Ocean Thermal Energy Conversion Otec

The surface of the ocean in the tropics has a temperature of 24 to 28 degrees Celsius. The water at depths greater than about 500 meters is at a temperature of about 4 degrees Celsius. This temperature difference of 20 to 24 degrees can be used to drive power-generating equipment. When the size of the ocean is taken into account, the total energy available is many times that used by humanity. 102

The efficiency of heat driven machines is dependent on the temperature difference between the heat source and the coolant. With the maximum 20-Kelvin difference available from seawater, the peak theoretical efficiency is 7%. The low efficiency results in a very large power plant if useful amounts of energy are to be recovered. A 100 MWe electric plant must have a coolant pipe 10 meters in diameter descending 400 to 500 meters. There must be millions of square meters of heat exchangers to extract the heat from the surface water. All this equipment must be protected from the corrosive effects of seawater and the growth of marine organisms. The problems of designing, operating, and maintaining such plants are formidable.103

The facilities used to harvest the ocean thermal energy are called OTEC plants (Ocean Thermal Energy Conversion). To achieve their maximum potential they must be located in the open ocean near the equator, far from any market. Several schemes to transmit their output to the market have been examined. The most practical appears to be: use the electric power to electrolyze water to produce hydrogen and oxygen. The oxygen can be vented and the hydrogen liquefied for shipment to market. This scheme allows the OTEC plant to produce a storable fuel that can be used for most energy needs.

Several small demonstration plants have been funded by the United States Department of Energy and the State of Hawaii. 104 These plants have proven the general technological feasibility of the concept.

101 http://www.damdecommissioning.org/bulletins/rrbl9.html

102 Penney, Terry R., and Baharathan, Desikan "Power from the Sea". Scientific American, Vol. 256, No. 1, January, 1987, Page 86

103 http://csf.colorado.edu/hypermail/eco-econ/dec96/0045.html

104 http://www.hawaii.gov/dbedt/ert/otec_hi.html

They have also helped to identify the following collection of severe problems. The optimum locations, far out at sea in the equatorial ocean, make direct transport of electrical energy to the markets difficult or impossible. The auxiliary equipment needed to produce fuels, such as hydrogen, will greatly increase the complexity of the facility. The plants will be very costly when configured to produce fuels. Biological fouling of the heat exchange equipment presents operational problems. Solutions, such as chlorination of the water, introduce significant environmental pollution. Because of the large area of heat exchangers required, storm resistance is difficult and costly to achieve. The environmental impact of a large number of these plants on the open ocean ecosystem is difficult to assess. The combined effect of cooling the surface and raising large amounts of deep water to the surface is likely to change the plant and animal life in the zones where the OTEC plants operate. These difficulties support the position that OTEC plants are unlikely to be a major long-term contributor to the world energy supply.

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Responses

  • ANKE
    Does ocean thermal energy conversion increase global warming?
    7 years ago

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