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      2.3.6.2 Tidal Energy

      This form of energy can be generated by using tidal range technologies, tidal current (often called tidal stream) technologies and hybrid technologies. Tidal range technologies produce electricity by making use of a barrage which can be a dam similar to that in hydropower or some other barrier. Devices used in tidal current technologies work similarly to a turbine deployed in wind energy to harness energy of tides, but these turbines use water to supply the captured energy. Technologies involved in tidal current are matured than wave energy owing to similarity of devices to wind turbine of the former. Tidal current energy converters are classified into three, which are further divided into six types (Wilberforce et al. 2019). These devices transform the kinetic energy of water into electricity. Tidal current energy could be mainly exploited along the coastal areas due to maximum availability of tides in these regions. Tidal stream energy is location‐dependent and at present, 106 locations are known in Europe which possess great potential for electricity production and 48 TWh/yr power can be generated using all these locations (Wilberforce et al. 2019).

      2.3.6.3 Ocean Thermal Energy Conversion (OTEC)

      As the name signifies, this form of energy is based on heat possessed by the ocean. This energy makes use of temperature difference existing between the water layers at 800–1000 m depth to drive the turbine and thereby produce electricity using OTEC technologies. For effective operation of technologies, temperature difference of approximately 20 °C is needed. Water vapors produced from hot sea layers act as working fluid which runs the turbines. Key techniques used in the OTEC plant are open cycle (function via water from the sea), closed cycle (operates using NH₃ as working fluid) and hybrid cycle (blend of both the cycles) (Wilberforce et al. 2019).

      2.3.6.4 Salinity Gradient Energy

      This energy originates because of difference in salt concentrations at the junction of river and ocean or sea. Energy generation from salinity gradient is a two‐stage process. The ‘pressure retarded osmosis’ is used where freshwater flows through a semi‐permeable membrane to increase the pressure in a reservoir of seawater and ‘reverse electro dialysis’ with ions of salt passing through alternating reservoirs of sea and river water. Recently, a research estimated the potential of salinity gradient energy as approximately 1650 TWh/yr. (Cornett 2008, p. 9). At present, the major challenge faced by utilization of this energy is its high cost of power generation. In 2009, this energy was harnessed for the first time at Tofte site (Nihous 2010).

      Currently, harnessing ocean energy using different technologies is in its infancy due to costly technologies, high capital cost, socio‐economic impact on shipping and tourism and environmental effects. There are several technical issues which are associated with this energy such as device manufacturing, installation, maintenance and grid and power transmission. Further, due to lack of studies which establish the impact of ocean energy exploitation on marine environment, aquatic ecosystem and water pollution due to corrosion of various device obstructs its development.

2.4 Future Fuel: Hydrogen

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.

      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 СКАЧАТЬ