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Solar Power

Solar cells (also called photovoltaics or "PV") absorb sunlight and convert it directly to electricity.

Solar cells are very thin (about 1/100th 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 electric current that flows from a battery. A four-inch silicon cell can produce about one watt of DC power.

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. More power can be generated by using larger surface area modules, and multiple arrays. 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, most solar cells turn between 12 and 20 percent of the sunlight that hits them into electricity. New cells have been tested at higher efficiencies.

The Two Types of Systems

There are two types of PV 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 ideal for 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.

In systems connected to the utility grid, the electricity from the PV 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 power demand, extra power is supplied by the utility and the building's electric meter runs forward to record that extra electricity used. But at times when the solar cells produce more than building's power demand, the additional power is fed back into the utility grid and the building's 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." Most states have statutes rules in place to regulate net metered systems.

Grid-Connected Photovoltaic System

Economics and Future Prospects
The PV industry has achieved impressive improvements in solar cell efficiencies and significant cost reductions. PV cells today achieve efficiencies between 12 and 20 percent, well above what they were just 15 years ago. The price of PV panels has declined from $100/W in the 1970s to the current price of approximately $3.00/W. The global PV industry is expanding rapidly; global manufacturing of solar cells stood at 58 megawatts (58,000,000 watts) per year in 1992 and has risen to over 1,600 megawatts (1,600,000,000 watts) per year in 2005 – an increase of almost 30% per annum over the past 15 years. Analysts believe that the PV industry will continue to see impressive gains in efficiencies and cost reductions as economies of scale come into play with larger production facilities.

Further Information
The US Department of Energy (DOE) conducts extensive research on PVs and carries out programs to advance the implementation of solar electricity. The Department's website includes background information (both general and technical) as well as information about the department's research and programs. There is a section on learning about PVs for teachers and students. www.eere.energy.gov/pv/

The National Renewable Energy Lab (NREL) in Colorado is one of the US DOE's laboratories. It carries out research and development on renewable energy and energy efficiency. NREL's website includes background information PVs, as well as an excellent online collection of photographs. www.nrel.gov