Photovoltaic Cell | Research & Encyclopedia Articles

Photovoltaics is the direct conversion of sunlight into electrical energy using solar cells. All energy on earth is received from the sun through its electromagnetic spectrum. At any one instant, the sun delivers 1,000 watts (kw) per square meter to the earth's surface. Most of this energy is absorbed as heat by the lithosphere (soil), hydrosphere (water), and atmosphere but photovoltaic cells (PVC) are capable of converting it into a non-polluting, ecologically sound, and dependable source of electrical power. Although the photovoltaic effect was discovered 150 years ago, economically viable applications were not possible until the recent development of efficient semiconductor material and processing methods.

The physics of converting sunlight into electricity is simple. Most photovoltaic cells are a standard negative/positive type with attached leads. The negative terminal lead is soldered on the light sensitive side of the cell, and the positive lead is attached to the back side of the cell. When simply exposed to light, each cell produces about the same voltage between the two terminals. But if the cell is exposed to light when a load such as a discharged battery or an electric motor is connected between the two terminals, the voltage difference causes a flow of electrons. This current is caused by the formation of hole-electron pairs by the absorbed light photons, and the amount of current is dependent on the amount of absorbed light, which is dependent in turn on the incident light intensity and the surface area of the absorbing photovoltaic cell.

There are two main types of solar cells: thick-film cells with a thickness greater than 25 microns of crystalline silicon and thin-film cells with a thickness of less than 10 microns. Thin-film cells are made of various materials, including amorphous silicon and copper indium diselenide, and by combining several varieties of these in tandem, each with unique absorbing characteristics, the solar flux can be more efficiently utilized. Because these cells use less material, they are less costly to produce and will probably replace thick-film cells.

Both thick-film and thin-film cells are classified by the materials from which they are made—as crystalline (a wafer sliced from a large ingot), amorphous (The condensed gaseous form of a semi-conductor material such as silicon), and polycrystalline. Material combinations known as compound semiconductors have been investigated in the 1990s. These are cell materials whose active layers are comprised of various semiconductor materials, such as gallium arsenide, copper sulfide, and cadmium telluride.

To increase voltage, multiple cells are connected in a series by attaching the positive lead of one cell to the negative lead of another. The series most commonly used for both commercial and domestic applications is known as a module and usually consists of 36 cells. One or more modules can be connected directly to a load, such as a battery, a water pump, or an exhaust fan. A typical photovoltaic system consists of the modules, a storage battery, a charge and voltage regulator, and a suitable load. Some compact high performance modules are designed to charge 12-volt batteries or directly power a 12-volt DC motor.

Photovoltaic modules may be installed on a standard ground mount or on a tracker. When installed on a standardground mount, the modules can be adjusted from 15–65 degrees at 5 degree increments. Ground mounts can support two to eight modules. The tracker utilizes a variable thermal expansion of gas, due to the changing solar exposure, and actually follows the sun at approximately 15 degrees plus latitude valve. To maximize efficiency, several modules are mounted on a tracker supported by a single pole.

The net cost per kilowatt hour is the most important factor in the future of photovoltaic cell production and application. In industrialized countries, the economic viability of this form of solar energy is determined by their cost relative to competitive energy sources, particularly fossil fuels and nuclear power, and the environmental impact of each source. The world market price for solar cells in 1993 was four dollars a watt, based on rated output, and prices are expected to drop 50% by the end of the decade, when most photovoltaic cells will be manufactured with thin-film technology.

Decentralized single dwellings, cattle ranches and tree farms remote from electric power lines, and small villages with limited power demands are one of the three market segments where photovoltaic cells can be utilized competitively. The consumer and leisure market is another, and solar cells are already widely used in boats, motor homes, and camp sites, as well as in calculators and other electronics. The third market is in industrial applications such as offshore buoys, lighthouses, illuminated road signs, and railroad and traffic signals.

The market for photovoltaic cells is increasing at an overall annual rate of about 10%. Current worldwide demand is estimated at about 100 megawatts of electrical power, and predictions for the year 2000 estimate a demand of several times this size. Growth is the fastest in the remote market; rural consumer applications are increasing at an annual rate of approximately 35%. There are millions of people around the world who are not served by electric utilities due to their remote location and the high cost of electrical transmission. These populations generally depend on a 12-volt automobile battery powered by a generator for their electrical needs, which include water pumping, lighting, and radio and television reception. The initial cost of a photovoltaic kit offsets the cost of owning and operating a generator within three years. As the life span of a module is usually 15 years, this option is much more economical in many of these situations.

Prior to 1989, the largest manufacturer of photovoltaic cells was ARCO Solar, a division of Atlantic Richfield Oil Company. In 1989, Siemens Solar Industries (SSI), which already had a joint manufacturing enterprise in Munich, Germany, with ARCO, had purchased ARCO Solar. SSI has manufacturing plants in Camarillo, California, and Munich, Germany.

Converting sunlight into electricity with photovoltaic cells is a versatile and simple process. Unlike diesel or gasoline generators, all-weather modules have no moving parts to wear out or break down, and they produce electricity without contributing to air pollution. Solar cells do not produce any noise and they do not require alternating-current power lines, since photovoltaic electricity is direct current. Maintenance is minimal and requires little technical skill; systems are easy to expand and there are no expensive fuels to purchase on a continuous basis. Photovoltaic cells are a cheap and dependable source of power for a variety of uses.

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