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Ocean Thermal Energy Conversion
DOI 10.1615/hedhme.a.000385
3.22 FLASH EVAPORATION
3.22.3 Ocean thermal energy conversion
H. El-Dessouky and H. Ettouney
Oceans cover more than two-thirds of the earth’s surface. The salt water of the oceans accounts for more than 96.5% of the total water available on the plant with a total volume of 1.338×109 km3. On daily basis the surface area of the oceans around the equatorial region receive large and nearly constant amount of solar energy. The depth of the surface layer varies between 35–100 m. Winds and waves provide good mixing within this layer and maintain a uniform temperature and water salinity. As a result, the surface temperature remains constant throughout the year and provides a sustainable source of energy.
Ocean thermal energy conversion (OTEC) converts the absorbed solar energy by the surface ocean water into electrical power. The temperature difference of the warm surface water and the cold ocean water may be used to operate power-producing cycles and desalinated water. It is necessary to have a temperature difference of more than 20 °C to generate significant amount of power. This temperature difference is found throughout the year in the area bound by the 32° N to the 25° S of the Equator. In this region, the surface seawater temperature remains constant over a range of 25–30 °C, while the deep seawater temperature at 800 to 1,000 m depth may vary over a range of 5–10 °C.
The power producing cycle of OTEC is based on evaporation of a low boiling liquid using the warm seawater; such liquids include ammonia and a number of refrigerants. The formed vapor should have high pressure, which is used to drive the power turbines and consequently generate electricity. The low-pressure vapor is then condensed using the low temperature ocean water, which is pumped from 1,000 m depth. The OTEC process can also be accompanied with a number of other applications including air conditioning, desalination, and cold-water aqua cultures.
There is enough solar energy received and stored in the warm tropical ocean surface layer to provide most, if not all, of present human energy needs. The OTEC has limited environmental effects, especially if the produced power is limited to 0.19 MW/km2, which corresponds to conversion of 0.07% of the average absorbed solar energy to electricity. The following sections cover various elements of the OTEC process and start with process features, historical overview, types of processes, the flashing process, heat transfer equipment, system model, and account of field data and conceptual designs.
A. Features of OTEC
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