Renewable sources of energy are clear, safe and mostly inexhaustible. They can also be referred to as solar energy sources, as they all depend upon the sun.

For example:

Solar power uses the sun's power directly
Wind turbines use the wind
Hydraulic power uses water
Geothermal power uses soil
Biofuels use nature

It has become essential to develop renewable energies in order to fight the greenhouse gas effect and start preparing for the replacement of fossil fuels (such as petrol).