Methods of harnessing solar energy:
Solar Cooker: Solar cooker is a simple device. It is composed of a rectangular box with a mirror at the top. The box has a glass cover. Mirror is kept in erect position so that reflected sunlight can go inside the box. Utensils are kept in the box. Glass cover on top prevents heat from escaping the box.
Solar Cell: A solar cell is also called photovoltaic cell. It converts solar energy into electrical energy. Electrical energy thus produced needs to be stored in inverters so that it can be used even during night.
Solar Furnace: Solar furnace is made of concave mirror. The heating device is kept at the focus of this furnace. Heat from sun is concentrated at the focus which helps in generating lot of heat. 
Solar furnace can be used to heat water and steam generated by this process can be used to generate electricity.
 Limitations of Solar Energy:
• Can be harnessed only at those places which get plenty of sunlight.
• Cannot be harnessed beyond certain latitudes.
• Cannot be harnessed during night.
• Current technologies are very costly.
Ways to overcome limitations: Batteries can be used to store electrical energy from photovoltaic cells. Cheaper technologies need to be discovered.

Conventional Source of Energy	Non Conventional Source of Energy
coal , petroleum, firewood, hydel energy	solar energy, wind energy, nuclear energy, biogas, tidal energy, thermal energy, wave energy

Methods to Harness Tidal Energy:
• For this, a cylindrical structure is vertically placed in coastal areas where tides come.
• A turbine is fitted at the top of this cylinder.
• When the tide comes, air gushes through the cylinder. This turns the turbine and electricity is generated.
• When the tide goes, air moves in opposite direction. This also turns the turbine and electricity is generated.

Following are the main reasons: because of which there is a need to harness non-conventional sources of energy:
 (a) Non-renewable fuels: Fossil fuels are the main energy sources for us. But it takes millions of years for the formation of fossil fuel. The rate at which we are using them means they are not going to last many years from now. They would be definitely exhausted sooner rather than later.
 (b) Polluting fuels: Burning of fossil fuels releases oxides of carbon, nitrogen and Sulphur. These are polluting gases. Carbon dioxide is a greenhouse gas. Carbon monoxide can be poisonous even in low concentration. Oxides of Sulphur and nitrogen cause acid rain. Air pollution also results in increased cases of respiratory problems.
 Following are various ways of harnessing energy from sea:
• Wave energy
• Tidal energy
• Ocean thermal energy 

It is true that energy from various sources have been derived from the sun. Following are
explanations for this:
(a) Plants harness solar energy to make biomass. This biomass was converted into fossil fuels. So, energy in fossil fuels came from sun.
(b) Firewood and gobar gas have got energy from sun; as explained in previous point.
(c) Water cycle on earth happens because of heat from the sun. So, kinetic energy in water is because of sun.
(d) Waves and ocean currents are caused because of heating from the sun. So, different kinds of energy from ocean have sun in their root.
(e) The earth was made after a piece from the sun took shape of this planet. So, geothermal energy or energy from nuclear power plant have come from the sun. 

Biomass is a fuel that is obtained from organic materials.
Principle of Biogas: Biogas is produced by decomposition of organic matter. Biogas is mainly composed of methane. It burns with negligible smoke and leaves no residue. It can be used as kitchen fuel. It can also be used for generating electricity.

Working of Biogas Plant:
• A mixture of farm waste is fed into the biogas plant. This waste is mixed to make a slurry.
• Slurry is then fed into the digester. Digester is an air-tight chamber and oxygen is not present in it.
• Anaerobic bacteria in digester carry out decomposition of slurry. This results in production of biogas.
• Biogas is sent out through an outlet so that it can be suitably used.
• Decomposed matter is taken out and it can be used as manure.