Science: And Now It Is Pond Power

Salty pools may soon provide some cheap energy

On a clear, still night last December, Israeli scientists and energy officials gathered in the tiny desert community of Ein Bokek near the southern shore of the Dead Sea. At the command of the Energy Minister, a switch was thrown and a battery of floodlights suddenly blazed in the darkness, casting an eerie bluish glow over the nearby hills. As onlookers cheered, one dazzled scientist exulted: "We have hit the skeptics pretty hard."

What was remarkable about the event —and what confounded the skeptics —was the fact that the electricity for the lights was generated not by a conventional or nuclear power plant, but by a shallow, briny pool of water—a solar pond. Easy to create, using existing technology, and apparently harmless to the environment, these ponds may be one of the brightest ideas yet for tapping the sun's radiation as an energy source.

Unlike some other solar energy schemes, the ponds can operate year round, on cloudy days as well as sunny ones, even at night. They are also cheap, consisting essentially of a sheltered, shallow body of salt water that acts as a solar collector, plus a heat-exchanging system and turbogenerator. The pond's operating principle is simple. When sunlight strikes a fresh-water pond, it heats the water and stirs up convection currents; cooler water sinks to the bottom while warmer water rises to the surface, where its heat quickly escapes into the atmosphere. In a solar pond, these currents are suppressed by dissolving salt near the bottom of the pond. That creates a layer of denser, heavier water that resists rising to the top even when it is heated by solar rays penetrating the pond. The lighter layer of water at the surface helps to contain the heat below by acting as an insulator. Since very little heat escapes, the temperature at the bottom of the pond quickly rises—to about 80° C (176° F) at Ein Bokek.

To produce electricity, hot water is pumped out of the bottom of the pond and funneled into the coiled tubes of a heat exchanger, or evaporator, which is surrounded by a low-boiling-point liquid similar to that used in refrigerators. The water's heat turns the liquid into a pressurized vapor, which is directed against the blades of a turbine designed to operate on such low-temperature gases. As the turbine spins, it drives an alternator, which produces an electric current. Completing the cycle, the vapor passes into another heat exchanger, or condenser, where it is rechilled (with the cooler water from the top of the pond) and changed back into a liquid. Then the liquid is returned to the evaporator to be reheated and vaporized again for another go-around. Nothing is wasted; the fluid is hermetically sealed for repeated use, and both the heating and cooling water are returned to their original layers in the pond.