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Photovoltaics: A Question and Answer Primer

Ingrid Melody

Of all the solar energy technologies, photovoltaics (PV) show the greatest promise for worldwide acceptance and application. Their universal appeal lies in the fact that they generate electricity from the sun. Working photovoltaics have no moving parts, are relatively simple in design, need very little maintenance, and are environmentally benign. They simply and silently produce electricity whenever they are exposed to light.

photovoltaics.

Q:What are photovoltaics?

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 of Florida's coast. The cells are wafer-thin 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 semiconductor material, such as silicon, liberate electrons from 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.

Q:How are photovoltaic cells made?

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. 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. For example, the Experimental Photovoltaic House at the Florida Solar Energy Center has 670 square feet of photovoltaic array on its south-facing roof. That array has produced 800 kilowatt hours of electricity each month for more than four years, providing 75 percent of the power needed in the building.

Q:Can PV modules power regular appliances?

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.

Q:Why aren't PV modules in widespread use?

Photovoltaic modules are currently too expensive to be cost-competitive with readily available utility power. However, PV costs are decreasing. When the first photovoltaic systems were used by NASA to power orbiting space satellites, the costs were as high as $1,000 per peak watt. (Peak watt is the amount of electricity produced by a PV cell when bright sunlight is available.) An individual can now purchase modules for $7 to $12 per peak watt. When photovoltaic module costs are reduced to about $1 per peak watt, they will be competitive for electricity production in residential settings. At that price, an installed PV system large enough to provide substantial amounts of residential power would cost about $10,000 ­­ a great deal of money, but not too much to pay for a power system with at least a 20-year life span and a probable payback time of about 10 years.

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

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. Some experts predict these problems will be solved by the early 1990s. More efficient cells also will help to lower the costs somewhat. The limit of efficiency for silicon PV cells is estimated to be about 25 percent. As they currently are manufactured, most PV cells operate at about 10 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.

Q:What research is being conducted on photovoltaic technology?

Presently, photovoltaic research is focused on two areas ­­ manufacturing and applications. Within the area of manufacturing, both methods and materials are being explored. Scientists are investigating the use of multicrystal and noncrystal silicon in PV cells. Semiconductor materials other than silicon also are receiving attention. Manufacturing methods being researched include new ways of purifying silicon to "solar grade," better methods of slicing cell wafers from silicon ingots, and more efficient production of cell material by casting it into blocks, drawing it out into ribbons or sheets, or depositing a thin film of the material on an inert base. Research on photovoltaic applications is both regional and national in scope. The U.S. Department of Energy has funded research centers in the Northeast, Southwest, and Southeast to study the application of photovoltaic power systems in these very different regions. The Florida Solar Energy Center operates the Photovoltaic Southeast Regional Experiment Station (SE RES). Using several prototype residences and test facilities at its Cape Canaveral site, along with many operating photovoltaic installations throughout the region, SE RES is investigating the amount and quality of power produced by both fixed and tracking PV systems, the effects of such systems when connected to utility grids, and the best PV system designs.

Q:What are the current uses of photovoltaics?

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.

Q:What future applications of photovoltaics are anticipated?

When system costs are reduced, several options will be feasible. Residences, such as the one at the Florida Solar Energy Center, may have their southfacing 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. Federal legislation has already been enacted to allow for such grid-interactive residential power systems. 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.

Q:What other issues must be resolved before residential PV systems become the norm?

Widespread residential use of photovoltaics will affect many sectors of society. Some will have to undergo significant change to allow for smooth incorporation of the new technology. Many of these issues are addressed in the SE RES program. The utilities will be greatly affected. Gridconnected PV systems will have to be designed to provide power compatible with that of the grid. Monitoring systems will need to be designed to measure system performance and its effects on the utility. Safety measures will need to be established to protect utility personnel and equipment. Times of peak power needs will have to be identified and utility power production adjusted to meet those needs. Equitable rates will need to be maintained for the purchase and sale of electricity by the utility and the PV system owner. Local governments will need to review their zoning regulations, assuring that PV-powered homes will have unobstructed access to sunlight; shade from adjacent buildings could render a roof-mounted photovoltaic system ineffective. The construction industry will have to determine and implement design and building techniques that will provide enough south-facing, correctly tilted building surface to make the maximum use of PV systems. In addition, they will have to design and build energy efficient homes that are integrated with the PV systems. To protect the public health and safety, the photovoltaic industry will have to meet minimum equipment standards, and PV installers will need to be correctly trained and licensed.

Q:What other sources of information on photovoltaics are available?

The following publications provide more detailed general information on photovoltaics. In photovoltaics, as in all developing technologies, new information and publications are constantly being produced. These are often announced and advertised in solar trade periodicals.

Selected References on Photovoltaics

Footnotes

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