in temperature differences between the ocean surface and the deep ocean and make

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1 OTEC OTEC, Ocean Thermal Energy Conversion, is designed to transform solar energy stored in temperature differences between the ocean surface and the deep ocean and make electricity and freshwater directly, if the OTEC facility is onshore, or generate a gaseous fuel such as methane or hydrogen that can be stored, if the facility is in the open ocean. A total of 10 TW of power might be generated from OTEC worldwide. Figure E shows the regions having significant OTEC potential. Fig. E Areas having greatest potential for OTEC have temperature differences greater than 20 C. These regions are concentrated between 20 of the Equator. (U.S. Department of Energy, National Renewable Energy Laboratory) OTEC is attractive especially to isolated Pacific islands, partly because of the possibility of generating huge volumes of freshwater through the open cycle process (see below and in the chapter) and because there is so much energy available (Fig. E22.3.1). Imported fuel is a great expense for such isolated places with no fossil fuel resources.

2 Energy, Ch. 22, extension 3 Ocean Thermal Energy Conversion 2 In open-cycle processes, seawater is flash-evaporated in a vacuum chamber, and later condensed on pipes carrying cold seawater. The operational plants are expensive; a coldwater pipe is needed to bring 4 m 3 /s/mw e through the plant, and this pipe must be very long and accounts for the major portion of the cost. Pumps comprise about 10% of the cost of a plant. A modern open cycle plant producing 7.8 MW and 24 million liters of fresh water daily might be built for an estimated cost of around $10,000/kW, with low operating costs of only 0.8 cents/kwh e and producing water for $0.25 per thousand liters. (62,76,86) It would have an assumed 30 year lifetime, and would take 3 to 5 years for construction. (62) Open-cycle machines are scalable to perhaps 10 MW; the closed cycle is scalable to 100 MW. The Natural Energy Laboratory estimates its closed cycle produced electricity for about $0.14/kWh. (97) With use of new aluminum heat exchangers instead of the current titanium, it estimates heat exchanger cost, a substantial fraction of the costs, would be only 20% of the present costs. (97) NELHA research A facility for OTEC research, the Natural Energy Laboratory of Hawaii Authority (NELHA) at Keahole Point on Hawaii, was begun in 1974 (97,98) with involvement of the Department of Energy s Solar Energy Research Institute (now the National Renewable Energy Laboratory). A lot of engineering research is necessary because the materials would be at sea or in deep water along shores, where corrosion and biofouling can be a significant problem. Tests of aluminum heat exchangers not as subject to corrosion problems as steel ones have the possibility of reducing costs significantly. The research was conducted at NELHA by ALCAN Aluminum of Canada and the Marconi Division of General Electric Company of Great Britain.

3 Energy, Ch. 22, extension 3 Ocean Thermal Energy Conversion 3 The Department of Energy with NELHA built the world s first real operational OTEC devices, the 50 kw (gross electricity output) mini-otec, the first OTEC plant with net energy gain, which operated off Hawaii in 1979 (Figure E22.3.2). It was a moored OTEC plant. The 1 MW e (gross) test OTEC-1 device operated off Hawaii in It showed the possibility of using a slow ship with onboard OTEC plant. Neither of these ever produced energy, but they did demonstrate feasibility. Fig. E A model design for a ship-based OTEC system. (U.S. Department of Energy) A third OTEC facility, giving 100 kw (gross electricity), was operated off Nauru in the Pacific Ocean from October 1981 to September 1982 by a Japanese group. Their coldwater pipe ran down to a depth of 580 meters below the ocean surface. Freon was the working fluid used in this facility, and a titanium heat exchanger was used. The actual output was 35 kw e. This was an important practical test. To make this technology

4 Energy, Ch. 22, extension 3 Ocean Thermal Energy Conversion 4 accessible to islands in the Pacific, the cost needs to come down to below $7200/kW. In the long range, the aim is to bring the cost below $3200/kW. The plants can be subsidized from fresh water production (open cycle), with the electricity as a bonus. (65) They also involve upwelling of lower oceanic waters, which are full of nutrients. Areas of natural upwelling are the most productive regions for fisheries in the world. This would increase local fish production, thus adding a valuable food resource and assisting the local economy. One could even build farms at the upwelling to minimize fishing costs. Fig. E The 210 kw Open-Cycle OTEC Experimental Facility that operated at NELHA from 1992 through (NELHA) There is no currently operating OTEC facility. Between 1993 and 1998, a plant ran at NELHA. It began as pure OTEC, pumping water from a depth of 600 m, where it is at 6 C. Later it became an open-cycle plant rated at 210 kw with 40 kw net electricity. (99) A vertical axis radial inflow turbine rotated at 30 Hz inside the top of the tower shown in

