THERMAL GRADIENTS

A steam engine or turbine, for example, works between the temperature of the superheated steam rushing in, and the temperature of the condenser, some 500 degrees C lower, where the steam is made to condense back into water. The human body, when viewed as a heat engine (the viewpoint, by the way, of the ruralizers who would replace industrial machinery by muscle power), works between the higher body warmth and the lower temperature of the air around it. In both examples the input energy is chemical: The higher temperature is produced by burning coal in one case, and digesting food in the other.

But a heat engine does not care how the temperature difference is produced, as long as it is there. Is there not one produced by nature ready to be tapped?

Yes, and it can be found in three places: on land, in the sea, and in the air. The last is not too promising, though tapping it has been proposed (AtE Aug.78). But the thermal gradients (degrees of temperature per vertical distance) in the earth and sea can be utilized.

Ocean Thermal Energy Conversion (OTEC) uses the difference between the warm upper layers of tropical oceans and the cold water from their depths. It is free solar energy that keeps the upper layers warm, so that the pitiful efficiency of 3 percent can be tolerated.

In the closed cycle version, a fluid with low boiling point (ammonia) circulates from an evaporator through a turbine to a condenser and back to the evaporator, which is kept warm by the surrounding tropical ocean, while the condenser is kept cold by the water pumped up from depths of 2,000 to 6,000 feet. (It is the energy lost in pumping up the cold water that makes OTEC so inefficient.)

In an open cycle, the working fluid is sea water itself. It is flashed into steam by a partial vacuum and runs a low-pressure turbine. The spent vapor is then cooled by a condenser, and the condensate is not, as in the open cycle, returned to the evaporator, but can be used as potable water.

In either case, the problems are bio-fouling (scaling of the walls of heat exchangers and pipes by marine organisms), corrosion, and above all, the constant forces of the tides and currents on the giant structures such as a 1,000 ft long cold-water pipe 100 ft in diameter, the mooring chain, and electrical transmission lines. Another basic question to be settled is whether these plants should produce electricity for transmission to the mailand, or manufacture energy-intensive products (ammonia, aluminum).

We have previously reported on this concept (Dec.73, Feb.77, Aug.78). The news is the start-up of the world's first OTEC plant for a 6-months test run. It has a capacity of 50 kW, and is built on a ship moored one mile offshore from the Natural Energy Laboratory of Hawaii at Keahole Point. It uses the closed cycle with ammonia, and its cold-water intake pipe is 2,170 ft long.

Meanwhile, tests with stresses on long, wide pipes suspended in the ocean have been carried out by Fluor Corporation's subsidiary Deep Oil Technology at Long Beach, Calif., and a T-2 tanker is being converted by the DoE to a 1 MW platform called OTEC-1 for deployment next spring.

The OTEC program was funded at $38 million in fiscal 1979, and the trend is up. The technical problems are still formidable; and the economy, with an estimated $1,500 to $1,700 per installed kW, is still out of sight. (For a conventional plant, with much of the money going for court costs and interest payments due to obstructionist delays, the cost is $900 to $1,100 per kW capacity.) It is claimed that by raising oil prices, OPEC will make OTEC economical. But have you noticed how the estimates of new energy sources keep increasing in parallel with the price of oil? (Oil from shale was thought to become economical if crude oil hit $15 a barrel.)

Yet the biggest obstacle, for the time being, lies elsewhere. The Nader-Commoner crowd and their partisans in the government doggedly oppose central power sources, solar or other; it does not fit in with their utopia of a de-industrialized America.

Solar energy? Of course! But not when, perish the thought, it produces power by the megawatt.

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