Oceans, covering more than 70% of the earth?s surface, are the world?s largest solar energy collector as well as the storage system. In a day tropical seas, spread over 60 million sq km, absorb solar radiation that equals heat content of 250 billion barrels of fossil oil. So, ocean thermal energy conversion (OTEC) can be an effective option for energy generation.
OTEC is a process that converts solar radiation to electric power by using the ocean?s natural gradient. It uses the temperature differences between deep and shallow waters. If the temperatures of the warm water surface and the cold deep water differ by about 20?C, the OTEC system can harness significant power.
According to experts, OTEC systems can produce about 1,000k million watts of base load power. If less than a tenth of one per cent of the solar energy stored in oceans can be converted into electricity, it would supply more than 20 times the total energy consumed in the US per day.
India has piloted a 1 mw floating OTEC plant near Tamil Nadu and the government continues to sponsor various research projects in developing floating OTEC facilities. Among other countries Japan is interested in funding researches in OTEC technology.
French physicist Jacques Arsene d?Arsonval first thought of tapping thermal energy from the oceans in 1881. Years later a d?Arsonval?s student, George Claude, built the first OTEC experimental plant in Cuba in 1930 for producing 22 kW of electricity with a low-pressure turbine. In 1935, Claude also constructed another plant aboard a 10,000-tonne cargo vessel off the coast of Brazil, but was destroyed by waves. Later in 1956, French scientists designed another 3-mw plant for Abidjan, Cote d?lvoire but couldn?t be completed. In 1962, J Hilbert Anderson and James H Anderson Jr focused on developing new and more efficient component design with a view to complete Claude?s unfinished agenda.
The US got into OTEC research in 1974 when the Natural Energy Laboratory of Hawaii Authority was set up at Keahole Point on the Kona coast. This laboratory is one of the world?s leading test facilities for OTEC technology. In 1978, Richard Meyer became a well-known figure among OTEC technologists. In 1979, a tiny OTEC generator was set up off the coast of Hawaii for producing 18kW power. Another plant, which continuously produced more than 50 kW power soon followed.
In 1984, the Solar Energy Research Institute (now known as the National Renewable Energy Laboratory) developed a vertical-spout evaporator to convert warm seawater into low-pressure steam for open cycle plants. Energy conversion efficiency, which was as high as 97%, was achieved on them. In May 1993 an open-cycle OTEC plant at Keahole Point produced 50,000 watt during a net power-producing experiment, breaking the record of 40,000 watt produced by the Japanese system in 1982.
Some proposed OTEC projects across the world include a small plant for the US Navy base on the British-administered island of Diego Garcia in the Indian Ocean. It is proposed that a 8-MW OTEC plant, backed up by a 2-mw gas turbine, would replace the existing 15-mw gas turbine plant. A US company has proposed building a 10-mw OTEC plant at Guam.
OTEC plants can be of open, closed or hybrid cycles. In an open cycle system, lowering the pressure above warm water turns it into vapour, effectively ?steam? which runs a turbine before it is re-condensed by cold water. In closed cycle and hybrid systems, the water heats and cools?vaporises and recondenses?an intermediary fluid/gas that powers a turbine within a closed sub-system, which enables much larger energy output.
Newer designs and material choices have reduced the capital investment costs of OTEC plants. Indian Ocean, Caribbean, South Pacific and the Hawaii regions are the most cost-effective sites for OTEC plants. More works are needed to reduce plant costs further. But certainly, harnessing the wealth of the oceans to replace the fossil fuel is an idea whose time has come.
Another hassle in the way of OTEC plants are laws and treaties governing the seas. The UN Convention on the Law of the Seas gives coastal nations rights over waters in the specified zones of varying legal authority along the coast. OTEC facilities which are stationary surface platforms and may be considered as artificial islands and, therefore, may invite legal problems. OTEC plants may be perceived as either a threat or potential partner to fisheries management or to future seabed mining controlled by the International Seabed Authority. World leaders need to put in place appropriate international law so that objective of tapping megawatts from oceans is not hindered.