Renewable sources of energy 

The energy source that are being produced continuously in nature are called renewable sources of energy. For example, solar energy, wind energy, biomass energy, tidal energy, wood, charcoal, many agricultural wastes, etc. These sources of energy will never get exhausted and can be used again and again. These energy sources can be replenished in a short period of time. 

Alternative energy sources 

Fossil fuels like coal, mineral oil, natural gas, etc. are running out because of their excessive use. We need to use other sources of energy to reserve them. These sources are called alternative sources of energy. Thus, the energy sources which can be used instead of non-renewable sources of energy are called alternative sources of energy. Renewable sources of energy are also called renewable sources of energy. For example, solar energy, wind energy, biomass energy, tidal energy, wood, charcoal, many agricultural wastes, etc. 

Bio-fuel 

The source of energy that can be obtained from biomass, wood, hay, straw, chaff etc. is called biofuel. 

a) Biomass 

Plants trap sunlight during photosynthesis. In this process, solar energy gets stored in the form of organic matter in plants. Thus, the wood, wood dust, crops, dried leaves, seaweed, animal wastes, etc. which are used to get energy are called biomass. The energy released from biomass through combustion

 or other chemical process is called biomass energy. Wood has been the most important source of energy from prehistoric time. It provides maximum energy. In village area of Nepal, people use wood as a source of heat. They cook their food on fire. Other biomass sources include agricultural wastes like straw, fruit pits, etc. 

b) Biogas 

The biomass can be converted into combustible gas i.e. biogas in a bio-gas plant. Biogas is a mixture of gases like methane, carbon dioxide, hydrogen, hydrogen sulphide, etc. which is used to get energy. The major constituent of biogas is methane (65%). 

Production of biogas 

Biogas is produced by the decomposition of the organic wastes like dung, vegetable waste, human excreta, etc. in a biogas plant. Biogas from the biogas plant can be supplied through pipes for cooking and lighting. 

Advantages of Biogas 

1. It produces more heat when burnt. 
2. It is a non-polluting fuel as it does not produce smoke. So, it helps in controlling pollution. 
3. It provides nutrient-rich manure for plants. 
4. Biogas can also be used to produce electricity for lighting purpose. 

Solar Energy

    

 The energy which can be obtained from the sun in form of heat and light is called solar energy. The earth receives an enormous amount of energy directly from the sun each day. Solar energy is in the form of solar radiation. It is possible to produce electricity from solar energy. 

Ways to capture solar energy for our use 

Solar energy is used in the devices like solar cooker, solar furnaces, solar water heater, solar electric power plants, etc. In the last decade, solar energy has also been used to power earth-orbiting satellites, cars, and even planes. Solar energy can be used directly in two main ways: 

i) Converting sunlight into electricity: A solar cell, also called photovoltaic cell (PV) cell collects light from the sun. It absorbs even a small fraction of the light energy. Some of the energy absorbed by the cell can move electrons from silicon atoms. These free electrons move to one side of the cell, creating a negative charge in one side and positive charge in another. When these two ends are connected in a circuit, it produces electricity. 

A group of solar cells connected together is called solar panel. Space ships and satellites use solar sells to convert sunlight into electricity., Some calculators are powered by solar cells.

ii) Converting sunlight into heat: Solar heating devices are designed in such a way that they collect more sunlight. Solar cooker, solar water heater, etc. convert the sunlight into heat. 

Applications of solar energy 

1. It is used for drying clothes, obtaining salt from sea water, etc. 
2. It provides warmth on the earth surface. 
3. Wind is caused due to unequal heating of the earth surface by the solar energy. 
4. It is used for the preservation of fruits, vegetables, etc. 
5. It can be used to cook foods in solar cookers and to heat water by solar water heaters. 

Advantages of solar energy 

1. It is the cleanest form of energy 
2. It does not cause air, water and soil pollution 
3. It is used for the production of large-scale electricity in solar power plants. 
4. It is freely available at day time in almost all the places. 

Hydropower 

Hydropower is the energy that comes from the force of moving water. It can be found in those areas where water flows at high speed from high ground to low ground. To generate hydroelectricity, water is collected in a dam. The collected water is passed through a tunnel. The force of water can be used to rotate turbines connected with the generator which produces electricity. Such electricity generated from the fast flowing water is called hydroelectricity. 

