What are Photovoltaics Panels and How they Work ??
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 directly through the collectors to be heated by the sun.
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. However, in recent years, people around the world have connected such systems to the main electricity grid. Most people install grid connected systems for environmental reasons and for independence of supply.
HOW DO PHOTOVOLTAICS WORK ?
Photovoltaic cells are usually made from layers of crystalline silicon doped with phosphorous or boron. Phosphorous has more electrons than boron, so the doping produces one crystal with more electrons than other which are called N-type or Ptype 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.
A stand-alone solar power system usually requires solar panels, inverter, charger, batteries, regulator, mounting and cabling. A backup generator may also be needed. Some inverters will perform charging, regulating and metering functions all together. In some cases where normal 240 v AC electricity is not required, an inverter is not needed. These systems run all appliances on extra low voltage DC power.
A grid-connected system requires:
- solar panels
- mounting and cabling to function properly
3 Types of photovoltaic solar cells
Monocrystalline Cells - Monocrystalline 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 m in length. Once formed, it is sliced into wafers about 0.2-0.4mm thick, layered to form p-n junctions and printed with collecting wires. Generally these cells have the highest efficiencies with Commercially produced monocrystalline panels have 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.
Polycrystalline Cells - Polycrystalline 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 photogenerated electrons to be collected by the p-n junction and grid before they reach a crystal boundary. Efficiencies of 12% are normal, although research cells have reached 21%.
Amorphous Cells - Single and polycrystalline 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%.