Question

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

Answer 1

Hint Using conventional energy sources releases carbon dioxide in the atmosphere. Conventional energy sources are also non-renewable and will be exhausted soon. So we need non-conventional sources.

Complete step by step answer
With the rise in the global population, the energy demand has increased and we need fuel to generate energy. Non-conventional sources of energy like coal and petroleum are non-renewable so we cannot keep on using them forever. They are being rapidly exhausted and we will run out of them very soon.
At the same time, the burning of coal and petroleum to power turbines releases carbon dioxide and other greenhouse gases in the atmosphere which causes global warming. The effect of global warming is seen in the form of climate changes all around the world in terms of more severe weather phenomena, natural disasters, etc.
So, we must harness non-conventional sources of energy such as solar, wind, hydro energy which are renewable and have fewer disadvantages if any. Since they are renewable, they can last us for longer periods for larger demands as well.
Energy can be harnessed from the sea in 2 main ways:
Tidal energy: Due to the gravitational pull of mainly the moon on the spinning earth, tides are created in the sea. Tidal energy is harnessed by constructing a dam across a narrow opening to the sea where the waves crash on the shore. A turbine fixed at the opening of the dam is powered by the waves and converts tidal energy stored in the waves to electricity.
Ocean thermal energy: The water at the surface of the sea or ocean is heated by the Sun is has a high temperature while the water in deeper sections is relatively cold compared to the surface. Ocean thermal plants can use this energy if the temperature difference between the water at the surface and water at depths is 20 K or more. The hot surface-water is used to boil a volatile liquid like ammonia which has a smaller boiling point than water. The vapours of the liquid are then used to run the turbine of a generator like water vapour powers a turbine. The cold water from the depth of the ocean is pumped upwards towards the surface and it condenses the liquid again and the cycle continues to power a turbine which can then be used to generate electricity.

Note
Conventional energy sources are called conventional because they have been used for a long time however their harmful effects are very damaging to our environment and recently there has been a big paradigm shift away from conventional energy sources to other non-conventional renewable sources of energy.