Ocean thermal energy conversion

OTEC relies on the principle exploited in most forms of electricity generation that a source of heat and a source of cold5 can be used to drive an engine. In the case of OTEC the source of heat is the surface of a tropical or subtropical sea while the source of cold is the deep sea.

The possibility of extracting energy from the sea in this way was recognised in the latter half of the nineteenth century and the first practical system was proposed by the French inventor D'Arsonval in 1881. D'Arsonval's system employed a closed cycle ammonia turbine and it was finally demonstrated in 1979 in a small pilot project of Hawaii. However a different, open cycle system was tested during the 1930s by another Frenchman, Georges Claude. Claude's system proved the theory on which it was based but was not successful commercially. Work was not revived on his system until late in the twentieth century.

Tropical oceans and seas have surface water temperatures of between 24°C and 33°C. Below 500 m, the temperature will drop to between 9°C and 5°C. This provides a maximum exploitable temperature difference of 28°C. In practice the temperature difference is likely to be closer to 20°C, providing a theoretical energy conversion efficiency of 6.7%.6 When account is taken of the need to pump cold water up from the depths, efficiency falls to 2-3%.

Though cool water may be available at 500 m, in practice the depth of 1000 m is normally considered necessary. If this is to be made accessible from land, a very long cold water pipe will be required to pump the deep water to the plant. This pipe will need to be 2000 m, or more, in length and if care is not taken, the cold water will become warmed before it reaches the plant. The alternative is to build an OTEC plant on a floating platform from which the cold water pipe stretches vertically downwards. Even with this arrangement, the cold water pipe will need to be 1000 m long.

Otec Plant
Figure 14.1 Schematic diagram of a floating OTEC plant

In order to generate 1MW of electricity, an OTEC plant requires 4 m3/s of warm seawater and 2m3/s of cold seawater. This will require a cold water pipe of around 11m in diameter to supply a 100 MW plant, the largest size considered practical.7 The discharge of mixed hot and cold water from a plant of this size would be equivalent to that of the Colorado River in the USA discharging into the Pacific Ocean. Such massive quantities of water could have significant environmental impact.

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