Energy from Sun

The energy from the sea can be obtained mainly in three forms:

1) Tidal energy

2) Wave energy

3) Ocean thermal energy

We will now describe these three means of obtaining energy from the sea in somewhat detail.

The raise of sea water due to gravitational pull of the moon is called “high tide” whereas the fall of sea water is called ” low tide “. The tidal waves in the sea build up and recede (rise and fall ) twice a day.The enormous movement of water between the high tides and low tides provides a very large source of energy in the coastal areas ofthe world.The tidal energy can be harnessed by constructing a tidal barrage or tidal dam across a narrow opening to the sea.

During high tide when the level of water in the sea is high ,sea water flows into the reservoir of the barrage and turns the turbines .The turbines then turn the generators

to produce electricity.And during the low tide ,when the level of sea-water is low ,the

sea water stored in the barrage reservoir is allowed to flow out into the sea.This flowing

water also turns the turbines and generates electricity.Thus as sea-water flows in and out of the tidal barrage during high and low tides ,it turns the turbines to generate electricity.

The tidal energy is not likely to be a potential source of energy in future because of the

following reasons:

1) There are very few sites around the world which are suitable for building tidal

barrages (or tidal dams).

2. The rise and fall of sea -water during high and low tides is not enough to generate electricity on a large scale.

Wave energy here means ‘sea waves energy ‘. Energy from the sea is also available in the form of sea waves.Due to the blowing of wind on the surface of sea,very fast sea-waves (or water waves) move on its surface.Due to their high speed ,sea -waves

have a lot of kinetic energy in them.The energy of moving sea-waves can be used to generate electricity. A wide variety of devices have been developed to trap sea- wave energy to turn turbines and drive generators for the production of electricity.

1) One idea is to set-up floating generators in the sea.These would move up and down with the sea-waves.This movement would drive the generators to produce electricity.

2) Another idea is to let the sea-waves move up and down inside large tubes. As the waves move up ,the air in the tubes is compressed. This compressed air can then be used to turn a turbine of a generator to produce electricity.

These ideas are only experimental in nature.Models have been made based on these

ideas but it will be many years before full-size wave-energy generators can be built to

harness the sea-waves energy on a large scale.The harnessing of sea-waves energy would be a viable proposition only at those places where sea-waves are very strong.

A very large area is sea is called an ocean .The water at the surface of an ocean gets heated by the heat of the sun and attains a higher temperature than the colder water at deeper levels in the ocean.So, there is always a temperature difference between the water”at the surface of ocean” and ” at deeper levels.”The energy available due to the

difference in the temperature of water at the surface of the ocean and at deeper levels is called ocean thermal energy (OTE). The Ocean thermal energy can be converted into a ” usable form” of energy like electricity.

This can be done as follows:

The device used to harness ocean thermal energy are called Ocean Thermal Energy Conversion power plants (or OTEC power plant). A temperature difference of 20 degree Celsius (or more) between the surface water of ocean and deeper water is needed for operating OTEC power plants.In one type of OTCE power plant, the warm surface water of ocean is used to boil a liquid like ammonia or a Chlorofluorocarbons (CFC). The high pressure vapors of the liquid (formed by boiling) are then used to turn the turbine of a generator and produce electricity.The colder water from the deeper ocean is pumped up to cool the used up vapors and convert them again into a liquid .This process is repeated again and again.

A great advantage of the ocean thermal energy is that it can be used continuously 24 hours a day throughout the year. Another advantage is that ocean thermal energy is

a renewable source of energy and its use does not cause any pollution.

Please note that wave energy and ocean thermal energy are the two forms in which solar energy manifests itself in ocean. Another point to be noted is that though the energy potential from the sea ( tidal energy, wave energy , and ocean thermal energy ) is very large but its large scale exploitation is difficult at the moment .

Energy from the Sea (Oceans). About 71% of the earth’s surface is covered by oceans. Energy can be harnessed from

Tides.

Sea-waves.

Temperature differences in the water.

We shall discuss each energy form in detail.

Tides are caused due the gravitational pull of the moon on the water in oceans and seas. The enormous movement of water between the high tides and low tides provides a very large source of energy in the coastal areas of the world. This tidal energy can be harnessed by constructing a dam or a barrages shown in the figure given above. The energy conversion involved in the process is given below:

During high tide, when the level of water in the sea is high, sea-water flows into the reservoir of the barrage and turns the turbines. The turbines then turn the generator shaft to produce electricity.

At high tide water flows from sea into reservoir and turns the turbine.

During low tide, the sea-water stored in the barrage reservoir is allowed to flow out into the sea. This flowing water also turns the turbines and generates electricity. Thus, as the sea-water flows in and out of the tidal barrage during high and low tides, the turbines rotate continuously to generate electricity.

At low tide stored water flows out from reservoir into sea and turns the turbine.

The tides do not possess enough energy to generate electricity on a large scale.

Only few sites are suitable for building such barrages (or tidal dams)