WAVE POWER

The energy density of waves is an order higher than that of ocean currents, which makes them better, but still not very good. With 10 kW/m available, a boom would have to be 60 miles long to intercept (not extract) the same power as that provided by a nuclear or fossil-fired plant unit (1,000 MW).

Since we wrote about Salter's Ducks and Cockerell rafts [Apr. 75], some highly ingenious designs have appeared; one that impresses us with its simplicity is a wave-operated pump developed by the Foundation of Ocean Research in San Diego. A long, vertical pipe flooded with water is suspended from a buoy on the surface. As the wave rises, the pipe and its water column (controlled by a simple check valve) are lifted upward, but when the water recedes again, only the pipe drops; the water column continues to flow upward because of its large mass and resulting inertia. (The inertia in the upward acceleration is much smaller because the entire system is being buoyed.) The water thus pumped by the waves is (after a pressurizing-stabilization stage) harnessed by a hydraulic turbine.

Which leaves only two energy-dense types of sea power: OTEC (Ocen Thermal Energy Conversion) and salinity. The former we have frequently written about, so briefly for new readers: OTEC runs on the temperature difference between the surface layers and the depths of the sea, using a liquid with low boiling point (ammonia) to drive a turbine, part of whose power is used to pump up cold water as a heat sink. In the "open cycle" version, sea water is, in a partial vacuum, flashed into steam by the warm surface water (but the low steam pressure requires extremely large turbines). The closed cycle version is the more advanced, and is now undergoing tests; for example, a 1 MW study project is being run by TRW Inc. off Hawaii.

At present, OTEC represents the only system that has some (rather faint) hope of economically producing electricity from solar energy mainly because the oceans collect solar energy on vast areas for free. But even if successful, its total amount produced in US waters is limited by what can be harnessed in a few plants off Florida, Hawaii and the Virgin Islands.

[More: Energy from the Waves by D. Ross (Pergamon Press, $8.25) has 115 pages, but covers much less than the survey "Ocean Energy: Forms and Prospects" by J.D. Isaacs and W.R. Schmitt, Science, 18 Jan 80. See also "Ocean Energy" special issue of Oceanus (a quarterly published by Woods Hole Oceanographic Inst., Mass.), Winter 1979/80. For more on wave-operated pump see article by Isaacs, Costel and Wick, Ocean Eng., v.3, p. 175 (1976).]