performance and efficiency of PV/T collectors (Huang et al. 2001; Tripanagnostopoulos et al. 2002; Zondag et al. 2003). Different materials for PV cell are chosen for research ranging from mono‐crystalline, poly‐crystalline, amorphous Si and thin‐film cells. Many options for collectors are also evaluated which are based on different size and shape of tube (square, rectangular, spiral, round hollow and flat) (Sandnes and Rekstad 2002). For the thermal collector part, different materials such as copper, aluminium, polymer, water and air are being evaluated by many researchers (Sandnes and Rekstad 2002; Tripanagnostopoulos et al. 2002).

In solar industry developments are also taking place to upgrade the solar heaters, improvisation in design and size of solar cells. Further, invention of new materials which can efficiently absorb the light has also been reported (Kannan and Vakeesan 2016).

      2.3.2 Wind Energy

      Harnessing energy from wind is one of the oldest technologies. Wind power has been the second dominated technology in the domain of renewable energy technologies in recent decades. Due to reduction in cost, its usage is increasing worldwide. Since 2000, wind power has increased at an average compound annual growth rate of >21% (IRENA 2019c). Using wind energy, electricity can be generated by two types of technologies, namely onshore and offshore. As per (IRENA 2020b), worldwide wind‐generation capacity including onshore and offshore has increased to 623 GW by 2019 as compared with 7.5 GW in 1997 which is a factor of >80 in the last 22 years. In future, the combination of wind and solar energy can transform the global energy sector. Around 35% of the total electricity requirement can be generated by wind power (onshore and offshore), and it can become one of the major sources of electricity generation by 2050.

      Advances in the wind industry can be realized from the milestone achievements in the last four decades. This industry has seen developments with respect to installations, advancement in technologies along with cost reduction. A milestone in the wind industry took place in 2018, with global installed wind capacity of 564 GW and requirement of 1.2 million man power was generated in the sector. In 2019, commercially available offshore wind turbines have reached 10 MW capacity (IRENA 2019a).

      Power generation from wind is also geographical location dependent as was solar power generation. Globally, speed of the wind varies in different locations and best locations are the remote ones. Apart from location, size of turbine and length of the blades determine the amount of electricity that can be produced from the wind energy. Further, output of the wind turbine is also proportional to rotor dimensions and cube of wind speed.

      2.3.2.1 Onshore Wind Energy Technology

      Innovative developments in design and size of the rotor have increased the capacity of wind turbine. At present, capacities of wind turbine have reached approximately 2 MW for the onshore and even more for the offshore (IRENA 2019a). To enhance the growth in onshore wind energy sector, various innovations, advancement in techniques and several practices are adopted in this area as discussed in the following sub‐sections.

       2.3.2.1.1 Turbine Size and Ratings

Important factors for the developments in wind turbine technologies are size of rotor diameter and hub height. Recently, various developments in technology have taken place to manufacture larger‐capacity turbines. These advances have increased the efficiency along with reduction in capital and operation costs. Large rotors increase the capacity of wind turbines even in low wind areas. By 2018, rotor diameter has increased to 110.4 m with wind turbine ratings of 2.6 MW from rotor diameter of 50.17 m with 1.0 MW capacity of wind turbine in 2000 (Pérez‐Collazo et al. 2015; IRENA 2019d; www.windpowermonthly.com). Expected capacity of the wind turbine by 2022–2025 is 5.8 MW with rotor diameter equal to 170 m. In a particular example, GE has come up with improved onshore turbine technologies rated at 4.8 MW and 5.3 MW, respectively (www.windpowermonthly.com).

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