نیروگاه تبدیل انرژی اقیانوسی (Ocean Thermal Energy Conversion (OTEC
Ocean Thermal Energy Conversion (OTEC)
Author: Thomas H. Daniel, Ph.D., The Natural Energy Laboratory of Hawaii Authority (NELHA). The world's largest solar collector absorbs a tremendous amount of the sun's energy, averaging about 65 million gigawatts (a gigawatt is one million kilowatts), or 570 quadrillion kW-hr per year - more than 5,000 times the amount of energy used in all forms by humans on the planet. A typical square mile of that collector - otherwise known as the surface waters of the Earth's vast oceans - absorbs an average of about 500 MW, or annually more energy than the equivalent of 2.6 million barrels of oil . The concept of ocean thermal energy conversion (OTEC) uses the natural difference that exists between warm tropical surface waters and those at depth. Since the ocean temperature changes little from night to day or - in the tropics -with the seasons, an OTEC power plant is able to generate electricity continuously, unlike many other renewable energy sources. This idea originated with a French physicist, Jacques D'Arsonval, in 1881. His pupil, Georges Claude, built the first plant at Matanzas Bay, Cuba in 1930, with a gross output of up to 22 kilowatts.
How it works
OTEC generates electricity by using the temperature difference of 20°C (36°F) or more that exists between warm tropical waters at the sun-warmed surface, and colder waters drawn from depths of about 1000 m. To convert this thermal gradient into electrical energy, the warm water can be used to heat and vaporize a liquid (known as a working fluid). The working fluid develops pressure as it is caused to evaporate. This expanding vapor runs through a turbine generator and is then condensed back into a liquid by cold water brought up from depth, and the cycle is repeated. There are potentially three basic types of OTEC power plants: closed-cycle, open-cycle, and various blendings of the two. All three types can be built on land, on offshore platforms fixed to the seafloor, on floating platforms anchored to the seafloor, or on ships that move from place to place [2,3,4].
Closed-Cycle Ocean Thermal Energy Conversion
In a closed-cycle OTEC process, first proposed in 1881 by French physicist Jacques D'Arsonval , warm surface water is vaporizes a working fluid (such as ammonia) in a heat exchanger (evaporator). The ammonia vapor is then condensed back to liquid by thermal contact with the cold water through another heat exchanger (condenser) and re-cycled. At all times, the working fluid remains in a closed system and is continuously circulated. Since ammonia vaporizes and condenses near atmospheric pressure at the available seawater temperatures, it provides a sufficient pressure drop across the turbine so that it can achieve relatively high efficiency at modest size compared to the open-cycle system (See More). Since this technology is essentially similiar to standard refrigeration systems, there is sufficient experience with the components to allow straightforward scale-up to commerical sizes.
The first electric 50-kilowatt closed-cycle OTEC
demonstration plant called "Mini-OTEC" deployed by the
National Energy Laboratory of Hawaii.
(Image courtesy of NELHA)
Closed-Cycle Ocean Thermal Energy Conversion (continued)
The heat exchangers (evaporator and condenser) are a large and crucial component of the closed-cycle power plant, both in terms of actual size and capital cost. Much of the work has been performed on alternative materials for OTEC heat exchangers, leading to the recent conclusion that inexpensive aluminum alloys may work as well as much more expensive titanium for this purpose. Though this process does not produce desalinated water as a direct byproduct, the cold water (warmed only about 4°C by the OTEC process) can condense large volumes of fresh water when it is passed through a heat exchanger in contact with the humid tropical atmosphere.
Other considerations associated with a closed-cycle OTEC power plant are the potential leakage of ammonia and the discharge of small amounts of chlorine that are added to the ocean water to prevent fouling of the heat exchangers. Practices developed over the past 100 years in the refrigeration industry can minimize ammonia leakage. Experiments at the Natural Energy Laboratory of Hawaii  have demonstrated that very small, environmentally benign, levels of chlorine can successfully control the micro-fouling that would dramatically diminish the efficiency of the heat exchangers at the small delta-T available for OTEC operation.
