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APPLICATION (S): Photovoltaic

DESCRIPTION:  Solar radiation can be converted directly into electricity using a semiconductor device known as a photovoltaic (PV) or solar cell. Silicon is the material most commonly used to make PV cells. PV cells make use of what is called photovoltaic effect in converting light energy to electrical energy. By diffusing phosphorus or boron into the silicon p- and n-type silicon can be created, each with its own electrical characteristics. A thin silicon wafer is divided into two layers. Both layers are provided with metallic contacts. When sunlight falls upon the solar cell some of it is absorbed producing a photo-voltage or potential difference between the layers as it releases electrons inside the silicon.

Light of certain wavelengths ionize the atoms in the silicon. The internal field produced by the junction separates some of the positive charges (“holes”) from the negative charges (electrons) within the photovoltaic cell. The holes are swept into the positive or p-layer and the electrons are swept into the negative or n-layer. Although these opposite charges are attracted to each other, they only recombine by passing through an external circuit outside the material because of the internal potential energy barrier. The size of the current depends upon the intensity of the incoming radiation. Not all the energy of the light is converted into electrical energy.

The amount of power available or the efficiency of the PV device is determined by;
•    The type and area of the material;
•    The intensity of the sunlight; and
•    The wavelength of the sunlight.

Currently single crystal silicon solar cells only converts a maximum of 25% of the solar energy it receives into electricity. This is because the radiation in the infrared region of the electromagnetic spectrum does not have enough energy to separate the positive and negative charges in the material.

Polycrystalline silicon solar cells have an efficiency of less than 20% at this time and amorphous silicon cells, are presently about 10% efficient, due to higher internal energy losses than single crystal silicon.

Because of the high cost of extending electrical transmission and distribution lines, photovoltaic systems are often used to power remote electrical devices for the following purposes communications, cathodic protection, battery charging, warning signals, lighting, refrigeration, remote switching monitoring and water pumping.





i.    Article(s):
ii.    Publication(s)
?    Renewable Energy Technologies: A review of status and costs of selected technologies. 1994. A world Bank Technical Paper number 240
iii.    Internet site(s):
?    The Australia Renewable Website (Austria Greenhouse Office)
?    EREN Website


i.    Capital or initial costs:
ii.    Operation and maintenance cost:
?    USc .05/kWh for small systems (1990)
?    USc .039-1.4/kWh for utility scale systems (1990)
iii.    Fuel energy costs: USc 5 – 15 kWh (projected 2000)




ORGANIZATION (S) PROMOTING TECHNOLOGY: US Department of Energy, International Energy Agency (IEA)

Category/ies:Solar Tech.
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