Ocean is a vast natural resource of water. These have also been examined in the search for alternate sources of energy-generation. For example, gravitational pull, by sun and moon causes tides, in which sea level rises and falls, two times in 24 hours. When it rises, sea water may be diverted through suitable channels, to inshore reservoirs driving the turbines, during its entry.
The stored water may gradually be released driving the turbines, again during the period of low tide. At present, tidal power plants are in operation in Russia, France, and Nova-Scotia etc.
For any power plant to be of practical use, fluctuation of at least m is needed between the high and low tides. Such situations exist only at about 1 5 localities in the world. The Bay of Fundi (USA) is one such site, where a Tidal Power Plant has been installed. Total tidal energy potential of the world has been estimated at 2 x 108 J/year. The world’s first Tidal Power Plant was installed in France in 1966 at Brittany.
I. Method of Harnessing Tidal Energy
In oceans, there is a large temperature difference between the upper and lower layers of sea water (about 5—10°C). This thermal gradient can be utilised to generate electricity with the help of some low boiling point working fluid (like liquid ammonia or propane), in reservoirs maintained on sea surface. This fluid picks up the heat and gets evaporated to develop high pressure which rotates the turbine. As the turbine rotates, the vapours are pushed down to lower temperature zone, where they cool, condense and are brought back to warm water zone to be recycled again. The conversion efficiency of this system [called as Ocean Thermal Energy Conversion (OTEC)] is only 2—3 percent, and so uneconomical, as it stays today.
The air currents rubbing against the surface of the ocean produce waves that are pushed towards the shore, where the energy contained in waves is dissipated out. The floating propellers, placed in shallow water, along the shore may be kept in a state of continuous motion by these water waves. Their kinetic energy may be used to drive turbo generators on floats or platforms, in shallow waves, to produce electricity.
India is surrounded by oceans from three sides. Hence, this country has great potentials of tidal energy and ocean thermal energy conversion.
The National Hydro-Power Corporation is attempting to install a Tidal Energy Power Plant in Kutch with a potentiality of about 900 MW. It is hoped that islands of Lakshadweep, Andaman and Nicobar may have appropriate sites of installations of Tidal and Ocean Thermal Energy conservation plants. It is estimated that about 9000 to 9500 MW of tidal energy can be derived from these areas.
III. Limitations and Environmental Consequences
(i)The system of Ocean Thermal Energy Conversion (OTEC), developed so far has very low efficiency of about 2 to 3 percent only.
(ii)Tidal Energy Plants can cause can endanger bird and fish populations and may cause trouble in navigation.
(iii)Installation of Ocean Thermal Energy Conversion plants may encourage pollution of the sea water.