Capacity to do work is known as energy. It is very important input for all our activities like cooking food, driving vehicle and running industries. The most important challenge that humanity is facing in 21^st century is availability of clean, secure and sustainable energy supply.

Energy sources are broadly divided into two categories. Renewable and renewable energy sources which are further divided into many sub categories:

These sources are regenerated by natural processes so that can be used indefinitely. These are:

(i)           Solar energy              (ii)     water               (iii)       wind

(iv)       Tidal                           (v)     biomass           (vi)       geothermal

(viii)     Ocean thermal energy