Solar energy is the energy obtained from the sunlight using proper technology. It is a natural and renewable source.

Solar energy can be harnessed using a solar cooker. It consists of an insulated metal or wooden box, with the inner walls painted black to avoid heat from dissipating out of the cooker. It also has a plane mirror which acts as reflector attached to the box to concentrate sunlight on the metal container having food. The food to be cooked is placed in a metal container and the black painted cooker is kept in box and covered with glass sheet. It is kept in sunlight so that strong beams of sunlight fall over the cooker top. The rays of sunlight pass through the glass

sheet cover and get absorbed by black surface and temperature in the box slowly rises up. This heat is used to cook the food.

Certain limitations of solar energy are as follows:

(a) It is unavailable during night or cloudy days.

(b) It is very costly.

To overcome these limitations, solar cells are used to store the energy during light unavailability.

OR

The matter present in the bodies of plants and animals is called biomass. It is used as a fuel. It includes wastes from trees and animals, dead bodies of trees and animals, urban wastes, excreta of living organisms and their bodies, etc.

The principle and working of a biogas plant are as follows:

Biogas is produced by the anaerobic fermentation of animal wastes like cow-dung (or plant waste) in the presence of water. This degradation is carried out by anaerobic micro-organisms called anaerobic bacteria in the presence of water but in the absence of oxygen.

A slurry of cow-dung and water is made in the mixing tank of the biogas plant. This slurry is made to flow into the digestor having no oxygen. Oxygen is removed to provide a suitable environment for the anaerobic bacteria. Anaerobic microbes decompose or break-down complex compounds of the cow-dung slurry into simpler molecules. It takes a few days for the decomposition process to be completed. Bio-gas is obtained after the process. The biogas is stored in the gas tank above the digestor from which it is drawn through pipes for usage.

The schematic diagram of a biogas plant is as follows: