Why is there a need for harnessing non-conventional sources of energy?

Why is there a need for harnessing non-conventional sources of energy? How can energy be harnessed from the sea in different ways?

There is a need for harnessing non-conventional sources of energy because of the following reasons:
(i) The demand for energy is increasing day by day to meet out the basic requirement of our changed life-styles, growing use of machines and industrialisation in order to improve our living standards.
(ii) The fossil fuels are non-renewable sources of energy and were formed over millions of years ago and there are only limited reserves of fossil fuels.
The energy from the sea can be harnessed in the different forms that are as follows:
(i) Tidal energy It is harnessed by constructing a dam across a narrow opening to the sea. A turbine fixed at the opening of the dam converts tidal energy to electricity.
(ii) Wave energy A wide variety of devices have been developed to trap huge waves near the seashore for rotation of turbine and production of electricity.
(iii) Ocean thermal energy The water at the surface of the sea or ocean is heated by the sun, while the water in deeper sections is relatively cold. This difference in temperature is exploited to obtain energy in ocean thermal energy conversion plants.
These plants can operate, if the temperature difference between the surface water and water at depths upto 2 km is 293 K (or 20 °C) or more. The warm surface water is used to boil a volatile liquid like ammonia. The vapours of the liquid are then used to run the turbine of the generator. The cold water from the depths of the oceans is used to condense vapour again to liquid.