Introduction

Photovoltaics

History

• The first photovoltaic effect was discovered in 1839 by Becquerel.
• By the twenties and thirties solar cells made from selenium and copper oxide were available, but they had an efficiency rate of less than 1%.
• In 1954 Bell labs discovered the silicon based p-n homojunction cell that is used now.
• Early on, the cells were mainly used for space applications.

Single-crystal cell

These are the most complicated and expensive to manufacture do to the size of crystal to grow as well as the process of cutting the large crystal into very thin sheets.

• Single-crystal cells are by far the most efficient with a rating of about 23%.
• Another advantage of these cells is that they are the most stable, loosing very little of their efficiency offer time.

Thin-film cell

• Instead of growing or forming crystals of photovaltaic material, cells can be made by directly depositing the material onto a thin substrate. As a result the cells are much thinner and require less material; therefore, more expensive material can economically be used.
• Overall, thin-film cells are inexpensive to manufacture.
• Since a wide variety of material is used to make thin-film cells, the efficiency varies between 7% and 11%, but most are unstable and the efficiency can deteriorate to half the original in a year before leveling off.

Application

• In order for each cell, which only produces 25-30 milliAmps/cm^2 at about .5 volts, to do anything useful, such as charging a battery, the cells are combined in series to form a module resulting in a higher voltage and a weak current. Then the modules are combined to form an array that provides the required voltage and current.
• To protect the modules, the cells are covered by either a plastic or glass covering. These protected arrays can have a life span of twenty to thirty years.
• As far as replacing large power plants with solar power fields, the move has been lethargic at best. During the oil crisis of the seventies, the government gave incentives to investors who financed research and prototype solar fields. This sparked an interest, but as soon as the crisis subsided the programs died out. Since then, support for solarvoltaic power on a large scale has been spuratic. The programs that have been run, mostly in California, have shown that the actual area of land used to produce power from solar cells is the same as any other power supply when taking into consideration the land mined for coal, the safety zones required for nuclear power, and the reservoir required for hydroelectric power. The maintenance required varies depending on the quality of the cells and the location with a danger comparable to that of a window washer. Still the initial cost of setting up the system remains a major set back. The situation is comparable to a car that never needed refueling but cost $200,000. The offer is temping but the initial cost would be hard for many to swallow. But even more inhibiting than the cost of the cells themselves is the cost of the batteries that store the power before the it ever reaches the main power grid. The life expectancy of the current batteries is five to seven years; with a life span of twenty to thirty years, the cells themselves would far out last even the pinkest of percussion bunnies. To conquer this hurdle other storage possibilities are being looked into. Among the possibilities are the following: converting the electricity into hydrogen gas that can be burned, pollution free, at a later time to fuel a steam turbine, using the direct power to compress air in large amounts that can be used to run air generators, to pump water into special hydroelectric facilities, or to improve the quality and cost of batteries that are used for mass storage of electricity.
• Although solar energy is not widely used for mass power, it is being used for products such as calculators where the product is used mainly in lighted areas. Solar cells are becoming increasingly popular to charge batteries in developing countries that do not have the power infrastructure of the United States and Europe. In such countries solar is more reliable and cost effective than diesel generators that require constant maintenance and fuel. Also these developing countries happen to be abundant in sunlight. Simple, cost-effective units can be implemented for each house to provide basic electrical needs like clean drinking water, mobile phone, radio, and television.
• In the U.S., California has taken the greatest interest in solar power, where among their programs includes the installation of a 128 kW covered parking lot at the Sacramento International Airport that uses solar arrays to cover the cars. The power from the parking lot provides enough electricity for fifty homes.
• A home solar power system is now available for %10,000 to %12,000. At that cost the cells would be worth approximately twenty years worth of electrical bills.
• With present technology, solar energy costs around 30 cents per kWh (coal is 6 cents kWh).

Future

• The incentive for a company to produce an inexpensive solar cell that had an efficiency rating of fifty percent or more is definitely there but can it be done? If a cell of those standards were to appear, it could create the spark necessary to make wide usage of solar power as profitable to investors as coal or nuclear power.
• Some cities are now starting to install solar arrays on the tops and sides of buildings to help with their electricity needs.
• The use of concentrator modules to focus large amounts of light, much like direct solar energy does, only onto high quality solarvoltaic modules instead of the heat exchange, has been tested with results around 30% efficient, but it also has similar problems as the direct solar devises.
• A more exotic solar possibility is using solar rays to directly convert sunlight and basic elements into fuel much like a plant does to survive. This however is only a dream as of now.

Bibliography