Name any three forms of energy which could be harnessed from the sea.

1. Tidal energy

2. Wave energy

3. Ocean thermal energy 

Explanation

Tidal energy: Tides occurs due to the rise and fall of sea level which is caused by the gravitational pull of the moon and the sun on the Earth. Coastlines experience either a daily diurnal or semi-diurnal tide consisting of one or two high and low tides respectively. If the difference between the high and low tides is large enough, then energy can be harnessed and used for electricity production

There are two main types of tidal energy:

1. Kinetic energy obtained from the currents of changing tides.

2. Potential energy obtained from changing heights between the high and low tide. 

Sea-waves energy: The wind blows due to the sun's energy sweeps over the ocean surface and produces waves. A large amount of kinetic energy is associated with the movement of sea waves. It is estimated that the power of waves striking coastlines around the world could generate 2-3 million megawatts. Although, there are only limited sites where this energy can be harnessed. Areas with an average density of 40 MW per kilometre of coastline are considered economically feasible for generating electricity from sea waves.

Ocean thermal energy: This type of energy arises because of the difference in temperature of ocean water. The upper surface of the ocean becomes hot because of the sun, and the surface deep inside remains colder. This form of energy is harnessed by devices known as "Ocean Thermal Energy Conversion" (OTEC). To use this setup, a difference of 20°C or more is required between surface water and water at a depth of 1000 m.