The world's first net power producing OTEC plant, called "Mini-OTEC," was deployed in 1979 on a barge off the Natural Energy Laboratory of Hawaii by the State of Hawaii, Lockheed Ocean Systems, and other private sector entities. This plant operated for three months, generating approximately 50 kilowatts of gross power with net power ranging from 10-17 kilowatts . Though only about 20% of Mini-OTEC's gross power was available for export, the net-to-gross ratio will approach 75% for plants larger than about 10 megawatts, making the process more commerically attractive.
Since the pressure drop across the turbine is the difference between the low pressure at which the water vaporizes and the lower pressure remaining after condensation, open-cycle systems require very large turbines to capture relatively small amounts of energy. Georges Claude, the inventor of the open-cycle process, calculated that a 6 MW turbine would need to be about 10 meters in diameter, and he could not design a realistic turbine larger than this. Recent re-evaluation of Claude's work  indicates that modern technology cannot improve significantly on his design, so it appears that the open-cycle turbines are limited to about 6 MW. The multiple turbines required for a commercial-sized OTEC plant will significantly increase its complexity and reduce its efficiency.
Less than one half of one percent of the incoming ocean water becomes steam, so large amounts of water must be pumped through the plant to create enough steam to run the large, low-pressure turbine. This does not substantially reduce the surplus or net electrical power, however, since pumping surface seawater requires little energy. In an ideal open-cycle plant, the vacuum pumps could be shut down after start-up, since all the water vaporized in the evaporator would be condensed in the condenser, leaving behind a vacuum. In the real world, however, both inevitable vacuum leaks and non-condensible gases dissolved in the surface and deep seawater necessitate continuous operation of the vacuum pumps. The overall thermal to electrical efficiency of these traditional open- and closed-cycle OTEC plants is very similar, approaching 2.5%. Though this is low compared to traditional power generation systems, the extent of the ocean thermal resource is sufficient to provide tremendous power outputs discussed in the introduction.
In 1993, the Pacific International Center for High Technology Research (PICHTR) designed, constructed, and operated a 210-kilowatt open-cycle OTEC plant at Keahole Point, Hawaii. When this demonstration plant was operational, it set the world record for OTEC power production at 255 kilowatts gross . The seawater pumps and vacuum systems consumed about 170 watts, so the nominal net output of this experimental plant was about 40 kilowatts. Following successful completion of experiments, the 210-kilowatt OTEC plant was shut down and demolished in January 1999 .
An alternative open-cycle process, called "Mist Lift" by its U.S. inventor, Stuart Ridgway, avoids the necessity of a large vapor turbine, but retains the potential to provide the inherent higher efficiency of the open-cycle. Ridgway proposes  to use the pressure difference in an open-cycle system to lift a mist of liquid water droplets entrained in a rising vapor stream to significant elevations. The liquid water would then be separated from the vapor and pulled by gravity down through a liquid or hydraulic turbine, which is much more compact and more easily scaled to large power outputs. Ridgway performed experiments at the National Energy Laboratory of Hawaii in the early 1980's  in which he generated appropriately-sized mist droplets and demonstrated that the vapor to droplet coupling was as his calculations predicted. Little further work has been performed on this process.
