Page 159 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

148 Fundamentals of Ocean Renewable Energy

FIG. 6.4 Closed cycle OTEC power plant. (Based on an image produced by DCNS.)

temperature, is discharged into the ocean. The warm water from the ocean
surface, at a slightly reduced temperature, is discharged into the upper ocean.
The world’s oldest OTEC power plant is of the closed cycle design—a
103 kW power plant at the Natural Energy Laboratory on Hawaii, built in 1979.
This power plant was land based, which lowers installation and maintenance
costs, and facilitates bringing the electricity to the shore, but floating OTEC
plants are generally considered to be more efficient and with fewer environmen-
tal concerns.

6.3.2 Open Cycle OTEC
In contrast to a closed cycle system, in an open cycle OTEC power plant, the
sea water itself is used as the ‘working fluid’. Although sea water has a much
higher boiling point than a fluid such as ammonia, warm surface water can
produce steam by reducing its pressure. This is achieved by pumping the surface
water into a low-pressure container (known as a flash evaporator), causing it to
boil. The vapour is used to drive a generator. This vapour is then condensed by
exposure to cold sea water that is pumped from deep in the ocean.
The largest OTEC project that has been built to date is of the open cycle
design—a 1 MW plant in Hawaii, which ran from 1993 to 1998 [14].