Solar photovoltaic (PV) technology is a convenient way of generating electricity directly from sunlight in a clear environment. Sunlight may be directly converted into electricity through the photovoltaic effect. These solar cells were primarily developed for providing power supply for space satellites in 1950s.

Typical Solar Cell

A typical solar cell consists of two very thin layers of material. The lower layer has atoms with single electrons in the outer orbital, which are easily lost.

The upper layer has atoms lacking one electron from their outer orbital and hence can readily gain electrons. The kinetic energy of light striking this “sandwich” dislodges electrons from the lower layer, which are trapped into the upper layer, thus creating an electric potential between the two layers.

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This potential provides the electric current through the rest of the circuit and which connects the upper side through a motor or other electrical circuit, which connects the upper side to the lower side. Thus, without any moving parts, solar cells convert light energy directly to electrical power. Their current life span is 20 years because they deteriorate due to exposure to weather.

Functioning

Semiconductor crystals can be made in such a way that one region is n-type and an adjacent region, p-type.

The charge separation is developed across the junction between them. P-type silicon conducts +ve charges while n-type silicon conducts -ve charges when light falls on the p-n junction, electrons from the valence band are promoted to the conduction band, thereby generating electron-hole pairs on both sides of the junction.

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Since the p-n junction is a barrier to both types of charge carriers, concentration of holes builds up on the p-side of the junction and a concentration of electrons builds up on the n-side of the junction. When these concentrations exceed the equilibrium concentration of the carriers in the corresponding parts of the semiconductor, a drift of holes towards A and a drift of electrons towards B takes place.

Therefore if, A and B are connected with a conductor, there occurs a flow of current from A to B and flowing of electrons from B to A.

Uses

1. The solar cells made of silicon have been used as a source of power in spacecraft and| satellites.

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2. A part from the electrical energy derived from solar energy by using solar cells cant stored by Charging Ni-Cd batteries or lead-acid batteries. Alternatively it can be used produced hydrogen by electrolysis of water.

3. Panels of solar cells are the most economical way of providing power at points that are far from utility lines, e.g., rural homes, light houses, irrigation pumps, radio- transmitters, offshore oil drilling platform, traffic signals etc.

4. The Japanese are developing solar powered airconditioner to be marked soon.

Limitation

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The silicon cell produces electricity reliably, but is quite expensive since very high-grade crystalline silicon is required. The huge capital cost is the major limitation for the large-scale of solar cells.

Uses

Photovoltaic systems are being used for small loads such as lighting, railway signaling, water pumping, and solar communications. Over 40,000 systems are deployed in almost; the states and Union territories of India.

Solar Pond

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It is possible to generate electrical power through the thermal route. Organic Ranking cycle can be used for this purpose. It is based on parabolic through concentrators.

Functioning

In this method, pioneered in Israel, an artificial pond is partially filled with brine and fresh water is placed over the brine. Brine, being denser than fresh water, remains at the bottom without any appreciable mixing. Sunlight passes through the fresh water but is absorbed and converted to heat in the brine. This fresh water then acts as an insulating blanket and holds the heat in.

The hot brine solution can be circulated through buildings for heating. Alternatively, it can be converted to electrical power by vaporizing the fluids with low boiling points and using the vapors to drive low-power turbo generators. Since the solar ponds also act as an efficient heat storage unit, it supplies power continuously.

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Solar Cooker

Heat generated from solar radiant energy can be used for cooking food. This energy is converted into thermal or heat energy with the help of solar collectors. A solar collector is nothing but an absorber made of highly heat conducting materials like copper, aluminum, steel etc.

These are coated with black coloured materials like black paint, oxides of certain materials such as chromium, copper, cobalt etc. These black-coating materials should have high absorption in the visible region of the solar spectrum, low emission at high temperature in the infrared region, high stability, low cost of production etc.

The temperature to be attained depends, apart from the thermal properties of these receivers the concentration of solar radiation falling on them. The collectors should be in the form of a flat surface.

The other components of a flat plate solar collector are insulating materials to reduce heat losses from the absorbing surfaces, front with highly transparent glazing’s to avoid contact of the absorber and the surrounding air, heat transfer materials like water, oil, air or any other fluid to be heated.

Outer box of the solar cooker is to enclose the absorber and insulating material and support to front glazing etc. In the case of concentrators, the reflecting mirrors with appropriate support structure and tracking mechanisms are the main components apart from the solar receiver.

Limitations

1. These have always to be kept in the open where sunlight is available during the cooking time.

2. In box type solar cookers, Indian chapattis cannot be prepared, neither are they meant for deep-frying.