Seite - 149 - in Water, Energy, and Environment - A Primer

Open-cycle technology directly vaporizes the warm seawater by introducing it into a low-pressure container which causes it to expand and boil (flash evaporation). The resulting ‘steam’ drives a low-pressure turbine-generator. This form of OTEC offers a significant benefit in addition to electricity: the steam is free of salt and other seawater contaminants, and can be condensed as freshwater. Inmany applications, particularly for isolated island locations, the desalinized fresh water may be more valuable than the electricity. A hybrid version of OTEC has alsobeendeveloped, incorporating features of both types. OTECfacilitiescanbelocated indeepwater (onships)butcan alsobeland-basedor locatednearshore.Thelatter locationsoffer several advantages: reducedexposure toextremeweather events, reducedcablingrequirements,andnoneedforexpensiveoffshore moorings in deep water. All locations require a long, large diameter pipe to deliver deep cold water, which can be problematic if not properly engineered. There have been several reports of suchpipes breaking loose andbeing lost. 8.7.4.3 Other coldwaterapplications The OTEC-extracted deep, cold water can also be used for non-power applications. These include use of the near-freezing water for air conditioning, chilled-water agriculture (which allows growth of crops not usually grown in tropical climates), and aquaculture, which makes productive use of the nutrient- rich content of the extracted deepwater. 8.7.4.4 OTECR&D Japan leads the world in OTEC R&D, centred at the Institute of Ocean Energy at Saga University. In 1981 a 120kW closed-cycle power plant went into operation on the Japanese island ofNauru. In 2013SagaUniversity, workingwith several Japanese companies, installed two 50kW OTEC units that are connected to the grid when not being used for research. Renewableenergy 149