5 Energy, Ch. 22, extension 3 Ocean Thermal Energy Conversion 5 Fig. E The fresh water production vapor to liquid surface heat exchanger is seen at the right of the picture. Figure E shows a schematic of the plant. Fig. E Schematic view of the technology used in the NELHA plant. (U.S. Department of Energy, National Renewable Energy Laboratory) The plant s water was used to cool the laboratory and commercial buildings, which saved considerable money. (97,99) The project successfully demonstrated about 28,000 liters per day of fresh water production with minimal power loss, and later showed an expanded freshwater output. The effluent water (12 C) had a high concentration of nutrients and was very clean biologically. Cold-water pipes were run above a garden, lowering the temperature and dripping condensed water from air onto the plants. Strawberries, zucchini, carrots, and lettuce grew very well there and were said to be much sweeter than usual. (99)

6 Energy, Ch. 22, extension 3 Ocean Thermal Energy Conversion 6 In 1984, the Hawaii Legislature funded the Hawaii Ocean Science and Technology (HOST) Park, which is adjacent to the NELHA site. It is host to businesses using the fruits of NELHA s research or doing parallel work. Since 1987 the still-cold water (mixed to 22 C) has been used to grow lobsters (in less than half the time needed in nature), salmon, other fish, and a special seaweed, nori (used in sushi) commercially. (97,99) The open cycle operation is close to economic viability. Even though the OTEC plant was closed, the cold water is still being brought up to support the businesses at HOST that grew around it. (100,101) It is hoped that eventually the cold water pipe will support a commercial OTEC facility. (101) Plant construction The advantage of an onshore plant is that buildings on land are not as difficult to protect from corrosion, storms, or accident as seaborne plants. However, land-based plants must deal with the surf turbulence and even in the best cases must construct kilometers-long cold water pipes to reach operating depths because of the slow downsloping of the underwater near shore decline. Seaborne plants have a much shorter cold water pipe, and can use an existing ship, but the pipe could be damaged in a storm. A ship must operate in a relatively threatening environment, and there is the problem of how to send energy to the shore for use. In this case, one could try to run a cable to shore (but this faces tremendous practical difficulties) or the energy could be stored in a fuel, which could be offloaded into a tanker and sent ashore. Alternatively, one could build a plant on the shelf just offshore. The facility could be built in a drydock much as oil rigs are, where construction costs are relatively cheap, and towed

7 Energy, Ch. 22, extension 3 Ocean Thermal Energy Conversion 7 into place. This eliminates the surf problem, but still entails the expense of a long pipe. Also, the plant must withstand ocean storms, and mooring is not feasible much below 2000 m depth. In addition, there is not the added problem of transporting the water and energy to shore. This appears the most commercially viable plant to some experts. (96) The Baltimore-based company, Sea Solar Power, believes that the NELHA research was tilted too much toward too large a scale. Their design uses basic principles of refrigeration instead of focusing on standard power plant design. (100) Sea Solar Power has developed a smaller system that uses a propylene vapor turbine instead of the ammonia vapor turbine used in the previous research. One problem with ammonia is the problem of tiny cracks allowing mixing with seawater, which eventually causes problems with heat transfer. Using money from the state of Maryland and later the Abell Foundation, Sea Solar Power designed both a shore-based 10 MW and sea-based 100 MW system. It is in the process of constructing a 10 MW OTEC plant near Agana, Guam. (100) Environmental considerations Large amounts of seawater would be needed to supply a large OTEC plant. The discharge expected from a 100 MW OTEC plant is about the size of the flow of the Colorado River into the Pacific Ocean. (96) If the cold seawater is taken from just under the boundary between temperature regimes and exhausted at the surface, it would have a different effect from exhausting the water just above the boundary layer (which would minimize whatever impact there was). In addition, as we mentioned in the Chapter, carbon dioxide in deep waters would be released when the water is brought to the surface. This release would be below 7 g/kwh

8 Energy, Ch. 22, extension 3 Ocean Thermal Energy Conversion 8 for an open cycle OTEC plant, compared to 100 times that much from fossil fuels. (96) Of course, there would be little or no release if a closed cycle is used. Depending on the destination of the seawater, climate changes could result. The ability to alter climate by building large numbers of OTEC installations could help humanity prevent another ice age (although this is not a present problem due to the huge fossil fuel input and global warming), if we can gain a better understanding of the interaction between the oceans and the atmosphere. (76) We have mentioned that the cold water exhausted on the surface will draw fish. However, the intake will kill fish eggs and larvae. (96) What balance exists between these effects is still speculative.