Some energy experts believe that if it could become cost-competitive with conventional power technologies, OTEC could produce giga-watts of electrical power. However managing costs is still a huge challenge. All OTEC plants require an expensive, large diameter intake pipe, which is sub-merged a kilometre or more into the ocean’s depths, to bring very cold water to the surface.
This cold sea water is an integral part of each of the three types of OTEC systems: closed- cycle, open-cycle, and hybrid. To operate, the cold sea water must be brought to the surface. This can be accomplished through direct pumping. A second method is to desalinate the sea water near the sea floor; this lowers its density where which will cause it to “float” up through a pipe to the surface.
Closed-Cycle:
Closed-cycle systems use fluid with a low boiling point, such as ammonia, to rotate a turbine to generate electricity. Warm surface sea- water is pumped through a heat exchanger where the low-boiling-point fluid is vapourized.
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The expanding vapor turns the turbo-generator. Then, cold, deep seawater—pumped through a second heat exchanger—condenses the vapour back into a liquid, which is then recycled through the system.
Open-Cycle:
Open-cycle OTEC uses the tropical oceans’ warm surface water to make electricity. When warm sea water is placed in a low-pressure container, it boils. The expanding steam drives a low-pressure turbine attached to an electrical generator.
The steam, which has left its salt and contaminants behind in the low-pressure container, is pure fresh water. It is condensed back into a liquid by exposure to cold temperatures from deep-ocean water. This method has the advantage of producing desalinization fresh water, suitable for drinking water or irrigation.
Hybrid:
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A hybrid cycle combines the features of both the closed-cycle and open-cycle systems. In a hybrid OTEC system, warm sea water enters a vacuum chamber where it is flash-evaporated into steam, similar to the open-cycle evaporation process.
The steam vapourizes the ammonia working fluid of a closed-cycle loop on the other side of an ammonia vapourizer. The vapourized fluid then drives a turbine to produce electricity. The steam condenses within the heat exchanger and provides desalinization fresh water.