• Clean energy production. OTEC has remarkably little adverse environmental impact, especially compared with other energy sources of comparable size. OTEC is inherently not exothermic, so it does not adversely contribute directly to global warming, as do, for example fossil fueled and nuclear plants. Nearly all human energy requirements can be supplied from this one source without significantly affecting the overall temperature structure of the ocean. Since the cold or mixed water will be discharged at depth, impacts on the atmospheric temperature or concentration of carbon dioxide, a greenhouse gas, will be minimal;
• Fresh water production. OTEC plants can produce fresh water as well as electrivity. Open-cycled and hybrid plants can directly produce fresh water as well as electricity and closed-cycle plants can produce similar volumes by condensation from the atmosphere. This is a significant advantage in island areas or deserts were fresh water is limited ;
• Continuous power. Unlike most other sources of renewable energy which vay with weather and time of day, OTEC power plants can produce electricity 24 hours a day, 365 days per year. Since the ocean doesn't change temperature at night, the solar energy stored in the seas is always available ;
• Energy independence. OTEC plants built on the coast or moored offshore could provide enough power and water to make tropical areas independent of costly fuel imports;
• Worldwide applicability. Production of fuel, such as hydrogen, by tropical OTEC plants can provide the benefits of low-cost OTEC power to the whole world ;
• Aquaculture enterprises. Deep seawater discharged from an OTEC plant is cold, rich in nutrients, relatively free of pathogens, and available in large quantity. This is an excellent medium for growing phytoplankton (microalgae), which in turn can support the production of a variety of commercially valuable fish and shellfish . Suitable mixing of the warm and cold water discharges, can provide large volume flows of seawater at any temperature between those of the surface and deep seawater, allowing temperature optimization throughout the growth cycle of cultured organisms -merely by turning a valve;
• Air-conditioning/refrigeration. The deep-ocean cold water can be used as a chiller fluid in air-conditioning systems. For example, only 1 m3 s-1 of 7°C deep ocean water is required to produce 5800 tons (roughly equivalent to 5,800 rooms) of air conditioning. This will typically require a pipeline about 1 m in diameter and the pumping power required will be about 360 kW, compared to 5000 kW for a conventional AC system. The investment payback period for a stand-alone air-conditioning system can be as little as 3 to 4 years, depending on the specifics of the pipeline installation. Combining the air-conditioning with OTEC and/or aquaculture systems can make the technology even more attractive. Cornell University installed a "Lake Cooling" system in 1999 that uses 100 m deep water from Cayuga Lake to cool the campus. This 20,000 ton system saves Cornell over 20 million kw-hrs annually, even though the air conditioning is only needed in the summer time. The savings would be even greater in the tropics where OTEC systems are viable. Space cooling is by far the most economically valuable use of deep cold seawater available now [18,19];
• Mineral extraction. OTEC systems could provide the opportunity to mine for some of the elements in the ocean water solution. In the past, most economic analyses showed that mining the ocean for trace elements dissolved in solution would be unprofitable because so much energy is required to pump the large volume of water needed and because it is so expensive to separate the minerals from seawater. However, because OTEC plants will already be pumping the water, the cost of the extraction process is the only remaining factor. Investigations are underway to determine the feasibility of combining the extraction of uranium dissolved in seawater with ocean energy production .
Drawbacks of OTEC power development include the following:
• Low efficiency. The small temperature difference between the heat source (warm surface water) and the heat sink (cold deep water) temperature gives OTEC plants a typical thermal to electrical energy conversion efficiency of less than 3 percent. The greater the temperature difference between the heat source and a heat sink, the greater the efficiency of an energy-conversion system. In comparison, conventional oil- or coal-fired steam plants, which may have temperature differences of 500°F, have thermal efficiencies around 30 to 35 percent. To compensate for its low thermal efficiency, an OTEC plant has to move a lot of water. That means OTEC plants have a large "hotel load." In other words, OTEC-generated electricity has a lot of work to do at the plant before any of it can be made available to the community power grid. For plants larger than about 10 megawatts, about 25% of the "gross" power will go to pump the water through the intake and discharge pipes of the OTEC system. Remember, however, that the ocean can provide effectively infinite amounts of the seawater "fuel" for free .
