Ocean thermal energy conversion (OTEC) is essentially a low temperature Rankine Cycle using the warm surface waters of the tropical and subtropical oceans as a heat source and the colder water from 500–1,000 m deep as the heat sink. With the surface water at a maximum of 27°C and the deeper water in the 5–10°C range, the maximum Carnot efficiency of the cycle is about 7 percent and the practical net station efficiency about 1.5–2 percent. Hence enormous quantities of water need to be circulated and large quantities of heat transferred even for plants in the 5-10 Mwe (net) range.
The OTEC concept was first proposed by the French physicist J. D’Arsonval in 1881. Two basic cycles have been considered. The closed cycle uses a low boiling working fluid (ammonia with 0.1–0.2 percent water is the clear choice on thermodynamic and thermal-hydraulic grounds) which is vaporized by warm sea water in the vaporizer, the vapor expanded in a turbine connected to an electrical generator, the exhaust vapor condensed by the cold sea water in the condenser, and the condensate returned to the vaporizer by a feed pump. The heat exchangers must be very large, but the turbine is small and the power cycle technology straightforward. Shell and tube, plate, and plate fin exchanger designs have been tested for this service.
The open cycle uses the water itself as the working fluid, a small fraction of the warm water being flashed to vapor at about 2,500 Pa absolute in a large vacuum chamber, the vapor then being expanded through a turbine before being condensed by cold water in a direct contact condenser at about 1,000 Pa. All of the equipment must be very large to accommodate the low density vapor and the technology has not been fully developed. A variant of the open cycle uses a surface condenser to condense the water vapor to produce potable water.
A number of OTEC pilot plants and test facilities have been built and operated, and there is a low but continuing level of interest, especially for isolated tropical islands with few other energy sources.
Avery, W. H. and Wu, C. (1994) Renewable Energy from the Ocean: A Guide to OTEC, Oxford University Press, New York.