Photovoltaic cells produce direct current (DC) electricity. About 40 cells are joined together in enclosed, protective casings called modules. About ten of these modules are mounted in one PV panel. These flatplate PV panels can be mounted facing south, or they can be mounted on a tracking device that follows the sun, allowing them to capture the most sunlight over the course of a day. About ten to 20 PV panels can provide enough power for a household; for large electric utility or industrial applications, hundreds of arrays can be interconnected to form a single, large PV system.
Two primary types of PV technologies available commercially are crystalline silicon and thin film. In crystalline-silicon technologies, individual PV cells are cut from ingots of crystalline silicon. In thin film PV technologies, the PV material is deposited on glass or thin metal that mechanically supports the module.
Thin film based modules are produced in sheets that are sized for specified electrical outputs.
Several companies have started integrating PV products into building materials. For example, PV shingles look like traditional asphalt shingles and can be installed by roofers. Other similar technologies are standing seam metal roofs incorporating PV and modules that look like slate roofing materials. Soon to be widely available is glass for windows and skylights that generates electricity. The benefit of these technologies is that they replace building materials that you would buy anyway.
The cost over 20 years will amount to between 20 and 40 cents per kilowatt hour. However, costs will vary quite a bit depending on your location, solar resources, and available subsidies.
In addition to PV modules, the components needed to complete a PV system may include a battery charge controller, batteries, an inverter or power control unit (for alternating current loads), safety disconnects and fuses, a grounding circuit, and wiring.
There are three factors to consider when determining whether your site is appropriate for photovoltaics.
Unlike utility power plants, which produce electricity constantly despite the time of day and year or the weather, the output of PV modules is directly related to time and weather. Where you live will affect the number of PV modules you will need for power, because different geographic regions experience different weather patterns. However, PV systems still produce electricity in bad weather. On cloudy days, they can produce up to 80% of their potential energy delivery; on hazy, humid days, about 50%; and on extremely overcast days, they still produce up to 30%. Seasonal variations affect the amount of sunlight available to power a PV system.
When designing a PV system, be sure your installer obtains data specific to your area. The National Oceanic and Atmospheric Administration began collecting solar data nearly 20 years ago.
The National Renewable Energy Laboratory’s (NREL’s) Renewable Resource Data Center can provide solar resource information, as can the Energy Efficiency and Renewable Energy Clearinghouse. Some state energy offices also have solar data collection programs to assist solar designers. Finally, books are available that contain solar data on most major cities in the United States.
The NREL has a Web site that will walk you through calculating your loads and sizing your PV system to meet your needs.