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K & C Stork Solar Power Consultants

About Solar Cells PDF Print E-mail

What Are Photovoltaics?
Photovoltaics are solar panels that convert sunlight directly into electricity. They should not be confused with panels used for solar water heating. Solar hot water systems are better Known as thermal collectors and usually circulate water or other liquid or gas through the collectors to be heated directly by the sun.

How Do Photovoltaics Work?
Photovoltaic cells are usually made from layers of crystalline silicon doped with phosphorous and boron. Phosphorous has more electrons than boron, so the doping produces one crystal with more electrons than other which are called N-type or P-type silicon respectively, When brought together, charged particles (electrons) move from one layer to the other. This creates an electric field between the layers and is called P-N junction. When energy particles in the form of photons (sunlight) strike electrons near the junction, electrons are released (photoelectric effect). The electric field at the P-N junction makes these electrons move across the border between the layers, since an electrical current is made of moving electrons, it can be said that light is converted directly into electricity.

Types Of Solar Cells
There are generally three types of photovoltaic cells available. Some cells may have advantages over others in certain circumstances. Conventional solar panels have 36 cells, which produce a maximum open circuit voltage output of 21 volts, a maximum power voltage of around 17 volts, and a nominal charging voltage of 14-15 volts to make them suitable for charging a 12 volt battery. This is the type of panel that should be used for most battery systems such as for home power.
Another type of panel, usually called self-regulating, is constructed with 30 or 32 cells. As a result, they have a lower peak power voltage around 14.5 volts making it harder to overcharge the batteries, However, this means that they do not produce a high enough voltage to allow for voltage losses in many systems. Most solar panels also produce less power when they heat up. With self regulating panels this means your batteries may not be fully charged when the panels are hot.

Undercharging may also be a problem under overcast conditions. Self regulating panels are only really suitable for small installations where low maintenance is a priority or when the battery capacity is very large.

  • Mono crystalline Cells
    Mono crystalline cells are cut from large single crystal ingots grown from molten silicon. The crystals are usually grown to about 10-15 cm in diameter and 1 metre in length. Once formed, it is sliced into waters about 0.2-0.8 mm thick, layered to form P-N junctions and printed with collecting wires. These cells require to be thick enough to trap all the photons.
    Generally these cells have the highest efficiencies with commercially produced mono crystalline panels having efficiencies between 12-17%. Laboratory cells can have efficiencies over 24%. However they require more energy to manufacture though the energy payback period is usually within 5 years.
  • Poly crystalline Cells
    Poly crystalline silicon is made by casting an ingot of silicon, resulting in many small crystals pieced together. These are cheaper to manufacture since it is easier to grow little crystals than larger ones. A disadvantage of poly crystalline cells is that the boundaries between the tiny crystals tend to trap electrons and act as barriers to electron movement or provide a path for electrical shorts across the cell. Manufacturers must ensure that the crystals are large enough for photo-generated electrons to be collected by the p-n junction and printed cell grid before they reach a crystal boundary. Efficiencies of 12% are normal, although research cells have reached 21 %.
  • Amorphous Cells
    Single and poly crystalline cells are produced from large blocks and then cut into wafers. However the only active part of a photovoltaic cells is the region near the p-n junction, a few millionths of a centimetre thick. Since it is impossible to cut anything this thin, much of the silicon in the cell is wasted. Amorphous cells use techniques such as the condensation of gaseous silicon to make cells whose thickness can be measured in numbers of atomic layers. The atoms in these layers are arranged randomly, and the cell is called an amorphous thin film cell. Though these cells are inexpensive, abandoning the crystal structure reduces their efficiency. About 12% is the best that has ever been achieved for multi-layer cells with average single layer efficiencies around 10%.
  • Using Solar Energy
    Until recently, solar power in conjunction with generators and wind turbines have been used mostly by people in remote areas who did not have access to the main electricity supply. Photovoltaics in these situations are often easier and cheaper to install than an extension to the grid, and besides, who wants to look at "sticks and wires" on your land! However, in recent years, people around the world have connected such systems to the existing main electricity grid. Most people install grid connected systems for environmental reasons and for independence of supply. Other reasons for installing such systems are for increasing the value of the home, and reducing their power bills accordingly. People who install these systems usually become aware how much energy they really use, and themselves become more energy efficient too.

    Some Interesting Solar Facts
    * One kW of photovoltaic energy can save one tonne of CO2 emissions every year.
    * The Mars buggy explored the red planet - over 200 million Km from the sun - using solar power.
    * The first applications of solar power were satellite systems.
    * The French physicist Becquerel "discovered" photovoltaics in the 19Th century.
    * Albert Einstein's Nobel Prize was given in part for his work on the photovoltaic effect.
    * Bell Laboratories made the first commercial silicon cell in 1954.
    * The sunlight hitting the earth's surface in one hour is enough to supply man's entire energy needs for a whole year.