esters and fatty acids.
2.3.6 Ocean Energy
As we know, 70% of the earth's surface is covered by water in different forms. Ocean energy also known as blue energy is present in the form of tidal, current, wave, temperature gradient and salinity gradient energy. Electricity could be generated using energy possessed by all the different types of ocean energy. IRENA 2020 report (IRENA 2020b) states that electricity generation capacity of ocean energy till 2019 was 500 MW, very small contribution compared with capacity of other alternative sources of energy. Approximately 1.7 GW is under development (Wilberforce et al. 2019). Compared with other alternative sources of energy, ocean energy is steady and predictable (especially tidal range technology) making it a right choice for power production in future (Crus 2008, pp. 70–75). Looking at the promising potential of ocean energy, various researches are going worldwide to see the feasibility of how to harness this energy (Wilberforce et al. 2019). Most of researches were aimed to pinpoint best possible locations for ocean energy exploitation, energy conversion efficiency of various technologies used in ocean energy sector and consequences of several technologies on the marine ecosystem as well as on the environment, as this is a raising concern about the usage of ocean energy technologies. This is a clean form of energy, GHGs emissions are negligible and assumed to be environment‐friendly. Despite all these factors, its share in energy sector is very small at present.
Around the globe, mainly four types of ocean energy are harnessed to generate electricity, namely wave, ocean thermal energy conversion (OTEC), tidal and salinity gradient energy. Out of all these forms, tidal range share with respect to generation capacity is approximately 99%. Currently, contribution of other forms is not significant but many projects are under development based on OTEC, tidal stream and wave energy (Kerr et al. 2015). On completion of various current projects share of OTEC, tidal stream and wave energy will become noticeable. Theoretical maximum capacity of all projects which are under construction will be 15 GW (Wilberforce et al. 2019). Major contributors in the commercialized marine technology are the United States and United Kingdom (UK) along with small share of South Korea, Ireland, The Netherlands and China.
2.3.6.1 Wave Energy
Wave energy convertor devices capture the energy possessed by the waves (kinetic and potential) and transform it to electric power. Wave energy convertors are classified into different types of categories based on the deployed location. Onshore devices are of two types ‘Oscillating water column’ (trap air to run a turbine) and ‘Overtopping devices’ which uses height differences of waves to generate electricity. ‘Oscillating wave surge converters’, ‘Point Absorber’ and ‘Submerged Pressure Differential devices’ come under the near‐shore location category. Offshore devices are ‘Attenuator’, ‘Bulge wave devices’ and ‘Rotating mass converters’. Each of these devices has its own characteristics (Wilberforce et al. 2019). Worldwide many companies are involved in research and development (R&D) to exploit wave energy so that this technology can be commercialized (Lorente et al. 2011; www.aquaret.com). Some studies have revealed that theoretical global capacity of wave energy was 32 PWh (petawatt‐hour) per annum (Mørk et al. 2010; Lagoun et al. 2014) which was double of the global electricity provided in the year 2008 (Leonard and Michaelides 2018). These studies have suggested potential of this energy in future.
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