• High capital costs for initial construction. About 75% of the capital cost of current OTEC designs will be for the deep seawater pipeline. These piplines must extend to 3,000 ft. depth and allow the pumping of very large volumes of water. A 100-megawatt plant, for example, will require about 215 m3 s-1 (3,400,000 gal/min) of deep seawater, necessitating a minimum pipe diameter of 10 m (32.8 ft.). Such large pipelines would currently be made of fiberglass-reinforced-plastic (FRP) or reinforced concrete pipe (RCP), both very expensive materials. If means can be found to install and operate the large pumps at the bottom end of the pipelines, inflatable pipes made of polyethylene or other flexible materials might allow dramatic reductions in materials and installation costs ;
• Potential ecological consequences. The flow of water from a 100-megawatt OTEC plant, for example, would equal the of a major river - equivalent to the nominal flow of the Colorado River into the Pacific Ocean (1/30 the Mississippi, or 1/10 the Danube, and 1/5 of the Nile). In fact, the discharge flow from 60,000 megawatts (0.6 percent of present world consumption) of OTEC plants would be equivalent to the combined discharge from all the rivers flowing into the Atlantic and Pacific Oceans . Since the salinity of the ocean is nearly uniform, these large discharges will not significantly affect the salinity of the receiving waters. The temperatures of the seawater discharges will be some 3°C (6°F) above or below their initial temperatures. If the warm and cold discharges are mixed, they will have an intermediate temperature near 18°C (64°F). In any event, the water will need to be discharged at a depth below the bottom of the surface layer in order to avoid contaminating the surface water intake. At that depth, somewhere below 100 m, the discharge will be denser than the water at that depth and will disperse gradually downward, having little impact on the surface layer where most life exists. The resulting changes in temperature could have an impact on the local ecology ;
• Siting considerations. OTEC plants must be located where a difference of at least 20°C (36°F) occurs year round - mostly limited to tropical waters . Ocean depths must be available fairly close to shore-based facilities for economic operation. Floating plant ships could provide more flexibility, serving as sources for fuel for distant regions ;
3. Ammonia storehouse; 4. Warm water supply;
5. Replacement of cold water; 6. Replacement of warm
water; 7. Condenser; 8. Turbine; 9. Replacement of
cold water. Conclusions
OTEC has tremendous potential to supply the world’s energy. This potential is estimated to be about 1013 watts of baseload power generation . However, OTEC systems must overcome the significant hurdle of high initial capital costs for construction and the perception of significant risk compared to conventional fossil fuel plants. These obstacles can be overcome only by progressing beyond the present experimental testing and evaluation of small-scale demonstration plants to the construction of pilot-sized and, eventually, commerical-sized plants to demonstrate economic feasibility. As a UN Development Program study determined, the confidence to build commercial-sized OTEC plants will not develop until investors have the demonstration of a 5-megawatt pilot plant operating for 5 years. This demonstration will require a significant investment with little potential near-term return.
For the near-term future development of OTEC systems, isolated niche markets with high conventional energy costs and a need for energy independence may provide a viable venue for market penetration in the size range of 1 MW to 15 MW. These may provide the demonstration required for penetration into larger markets where economically competitive plants of 50 - 400 MW will be viable.
It appears that OTEC technology might become more financially competitive if it could capitalize on the many value-added byproducts that can be produced from the deep seawater. Though many of these aquaculture and energy-related byproducts appear promising, insufficient data and economic models have thus far been developed to convince potential investors that the overall system will be profitable. Such data are now being developed at the Natural Energy Laboratory of Hawaii Authority at much smaller scale than that required for OTEC development.
1. Average absorbed = 400 cal/cm2/da (Knauss, p. 28) = 194 W/m2. Ocean surface
area = 3.35328 x 1014 m2, so average total absorbed = 6.5 x 1016 watts. Alternatively,
this comes to 5.7 x 1017 kW-hr/yr. From WorldWatch 1997 data, human energy
consumption is about 1.07 x 1014 kW-hr/yr, so the annual input is about 5,330 times the
annual consumption. The ocean surface area = 129,400,000 sq. mi., so the average
input is 5.023 x 108 watts/sq. mi. (~500 MW/sq. mi.). This is equivalent to 4.4. billion
kW-hr/yr, or 2.59 MBOE.
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