SOLAR TODAY - Fall/Winter 2009

solar electricity

System components must be matched. A 3-k W inverter can’t handle the power produced by a 5-k W DC array. If you install a 3-k W array, you may consider putting in a 5-kW inverter to allow expansion of the array later. Also be aware of economies of scale. Labor costs may be lower, per unit, for a large system. If a 5-k W system can be installed in a day, you won’t save labor costs by buying a 3-k W system, which may also take a day.

Net metering >> Your utility company may not offer net metering. If it does, it will do so under terms usually determined by state law or by your public utility commission. The rate the utility will pay for power you sell will therefore vary widely state to state and city to city. Some utilities may pay you more for power during high-load periods, as in summertime late afternoons, when heavy use of air conditioners creates high demand. Check with your utility company for details or look up your local information at dsireusa.org.

Maintenance >> Modules are tough and usually carry a 20- to 25-year warranty. They need to be cleaned occasionally, which is usually a matter of hosing off dust and leaves. If appropriate, consider how you’ll clear snow off the modules. The inverter processes hundreds of volts for several hours a day. It needs to be mounted in a cool, shaded place. Inverter life expectancy is about 10 to 15 years.

PHoto b Y lAVonne e WinG / Pix YJACk PreSS

Installation location is critical to PV performance. The array should face the sun. This usually means due south, though if you have a heavy air-conditioning load in the late afternoon, you may want to point the array southwest. The array should not be shaded by trees, towers or chimneys during any part of its productive day. The array should be tilted upward at the correct angle to optimize seasonal exposure — typically at the angle of your latitude, so it gets sunlight at a right angle at spring and fall equinoxes. Some arrays can be adjusted for the seasonal sun angle. The ideal location is on a south-facing pitched roof. If the array needs to be elevated above the roof surface, it places additional torsional loads on the roof structure during wind storms.

Since 1999, Rex and LaVonne Ewing’s off-grid mountain home has been powered entirely

by solar and wind energy. The ground-mounted 1,940-watt photovoltaic array is supple-

mented by a 1,000-watt wind turbine. Domestic water is heated by a roof-mounted solar

collector array of 30 evacuated tubes, with an on-demand water heater as backup.

Thin-film modules are made by depositing or printing photosensitive materials on a glass, metal or plastic substrate — even on roofing tile. They’re considerably less efficient, so you may need to give them a lot of space to generate the same amount of power as a smaller single-crystal or multicrystalline system. Thin-film modules are relatively inexpensive and, depending on the substrate, can be very robust and flexible. One practical use is to glue a flexible-plastic thin-film module directly to a metal roof.

off the Grid

If you plan to live far from the main road, it may be too expensive to run a power line in from a utility pole. In that case, you’ll need to generate your own electricity and store it in a bank of batteries for use at night. Off-grid homes use massive heavy-duty, deep-cycle batteries — similar to the batteries used in forklift trucks. Batteries should be housed in a stable, weather-proof space, insulated against very hot

and very cold weather but isolated from living spaces.

Batteries are charged with DC from a charge controller. The charge controller can take power directly from a PV array, from a wind turbine, from a diesel generator or from a hydroelectric source — anything that can feed steady DC can be a charging source. Current taken off the battery goes to the inverter for household use.

If you live on the grid, a battery storage system is usually unnecessary. It can, however, substitute for a back-up generator, assuring that you’ll have power and heat during utility company outages. That means your small business can have reliable refrigeration in hot-weather brown-outs, and you can operate a fan-driven furnace when an ice storm takes out power lines. GS

This article is adapted from the Solar Energy Resource Guide 2008, published by the NorCal Solar Energy Assciation (norcal solar.org), a chapter of the American SolarEnergy Society.