Desalinated Water

Desalinated water can be produced in open- or hybrid-cycle plants using surface condensers. In a surface condenser, the spent steam is condensed by indirect contact with the cold seawater. This condensate is relatively free of impurities and can be collected and sold to local communities where natural freshwater supplies for agriculture or drinking are limited. System analysis indicates that a 2-megawatt (electric) (net) plant could produce about 4300 cubic meters of desalinated water each day (Block and Lalenzuela 1985).

The large surface condensers required to condense the entire steam flow increase the size and cost of an open-cycle plant. A surface condenser can be used to recover part of the steam in the cycle and to reduce the overall size of the heat exchangers; the rest of the steam can be passed through the less costly and more efficient direct-contact condenser stages. A second-stage direct-contact condenser concentrates the noncondensable gases and makes it possible to use a smaller vacuum exhaust system, thereby increasing the plant's net power.

One way to produce large quantities of desalinated water without incurring the cost of an open-cycle turbine is to use a hybrid system. In a hybrid system, desalinated water is produced by vacuum flash distillation and power is produced by a closed-cycle loop. Other schemes that use discharge waters from OTEC systems to produce desalinated water have also been considered.