ENERGY FROM THE OCEANS

The earth's oceans cover the great majority of its surface, and they absorb solar energy. The temperature of the surface layer in tropical oceans is about 26°C; the deep water is as cold as 5°C. Multiply this temperature differential by the billions of tons of water involved, and you arrive at energies compared to which a few hundred thousand hydrogen bombs are chickenfeed.

Can this energy be harnessed? The obvious difficulty is the small temperature differential, which in a sense is analogous to the height differential of water driving an hydraulic turbine. A lot of water flowing slowly may have the same (kinetic) energy as a little water tumbling down at a high velocity, but it is the latter case that makes the energy easy to harness: Dams are built to realize a large difference in water levels.

Analagously, a heat engine usually needs a high temperature differential; that is why the steam in the boilers of a power plant is superheated to some 500°C before it goes into the turbine, and condensed to liquid water at the turbine exhaust. The temperature differential in electric power plants amounts to as much as 500° C. But can you make a heat engine run on a 20° C heat differential?

Yes, if you sacrifice efficiency (as you would have to with an hydraulic turbine given only a one foot feedstock). In fact, the efficiency will go down from almost 40% to a mere ago; but it can be done, by using a working fluid that is superheated (hotter than the boiling point) at 25° C --ammonia, for example. A specially designed turbine would run on the pressure difference of the gas boiled by the surrounding warm sea water and condensed by the cold water brought up by an intake pipe from a depth of several hundred meters. (The low efficiency of the system is, in part, due to the power needed to pump the cold water). The turbine would produce electric power in a conventional generator either for transmission to the shore, or even better, for distilling and electrolyzing sea water in order to produce hydrogen which would then be shipped (or piped) to the mainland as the fuel of the hydrogen economy (Sept. AtE).

Even at a low 3% conversion efficiency, the system would produce energy at about half of the cost of nuclear produced electricity, and at about four times the cost of coal as priced at the mine mouth; and its calculated cost has already been overtaken by the cost of foreign petroleum and natural gas (US oil and gas are still kept at artificially low prices at press time).

The minor problems, such as corrosion by sea water, microbial fouling, and plant anchoring on the open seaihave all been shown surmountable.

Particularly noteworthy is the environmental impact of such solar sea power plants, which is virtually absent: If the world population in the year 2000 were to use energy at the present per capita US rate (which it will not, of course, by far), then these systems could not only provide the total need, but they would only lower the surface temperature of tropical oceans by less than 1 degree C.

A basic article on the system has been published by A. Lavi and C. Zener, both of the Carnegie Mellon University, in the October issue of the IEEE Spectnum (available in engineering libraries, or for $3 from IEEE, 345 E. 47 St., New York, NY 10017).

The US is again lucky: It has territories near the ideal areas for using solar sea power plants, in Hawaii and the Virgin Islands. A proposal for a small prototype plant (1 to 10 MW) at either of these locations has been submitted to the NSF by Prof. Lavi and collaborators. It is now up to the NSF, so fond of funding "societally relevant" projects, to put some of its money to unusually good use.