Notes

c) Hydro power plants :-
In hydro power plants water from rivers are stored by constructing dams. The water from the dam flows down through pipes and rotates the turbines of generators to produce electricity.
Advantages :-
i) Flowing water is a renewable source of energy.
ii) The electricity produced does not cause pollution.
iii) The water stored in dams can also be used to control floods and
for irrigation.
Disadvantages :-
i) The initial cost is high.
ii) Large areas of land gets submerged and the decomposition of
vegetation produces methane gas which is a green house gas.
iii) It causes displacement of people from large areas of land.

b) Energy from the sea :-
Energy from the sea is obtained in three different forms. They are Tidal energy, Sea wave energy and Ocean thermal energy.
i) Tidal energy :-
The periodic rise and fall of sea level due to gravitational attraction of the moon causes tides. A dam is constructed at a narrow opening between the land and sea. The movement of water during high tide and low tide can be used to rotate the turbines of generators to produce electricity.
ii) Sea wave energy :-
When strong wind blows over the sea it produces huge waves. The kinetic energy of the moving waves can be used to rotate the turbines of generators to produce electricity.
iii) Ocean thermal energy :-
There is a temperature difference between the warm surface water and the cold water at the bottom of the oceans. This difference is about 20°C. The warm surface water is used to boil liquid ammonia and the vapour is used to rotate the turbines of generators to produce electricity. The cold water from the bottom is then pumped up to cool the vapour back to liquid.