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using natural gas and coal, respectively. Using a mature process i.e. steam reforming, 96% of hydrogen is generated industrially (Balat 2008). Other two processes which utilize nuclear energy for hydrogen production are thermochemical water splitting and high‐temperature electrolysis (Acar and Dincer 2014). Electrolysis method can either use solar photovoltaic or wind as energy source for H₂ production. Biomass gasification and bio‐hydrogen are two green methods which utilize biomass remains, agricultural, human and animal wastes and other biodegradable waste materials. Figure 2.5 shows how hydrogen can be produced from various processes by consuming different renewable energy sources with respect to near‐, medium‐ and long‐term objective. Producing hydrogen from renewable sources is at developing stages, and status of technology used in many processes is at R&D or early R&D level (Pareek et al. 2020).

Figure 2.5 Hydrogen production methods using renewable energy sources.

      Source: Reprinted from Ref. Pareek et al. 2020 with Licence under http://creativecommons.org.

      2.4.1 Hydrogen Production Methods Using Renewable Sources

      2.4.1.1 Renewable Electrolysis

      This technique uses electric current, generated via solar PV or wind energy to split water into its constituent elements (hydrogen and oxygen). Utilization of solar PV for hydrogen production is the costliest method using current technologies. However, decreasing cost of solar PV due to advances in technology will decrease the cost of this process in future (Karlsson and Oparaocha 2009). Exploiting wind energy for electricity production needed for electrolysis is also a sustainable method; however, wind turbine and electrolyser cost is again a matter of concern. This is again a costly method compared with methods which are fossil fuels based.

      2.4.1.2 Biomass Gasification

      Biomass gasification process, carried out in a gasifier, uses steam and oxygen at high temperature and pressure to transform different types of biomass residue into a mixture of gases such as hydrogen, carbon monoxide and carbon dioxide. This method has an advantage of consuming biomass remains and wastes from animal, municipal and agricultural along with clean form of energy generation. However, large‐scale hydrogen production using this process at affordable cost is not viable, at present. By this process, overall emission of CO₂ decreases compared with methods which utilize fossil fuels. Main challenge faced by this technique is availability of massive amounts of natural resources and land, needed for growing feedstock required for the process (Acar and Dincer 2014).

      2.4.1.3 Thermochemical Water Splitting

      Hydrogen production from this method needs very high temperature heat at 500–2000 °C which can be produced from CSP. This is a closed‐loop reaction in which only water is utilized and splits into hydrogen and oxygen and rest of the chemicals are recycled (Acar and Dincer 2014). It is a cost‐effective technology with high efficiency and apt for mass‐scale production (Wang et al. 2019). However, high cost of solar concentrators and heat transfer medium renders this technique to reach at commercial level, currently. With advances in the CSP, this process can be commercialized in future.

      2.4.1.4 Bio‐Hydrogen Production

      This is a biological way of producing hydrogen via microorganisms which are normally present in aqueous atmosphere at normal temperature and pressure conditions (Kotay and Das 2008). Last two decades have seen tremendous input at R&D level to advance various technologies needed in this process to make these methods economically viable. This process consumes bio‐wastes and offers an interesting path for their minimization along with energy generation. Energy from this process can be harnessed locally due to availability of primary resources on which microorganisms will act upon, thus making possible production of distributed hydrogen. Bio‐hydrogen generation processes have further categories, namely direct bio‐photolysis, indirect bio‐photolysis, photo fermentation, dark fermentation and bio‐catalysed electrolysis (Pareek et al. 2020). All these methods need further advances due to their poor hydrogen conversion efficiency and high cost, at present.

2.5 Challenges

      2.5.1 Efficiency

      Energy conversion efficiency of various technologies which utilize renewable energy sources is still small except hydropower (Sahli et al. 2018). Advances are taking place worldwide to increase the efficiency. Since, solar and wind sectors are expanding at much larger scale compared with other energy sources. These two are further discussed with respect to efficiency increment.

      In solar industry, scientific community is evaluating various new materials to produce electricity more efficiently. Thin‐film solar cells are cheaper in cost and flexible in nature; however, efficiency of these cells is still a matter of concern for researchers so experimental studies are carried out on the range of materials such as amorphous silicon, CdS/CdTe and CIS to overcome this barrier (El Chaar et al. 2011). To improve the efficiency of organic and polymer solar cells, use of a controlled layer of multi‐wall carbon nanotubes is also reported (Capasso et al. 2014) which improves the efficiency of these cells from 8 to 10%. DSSCs also fall in the same category of low efficiency which is again between 8 and 12%. To increase the efficiency of PV system, generated current is transferred to grid systems. This grid is either mounted on ground or built on the roof of a building so that PV system provides better efficiency as it mainly depends on the intensity of solar radiation. On an average, capacity of PV panel ranges from 10 to 60 MW. Lifetime of a well‐developed PV panel is approximately 10 years at 90% capacity, and it increases to 25 years with a 10% decrease in capacity (Devabhaktuni et al. 2013).

      A major challenge in CPV system is reduction in efficiency of PV due to increase in temperature while concentrating light as conversion efficiency of CPV is still small. Remaining solar radiation gets converted into thermal energy which raises the temperature of junction in the cell, thereby decreasing efficiency. Lifetime of PV also decreases due to increase in temperature. This leads to overall degradation in performance of PV system (Pandey et al. 2016). So, more studies are required to explore ways to decrease temperature of the CPV system.

In wind energy sector, replacement of components of wind turbine system with latest components having cost‐effective technology will increase the efficiency of the existing wind farm. In addition to this, repowering of onshore wind assets can enhance operating lifetime of the current wind project. Repowering can be either full or it can be partial.

      2.5.2 Large‐Scale Production

      Large‐scale production of solar PV systems is at infant stage due to low efficiency and high cost. Government is supporting these installations to promote renewable energy sources. Technically solar PV systems are viable but not economical. So, this area needs more R&D and innovative methods to increase the production at large scale.

      One of the major challenges in deployment of offshore wind energy at a large scale is its high cost of installations and maintenance. With the latest technology developments in the field of manufacturing, installations and operations, its cost has significantly decreased. By 2050, cost of offshore wind energy is expected to decrease by a large factor, and it can directly compete with electricity generation using fossil fuels (IRENA 2019d). So, this challenge could be tackled by the wind industry based on the advanced technologies developed by scientific research community.

      Other than solar and wind sector, all alternative sources of energy technologies are not economically viable compared with conventional sources of energy. Thus, large‐scale