A good source of energy fulfils the following criteria:

(I)It produces a lot of heat per unit mass.

(II) It does a huge amount of work per unit mass.

(III) It is easily accessible.

(IV) It is easy to store and transport.

(V)It is economical.

(VI)It produces less amount of smoke.

A good fuel produces a huge amount of heat on burning, does not produce a lot of smoke, and is easily available.

Natural gas can be used for heating and cooking food because it is a clean source of energy. It does not produce huge amount of smoke on burning. Although it is highly inflammable, it is easy to use, transport, and it produces a huge amount of heat on burning.

The disadvantages of fossil fuels are:
→ Burning of coal and petroleum produces a lot of pollutants causing air pollution.
→ Fossil fuels release oxides of carbon, nitrogen, sulphur, etc. that cause acid rain, which affects the soil fertility and potable water.
→ Burning of fossil fuels produce gases such as carbon dioxide that causes global warming.

Fossil fuels which are traditionally used by human beings everywhere as an energy sources are non-renewable sources of energy. These sources of energy are limited and will disappear after some time. They are being consumed at a large rate. Therefore, we should conserve the energy sources. Hence, we should look for alternate sources of energy.

Earlier, the windmills were used to harness wind energy to do mechanical work such as lifting/drawing water from a well. Today, windmills are used to generate electricity.In windmills, the kinetic energy of wind is harnessed and converted into electricity.
Water energy which was used for transportation before is now a good source to generate electricity. Dams has been constructed on river for generating electricity. Waterfalls were used as a source of potential energy which was converted to electricity with the help of turbines.

A concave mirror is used in a solar cooker as it uses heat of the sunlight to cook food. The mirror focuses all the incident sunlight at a point. The temperature at that point increases, thereby cooking and heating the food placed at that point.

The forms of energy that can be obtained from the ocean are tidal energy, wave energy, and ocean thermal energy. There are several limitations in order to harness these energies.

(i) Tidal energy depends on the relative positioning of the Earth, moon, and the Sun.

(ii) High dams are required to be built to convert tidal energy into electricity.

(iii) Very strong waves are required to obtain electricity from wave energy.

(iv) To harness ocean thermal energy efficiently, the difference in the temperature of surface water (hot) and the water at depth (cold) must be 20ºC or more.