Solar Energy

Powering a House

Now that we have our PV module, what do we do with it? What would you have to do to power your house with solar energy? Although it's not as simple as just slapping some modules on your roof, it's not extremely difficult to do, either.

First of all, not every roof has the correct orientation or angle of inclination to take advantage of the sun's energy. Non-tracking PV systems in the Northern Hemisphere should point toward true south (this is the orientation). They should be inclined at an angle equal to the area's latitude to absorb the maximum amount of energy year-round.

A different orientation and/or inclination could be used if you want to maximize energy production for the morning or afternoon, and/or the summer or winter. Of course, the modules should never be shaded by nearby trees or buildings, no matter the time of day or the time of year. In a PV module, even if just one of its 36 cells is shaded, power production will be reduced by more than half.

If you have a house with an unshaded, south-facing roof, you need to decide what size system you need. This is complicated by the facts that your electricity production depends on the weather, which is never completely predictable, and that your electricity demand will also vary. These hurdles are fairly easy to clear. Meteorological data gives average monthly sunlight levels for different geographical areas.

This takes into account rainfall and cloudy days, as well as altitude, humidity, and other more subtle factors. You should design for the worst month, so that you'll have enough electricity all year. With that data, and knowing your average household demand (your utility bill conveniently lets you know how much energy you use every month),there are simple methods you can use to determine just how many PV modules you'll need. You'll also need to decide on a system voltage, which you can control by deciding how many modules to wire in series.

Obstacles

You may have already guessed a couple of problems that we'll have to solve. First, what do we do when the sun isn't shining? Certainly, no one would accept only having electricity during the day, and then only on clear days, if they have a choice. We need energy storage -- batteries.

Unfortunately, batteries add a lot of cost and maintenance to the PV system. Currently, however, it's a necessity if you want to be completely independent. One way around the problem is to connect your house to the utility grid, buying power when you need it and selling to them when you produce more than you need.

This way, the utility acts as a practically infinite storage system. The utility has to agree, of course, and in most cases will buy power from you at a much lower price than their own selling price. You will also need special equipment to make sure that the power you sell to your utility is synchronous with theirs -- that it shares the same sinusoidal waveform and frequency.

Safety is an issue as well. The utility has to make sure that if there's a power outage in your neighborhood, your PV system won't try to feed electricity into lines that a lineman may think is dead. This is called islanding.

If you decide to use batteries, keep in mind that they will have to be maintained, and then replaced after a certain number of years. The PV modules should last 20 years or more, but batteries just don't have that kind of useful life. Batteries in PV systems can also be very dangerous because of the energy they store and the acidic electrolytes they contain, so you'll need a well-ventilated, non-metallic enclosure for them.