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Bulletin Board
Rants, Raves, Interesting Science & Awful Puns
December 19, 1997

Photovoltaics

Misplaced Hopes

A misunderstanding being vigorously promoted by the solar lobby (for example Al Gore in his book, Earth in the Balance, 1992) is that with government backing and subsidies to promote development, the price of photovoltaics would plummet and open up the same kind of vast market for solar cells as has happened with computer chips in recent decades. Consideration of the different roles played by the semiconductors in these two applications shows, however, that such expectations are unfounded.

It has remained a consistent trend for a surprisingly long time that the speed of commercially available semiconductors and the amount of memory on a chip has doubled, while at the same time the price per transistor has halved, about every eighteen months. These benefits all follow from improving technology that enables transistors to be made smaller, thus fitting more of them onto a chip. In other words, performance per unit cost increases because more and more of what computers do--computing--gets done on smaller and smaller areas of silicon. But the function of solar cells is to collect sunlight; to collect a given amount, you need a given amount of area. The technology that enabled computer circuits to shrink to such tiny sizes gains you nothing because you'd need correspondingly more of them to do the same job.

The only area with room for improvement would be raising the conversion efficiency of sunlight to electricity. The progress made in semiconductor computing circuits is irrelevant to this function, and there are reasons for supposing that the 20% or so achieved by the best systems currently is pushing the limit. But even in the wildest flights of fancy, with absolute perfection and no losses at all, you can't go beyond 100%.

More information on this appears in the October 1997 issue of The Energy Advocate newsletter. (For details of EA, see the March 26 posting in the ENERGY section of the Bulletin Board Archives.)

 
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