Produce your own electric power - photovoltaic power - Brief Article

Sunset, Oct, 2001 by Peter O. Whiteley

Photovoltaics explained: How you can convert to solar

The quietest power plant on Earth costs nothing to run, has no moving parts, requires no maintenance, and produces no pollution. It's an array of photovoltaic cells (a grouping of panels containing many roughly palm-size cells) installed on your roof and powered by the sun. Photovoltaics offer the possibility of freedom from public power supplies and their uncertainties. For about $8,500 in installation costs, you can make a start on energy independence. If you're willing to spend more, you can probably generate all the power you need.

Though initially expensive, costs are declining and cell technology is evolving rapidly The photovoltaic panels available today can be installed on most roofs that have an unobstructed, sunny area. Most roofs can tolerate the weight of the panels without reinforcement.

On or off the grid

Some photovoltaic systems turn your home into a generating plant that feeds surplus electricity back into the power grid, while others allow you to be totally independent--"off the grid." The latter systems cost more because they require heavy-duty batteries for storing the electricity but they make sense for remote locations and for some people who work at home.

A grid-tied system can actually generate savings as well as power. During the daytime, when the demand for electricity from businesses is high, most homes are quiet--family members are at school and work. The photovoltaic system, however, continues to produce power. A gridtied system can send the surplus energy it produces back into the power grid, and the homeowner will receive credit for it.

How a photovoltaic system works

Typically, the cells are made of chemically treated silicon covered with an upper layer of clear protective material, such as glass. Each solar cell converts sunlight into a small amount of electricity.

Three types of photovoltaic cells are in production: single crystal, polycrystalline, and amorphous. Single crystal and polycrystalline cells are "loaves" of silicon that are sliced into thin wafers. For the amorphous system, a thin film of vaporized silicon is deposited on glass or stainless steel. Amorphous cells cost less to produce but are slightly less efficient at converting sunlight to electricity.

All photovoltaic panels create direct-current power, which must be converted to alternating current to match and synchronize with the type of power delivered by utility companies. To do this, every photovoltaic system requires an inverter that's sized to its output.

Most systems also include storage, but grid-tied installations don't require it. Systems without batteries can use the existing electrical panel to distribute solar power to the house or to send it back to the utility with the aid of a reverse-direction meter. Grid-tied systems with battery storage need an electric subpanel that operates selected circuits in the house during a utility power outage. (The batteries, which have greater storage capacity and longer life than car batteries, must be replaced about every eight years.)

Arrays

At present there are four basic ways in which photovoltaic cells are arrayed. The most popular and easiest to add to an existing roof are metal-framed modules, which mount to brackets installed on an existing roof. There are many manufacturers of these systems.

Two other systems can be integrated into a roof, doubling as roofing material: photovoltaic shingles, available from Atlantis Energy (916/920-9500) and Uni-Solar (248/362-4170), and standing-seam metal roofing with amorphous-silicon solar cells bonded to the surface (also from Uni-Solar).

The fourth system, which uses cells sandwiched between layers of glass, is offered by Four Seasons Sunrooms (631/563-4000). Other products are on the horizon but are not currently in national distribution for residential use.

Sizing and installation

There is no universal photovoltaic system. The amount of energy needed varies from household to household, depending on the types of appliances, lighting, electronic devices, and patterns of living. It has little to do with a home's square footage.

However, you can reduce the number of panels needed--and thus the cost of your installation--by lowering the amount of energy your household requires. Do this through conservation and the use of energy-saving products such as added insulation, double-glazed windows, compact fluorescent light-bulbs, and appliances that qualify for Energy Star rating. (See the June 2001 Sunset for a more comprehensive list of ways to save energy, or go to www.sunset.com/home/energywise. html.)

Monitor your electric bill for the total kilowatt hours (kWh) used each month. If you're still exceeding baseline usage (noted on your bill), you're paying a premium. The most practical, affordable photovoltaic systems target this above-baseline usage.

The greater access you have to the sun, the more cost-effective your system will be. The biggest variant is location. For instance, the Southwest gets about twice the amount of midwinter sun (and requires smaller solar power systems) as the Northwest. Other considerations are shade and roof orientation.