PRACTICAL PHOTOVOLTAICS

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Photovoltaic panels are, for technical purposes, rated in volt/amps rather than watts. One volt at 1 amp equals 1 volt/amp . . . consequently, 12 volts at 1 amp will work out to 12 volt/amps. Of course, the motor used in a pumping system may be rated in watts, but it should also have separate voltage and amperage ratings.

Let's say that we want to use a motor that has a listed capacity of 12 volts at 5 amps. First, keep in mind that most commercial panels are standardized at 12 volts, so the voltage will likely match. Next, let's assume that we have a 2-amp panel, which results in a collector with a 24volt/amp rating (12 volts times 2 amps equals 24-volt/amps). The requirement of the motor is 60 volt/amps (12 volts times 5 amps). Thus we'll need three panels hooked in parallel , as shown in Fig. 2, to operate our pump properly. And obviously, in order to expand this system, we need only add more panels in parallel to achieve any current level necessary for a specific job.

HIGH-VOLTAGE SYSTEMS

Thus far we've discussed only 12-volt systems, and you're probably wondering whether a low-voltage collector can be practical. Well, there are numerous devices that operate on 12 volts—auto radios and stereos, small motors, and recreational-vehicle refrigerators are just a few examples—but there are also a number of appliances that just won't work at such a limited potential.

However, the voltage of a photovoltaic system can be increased by connecting panels in series, as shown in Fig. 3. You must remember, though, that amperage will seek the level of the weakest panel in the group. Therefore, if you hook a 1-amp and a 2-amp panel In series, the resultant amperage will be only I . . . but if two panels of 2-amp rating are hooked in series, the output will be 2 amps.

Obviously, connecting panels in series can make solar electricity much more versatile than can a simple parallel setup. There's a great deal of 24-volt, 36-volt, and 48-volt equipment available. Furthermore, if you hook nine panels in series, you'll have a 110-volt DC unit . . . and many common 110volt AC appliances will operate on DC current (small hand tools, kitchen gadgets, heating elements, light bulbs, and radios and televisions that specifically claim AC/DC; compatibility).

In addition, groups of series collectors can be wired in parallel . . . as long as they're stacked in incrementsof the total voltage . Notice, in Fig. 4, that three panels at a time must be added to a 36-volt system . . . no more, no less!

SAVING FOR A RAINY DAY

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