Advantages of hydroelectricity 

1. Hydroelectricity does not cause air pollution 
2. It can be transmitted over long distances in extremely small interval of time. 
3. It can be converted into different forms of energy like heat, light, etc. by passing through suitable devices. 
4. Water used to generate hydroelectricity can be reused for other purposes like irrigation. 
5. It is cheaper in long term use though the installation cost of a hydropower plant is very high. 

Wind energy 

The energy obtained from the moving wind is called wind energy. This energy is a result of unequal heating of the earth surface and the atmosphere. It is one of the oldest and cleanest forms of energy. A large amount of energy can be produced from the windmill. It depends upon the weather and location. Wind energy is used for grinding grain, pumping water, or generating electricity. The kinetic energy of wind rotates the turbines connected with generators which generate electricity. Wind power plants or wind farms are a collection of many wind turbines to generate electricity. 

Applications of wind energy 

1. Wind energy is used to propel sailboats. 
2. Wind energy rotates the turbines connected with generator to generate electricity. 
3. It is used to pump water from well. 
4. It is used to grind wheat and other grains in grinding mills. 

Advantages 

1. Wind is a renewable source of energy, we never run out of it. 
2. It is a clean source of energy. It does not cause pollution. 
3. It is free of the cost and is reliable. 
4. Wind farms can be built off-shore. 

Tidal energy 

The gravitational pull of the moon and the sun in ocean water causes rise in water levels called tides. The energy generated in oceans or seas when the tides rise and fall is called tidal energy. It is a renewable sources of energy. 

Ways to get tidal energy 

Tidal powers stations are constructed near narrow bays, where the water level rises up and then falls down during the tides. Gates of the power generating station are opened during high tides and water is allowed to pass into the storage dam. When the level of water falls, the gates are closed. This collected water is allowed to fall over the turbines. When the turbines connected with generator are rotated, electricity gets generated. 

Advantages of tidal energy 

1. Gravitational pull of the moon and the sun causes tides in the ocean. So, it is an inexhaustible energy sources. 
2. Tides in the ocean are predictable. So, it is more reliable and uniform source of energy. 
3. It is a clean source of energy. 
4. The cost of generating electricity from tides is very low. So, it is a cheap energy source. 

Geothermal energy 

The word geothermal comes from the Greek words 'geo', meaning earth and 'therme', meaning heat. Thus, the heat energy obtained from the hot rocks deep inside the earth's crust is called geothermal energy. Geothermal energy comes from the heat within the earth. The hottest geothermal regions are found along major plate boundaries where earthquakes and volcanoes are concentrated. Temperature increase below the Earth's surface at a rate of about 30 degree celcius per kilometer in the first 10 kilometer below the earth surface. But in the volcanic and earthquake active region it increases at a rate of about 80 degree celcius per km. 

Natural ways by which the geothermal energy comes on the earth surface 

i) Hot spring: In the arenas where there is underground water, the magma heats the water and creates hot spring. 

ii) Volcanic eruption: During volcanic eruption, hot molten mass i.e., magma inside the earth crust comes on the earth surface in the form of lava. 

Ways to get geothermal energy for our use 

Geothermal energy is used for heating homes and for generating electricity from geothermal power plants. 

Geothermal power plants: To generate electricity from a geothermal power plant, large holes have to be dug into the earth until a geothermal hotspot is found. Metallic pipes are inserted inside these holes through which water is sent and steam output is obtained. They are called injection well and production well. 

i) Injection well: Water is sent through the injection well and it reaches the rocks. 

ii) Production well: Hot water from production wells explosively boils into steam. When it is released from the underground pressure of the the reservoir, the force of that steam is used to rotate the turbine connected with generator. 

Advantages of geothermal energy 

1. Geothermal energy does not depend on the weather conditions. It can be used to produce electricity for 24 hours a day. 
2. A geothermal power plant does not cause pollution.
3. Geothermal power plants have very low running costs. 
4. Geothermal power plants do not occupy too much space. They help in protecting natural environment.