This article is the second in a series outlining the technological and economic obstacles to the widespread use of solar power.
The best use of photovoltaic (PV) cells, the key element in solar power, is actually to charge batteries, for reasons that need not concern us here. But it is important not to overcharge batteries. The charging system must be voltage-regulated and properly adjusted for the ambient temperature of the batteries.
PV cells produce DC (direct current) voltage, and much equipment in a house is designed for DC–all electronic devices, including TV sets, radios, computers, printers, fax machines, cordless phones, VCRs, and everything else you can name. Many motors (such as electric drills) can also run on DC.
However, the voltages are not all the same. Some electronic devices immediately convert the AC (alternating current) from the power line into about 300 volts DC. However, to run a computer requires several different voltages, including 12 volts DC (positive and negative), 5 volts (positive and negative), and positive 3.1 volts (or lower, depending on the central processing unit).
In other words, no single DC voltage suggests itself as being the best for household use.
All of the voltage conversion takes place via AC anyway, and everything in the house is designed around 115 VAC (input voltage), so it is most convenient simply to use something to convert the PV system’s DC into 115 volts AC. That device is called an inverter.
You can buy inverters that convert your car battery’s 12 volts into 115 VAC (100 watts to as much as 500 watts of power), but the voltage produced is of pretty poor quality. If you buy a utility-grade inverter to handle (say) 5,000 watts, you’ll pay a pretty penny for it.
In some cases, lighting can be done with 12 volt lamps, but the wiring has to be heavier. For example, a 100 watt lamp operating at 115 volts requires a current of 0.87 amperes running through the wires. A 100 watt lamp designed for 12 volts will draw 10 times as much current through the wire, 8.7 amperes.
Thus a mere two 100 watt lamps operating off a 12 volt battery would blow a 15 ampere circuit breaker. This is another reason the DC voltage off the PV-charged batteries has to be inverted to 115 VAC.
On the Grid
Using PV cells in a residence that is connected to the electrical grid is another matter and presents difficulties of its own.
The simple-minded view is that you can simply use power from your PV array when the sun is shining, and you can sell any excess electricity to the power company–the watt-hour meter automatically runs backwards when the power comes from the house. At night or in cloudy weather, you can get power from the utility.
In this way, the idea goes, you can avoid buying and maintaining storage batteries. Alas, things are never that simple.
How would you actually go about selling electricity to the utility?
Suppose, for example, your solar gadgets produced exactly as much electricity during the year as you used. Even suppose that the solar gadgets produced exactly as much power as you required just when you wanted the power. Your house is connected to the grid via some wires, the wires are connected to distribution lines to a substation, and the substation is connected to transmission lines, in turn connected to not just one but numerous power stations.
More realistically, suppose you generate exactly as much energy as you use during the year, but sometimes generate excess power and at others fall short. You are sometimes buying energy from the utility and sometimes selling energy to the utility–both at the retail price. If there is a standard watt-hour meter, like that shown in the accompanying photo, it runs forward when you draw power from the line and backward when you provide power to the line.
Using the watt-hour meter for both purchases and sales is called net metering. A PV array is used for exactly that purpose. The owner sells electricity to the utility at the retail price. Normally, the wholesale price for electricity is about 2 cents per kWh, whereas the retail price is closer to 10 cents per kWh.
At the end of the year, if the PV system has generated as much electricity as the customer has used, the meter reads zero and the customer pays the utility nothing. But somebody has to pay for all that equipment and labor, not only for reasons of morality but also from a standpoint of practicality.
Howard Hayden ([email protected]) is professor emeritus of physics at the University of Connecticut and adjunct professor at Colorado State University at Pueblo. He writes a monthly energy newsletter available for a $35 annual subscription at The Energy Advocate, PO Box 7595, Pueblo West CO 81007.