![Map of ocean temperatures [°C].](/data/web/img-water-energy/marine-current-power-and-ocean-thermal-energy/mapa-tepelna-energie-mori.jpg "Map of ocean temperatures [°C].")
Marine Current Power and Ocean Thermal Energy
3 min read
Marine Current Power
Ocean currents all over the globe carry huge amounts of kinetic energy and therefore represent a potentially valuable renewable energy source. Their relatively persistent flow makes some ocean currents particularly interesting for electricity generation. Several countries are researching technologies for harnessing this energy.
Ocean currents are driven by several factors, including winds, differences in water temperature and salinity, tides and the Earth’s rotation. Their direction and speed are also influenced by the shape of ocean basins and seabed topography. Some major ocean currents are relatively persistent and predictable, making them potentially suitable for energy generation. The best-known example is the Gulf Stream, which flows along the east coast of North America and across the North Atlantic. There are many other ocean currents, like the California Current or the Humboldt Current, to name just a few.
Due to the much higher density of water, ocean currents can provide considerable energy even at relatively low flow velocities. Technologies for harnessing this energy are being researched and developed in several countries, including the USA, Japan and China. Many concepts use underwater turbines that operate on a principle similar to wind turbines, but rotate much more slowly.
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The Gulf Stream carries an enormous amount of kinetic energy, but only a small fraction of it could realistically be extracted without significantly affecting the current.
OTEC — Ocean Thermal Energy Conversion
Video: OTEC system diagrams. Closed-cycle OTEC system (left); open-cycle OTEC system (right).
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The world’s oceans contain huge amounts of thermal energy. Ocean Thermal Energy Conversion (OTEC) exploits the temperature difference between warm surface seawater and cold deep seawater to generate electricity. A temperature difference of about 20 °C is generally required, which makes tropical and subtropical oceans particularly suitable. In a closed-cycle system, warm seawater heats and evaporates a low-boiling-point working fluid, such as ammonia, in a heat exchanger. The vapour then drives a low-pressure turbine connected to a generator. After passing through the turbine, the working fluid is condensed using cold deep seawater and returned to the cycle. In an open-cycle system, warm seawater itself is flash-evaporated under low pressure. The resulting steam drives a low-pressure turbine and is then condensed using cold deep seawater. Since the salt remains behind during evaporation, the condensed steam can also provide fresh water. Hybrid OTEC systems combine features of the open and closed cycles.
Jules Verne described the idea of harnessing ocean temperature differences as a source of energy in his 1870 novel Twenty Thousand Leagues Under the Sea.
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