solar cell

Introduction to solar cell applications

Alongside a variety of consumer products - electronic watches, calculators, power for leisure equipment and tourism - there is an extensive range of applications where solar cells are already viewed as the best option for electricity supply. These applications are usually stand-alone, and exploit the following advantages of photovoltaic electricity:
• There are no fuel costs or fuel supply problems
• The equipment can usually operate unattended
• Solar cells are very reliable and require little maintenance
At the other end of the scale are grid-connected systems which are now being seriously considered to supplement the conventional power generation in many industrialised countries. Although they have yet to become viable on economic grounds, the participation of PV in large-scale power generation is viewed with increasing prominence as a means of halting the adverse environmental effects of conventional energy sources.
Rural electrification

The provision of electricity to rural areas derives important social and economic benefits to remote communities throughout the world. Power supply to remote houses or villages, electrification of the health care facilities, irrigation and water supply and treatment are just few examples of such applications.
The potential for PV powered rural applications is enormous. The UN estimates that two million villages within 20 of the equator have neither grid electricity nor easy access to fossil fuel. It is also estimated that 80% of all people worldwide do not have electricity, with a large number of these people living in climates ideally suited to PV applications. Even in Europe, several hundred thousand houses in permanent occupation (and yet more holiday homes) do not have access to grid electricity.
The economics of PV systems compares favourably with the usual alternative forms of rural electricity supply, grid extension and diesel generators. The extension and subsequent maintenance of transmission lines over long distances of often a difficult terrain is expensive, particularly if the loads are relatively small. Regular fuel supply to diesel generators, on the other hand, often present problems in rural areas, in addition to the maintenance of the generating equipment.

Water pumping

More than 10,000 PV powered water pumps are known to be successfully operating throughout the world. Solar pumps are used principally for two applications: village water supply (including livestock watering), and irrigation. Since villages need a steady supply of water, provision has to be made for water storage for periods of low insolation. In contrast, crops have variable water requirements during the year which can often be met by supplying water directly to produce without the need for a storage tank.
Deep well solar pump in Arizona. Courtesy of Paul Maycock PV Energy Systems

Domestic supply

Stand-alone PV domestic supply systems are commonly encountered in developing countries and remote locations in industrialised countries. The size range varies from 50 Wp to 5 kWp depending on the existing standard of living. Typically larger systems are used in remote locations or island communities of developed countries where household appliances include refrigeration, washing machine, television and lighting. In developing regions large systems (5 kWp) are typically found for village supply while small systems (20-200 Wp) are used for lighting, radio and television in individual houses.
Solar-powered house in Main, USA. Courtesy of Paul Maycock
Professional applications

For some time, photovoltaic modules have proved to be a good source of power for high-reliability remote industrial use in inaccessible locations, or where the small amount of power required is more economically met from a stand-alone PV system than from mains electricity. Examples of these applications include:
Ocean navigation aids: many lighthouses and most buoys are now powered by solar cells.
PV powered navigation aid in Saint Lawrence river. Courtesy of Paul Maycock PV Energy Systems



Telecommunication systems: radio transceivers on mountain tops, or telephone boxes in the country can often be solar powered.
Courtesy of Paul Maycock PV Energy Systems

Remote monitoring and control: scientific research stations, seismic recording, weather stations, etc. use very little power which, in combination with a dependable battery, is provided reliably by a small PV module.
Solar-powered weather station. Photo courtesy of IT Power Ltd



Cathodic protection: this is a method for shielding metalwork from corrosion, for example, pipelines and other metal structures. A PV system is well suited to this application since a DC source of power is required in remote locations along the path of a pipelin

SumPhotovoltaics 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 operation of a photovoltaic cell. (McGraw- Hill Inc. Reproduced by permission.)
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.
See Also
Alternative Energy Sources; Alternative Fuels; Energy and the Environment; Energy Policy
Resources

Photovoltaic Cell
As far back as the 1800s scientists realized that, through certain chemical reactions, sunlight could be converted into electricity. The first experimenter to successfully accomplish this feat was A. E. Becquerel (1820-1891), who built a device that could measure the intensity of light by observing the strength of an electric current produced between two metal plates. Later scientists discovered that the metal selenium was particularly sensitive to sunlight, and during the 1880s Charles Fritts constructed the first selenium solar cell. Fritts's device was woefully inefficient, however, converting less than one percent of the received light into usable electricity.
The Fritts selenium solar cell was mostly forgotten until the 1960s when the drive to produce an efficient solar cell was renewed. It was known that the key to the photovoltaic cell was in creating a semiconductor that would release electrons when exposed to radiation within the visible spectrum. During this time researchers at the Bell Telephone Laboratories had been developing similar semiconductors to be used in communications systems. Quite by accident, Bell scientists Calvin Fuller and Daryl Chapin (who had been working independently on projects unrelated to solar cells) found the perfect semiconductor: a hybridized crystal called a "doped" cell made of phosphorous and boron. The first solar cells using these new crystals debuted in 1954 and yielded a conversion efficiency of nearly six percent. Later improvements in the design increased the efficiency to almost fifteen percent, a high mark by even today's standards.
In 1957 Bell Telephone used a silicon solar cell to power a telephone repeater station in Georgia. The process was considered a success, though it was still too inefficient to penetrate the general marketplace. The first real application of silicon solar cells came in 1958, when a solar array was used to provide electricity for the radio transmitter of Vanguard 1, the second American satellite to orbit the earth. Solar cells have been used on almost every satellite launched since.
Once space exploration had proven their efficacy, photovoltaic cells began to appear more and more frequently in homes. Just about any small appliance can be adapted to run on solar power, but by far the most successful have been watches and pocket calculators. These devices generally use a solar cell to charge a nickel-cadmium battery, so that they can be used even during periods of dim light.
Even in the early days of solar technology scientists envisioned vast photovoltaic arrays that could power entire cities. Those early dreams have been realized to a small extent by many homeowners who have installed solar panels in their homes. The usefulness of these panels is limited, however, by their relatively low rate of efficiency, as well as by the limited hours of available sunlight. It is unlikely that solar power will replace that gleaned from fossil fuels even in the distant future; still, the production of solar energy has increased steadily, approximately doubling each year. At that rate, it is conceivable that, by the year 2000, ten percent of the energy produced in the world will be produced by solar cells.


Health care

Extensive vaccination programmes are in progress throughout the developing world in the fight against common diseases. To be effective, these programmes must provide immunisation services to rural areas. All vaccines have to be kept within a strict temperature range throughout transportation and storage. The provision of refrigeration for this aim is known as the vaccine cold chain.
Mobile solar vaccination cooler. Courtesy of Neste Advanced Power Systems (NAPS)

Lighting

In terms of the number of installations, lighting is presently the biggest application of photovoltaics, with tens of thousands of units installed world-wide. They are mainly used to provide lighting for domestic or community buildings, such as schools or health centres. PV is also being increasingly used for lighting streets and tunnels, and for security lighting.