PRACTICAL PHOTOVOLTAICS

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Since each unit is capable of producing only about 1/2 volt, the cells must be connected in a series circuit in order to increase the voltage to a useful level. (The procedure is similar to stacking flashlight batteries.) Hence, 24 cells will, in theory, give a total output of 12 volts. In actual practice, however, each cell's output is closer to 0.46 volt, so 26 cells are required to produce a full 12 volts. And, though amperage varies from manufacturer to manufacturer (depending on the efficiency and size of the cells), a typical 12-volt panel might produce 2 amperes.

If left unprotected , silicon photovoltaic cells would be susceptible to damage from moisture and airborne contaminants. So, after they're wired together, the wafers are laid face down on a sheet of safety glass. A piece of plastic (such as Mylar) is then stretched across the back of the assembly and heat-bonded. Last of all, the 3/8"-thick panel is crimped into a metal frame. . . both to protect the glass and to help conduct heat away from the cells. A perfect seal is then insured by applying a liberal bead of silicone sealant along the joined edges.

DOLLARS PER WATT

With the background information pretty much taken care of, let's examine a couple of the practical aspects of solar electricity: the size and type of setup you might consider installing. And, since most folks will allow cost to determine just how far they go in developing a photovoltaic system, let's start by talking dollars and cents.

Currently, the market prices for panels vary from $10 to $20 per watt of capacity .. . that is, a 30-watt panel would cost between $300 and $600. But remember . . . that's for prime, first-quality collectors.

There are ways, fortunately, to purchase panels for less money. One possibility is to buy surplus equipment. Because the photovoltaic industry is expanding so rapidly, today's top seller may be replaced by an improved version at any time, and the obsolete units often sell for less than $10 per watt. Look for existing photovoltaic systems that are being updated—the Department of Energy has a few scattered throughout the country—or check directly with manufacturers to find out whether they have any unsold obsolete panels in stock.

"Manufacturer's seconds" (any panels whose performance isn't up to one or more of the maker's specifications) can also be bought at reduced prices. The most frequently found defect in such units is the production of a lower current output than was expected . . . although in rare cases a defective cell may reduce panel voltage, too. You might even be able—if you can do business with a vendor who has a government contract—to acquire panels that have been rejected merely for cosmetic reasons (such as discoloring or blemishes), which in no way affect the performance of the units!

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