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).
2.3.2.1.2 Design and Materials of Rotor Blade
Two types of wind turbine designs are horizontal axis wind turbine and vertical axis wind turbine, and the former type dominates in the wind turbine industry. Nowadays, focussed research is carried out to improve the aerodynamic profile of blades as well as enhancement in the quality of materials so that energy production can be increased along with cost reduction of operation and maintenance. Advances in the manufacturing of composite materials leads to better performances particularly in rough and corrosive environments of sea and deserts (Windtrust 2016). Blades made up of composite materials can spin faster and capture winds even at lower speed.
2.3.2.1.3 Power Electronics Optimization
To cut the cost of wind turbine installation and operation, optimized design of the various parts of power electronics has been used by different manufactures via concentrating on the several features such as humidity protection, scalability and decreasing the number of components. These features lead to minimizing the failure of power modules, creation of smart and innovative power modules having power density of nearly 30% more than their predecessor and better performance of power electronics by reducing the number of active elements in power modules (Windtrust 2016).
2.3.2.1.4 Smart Wind Turbines
These turbines are equipped with novel technologies which control and monitor the turbine. These turbines possess the advanced mechanism to forecast using big data and artificial intelligence along with the automatic regulations of turbines which results in higher energy output (Windtrust 2016). In addition, these digitized turbines also reduce the maintenance costs (www.woodmac.com). Recently, GE Japan has exemplified the manufacturing of such a smart turbine with the help of artificial intelligence which has higher efficiency, low maintenance costs (20% lesser) and higher output (5% more) (IRENA 2019e).
2.3.2.1.5 Recycling of Materials
In recent times, recycling of various materials in the wind energy sector has become popular due to reduction in cost for power generation. By increasing the three R's i.e. reduce, reuse and recycle of raw materials, residues, metals and other resources in the wind sector will cut down the overall cost of electricity generation. At present, about 2.5 million tonnes of composite materials are used in the wind energy sector. In the next five years, around 12,000 wind turbines will be out of service in Europe (www.recycling‐magazine.com) which will produce large amounts of materials that need to be recycled. For the recycling of materials, mechanical process i.e. cutting the turbine blades into smaller slices for easy transport or thermal processes such as combustion or pyrolysis provide viable options (WindEurope 2017). For the circular economy along with the production of new blades, reuse of decommissioned blades after some processing should be considered (WindEurope 2017). For example, the Dreamwind project (Designing REcyclable Advanced Materials for WIND energy) is working towards the