PRACTICAL PHOTOVOLTAICS

Enlarge Image Image Gallery CLOCKWISE FROM ABOVE: One panel of photovoltaic cells?such as this ARCO Solar installation on the roof of the Pratts' residence?can supply a goodly share of a homestead's electrical needs. . . . A power inverter can change 12-volt DC current into 110-volt AC power. . . . Richard Pratt uses a 12-volt Sears electric rototiller (unfortunately, this model has been discontinued) when he cultivates his garden. . . . A solar-powered chain saw quietly cut all the Pratts' firewood last winter. . . . Richard and Maureen irrigate their vegetable patch with the help of this sun-energised Teel water pump

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Issue # 70 - July/August 1981 PRACTICAL PHOTOVOLTAICS

You can get

by T.J. Byers

You can get affordable electricity (and achieve independence) with sunbeams . . . today!

A short while ago, most North Americans thought solar energy too exotic to be put to any practical use . . . but in the past few years our worsening energy situation has changed the picture considerably. Sun power is now becoming a standard means of heating homes and domestic water . . . and its popularity is no doubt due—at least in part—to the feelings of self-sufficiency and wellbeing that an "independent" energy source can provide. In fact, harnessing the sun has become so widely accepted that a number of regional governments (California's San Diego County, for example) now require that solar features be incorporated in all new construction.

However, when someone mentions producing electricity from sunlight, most of us are probably still inclined to be skeptical . . . assuming that photovoltaic setups are better left to the latest Buck Rogers episodes. "Not so!" I say. Because, for many people, practical solar electric power is here today . . . and at affordable prices! And how, you may ask, do I know? Well, for one thing, a bank of solar cells provides all the electricity used in my family's home!

HOW IT'S DONE

Changing sunlight (photons) into electricity (electrons)—the process called photovoltaic conversion—was pioneered by Bell Laboratories in the mid-fifties. And the silicon solar cells that Bell first devel oped for the space program are still the workhorses of the industry.

The cells are sliced from a cylinder of ultrapure silicon crystal . . . which is nothing more than ( highly ) refined sand. Every wafer is then chemically treated and processed to form a semiconductor junction (the technique is similar to that used in the fabrication of common transistors). It's within this thin semiconductor junction that electricity is generated.

And just how is the power produced? Well, photons strike the junction, liberating electrons (the action involves a mechanism that can be fully explained only by an excursion into quantum physics that I'd rather not make). The freed electrons are then collected by a conductive grid placed over the face of the cell. When a wire is connected from the front grid to the back of the cell, current flows.

Each cell generates about 1/2 volt of DC electricity, while the amount of current (amperage) depends upon the number of free electrons—which is proportional to light intensity—and the size of the cell.

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