Ocean Thermal Energy Conversion

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Ocean Thermal Energy Conversion (OTEC) Oceans cover over 70% of the earth’s surface area and are a huge source of thermal energy. Ocean Thermal Energy Conversion (OTEC) takes advantage of the temperature difference between the warm surface waters and cold deep waters of oceans to drive a steam turbine and produce electricity. Tropical waters are especially suitable because of the higher surface temperatures (Figure 10-12). Ocean waters between the tropics of Cancer and Capricorn (latitudes between 23°N and 23°S) typically have highest surface temperatures around 25°C-30oC (77°F-86oF), while the water below 900 meters is only 5°C (41°F). The temperature differences are very steady over time between days and nights and from season to season. French engineer Jacques D’Arsonval was the first to note the huge potential for using thermal gradients in the ocean to produce power. His proposed system was not implemented until 1930 when his student, George Claude, built the first OTEC plant at Mantanzas Bay, off the coast of Cuba. Although the system functioned, the work needed to run the pumps and other devices (work input) was more than the work output. Furthermore, the difficulties in lubricating underwater pipes, salt corrosion, and growth of algae (biofouling) made the system inoperable and the project was abandoned. Many of the technical difficulties facing Claude have since been resolved. Recently, there has been a renewed interest in OTEC plants and demonstration plants have been constructed near Kailua-Kona on the island of Hawaii. In the United States, Florida and Puerto Rico could also utilize OTEC Technology. The Indian government has also actively sought to invest in this technology and has built a 1-MW floating OTEC plant. OTEC plants can be constructed to operate in either closed or open cycles. In open-cycle systems, water itself is the working fluid. Open-cycle systems work by boiling the surface water at very low pressures (flash evaporation). After expansion through the turbine, steam is condensed back to liquid water via a condenser, which is then cooled by deep seawater before being discharged. The output of the condenser is desalinated water, one of the desirable byproducts of the OTEC system. Closed-cycle systems use the ocean’s warm surface water to vaporize a working fluid, such as ammonia or another refrigerant. The vapor expands through a turbine, turning a generator to produce electricity. The vapor is eventually condensed by transferring its heat to the cold water of the deep ocean, before being pumped back to the evaporator to complete the cycle. A schematic of a Figure 10-12 The map shows the contours of oceanic temperature differences between the surface and 1,000-m depth. Orange and red colors are regions with a DT of at least 20°C. Figure 10-13 Closed-Cycle OTEC 15 Taylor, Craig, et al., “Concentrating Solar Power in 2001,” SolarPaces Corp, 2001. 229 Chapter 10 - Solar Energy closed-cycle OTEC plant is shown in Figure 10-13. In addition to open and closed cycles, OTEC can also be designed as a hybrid. In a hybrid cycle, steam is generated by flash evaporation of the warm surface water and then used as a heat source for a closed cycle. Example 10-1: What is the maximum theoretical efficiency of an Ocean Thermal Energy Conversion (OTEC) plant? Solution: The maximum efficiency is limited to its Carnot efficiency, which can be calculated as: η Ideal, OTEC = 1- = 6.71%(273+5)(273+25) Because of the power required for pumping cold temperature water from the bottom of the ocean and other frictional losses, the efficiency of a practical OTEC device is lower than this value at around 2-3%; this is far below the 30-35% thermal efficiencies commonly found in conventional oil- or coal-fired steam plants. Unlike conventional plants, however, there are no fuel costs. Advantages and Disadvantages OTEC could potentially fulfill a major part of our energy demand with few adverse environmental effects. Because of the huge water resources at our disposal, the technology potential could be significant, especially for those countries that have few fossil resources. Among features that distinguish OTEC from other renewable sources of energy is the wide range of side benefits it offers. Besides desalination, OTEC can provide industrial cooling, space air conditioning, and can allow chilled soil agriculture and aquaculture.16 Cooling – After steam is expanded in the turbine, it is condensed in the condenser. The condensate is relatively free of impurities and can be used in a number of applications such as air conditioning, desalination, and chilled soil agriculture. Aquaculture – Deep water is not only clean, but also has a significantly higher amount of nutrients that allow enhanced growth of marine algae and many marine organisms, such as salmon and lobster, that could not otherwise grow in a tropical environment. Sink - Deep seawater is a huge heat sink for many industrial applications that require dumping of waste heat. The environmental impact associated with OTEC operation is relatively minimal and limited to some changes in local marine life and the release of CO2 as the cold water from the deep oceans are brought to the warm 16 Daniel, H. T., “Ocean Thermal Energy Conversion: An Extensive, Environmentally Benign Source of Energy for the Future,” Natural Energy Laboratory of Hawaii Authority, Kailua-Kona, HI 96740. 230 water surface. This release is however negligible compared to the amount of CO2 release that would result from the use of fossil fuel to generate a comparable amount of energy. The major obstacle to OTEC development is the initial cost and limit of suitable sites near shorelines. The OTEC cold water pipes make up almost all of the costs, so OTEC sites cannot be more than a few kilometers away from the shoreline. Most suitable sites are in areas away from population centers which make the power transmission costs even higher. It is likely that the best solution is to use the electricity generated by offshore OTEC platforms to produce alternative fuels such as methanol and hydrogen which can be loaded onto tankers or flow through pipes.17 It has also been suggested that the solar thermal energy stored in the reservoirs behind hydroelectric dams (thermocline) be used to increase the power output of conventional hydroelectric plants. The data suggests that, for typical thermoclines of 15°C, heat engines can effectively increase the gravitational potential (head) by tens to hundreds of meters and thus can double the hydropower efficiency for many hydroelectric power plants.18

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