OTEC concepts have been developed using two basic types of cycles. Closed cycle plants use a low boiling point working fluid such as ammonia. The working fluid is heated and vaporized by the warm surface water, expanded in a turbine generator, and condensed by deep cold water. Open cycle plants use seawater as the working fluid. The warm surface water is flashed to low-pressure steam, expanded in a turbine generator, and condensed by the deep cold water. The
relatively small temperature difference between warm and cold seawater, and the resultant low system efficiency, require the pumping of enormous amounts of water for power
generation. A 100-MW plant would require pumping about 75 million gpm of both warm and cold seawater. The cold water intake pipe for a 100-MW plant would range in length from 2,000 to 3,300 ft (Cavanagh et al. 1993).