PRACTICAL PHOTOVOLTAICS

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But—most would-be photovoltaic purchasers ask—what happens when the sun doesn't come out? Well, believe it or not, solar-electric collectors do produce power on cloudy days . . . but at only about 50% of their "normal" rate. And, of course, the semiconductors take a well-earned rest each evening. Consequently, while a power storage system isn't absolutely necessary, it certainly can be useful.

There are numerous ways of storing electricity, but the lead/acid battery is the least expensive and most widely available. And, although we've referred to panel output as 12 volts up to this point for the sake of convenience, nearly all manufacturers have had the foresight to design panels that put out the 14 to 16 volts necessary to charge a 12-volt storage battery.

The batteries needed for photovoltaic systems are the deep-cycle type commonly employed to provide storage for windplants and power for electric vehicles. They're different from standard automobile batteries in that they were designed to withstand numerous discharging/charging cycles. The storage cells are available in 2-volt, 6-volt, or 12-volt units.

In the same way that panels can be grouped to produce the output required, any number of batteries may be connected in parallel . . . as long as they match the voltage of the system . And when wired in series, the batteries again need to match the panel voltage (see the example in Fig. 5).

To prevent the deep-cycle units from overcharging, a specially designed regulator can be placed in line with the electricitygenerating circuit. However, if you set up a well-balanced system—one that consumes the same amount of power as it generates—a charge regulator shouldn't be necessary.

AC TOO?

Some electrical devices—such as powerful electric motors and color television sets-can't be operated on direct current. Fortunately, you can produce alternating current from photovoltaic DC power by using an inverter. Such a device employs a pair of switching transistors to change the direction of the current 60 times per second. This form of electricity is known as 60-cycle (or-hertz) alternating current.

The AC voltage is then run through a step-up transformer to yield the equivalent of household current. Although you can find inverters in a wide variety of power ranges, units of less than 500 watts are usually the easiest to locate and the least expensive. The smaller inverters can get by with 12-volt input current (as shown in Fig. 6), while higher-wattage inverters generally require substantially greater input voltages to overcome internal power losses caused by the larger unit's heavier circuitry. For example, a 48-volt input is common for a 2.5KW inverter. Consequently, when you're mapping out an expandable photovoltaic installation, you need to design your series/parallel arrangement to match any future inverter purchases. My own system now consists of a 12-volt parallel setup with six panels, batteries, and a couple of small 12-volt to 110-volt AC inverters . . . but I plan eventually to expand it in order to have a 110-volt DC array.

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