SOLAR PONDS

The biggest drawback of solar energy is its diluteness - only 1.36 kW/m2 in space (at earth distance), where there is no night and day and no atmosphere (let alone clouds). Large collecting areas are thus needed to generate the power that can be generated in compact facilities by other methods. There are, however, some applications where solar energy can be economically harnessed - those where not much energy is needed and sufficient collecting area is available. Heating and cooling homes is a possibility, though often only on the economic fringe. Heating water to boil a secondary working fluid for turbine-generated electric power is technically feasible, but the cost is about double that of conventionally generated electricity, and other methods are even more costly.

But what about the industries that merely need hot water, not steam? For example, Sohio Petroleum operates a uranium mine and processing plant near Grants, N.M., and the processing plant requires hot water (140°F) at the rate of 500 gallons per minute, 24 hours a day. To heat the water, it has been burning oil, tens of thousands of barrels a year.

But it is about to substitute solar energy. The basic idea of the project, designed by the Lawrence Livermore Lab with ERDA support, is to expose the water in shallow ponds (200 by 12 ft., 3 to 4 inches deep); if this year's experiments are successful, the system of ponds will cover an area of 6 acres (no way to avoid a large area). To prevent evaporation, each pond is in a long, flat, plastic bag, and to prevent heat loss into the earth, the bag lies on an insulation layer topped off by black plastic. Each pond lies under an arch of transparent, semi-rigid plastic to create a greenhouse effect.

The water heated during the night will be pumped into an underground storage reservoir large enough to supply the plant with hot water for the night, and beyond that, for one to three sunless days. Construction began last month, and the system is expected to provide all the hot water needed during the summer months; in winter, it will preheat the water before a fuel-burning heater takes over. The saving will be about 20,000 barrels of oil a year.

LLL has been experimenting with smaller (50 ft) ponds for some time and has demonstrated their feasibility; in Livermore near San Francisco, where conditions are more adverse than in New Mexico, they heated water to 135°F on hot summer days, and to 85°F on chilly, but sunny winter days. No tough technical problems are therefore expected. The real problems, quite unknown to the dreamers who sport "Go solar" buttons, are economic: Do the insulation layers under the ponds prevent enough heat loss to justify their expense? Is a semi-rigid, durable "greenhouse" roof in the long run more costeffective than "disposable," cheap, soft plastic held up by air pressure?

Some of the answers will be known by this time next year, and Sohio will then decide whether to expand the initial system of three ponds to the full six-acre assembly.