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Investor Presentaiton

Energies 2019, 12, 3658 8 of 37 3.1.3. Ocean Current Power The ocean current is the movement of seawater in the open sea generated by forces acting upon the water, including wind, the Coriolis effect, temperature and salinity differences, and so on. Compared with tidal currents, ocean currents are generally slower, relatively constant, flow in only one direction, and fluctuate seasonally. The currents off South Africa (Agulhas/Mozambique), the Kuroshio Current (off East Asia), the Gulf Stream (off eastern North America), and the East Australian Current are locations with potential ocean currents already identified [59]. Yang et al. [60] estimated a theoretical potential of about 163 TWh/yr for the Gulf Stream system, considering the entire area of the Gulf Stream within 200 miles of the US coastline between Florida and North Carolina as the extraction region. Besides that, they calculated a technical potential of about 49 TWh/yr, assuming a power conversion efficiency of 30%. Chang et al. [61] identified suitable sites for ocean current energy extraction near the coastlines of Japan, Vietnam, Taiwan, and the Philippines. Goundar and Ahmed [62] evaluated marine current resources for Fiji, presenting a peak velocity of about 2.5 m/s. 3.1.4. Ocean Thermal Energy The resulting temperature difference between the upper layers and the colder layers of seawater-usually at a depth of more than 1000 m-can be converted through different oceanic thermal energy conversion (OTEC) methods [63]. In practice, a minimum temperature difference of 20 °C (or K) is required for the use of the temperature gradient in the generation of electricity. The tropical latitudes (0° to 30°) in both hemispheres, including the western and eastern coasts of the Americas, many islands of the Caribbean and Pacific, and the coasts of Africa and India, are the places with the greatest potential [59]. Although there is a little variation in the temperature gradient from summer to winter, the thermal gradient feature is continuously available. It is estimated that about 44,000 TWh/yr (159 EJ/yr) to 88,000 TWh/yr (318 EJ/yr) of power could be generated through OTEC devices [34,64]. Rajagopalan and Nihous [65] estimated that an annual OTEC net power of about 7 TW could be obtained, considering the small effect on the oceanic temperature field. Thus, ocean thermal energy has the highest potential among ocean renewable energy sources. However, the energy density of OTEC systems is quite low compared with that of other sources, such as waves and tidal currents. This issue may affect the low-cost OTEC operation [63], requiring further investigation. 3.1.5. Salinity Gradient Power The salinity gradient power (osmotic power) is the potential energy from the difference in the salt concentrations of seawater and freshwater. Energy is released due to the mixing of fresh water with seawater. The entropy of the freshwater-seawater mixture can be exploited as pressure by using the semipermeable membrane. This pressure can be converted into the desired energy form. The freshwater rivers discharging into saltwater are distributed globally, with a volume of about 44,500 km³/yr. Assuming that only 20% of this discharge can be used for salinity gradient energy generation, the overall potential is approximately 2000 TWh/yr (7.2 EJ/yr) [44]. Skramesto et al. [66] estimated a technical potential of 1650 TWh/yr (5.9 EJ/yr) for the production of salinity gradient energy. Recently, in [67] it was shown that, practically, 625 TWh/yr of salinity gradient energy is globally extractable from river mouths. Some examples of the regional assessment of the salinity gradient potential can be found in Colombia [68], remote regions of Quebec [69], and the hypersaline Urmia Lake of Iran [70]. The potential for salinity gradient energy extraction from some major world rivers is presented in [71]. Figure 3 shows a summary of the potential of ocean renewable energy resources based on the references presented in this paper. The bars illustrate the range of estimated resource potential. Note that the technical potential of ocean current is shown as presented in [60] for the Gulf Stream.
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