Question: What are Photovoltaics?

Answer: Photovoltaics are solar cells that produce electricity directly from sunlight. They are usually made of silicon-the same material that makes up the common beach sand. The cells are wafer-think circles or rectangles, about three to four inches across.

Solar cells operate according to what is called the photovoltaic effect, ("photo"-light, "voltaic"-electricity). In the photovoltaic effect, "bullets" of sunlight-photons-striking the surface of semi-conductor material such as silicon, liberate electrons from the material's atoms. Certain chemicals added to the material's atoms. Certain chemicals added to the material's composition help establish a path for the freed electrons. This creates an electrical current. Through the photovoltaic effect, a typical four-inch silicon solar cell produces about one watt of direct current electricity.

Question: How are photovoltaic cells made?

Answer: In the most common cell production process, very pure silicon is reduced to its molten form. Through a painstaking and time-consuming process, the silicon is re-formed into a solid, single-crystal cylinder called an ingot. Extremely thin slices cut from the ingot are chemically treated to form photovoltaic cells-sometimes referred to as solar batteries. Wires attached to the negative and positive surfaces of the cell complete the electrical circuit. Direct current electricity flows through the circuit when the cell is exposed to light.

Silicon photovoltaic cells convert sunlight directly to electricity.

For efficiency and practicality, multiple cells are wired together in a series/parallel fashion and placed in a glass-covered housing called a module. The modules themselves can then be wired together into arrays.

PV arrays can produce as much direct current electricity as desired through the addition of more modules.

Question: Can PV modules power regular appliances?

Answer: Photovoltaic modules and arrays produce direct current (DC) electricity. Because most appliances and equipment are designed to be powered by alternating current (AC), PV-produced electricity must be converted. This is accomplished by an inverter.

Most of these solid state devices convert DC current to an AC current compatible with that sent over utility grids. As a result, PV installations may be interconnected with a utility grid, sending power onto the grid whenever there is an excess, and drawing electricity from the utility when sunlight is not available. Most inverters have a fail-safe relay that disconnects the PV system from the utility grid whenever the grid fails, ensuring the safety of utility repair personnel.

Question: Why are PV cells so expensive and how can the cost be reduced?

Answer: Material and manufacturing costs are the two major factors that influence the price of photovoltaic cells. Even though silicon is the second most abundant material on earth, the silicon used for PV cells must be very pure; refining high-grade silicon to remove most of its impurities is an expensive process. In addition, the manufacture of PV cells at present is labor and capital intensive, although methods of automation have been undertaken.

How quickly Photovoltaics become cost-effective depends on whether research resolves these material and production problems.

More efficient cells also will help to lower the costs somewhat. The limit of efficiency for silicon PV cells is estimated to be about 30 percent. As they currently are manufactured, most PV cells operate at about 12 percent efficiency. When the cells and systems can be made to operate at higher efficiency levels, the cost of a system may be lower because fewer cells will be needed to generate the desired amount of electricity.

Question: What are the current uses of Photovoltaics?

Answer: Many remote uses of Photovoltaics are cost-effective and practical now. Photovoltaics are generating power for both on-and off-shore traffic control systems, crop irrigations systems, bridge corrosion inhibitors and radio relay stations. They are also providing electricity to remote cabins, villages, medical centers and other isolated sites where the cost of Photovoltaics is less than the expense of extending cables from utility power grids or producing diesel-generated electricity.

Question: What future applications of Photovoltaics are anticipated?

Answer: When system costs are reduced, several options will be feasible. Residences, such may have their south-facing roofs covered with photovoltaic modules, either as an integral part of the roof structure or mounted on supports designed for that purpose.

Such residential PV systems will probably be connected to the utility grid as well as the home. In that way, excess power would be sent onto the grid for credit during sunny periods, and power would be drawn from the utility at night and on cloudy days.

In another option, clusters of homes and businesses may jointly own or share a common photovoltaic array located at a central site. Such centralized installations also could be owned and operated by a utility company. Because maintenance needs are generally low for photovoltaic systems, on-site crews and auxiliary equipment could be kept to a minimum, cutting utility operating costs.