Ocean Thermal Energy Conversion (OTEC)

Background

Author: Thomas H. Daniel, Ph.D., The Natural Energy Laboratory of Hawaii Authority (NELHA). The world's largest solar collector absorbs a tremendous amount of the sun's energy, averaging about 65 million gigawatts (a gigawatt is one million kilowatts), or 570 quadrillion kW-hr per year - more than 5,000 times the amount of energy used in all forms by humans on the planet. A typical square mile of that collector - otherwise known as the surface waters of the Earth's vast oceans - absorbs an average of about 500 MW, or annually more energy than the equivalent of 2.6 million barrels of oil [1]. The concept of ocean thermal energy conversion (OTEC) uses the natural difference that exists between warm tropical surface waters and those at depth. Since the ocean temperature changes little from night to day or - in the tropics -with the seasons, an OTEC power plant is able to generate electricity continuously, unlike many other renewable energy sources. This idea originated with a French physicist, Jacques D'Arsonval, in 1881. His pupil, Georges Claude, built the first plant at Matanzas Bay, Cuba in 1930, with a gross output of up to 22 kilowatts.

How it works

OTEC generates electricity by using the temperature difference of 20°C (36°F) or more that exists between warm tropical waters at the sun-warmed surface, and colder waters drawn from depths of about 1000 m. To convert this thermal gradient into electrical energy, the warm water can be used to heat and vaporize a liquid (known as a working fluid). The working fluid develops pressure as it is caused to evaporate. This expanding vapor runs through a turbine generator and is then condensed back into a liquid by cold water brought up from depth, and the cycle is repeated. There are potentially three basic types of OTEC power plants: closed-cycle, open-cycle, and various blendings of the two. All three types can be built on land, on offshore platforms fixed to the seafloor, on floating platforms anchored to the seafloor, or on ships that move from place to place [2,3,4].

Closed-Cycle Ocean Thermal Energy Conversion

In a closed-cycle OTEC process, first proposed in 1881 by French physicist Jacques D'Arsonval [5], warm surface water is vaporizes a working fluid (such as ammonia) in a heat exchanger (evaporator). The ammonia vapor is then condensed back to liquid by thermal contact with the cold water through another heat exchanger (condenser) and re-cycled. At all times, the working fluid remains in a closed system and is continuously circulated. Since ammonia vaporizes and condenses near atmospheric pressure at the available seawater temperatures, it provides a sufficient pressure drop across the turbine so that it can achieve relatively high efficiency at modest size compared to the open-cycle system (See More). Since this technology is essentially similiar to standard refrigeration systems, there is sufficient experience with the components to allow straightforward scale-up to commerical sizes.

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