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Capturing Ocean Heat

Ocean thermal energy conversion offers a timely renewable alternative.

Fortnightly Magazine - October 2009

surface water) and the sink (cold deep water) temperature gives OTEC plants a typical thermal-to-electrical energy conversion efficiency of less than 3 percent. In comparison, conventional oil- or coal-fired steam plants, which may have temperature differences of over 200C, have thermal efficiencies around 30 to 35 percent. To compensate for its low thermal efficiency, an OTEC plant has to move significant quantities of water, which increases the power it needs to feed back into the plant’s pumps before any OTEC-generated electricity can be made available to the power grid. For plants larger than about 10 MW, about 25- to 40-percent of the generated power will go to pump the water through the intake and discharge pipes.

Another major challenge OTEC faces is the high capital costs for initial construction. About half of the capital cost of current OTEC designs will be for the heat exchanges, followed by the costs involved with the platform and its moorings, and the sea water pumps and deep seawater pipes, which must extend to around 1,000 m (3,300 ft.) and withstand the pumping of very large volumes of water. For example, a 100-MW OTEC plant will require about 215 m 3/s (3.4 million gal/min) of deep sea water, necessitating a minimum pipe diameter of 10 m (32.8 ft.). Such large pipelines would be composed of very expensive materials. In addition, the very large pumps, heat exchangers, and low-efficiency turbines all will add considerably to the construction cost. However, it should be noted that these low-efficiency turbines also could be retrofitted to existing power plants in order to increase their power output and reduce thermal pollution, thus increasing their overall efficiency and profitability.

OTEC plants pose potential ecological consequences. A 100-MW OTEC plant would pump a volume of water similar to that of a major river ( i.e., equivalent to the nominal flow of the Colorado River into the Pacific Ocean, or about 3 percent of the Mississippi, 10 percent of the Danube, or 20 percent of the Nile). This will require ensuring that sea water discharges occur at a depth below the bottom of the surface thermocline layer in order to avoid contaminating the surface water and causing poten- tial negative impacts on the local ecology, as well as on the overall thermal efficiency of the OTEC plant.

Perhaps the biggest challenge to eventual OTEC deployment is making the financial case—will it be economical to build and operate? Having in place a consistent regulatory infrastructure, which provides necessary predictability through an orderly, timely, and efficient review of OTEC license applications and operations, along with a supportive legal climate and financial support from the federal government, at least for the initial plants until the technology is proven, will help assure the economic viability of the technology in its infancy. Absent a legal framework and supporting regulatory infrastructure, financing and insuring commercial OTEC operations in the United States may be im-possible.

Path Forward

There are several areas that need to be addressed if OTEC is to become a viable energy option. Unresolved challenges in any of these