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      2.2.3 Hydropower Energy

Hydropower has a wide resource availability coverage, producing about 17.2% of the nation's electricity, in 2019, [11]. Hydropower is recognized for being the highest density of energy resources; it is ranked first with regards to installed production capacity from renewables. The main goal of hydropower in the global energy supply is in delivering centralized power generation, moreover, hydropower plants (HPPs) has the ability to function in isolation and supply independent systems, usually in rural and remote regions [28].

      Hydropower describes the energy obtained due to the movement of water. Flowing water generates energy that can be transformed into electrical power with the use of turbines. The most widespread form of hydropower is dams, even although advanced forms of utilizing wave and tidal power are becoming more common. Hydropower is produced from water flowing in the hydrological cycle, propelled by solar radiation. It is the movement of water propelled by the force of gravity to move from higher to lower elevations that could be utilized to produce hydropower. HPPs span a wide range of scales, from a few watts to several GW. Four broad hydropower typologies are present today [29, 30]:

       Run‐of‐river hydropower: a facility that channels flowing water from a river to spin a turbine. Normally, a run‐of‐river case will have minimum or no storage possibility. Run‐of‐river delivers a constant generation of electrical power with limited flexibility of operation for daily changes in demand by water flow management.

       Storage hydropower: a large system that utilizes a dam to store water in a reservoir. Electrical power is generated by releasing water from the reservoir by a turbine, which operates a generator. Storage hydropower delivers the base load and can be shut down and operated at short notice depending on the demands of the system. It could deliver sufficient storage capacity to function without the need for the hydrological inflow for several weeks or possibly months.

       Pumped‐storage hydropower: delivers peak‐load supply, taking advantage of water which is circulated between a lower and upper reservoir by pumps, that use the additional energy from the system at periods of low demand. When energy demand is high, water is released back to the lower reservoir by turbines to generate electrical power.

       Offshore hydropower: less recognized relative to the others but an increasing group of technologies that utilize tidal currents or the energy of waves to produce electrical power from seawater.

An estimated 21.8 GW of hydropower capacity was employed, with pumped storage, amounting to an installed capacity of 1292 GW worldwide. The total installed capacity has risen by 39% from 2005 to 2015, with an average rate of approximately 2.4% each year. World leaders in producing hydro power are China, Brazil, the United States, Canada, the Russian Federation, India and Norway, which was responsible for almost 68% of installed capacity at the end of 2019, as illustrated in Figure 2.10, [12]. Hydropower is a proven and well‐advanced technology based on more than 100 years of experience. Presently, hydropower is a relatively flexible power technology with one of the best conversion efficiencies relative to other energy sources because of its direct transformation of hydraulic energy to electrical energy. Yet, more improvements can be made by refining operations, reducing environmental impacts, adapting to new social and environmental needs and advancing more robust and cost‐effective technological solutions. Figure 2.11 shows evolution of world hydropower generation since 1980, [31]. From 1999 through 2005, hydropower advancement was relatively halted globally, showing the influence of the World Commission on Dams (WCD), which was organized to review the improvement of large dam performance and come up with guidelines for the integration of new dams. From 2005 onwards (green arrow), hydropower development has seen an upswing in development due to the expansion and use of the Hydropower Sustainability Assessment Protocol [32].

Figure 2.10 Hydropower generation by top 10 countries in 2019. Adapted from Ref Num [12].

Figure 2.11 The evolution of world hydropower generation since 1980.

      2.2.4 Marine Energy

Ocean and marine energy denote the diverse forms of renewable energy acquired from the ocean. Two kinds of ocean energy exist: mechanical and thermal. The movement of the earth and the moon's gravitational force leads to the mechanical forces experienced. The movements of the earth create wind in the oceans that ultimately produces waves and the gravitational pull of the moon causes the initiation of coastal tides and currents. Thermal energy is acquired from the sun, which increases the temperature of the ocean yet the depths are still of a lower temperature. Therefore, the temperature difference experienced allows for the energy to become trapped and converted to useful energy, typically, electrical energy. Five kinds of ocean energy transformation take place: wave energy, tidal energy, marine current energy, and ocean thermal energy conversion [33–37].

       Wave energy is produced by the motion of a device either floating on the surface of the ocean or fixed to the ocean floor and there are several methods for transforming wave energy to electric energy. Wave energy is recognized as the most commercially developed of the ocean energy technologies yet is still far from where it could be practically.

       Tidal energy, the tidal cycle takes place every 12 hours as a result of the gravitational force of the moon. The difference in water height from low and high tide is potential energy. Comparable to traditional hydropower produced from dams, tidal water can be trapped in a barrage across an estuary at periods of high tide and forced by a hydro‐turbine at periods of low tide.

       Current energy, marine current is ocean water moving in one direction. Kinetic energy of the marine current can be trapped with submerged turbines that are relatively comparable to wind turbines, where the marine current forces the rotor blades to move to produce electrical energy.

       Ocean thermal energy conversion (OTEC), utilizes ocean temperature variations from the surface to depths lower than 1000 m, to obtain energy. Research focuses on two types of OTEC technologies to obtain thermal energy and transform it to electrical energy: closed and open cycles.

       Salinity gradient power is the energy generated from the variation in salt concentration between two fluids, usually fresh and saltwater, e.g. when a river flows into the sea. Collision of fresh and saltwater delivers large amounts of energy, which this technology strives to capture.

      Tidal power stations generate tens to hundreds of MW similar to hydropower stations, wave energy converters (WECs) from some kW to MW, salinity gradient power stations from some kW to MW, ocean thermal energy converters (OTECs) from kW to MW and ocean thermo‐electric generators (OTEGs) from some watts to kW.

      Ocean energy is still in the process of development, and intensive research is required, progress and demonstration efforts needed for learning and cost reduction before it can contribute to the energy supply. Therefore, the ocean energy market is still in its infancy, and the sector must address many issues to confirm the reliability and affordability of its technologies. A number of barriers are present in ocean energy technologies, which include obtaining site permits, the environmental influence of technology implementation, and grid connectivity for transporting the energy generated.

In 2019, the global installed capacity of ocean energies was 536 MW, a rise of 32 MW relative to 2014. Ocean energies amount to 0.03% of the global renewable energies installed capacity. However, there is huge potential to increase

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