The planet earth's surface is about two thirds water. These are vast areas of renewable, alternative energy. Ocean thermal energy conversion (OTEC) is the way that we can harness this energy for our purposes. Since the real source of energy from the oceans is the sun, OTEC systems are renewable are nonpolluting, and have no fuel costs.
OTEC power systems use the difference in the water temperature between warm surface water and colder waters at the ocean depths, to make electricity. A temperature of 20C or more between the surface water and deep waters is required.
The idea dates back to 1881, when a French engineer, Jacques D'Arsonval first envisioned OTEC. His student, Georges Claud, developed the first working OTEC system in 1930.
OTEC plants can be on-shore or off-shore, with the electricity transmitted to shore by electrical cables. (Figure 1)
There are two types of OTEC plants. Open-cycle, closed-cycle. Open-cycle use the fact that water boils at temperatures below its normal boiling point when under lower pressures. They convert warm surface water into steam in a vacuum, then they use this steam to turn a turbine that powers an electrical generator. Cold water is piped up from the ocean to cool the steam. This condensed steam becomes desalinated and can be used for drinking and irrigation.
Closed-cycle systems use warm surface water and pipe it through a heat exchanger to boil fluids such as ammonia and chloroflorocarbon that have a low boiling point. The vapor that is given off turns a turbine/generator making electricity. The cooling process is the same as the open-cycle. Cold water condenses the vapor which is passed back to the heat exchanger.
OTEC power plants differ from other alliterative energy sources such as solar and wind. OTEC systems can produce electricity 24 hours a day, 365 days a year. Fresh water production is one of the potential by-products of OTEC. The cold ocean water can be used for fish farming and air conditioning and refrigeration, since it is rich with nutrients and pathogen free.
OTEC systems is a relatively non-polluting source of energy compared to other sources such as fossil fuels and nuclear power. Cold water that is released at the surface gives out trapped carbon dioxide, but only at low levels of about 4% of theses from a fossil fuel plant.
Right now, OTEC systems are so expensive to build that they are restricted to demonstration and experimental units. Still, there is no fuel cost, and they produce a useful by product. Standardizing plant design and more experience in building these systems could bring the cost way down in the future.
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