THERMAL ENERGY FROM THE OCEAN ?

Physicists would say that a heat engine is a simple thing.

What’s needed is a cylinder with a movable piston closing one end, the working substance (a gas or liquid that will evaporate) inside, a big tub of something hot, and a big tub of something cold.

Push the piston in, put the cylinder in the hot tub and let the gas expand at the hot temperature, letting the push of the piston drive the wheels of the railroad engine or the rotor of a dynamo. Take it out and let the gas expand, growing cooler as it does so, until it's at the temperature of the cold tub.

Put it into the cold tub and push the cylinder in – that takes work, but less than was obtained by the expansion because the gas is now held at the cold temperature and the pressure is less. Then start all over again.

The physicist is correct, of course, but impractical. It takes an engineer like Bob Cohen, speaking to the Boulder Rotary Club this month, to put flesh on those physical bones and really make things work. Still, the essential conditions remain – a big tub of something hot, a big tub of something cold, and a working substance that can expand and contract.

Cohen's interest for many years has been the possibility of using the ocean as the source of those big tubs.

"Solar radiation falls on the ocean surface, heating it, and the motion of the water mixes the surface water with the water just below the surface to form a ‘mixed layer,’” close to the top, he said. Below the mixed layer, the water doesn't warm from the sun's radiation, and at one kilometer down, ocean water is a steady 5 degrees Celsius. Those are the two tubs: the warm mixed layer at the surface, the cold reservoir a kilometer down.

"The French scientist d'Arsonval observed as early as 1881 that these two volumes of ocean water could in principle be used for a heat engine," said Cohen. The devil, of course, is in the engineering details, and it has taken well over a century to arrive at something promising.

Ammonia is a good choice as a working substance for a thermal engine operating between the temperatures of the working layer and the cold below, and in 1974, TRW and Lockheed developed commercially feasible concepts for an engine using the thermal energy of the ocean. Working models have been constructed, but the costs of construction are still too high in comparison with the savings from not using fossil fuels, so the engines remain in test mode.

Lockheed has built a small plant in Hawaii that generates 15 kilowatts net power, and in 1981 the Japanese built a 34 kilowatt plant on land, piping the warm and cold water from intakes at sea.

The most promising area of the ocean for such a plant is near the equator, where the mixed layer temperature is highest. On an island, the economic competition is fossil fuel, which must be shipped in, giving the ocean thermal engine a better price break. Alternatively, ocean thermal plants could be mounted on floating platforms or "plantships," possibly generating electricity. The power could be sent to shore by underwater cables or be used on the ship to obtain hydrogen by electrolysis – a portable energy source when bottled – or to refine aluminum, a process requiring large amounts of electrical energy.

Cohen reported that Lockheed Martin is now doing further exploration of using thermal ocean energy through its own funding – possibly a harbinger of future development of a physical phenomenon that holds promise of cleaner energy for civilization.

For more information on Rotary, see www.boulderrotary.org or www.rotary.org.

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