Ocean Thermal Energy Conversion (OTEC) is a means of converting into useful energy, the temperature difference between the surface water of the oceans in tropical and sub-tropical areas, and water at a depth of approximately 1 000 metres, which comes from the polar regions.

OTEC plants can be land-based, floating or grazing.

Floating plants have the advantage that the cold water pipe is shorter, reaching directly down to the cold resource, but the power generated has to be brought ashore, and moorings are likely to be in water depths of, typically, 2 000 metres. The development of High Voltage DC transmission offers substantial advantage to floating OTEC, and the increasing depths for offshore oil and gas production over the last decade mean that mooring can now be classed as ‘current technology’- but remains a significant cost item for floating OTEC.

Land-based plants have the advantage of not requiring a power transmission cable to the shore, and subsequently, no mooring costs are incurred. However, the cold water pipe has to cross the surf zone and then follow the seabed until the depth reaches approximately 1 000 metres – resulting in a much longer pipe which much greater friction losses, and greater warming of the cold water before it reaches the heat exchanger, both resulting in lower efficiency.

A grazing plant would allow OTEC energy use in highly-developed economies which lie in the world’s temperate zones. In this case, the OTEC grazing plant is free to drift in areas of ocean with a high temperature difference, the power being used to split sea water into liquid hydrogen and liquid oxygen. The hydrogen, and in cases where it is economic, also the oxygen, would be offloaded into shuttle tankers which would take the product to energy-hungry countries.

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Benefits of OTEC technology

• The requirement for OTEC to function is the thermal differential resource of the ocean. This resource is available 24 hours a day with only modest variation from summer to winter.
• OTEC is Environmentally benign, and some floating OTEC plants would actually result in net CO2 absorption.
• Additional products which can readily be derived – food (aquaculture and agriculture); pharmaceuticals; potable water; air conditioning; etc. Many of these arise from the pathogen-free, nutrient-rich, deep cold water. OTEC is therefore the basis for a whole family of Deep Ocean Water Applications (DOWA), which can additionally benefit the cost of generated electricity. Potable water production alone can reduce electricity generating costs by up to one-third, and is itself in very considerable demand in most areas where OTEC can operate.

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