SOLAR CELLS

SOLAR ENERGY CONVERSION
Solar energy conversion is the process of conversion of direct sunlight into more useful forms. This solar energy conversion occurs by the following two mechanisms.

1. Thermal conversion
2. Photo conversion

Thermal conversion
Thermal conversion involves absorption of thermal energy in the form of IR radiation. Solar energy is an important source for low- temperature heat (temperature below 1000 C) which is useful for heating buildings, water and refrigeration.

Methods of thermal conversion
1. Solar heat collectors
2. Solar water heater

1. Solar heat collectors
Solar heat collectors consist of natural materials like stones, bricks (or) materials like glass, which can absorb heat during the day time and release it slowly at night.

Uses
It is generally used in cold places, where houses are kept in hot condition using solar heat collectors.

2. Solar water heater
It consists of an insulated box inside of which is painted with black paint. It is also provided with a glass lid to receive and store solar heat. Inside the box it has black painted copper coil, through which cold water is allowed to flow in, which gets heated up and flows out into a storage tank. From the storage tank water is then supplied through pipes.

PHOTOCONVERSION
Photoconversion involves conversion of light energy directly into electrical energy.

Methods of photoconversion
Photogalvanic cell or Solar cell

PHOTOGALVANIC CELL OR SOLAR CELL
Definition
Photogalvanic cell is the one, which converts the solar energy (energy obtained from the sun) directly into electrical energy.

Principle
The basic principle involved in the solar cells is based on the photovoltaic (PV) effect. When the solar rays fall on a two layer is produced. This potential difference causes flow of electrons and produces electricity.

Solar cells consist of a p-type Semiconductor (such as Si doped with B) and n-type semiconductor (such as Si doped with P). They are in close contact with each other.

Working
When the solar rays fall on the top layer of p-type semiconductor, the electrons from the valence band get promoted to the conduction band and cross the p-n junction into n-type semiconductor. There by potential difference between two layers is created which causes flow of electrons (i.e., an electric current). The potential difference and hence current increases as more solar rays falls on the surface of the top layer.
Thus when this p and n layers are connected to an external circuit, electrons flow from n-layer to p-layer, and hence current is generated.

Applications of Solar cells

1. Lighting purpose
Solar cells can be used for lighting purpose. Now a days electrical street lights are replaced by solar street lights.

2. Solar pumps run by solar battery
When a large number of solar cells are connected in series it form a solar battery. Solar battery produces more electricity which is enough to run, water pump, street light, etc.
3. They can be used to produce hydrogen by electrolysis of water. The liberated hydrogen can be used in H2 – O2 fuel cells.
4. Solar cells are used in boilers to produce hot water for domestic and industrial uses.
5. Solar cells are used in calculators, electronic watches, radios and TVs.
6. Solar cells are superior to other type of cells, because these are nonpolluting and eco-friendly.
7. Solar energy can be stored in Ni-Cd batteries and lead-acid batteries.
8. Solar cells can be used to drive vehicles.
9. Solar cells, made of silicon, are used as a source of electricity in space craft and satellites.