Solar cells (also called photovoltaics) absorb sunlight and convert it directly to electricity. Solar cells are very thin (about 1/100^th of an inch thick). Most are rectangular or circular wafers made of silicon (sand), but some consist of a thin film that is mounted on glass or thin metal.

When sunlight hits the cell, electrons are released. The electrons then flow onto wires, forming direct current (DC), which is the same kind of current that flows from a regular battery. A four-inch silicon cell can produce about one watt of DC electricity.

A number of cells (usually 20 or more) or a film can be mounted within a frame under a transparent glass or plastic covering to form a module. Modules can be connected to other modules to form an array. The larger the area of photovoltaics in each module, or the more arrays that are used, the more electrical power you will have.

Solar modules can be free-standing units, but there are also building-integrated solar products, such as solar roof shingles and opaque glass photovoltaic facades. When these products replace conventional building materials, it reduces the net cost of incorporating solar electricity into a building.

The DC power from solar cells can be used directly to operate many household items. Alternatively, using a device called an inverter, the DC current can be converted to alternating current (AC) for standard household appliances. At present, solar cells turn between 12 and 20 percent of the sunlight that hits them into electricity. New cells have been tested at much higher efficiencies and they should be available in the near future.

There are two types of photovoltaic systems: (1) stand-alone systems and (2) systems that are connected to the electric power lines of the utility grid. Stand-alone, or independent, systems are especially well matched to settings far from electric power lines. They also work well with portable road signs and other devices that are used in locations where it would be costly or inconvenient to connect them to the utility grid.

Homeowners with stand-alone systems are completely independent of the utility gird, relying on their own power systems to meet all their electricity needs. They connect their solar cells to batteries that store electricity for use when the sun is not shining. In some physically isolated settings, it can cost a homeowner $10,000 or more to connect to the utility grid, so a solar electricity system can be quite cost effective.

Most people in the Northeast who purchase a solar system will want to choose a utility-connected one. In these cases, the electricity from the system supplements what is available to the building from the electric utility. When the solar cells do not provide sufficient electricity for the building's users, extra electricity is supplied by the utility and the buildingÕs electric meter runs forward to record that extra electricity used. But at times when the solar cells produce more than enough electricity for the buildingÕs users, the additional power is fed back into the utility grid and the buildingÕs electric meter runs backwards, recording the "sale" of the electricity to the utility. This arrangement, in which the electric meter runs both forward and backward, is called net metering. In the Northeast, net metering is supported by legislation in all New England states, Maryland, New York, and Pennsylvania.

Photovoltaic systems, whether grid-connected or grid-independent, can also be configured to be either fixed or tracking systems. Fixed systems are mounted at a set angle in such a way that the angle cannot be adjusted. The fixed angle is selected based on geographic location. In the Northeast, the optimal angle for a fixed system is typically the locationÕs latitude or latitude minus 15 degrees.

Tracking systems come in either single-axis or two-axis designs. Single-axis trackers move throughout the day, following the path of the sun across the horizon. Two-axis tracking systems not only follow the path of the sun throughout the day, but they also adjust their horizontal angle throughout the year in response to the position of the sun in the sky as it changes from season to season. Although tracking systems generally produce more energy, they are more expensive and require more maintenance than fixed angle systems.

The photovoltaic industry has achieved both impressive improvements in solar cell efficiencies and significant cost reductions. TodayÕs photovoltaic cells achieve efficiencies between 12 and 20 percent, well above what they were just ten years ago. The price of photovoltaic panels has declined from $100/watt in the 1970s to the current price of approximately $3.00/watt. The global photovoltaic industry is expanding rapidly; global manufacturing of solar cells stood at 58 megawatts (5,800,000 watts) per year in 1992 and is now approximately 153 megawatts per year. Analysts believe that the photovoltaic industry will continue to see impressive gains in efficiencies and cost reductions as economies of scale come into play with larger production facilities.

Although the current market for photovoltaic technology is primarily in consumer products and remote power applications, the grid-connected market is the fastest growing market segment. Countries like Japan and Germany have programs similar to the United States Department of EnergyÕs Million Solar Roofs initiative, which is helping to expand grid-connected markets. However, the price of electricity produced from solar cells is still significantly more expensive than it is from fossil fuels like coal and oil, especially when environmental costs are not considered. The competitiveness of solar-generated electricity in grid-connected applications is largely a function of electricity rates, which vary from region to region across the country. Given the high cost of power in the Northeast, photovoltaics will likely become cost-effective in grid-connected applications in the region before it does in other parts of the nation. In fact, given the high cost of power during the peak demand periods during the summer as well as the available sunshine, photovoltaics can be cost-competitive for utility companies today, especially in areas where photovoltaic installations can defer or avoid costly upgrades to the transmission and distribution system.

Of all the renewable energy sources available, solar cells have the smallest environmental impacts. Electricity produced from photovoltaic cells does not result in air or water pollution, deplete natural resources, or endanger animal or human health. The only potential negative impacts are associated with some toxic chemicals, like cadmium and arsenic, that are used in the production process. These environmental impacts are minor and can be easily controlled through recycling and proper disposal.

• The Energy Efficiency and Renewable Energy Network of the US Department of Energy (www.eren.doe.gov).
• The Solar Energy Industries Association (www.seia.org).
• The National Renewable Energy Laboratory (www.nrel.gov).
• The American Solar Energy Society (www.ases.org).
• The Interstate Renewable Energy Council (http://irecusa.org) 

  Financing Information

  • Database for State Incentives for Renewable Energy (DSIRE)
   

   

Excerpted from Photovoltaics: Basic Design Principles and Guidelinesby the US Department of Energy’s Energy Efficiency and Renewable Energy Network

A photovoltaic (PV) system designer can conduct a detailed site assessment for you. But you should first consider these three factors:

Systems installed in the United States must have a southern exposure. For maximum daily power outlet, solar modules should be exposed to the sun for as much of the day as possible, especially during the peak sun hours of 10 a.m. to 3 p.m.

The southern exposure must be free of obstructions such as trees, mountains, and buildings that might shade the modules. Consider both summer and winter paths of the sun.

Choosing a PV professional will be one of your most important decisions. If you choose a competent dealer, you won't need to know all the details of designing, purchasing, and installing your PV system. Instead, you can rely on the dealer's expertise to design and install a system that meets your needs. However, just like buying a car or a television, you must have confidence in the dealer's products and services and be an informed consumer.

Professional credentials are one indication of a PV dealer's knowledge and qualifications. Ask dealers what PV-related courses they have taken, certifications they have earned, and licenses they have received.

A second consideration is the dealer's experience in the field. How long has the company been in business? The local Better Business Bureau can advise you whether any customers have registered complaints about the dealer. You should also ask the dealer how many systems like yours he or she has designed and installed. Ask to see installations, and talk with owners of systems similar to the one you want to purchase.

A third consideration in selecting a system installer is the variety and quality of products offered for each component of the system. Because PV systems are often designed for a specific site, one company's products may not be appropriate for all applications. A variety of product options will help ensure that the most appropriate components are available for your system. When a dealer recommends a product, ask what the recommendation is based on, whether there are consumer or independent testing facility reports you can read, and whether the products are listed with Underwriters Laboratories (UL).

Fourth, consider the service agreements and performance guarantees the dealer provides and the warranties given by the product manufacturers. No system is maintenance-free, nor will all components function flawlessly forever. When problems emerge with your system, what services will the dealer provide? What warranties do the manufacturers provide? What costs should you expect to pay, and which costs will be assumed by the dealer and/or the manufacturer?

• Search for installers in the Sustainable Yellow Pages