RENEWABLE ENERGY 71 MR. ABHINAV BHASKAR; RESEARCH ASSOCIATE, THE ENERGY AND RESOURCES INSTITUTE, NEW DELHI Ocean thermal energy conversion Ocean thermal energy conversion technology uses the difference in the temperature between the warm surface water and cool deep water to run a turbine and produce electricity. Water, as we all know has a high specific heat and this enables it to store the solar thermal energy as sensible heat. 70% of earth’s surface area is occupied by the oceans and this makes them a huge store house of incident solar thermal radiation on them. Working Principle The figure below shows the basic schematic of a closed cycle OTEC power plant. Warm sea water is pumped in the heat exchanger to convert the working fluid from liquid into high pressure vapour. Organic working fluids with low boiling temperatures can be used in the system .High pressure vapour runs an organic rankine cycle turbine and produces mechanical power which is converted into electricity. Cold water from the ocean depths is pumped to assist the condensation of vaporized working fluid. The net power generated is the difference between the energy generated and the energy consumed to run the pumps .Ammonia which has a low boiling point can also be used as a working fluid. In an open cycle OTEC power plant hot water is directly used as the working fluid. Benefits OTEC is more beneficial than other renewable sources of energy as it’s not an intermittent source of power and can supply base load power 24X7. There are other allied benefits of OTEC power apart from energetica INDIA · JAN | FEB16 power generation. For example, spent cold seawater from an OTEC plant can chill fresh water in a heat exchanger or flow directly into a cooling system. Simple systems of this type have airconditioned buildings at the Natural Energy Laboratory for several years. OTEC technology also supports chilled-soil agriculture. When cold seawater flows through underground pipes, it chills the surrounding soil. The temperature difference between plant roots in the cool soil and plant leaves in the warm air allows many plants that evolved in temperate climates to be grown in the subtropics. Aquaculture is perhaps the most well-known by-product of OTEC. Cold-water delicacies, such as salmon and lobster, thrive in the nutrient-rich, deep seawater culled from the OTEC process. Microalgae such as Spirulina, a health food supplement, also can be cultivated in the deep-ocean water. Finally, an advantage of open or hybrid-cycle OTEC plants is the production of fresh water from seawater. Theoretically, an OTEC plant that generates 2 megawatts of net electricity could produce about 14,118.3 cubic feet (4,300 cubic meters) of desalinated water each day. Deployment status The idea to use the ocean thermal energy was first proposed by French physicist, Jacques Arsene d’Arsonval in the year 1881 . His proposal was taken forward by his student Georges Claude. He built the first OTEC plant in Cuba in the year 1930. The system produced 22 kilowatts of electricity with a low-pressure turbine. In 1935, Claude constructed another plant aboard a 10,000-ton cargo vessel moored off the coast of Brazil. However, weather and waves destroyed both plants before they became net power generators. (Net power is the amount of power generated after subtracting power needed to run the system.) In 1956, French scientists designed another 3-megawatt OTEC plant for Abidjan, Ivory Coast, West Africa. The plant was never completed, however, because it was too expensive. The United States became involved in OTEC research in 1974 with the establishment of the Natural Energy Laboratory of Hawaii Authority. The laboratory has become one of the world’s leading test facilities for OTEC technology. Besides the united stated Lockheed martin is exploring the possibility of deploying a 10 MW OTEC power plant in South China Sea . Recently the Indian navy announced its plan to use OTEC power plant to power its naval base in the Bay of Bengal. Feasibility study has been carried out and the results are very promising. The cost of generating power from OTEC would be same as the electricity generation cost from diesel generators. Problems Deterrents High investment costs associated with OTEC power plants act as a deterrent to the wide scale deployment of this technology. Investments required to lay a cold water pipe in the ocean are quiet high. Ecological impacts on the aquatic life are also a concern for wide scale deployment of this technology. The pumping power required to pump cold water from the depths reducing the net power generated from the OTEC power plants. OTEC power plants operate optimally at a temperature difference of 20 degree Celsius. This temperature difference is available only in the tropical areas. Western developed countries like Europe, most parts of the US and Japan have very limited scope for development of this technology. The figure below shows the temperature difference between surface water and water at a depth of 500-1000 meters .OTEC has a potential of 180000 MW in India. Small demonstration unit has been built by a start up from Netherlands, called Blue rise.
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