Engineers have invented a method of using the heat of sea water in the tropics to generate electricity and produce fresh water at the same time without consuming outside sources of energy, according to an announcement from Argonne National Laboratory.

A small, experimental version of such a facility has already been shown to work and the researchers estimate that a full-size version could produce 10 megawatts of electricity and 5 million gallons of fresh water daily -- enough for a city of 10,000 to 20,000.

The research is being done jointly at the Natural Energy Laboratory in Hawaii by Argonne, which is based in Illinois, and the Solar Energy Research Institute of Golden, Colo.

The method's energy source is solar heat in the top layers of tropical ocean water. This water, already about 78 degrees Fahrenheit, is pumped into a vacuum chamber where, because low air pressure lowers water's boiling point, about 1 percent of the water flashes instantly into steam. The steam drives turbines, as in a conventional power plant. The spent steam condenses as fresh water. To speed the condensation, the steam pipes are cooled with 44-degree sea water pumped from half a mile below the surface.

The engineers say the power generated by the turbines is more than enough to pump the sea water and maintain the low pressure in the vacuum chamber.

"This means that in addition to providing electricity, future OTEC {ocean thermal energy conversion} plants could provide fresh water to islands and coastal cities," said Argonne's Anthony Thomas.