Renewable energy sources

RENEWABLE ENERGY SOURCES:
Renewable energy sources are natural resources that can be regenerated continuously and are inexhaustible. They can be repeatedly used.

SOLAR ENERGY:
The energy that we get directly from the sun is called solar energy. Nuclear fusion occurring in the sun releases enormous amount of energy in the form of heat and light. Several techniques are available for collecting, converting and using solar energy.

METHODS OF HARVESTING SOLAR ENERGY
Listed below are the various devices used to harvest solar energy.

1. Solar cells or Photovoltaic cells or PV cells: Solar cells consist of a p-type semiconductor (Silicon doped with Boron) and n-type semiconductor (Silicon doped with Phosphorus) in close contact with each other. When solar rays fall on the top p-type semiconductor, the electrons from the valence band move to the conduction band and cross the p-type junction into the n-type semiconductor. A potential difference is created causing an electric current to flow. These cells are widely used in calculators, electronic watches, street lights, water pumps  to radio and television.
2. Solar battery: When several solar cells are connected in series, it forms a solar battery. These solar batteries generate enough electricity to run water pumps, street lights, etc. They are mainly used in remote areas where electricity supply is a problem.
3. Solar heat collectors: Solar heat collectors consist of natural materials like stones, bricks, or materials like glass, which absorb heat during the day time and release it slowly in the night. It is generally used in cold places where houses are kept in hot condition using solar heat collectors.
4. Solar water Heaters: It consists of an insulated box, in which is painted black on  the inside. It is provided with a glass lid to receive and store solar heat. The box contains a copper coil painted in black through which cold water flows in, gets heated in the copper coil and flows into a storage tank. Finally, water from storage tanks is supplied.
5. Solar energy is currently used to power satellites, watches, calculators, etc
6. A few applications of solar energy are: (i) solar water heater, (ii) solar cooker, (iii) solar dryer and (v) solar refrigerator.

WIND ENERGY:
Wind is defined as moving air. Energy recovered from the force of wind is called wind energy. Wind energy is harnessed by the use of wind mills.

1. Wind mills: The  force of blowing wind strikes the blades of the wind mill thereby causing it to rotate continuously. This rotational energy of the blades is used to drive several machines like water pump, flour mill and electric generators.
2. Wind farms: Several wind mills joined together in a definite pattern forms a wind farm. Wind farms  generate large amounts of electricity.

CONDITION The minimum speed required for satisfactory working of a wind generator is 15 kmph

ADVANTAGES:

1. It does not cause any air pollution
2. It is very cheap

OCEAN ENERGY
Ocean can be used for generating electricity:

1. Tidal energy: Ocean tides produced by virtue of gravitational force of sun and moon possess enormous amounts of energy. Tidal energy can be harnessed by constructing a tidal barrage.
   
   During high tide, sea-water is allowed to flow into reservoir of the barrage and rotate the turbine thereby producing electricity.
   During low tide, when sea level is low, sea water stored in the barrage reservoir is allowed to flow into the sea thereby rotating the turbine again.
   
   
2. Ocean thermal energy There is a large temperature difference between surface level and deep water level of tropical oceans. This temperature difference can be utilized to generate electricity. This energy is called ocean thermal energy. A necessary condition is that temperature difference of at least 20C is required between surface water and deep water. The technique used here is that the relatively hot surface temperature of the ocean is used to boil a low boiling liquid like ammonia. The high vapor  pressure of the liquid formed by boiling is used to turn the blades in a turbine and generate electricity. The cool waters in the deep sea are used to cool and condense the vapor into liquid.

Geo-thermal energy:
Temperature of the earth increases at the rate of  20 - 75C per km. Down below the earths surface, high pressure and high temperature steam fields exist in many places. The energy harnessed from high temperature present inside the earth's surface is called geothermal energy.

1. Natural Geysers: In some places, steam comes out of ground through cracks naturally in the form of natural geysers.
2. Artificial Geysers: Sometimes, a hole is drilled up to the hot region and by sending a pipe in it, the steam is made to rush-out through the pipe with very high pressure.

The steam coming out of natural or artificial geysers is made to rotate the turbine of a generator to produce electricity.

BIOMASS ENERGYBiomass is organic matter produced by plants or animals. It is used as a source of energy. Biomass is generally burnt for heating, cooling and industrial purposes.
Ex: wood, crop residues, seeds, cattle dung, sewage, agricultural wastes, etc.
Biomass may be converted into energy in any of the following types:
1.   Biogas: Biogas is a mixture of gases such as methane, carbondioxide, hydrogen sulphide, etc. It contains about 65% of methane gas as a major constituent. Biogas is obtained by the anaerobic fermentation of animal dung or plant wastes in the presence of water.

2.   Biofuels: Biofuels are the fuels, obtained by the fermentation of biomass. Examples are Ethanol, Methanol.

                 (i)   Ethanol: Ethanol can be produced by sugarcane. Its calorific value is less when compared to petrol and therefor produces much less heat than petrol.

                 (ii)   Methanol: Methanol can be easily obtained from ethanol or sugar containing plants. Its calorific value is also too low when compared to gasoline and petrol.

                 (iii)   Gasohol: Gasohol is a mixture of ethanol and gasoline.

3.  Hydrogen fuel: Hydrogen can be produced by thermal dissociation or photolysis or electrolysis of water. It possesses high calorific value. It is non polluting as the product of combustion is water.

Disadvantages of Hydrogen fuel

(i)  Hydrogen is highly flammable and explosive in nature.

(ii)  Safe handling is required

(iii)  It is difficult to store and transport.