SUN SOLUTIONS

Solar Electricity

Introduction
Solar electricity is generated when light falls on specially designed devices, usually made from silicon based materials. These devices, known as photovoltaic (PV) cells, convert the photon energy of light into the electron energy of electricity. The sun offers tremendous potential for use as a fuel on earth.

The United States receives more energy in the form of sunlight in less than 40 minutes than from all the fossil fuels we burn every year.

The Technology
Though today's PV industry is actually a product of twentieth century solid state physics, it's beginnings can be traced back to 1839. In that year a Frenchman named Becquerel observed that light falling on certain materials created a small electric charge--in other words, it created electricity. He further discovered that the amount of electricity produced varied with the amount of light and intensity. This process is called the photovoltaic effect--photo for light and voltaic for voltage.

To increase the power of PV cells, they are usually wired to other cells. Twenty or more are typically packaged together with a transparent cover such as glass or plastic and encased in a water-tight seal to form a module or panel. Those panels can then be wired together to provide enough power for almost any need.

A variety of semiconductor materials are used in making photovoltaic cells. Typical materials include single and polycrystalline silicon, thin-film amorphous silicon and several advanced technology materials. In addition to having the proper physical properties to covert solar to electricity, silicon is an excellent choice for PV cells because it is so abundant--it comprises 28% of the earth's crust and its main component is sand.

During the past few years, the efficiency of PV cells has increased tremendously and laboratory tests have converted more than 35% of the sunlight to energy using concentrators or solar cells. New cell materials are more reliable and efficient, and offer promise of continued efficiency improvements.

Types of PV Systems
In general, there are two types of PV systems: stand-alone and utility connected. Because current PV costs for uses in conjunction with utility systems run four to five times higher than current electricity costs, most PV applications today are in stand-alone systems in areas where electric power is not readily available. However, many utilities now are exploring the use of PV because of other benefits such as pollution-free operation.

As its name implies, a stand-alone system provides power without any other power source. In many cases, PV panels charge batteries to provide storage of the power. There are thousands of stand-alone systems around the world today used for a variety of applications.

Telecommunications and water pumping are two very common uses of photovoltaics. This power is used for microwave repeaters, mobile radios, satellite ground stations, telemetry, television and other communications systems. Another popular use today involves portable communications units for the military. PV panels are used to power pumps which deliver water for humans and animals to drink or for crop irrigation.

There are dozens of other uses for PV power that are cost effective and reliable like indoor and outdoor lighting systems (bus stops or buildings or national park camp sites), rust and corrosion protection (to protect oil pipelines and bridges), navigational aids (signals to direct ships at night or in dangerous waters), transportation aids (emergency telephone systems on highways, traffic signs and signals in rural or remote areas), and power consumer products (such as calculators, watches or radios).

In utility-connected systems, the electric company acts as the backup power source. For example, PV panels on a building can be used to provide electricity for lights, run appliances or to charge batteries that provide electricity for a building at night. However, the PV system also is connected to the local electric company. If the PV panels produce more electricity than the building needs, the extra electricity is sold to the electric company. A 1978 federal law called PURPA--the Public Utilities Regulatory Power Act--requires that utilities buy privately produced power. When the PV cells are not producing enough power, such as during very cloudy days, PV cells still produce power due to the fact that some light still gets through (that's why you can still see on cloudy days).

PV cells also are used in concentrating systems. Optical concentrators, such as a parabolic trough, can increase the amount of sunlight striking a PV cell by up to 1000 times.

Economics
There are many thousands of homes in the United States that use photovoltaic cells for power. Most of these homes are in remote areas, and the costs of these PV systems are equal or less than the cost of running power lines from the local electricity company. Because PV cells produce DC power, a complete system for residential use also can include an inverter that changes the DC current to AC which is the type of electricity needed to operate standard home appliances. In most cases, residential systems also include batteries to store power for up to five days of bad weather.

Many people use photovoltaics to power solar calculators. Now a wide range of products are available from PV-powered vent fans for the attic, fence chargers for farmers and gardeners and walkway, security and patio lights for homes and businesses. They are cost effective because in most cases they are less expensive than running power wires to these products.

There are also a number of large PV systems being reviewed by utility companies. They are exploring several ways to use photovoltaics. Among the most cost-effective applications are stand-alone systems where the utility can avoid stringing wires or installing transformers for small loads. Systems such as warning lights on transmission towers, environmental monitoring, data collection and sectionalizing switches are among those most frequently used.

Other options utilities are considering include supplementary power at overloaded substations, and rooftop systems on commercial and residential buildings that experience a daytime peak in their demand during the summer. For many of these systems, the benefits of using PV may well justify today's system costs.

The Sun Solutions brochure also contains information on solar water heating, solar pool heating, solar thermal power, passive solar energy and energy conservation. If you'd like to view the entire document, click here.

This document was developed by:

Solar Energy Research and Education Foundation
Solar Energy Industries Association
1616 H Street NW, #800
Washington, DC 20006
Phone: (202) 628-7745
Fax: (202) 628-7779