What is OTEC

A.� OTEC stands for ocean thermal energy conversion. Electricity is generated using the large temperature gap between warm surface and cold deep-sea water.

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Q.� How does OTEC work

A.� There are a couple of different systems but their underlying principles are the same.

Heat is transferred from the warm waters to evaporate a liquid: either something with a low boiling-point, such as ammonia, or the warm sea water itself is evaporated.

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The resulting steam expands and is forced through turbines attached to generators, which create electricity. The vapour is caught in storage tanks, where it is cooled back into a liquid by the cold seawater, and the whole process starts again.

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If warm seawater, rather than ammonia is used, the cooled water is either returned to the sea or, since the evaporation process has removed the salt, is used as fresh water.

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Q.� How long has OTEC been around

A.� George Claude, a French engineer, working on the coast of Cuba, built the first ocean thermal energy converter in 1929. His machine was almost identical in principal to current versions, but had to eventually be abandoned because it was too labour-intensive to operate. One of the main problems was maintaining a supply of deep, cold water from the sea because storms kept damaging the pipe that fed the water up from the depths.

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Q.� Given that two thirds of the world is composed of ocean, why isn't OTEC electricity widely used

A.� As with many forms of alternative energy systems, high costs are the main stumbling-block. At the moment only small scale OTECs have been built which can only generate relatively small amounts of electricity. Currently electricity generated by traditional methods, mainly fossil fuels, is much cheaper.

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Q.� How much difference in the temperatures is required

A.� This is another hurdle in generating OTEC energy on a large scale.

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The system requires a difference of around 20 degrees Centigrade between the warm surface water and the cold deep-sea water. Coastal areas where this kind of temperature difference exists year round are few.

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Do you want to know how a form of alternative energy works Click here to ask.

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by Lisa Cardy