What is OTEC?
OTEC is an application of solar energy that exploits the heat that the ocean captures from the sun’s rays. It possesses huge environmental advantages over fossil fuels and nuclear power; avoids land-use problems associated with renewable energy technologies such as solar, wind, biomass, and hydroelectric power; and has the potential to produce far more useful and affordable energy than could be obtained from other renewable sources.
OTEC is a technology for converting some of the energy that the tropical oceans absorb from the sun, first into electricity and then into fuels. During an average day, the 60 million square kilometers of surface waters of the tropical oceans (located approximately 10 degrees north to 10 degrees south of the equator) absorb one quadrillion megajoules of solar energy-equivalent to the energy that would be released by the combustion of 170 billion barrels of oil per day. The surface waters are a warm-water reservoir 35 to 100 meters deep that is maintained night and day at a temperature of 25 to 28 degrees celsius (°C).
Below about 800 meters, an enormous source of ice-cold water, which is fed by currents flowing along the ocean bottom from the northern and southern polar regions, is maintained at about 4°C.
OTEC uses this temperature difference to generate electricity. Warm water is drawn from the surface layer into a heat exchanger to vapourise a 'working fluid' with a boiling point of about -30°C. The vapour drives a turbine attached to an electric generator. Exhaust vapour from the turbine is subsequently condensed in a second heat exchanger, which is cooled by water pumped from the cold water source below. The condensed vapour is then returned to the boiler to complete a cycle that will generate electricity 24 hours a day throughout the year.
A BRIEF HISTORY OF OTEC
The concept of OTEC was developed by Jacques D’Arsonval in France in the 1880's, after an idea presented by Jules Verne in his novel Twenty Thousand Leagues Under the Sea (published in 1869). His disciple, engineer and entrepreneur Dr. Georges Claude, who improved the air liquefaction process, and invented neon lighting, tested the concept in a cold lake in Ougreé, Belgium, using the heated discharge from a factory which provided a temperature differential similar to that required by OTEC. Based on his success at Ougreé, Dr. Claude then built the first OTEC plant in Matanzas, Cuba in 1930. The plant was destroyed by a hurricane during initial testing, but did operate for a few days. Some persons claim that Dr. Claude attained net power generation, but this cannot be verified with available information.
Installation of the cold water pipe by Dr. Claude's team in Matanzas, Cuba- a basic installation method for underwater pipelines still used today
Subsequently, and inspired by what he considered a success during his short-lived Cuban experiment, Dr. Claude attempted to use OTEC to manufacture ice in Brazil, but the facility was also damaged in a storm. In the mid 1950's French engineers again attempted to build an OTEC plant in Abidjan, Ivory Coast, but the plant proved too costly and was never built.
In 1950’s Norwegian-American engineer Bryn Beorse, who had studied the French work, and Professor Everett D. Howe founded the Sea Water Conversion Laboratory at University of California and obtained some government funds for research. An open-cycle plant was proposed for water desalination in California (the temperature differential needed to produce desalinated water is less than what is required to produce power) but government was not receptive.
During the energy crisis of the mid 1970's, interest in OTEC was renewed in the United States and elsewhere. The U.S. government launched various R&D programs that included performance tests, preliminary designs and demonstration plants. Major efforts include preliminary designs for OTEC production plants by the Applied Physics Laboratory of Johns Hopkins University, General Electric, and TRW Corporation; heat exchangers performance tests by the Argonne National Laboratory, and demonstration plants in Hawaii (Mini-OTEC and OTEC-1).
Other major R&D efforts during this period include the Toshiba/Tokyo Electric Power 100-kW closed cycle land-based plant at the Republic of Nauru, and the studies completed at the Natural Energy Laboratory of Hawaii (NELHA), which led to the construction and operation of a 210-kW open-cycle pilot plant for the co-production of electric power and potable water.
Proposals for 40 MW net demonstration systems were requested by the Department of Energy. The Puerto Rico Electric Power Authority (PREPA) was among the proposers and conducted several detailed studies on the feasibility of OTEC.
Paper presenting one of the proposals submitted by PREPA to the US Department of Energy
Later, when oil prices were reduced, the Federal government lost interest in the program, although the state of Hawaii has continued to investigate applications of deep ocean water. A Japanese consortium later built a land-based OTEC facility in the island of Nauru in the Pacific. However, both were research units too small to be scaled to commercial sized systems. India reportedly tested a 1 MW pilot plant.