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WEEK 03: GENERATION: ALTERNATIVE SOURCES: OCEAN THERMAL ENERGY CONVERSION Thermoelectricity & Thermionics Sections: Introduction | Salter's Ducks | Oscillating Water Column | Tidal Power | Challenges Ocean Thermal Energy Conversion (OTEC)1 is a process for converting the solar energy absorbed by tropical oceans into electricity, using a heat engine, with warm surface water serving as the heat source and deep water as the heat sink. The process has aroused considerable interest because it utilizes a renewable resource, ocean water, and has a benign environmental impact. OTEC experts believe the process is feasible for small (100-MW) stationary plants sited on shore and for larger (400-MW) plants housed on ships cruising equatorial waters. Such plantships could also extract chemicals and minerals from seawater, as well as hydrogen for use as a fuel. This water could serve as a medium for culturing fish. The French engineer Georges Claude experimented with OTEC technology as early as 1929. His system used low-pressure steam derived from seawater as the working fluid in an open-cycle method, where warm surface water was drawn into an evaporator and the steam produced was used to drive a turbo generator. Today's OTEC technology generates electricity through a closed-cycle heat engine, with ammonia as the working fluid. Typically, surface water at 28 degrees C (82.4 degrees F) is pumped into evaporator tubes, and liquid ammonia is sprayed on the tubes' outer surfaces. The vaporized ammonia drives a turbine and is then returned to a liquid state in a condenser cooled by water at 4.5 degrees C (40.1 degrees F) pumped from a depth of about 1,000 m (3,300 ft). In 1979 a privately funded 50-kW plant, the Mini-Otec,2 was built on a ship anchored off the coast of Hawaii. It proved successful, although its net generating yield was only about 20%; 80% of the electricity generated was needed to operate the plant. OTEC-1, a modified U.S. navy tanker, successfully conducted tests of an evaporator and condenser in Hawaiian waters in 1980. A Japanese-built OTEC plant on the island of Nauru in the Pacific Ocean underwent field tests in the early 1980s. There are still significant unresolved problems. For plants housed aboard ship the designs of the cold-water pipe, the platform, and the mooring system are crucial because of the complexity of the structures and the immensity of ocean forces. Both sea- and land-based plants must deal with the possibility of the "biofouling" of heat-exchanger surfaces by marine organisms. No tests have yet been conducted over periods long enough to prove the resistance of various designs and materials to oceanic currents, waves, and marine life. The warm water needed to generate 1 kW is 4 liters/sec, and water volumes become enormous when, for instance, a 100-MW plant is considered. Capital costs for building such a plant are high enough to discourage the many developing countries that possess excellent thermal resources and could profitably use OTEC technology, rather than consume expensive fossil fuels for electricity generation. 1. A. Lavi. The 1998 Grolier Multimedia Encyclopedia: Ocean Thermal Energy Conversion , Grolier Interactive Inc: 1997. |
