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How Can We Use Ocean Energy to Generate Electricity?
The energy from the sea can be obtained mainly in three forms :
- Tidal energy,
- Wave energy, and
- Ocean thermal energy.
We will now describe these three means of obtaining energy from the sea in somewhat detail.
1. Tidal Energy
The rise 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 of the world. The tidal energy can be harnessed by constructing a tidal barrage or tidal dam across a narrow opening to the sea (see Figure).
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 [see Figure (a)]. 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 [see Figure (b)].
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 :
- There are very few sites around the world which are suitable for building tidal barrages (or tidal dams).
- The rise and fall of sea-water during high and low tides is not enough to generate electricity on a large scale.
2. Wave Energy
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.
- 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.
- 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.
3. Ocean Thermal Energy
A very large area of 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 devices used to harness ocean thermal energy are called Ocean Thermal Energy Conversion power plants (or OTEC power plants). A temperature difference of 20°C (or more) between the surface water of ocean and deeper water is needed for operating OTEC power plants.
In one type of OTEC power plant, the warm surface water of ocean is used to boil a liquid like ammonia or a chlorofluorocarbon (CFC). The high pressure vapours 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 vapours 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 oceans. 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.
Geothermal Energy
‘Geo’ means ‘earth’ and ‘thermal’ means ‘heat’. Thus, geothermal energy is the heat energy from hot rocks present inside the earth. This heat can be used as a source of energy to produce electricity. Please note that geothermal energy is one of the few sources of energy that do not come directly or indirectly from solar energy (or sun’s energy).
At some places in the world, the rocks at some depth below the surface of the earth are very, very hot. This heat comes from the fission of radioactive materials which are naturally present in these rocks. The places where very hot rocks occur at some depth below the surface of earth are called ‘hot spots’, and are sources of geothermal energy. The geothermal energy is harnessed as follows :
The extremely hot rocks present below the surface of earth heat the underground water and turn it into steam (see Figure). As more and more steam is formed between the rocks, it gets compressed to high pressures. A hole is drilled into the earth up to the hot rocks and a pipe is put into it. The steam present around the hot rocks comes up through the pipe at high pressure. This high pressure steam turns the turbine of a generator to produce electricity.
Sometimes two holes are drilled into the earth in the region of hot rocks and two pipes are put into them. Cold water is pumped in through one of the pipes. This cold water is turned into steam by the hot rocks. The steam thus formed comes out through the other pipe and used to generate electricity.
Some of the advantages of using geothermal energy are as follows : It is economical to use geothermal energy. This is because the cost of electricity produced by using geothermal energy is almost half of that produced from conventional energy sources. Another advantage is that the use of geothermal energy does not cause any pollution. So, it is a clean and environment friendly source of energy.
Some of the disadvantages of geothermal energy are as follows : Geothermal energy is not available everywhere. It is available only in those areas where there are hot rocks near the earth’s surface. Another disadvantage is that deep drilling in the earth to obtain geothermal energy is technically very difficult and expensive.
In our country there are a very limited number of places where geothermal energy can be harnessed on a commercial scale. Two places where geothermal energy can be exploited on commercial scale are located in Madhya Pradesh and Himachal Pradesh. In USA and Newzealand, a number of geothermal energy power plants are working successfully.