Fossil fuels come from the very old life forms that have decomposed over a long period of time under the earth's crust. Fossil fuel is a general term for the buried combustible deposits, formed from the decomposition of organic matter (such as plants and animals) when subjected to high heat and pressure in the earth's crust over hundreds of millions of years. The three most important fossil fuels are coal, petroleum or crude oil, and natural gas.

Different parts of Earth receive different intensity of sunlight and also different landforms have different ability to heat-up. The air mass above hotter regions develop low pressure and that above colder regions develop high pressure. Wind blows from high pressure region to low pressure region. Thus sunlight is the ultimate source of energy for winds.

A slurry of cow dung and water is made in the mixing tank from where it is fed into the digester. As the starting material is cow dung, hence the name gobar gas.

Advantages of hydel power generation are: 
Pollution is rarely created.
Reduces greenhouse emissions.
Relatively low maintenance costs.
It is renewable.

Hydel power plants cover large area and hence rehabilitation of people is done.

Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles (neutrons or protons). Nuclear fusion occurs in sun. As a result, the sun forms this immense source of energy.

An ideal source of energy should possess the following characteristics. 
(a) It should be capable of giving an adequate amount of useful energy. 
(b) It should be convenient to transport, store and use.
(c) It should be economical.
(d) It should be capable of supplying the desired quantity of energy at a steady rate over a long period of time and it should be pollution free.

Solar cells are used in traffic signals,calculators and artificial satellite.Assemblies of photovoltaic cells are used to make solar modules which generate electrical power from sunlight.

Combustible material is the substance that can be burned to provide heat or power. Oxygen cannot be burned to produce heat. In fact, other substances are burned in the presence of oxygen to produce heat energy. Hydrogen, butane, and methane are fuels used to produce heat.

The major demerits of nuclear power plant are as follows:

-Disposal and storage of nuclear waste
-Uranium decomposes into harmful sub atomic masses
-Accidental leakage of radiation  
These limitation makes construction of nuclear power plant difficult.
Production of huge amount of energy is an advantage of nuclear plants. 

Tidal energy uses the difference in sea level between high tides and low tides to produce energy. Wave energy uses the kinetic energy of the wave to produce energy. Ocean thermal energy uses the temperature difference between top surface and water at depth to evaporate ammonia, which is further used to run the turbine.