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Use of fossil fuels to meet the energy demands of world population is consistently increasing the amount of GHGs which ultimately changes the earth's environment. Temperature of the earth is increasing at a faster rate due to GHGs emission. Replacing fossil fuels with such an energy source which accomplishes energy requirements along with reducing the percentage of GHGs in earth's environment is the need of present era. So, hydrogen seems to be the future fuel as it meets both the requirements (Pareek et al. 2020). Due to its high calorific value and no emission of GHGs, it is considered as a clean, green and sustainable fuel as long as it is produced from renewable sources of energy. It also possesses high energy and low volumetric density which further enhances its prospects as future fuel. Moreover, it decreases the dependency on oil which is mainly imported and thus provides energy security. Although hydrogen is present in abundant amount in the universe, it only exists in amalgamation with other elements such as oxygen, carbon and nitrogen. Splitting hydrogen from other elements by a process using some other source of energy (renewable and non‐renewable) provides an alternative for this high energy‐carrier fuel. Producing hydrogen in pure form using renewable sources of energy from a cost‐effective method along with possessing high energy conversion efficiency is the main challenge in exploiting it as a fuel (Pareek et al. 2020). Currently, it is majorly produced from non‐renewable sources due to easy and cost‐effective methods. Steam reforming and gasification are two such methods which are used to produce hydrogen 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.