Current Status and Future Prospects of Ocean Thermal Energy Conversion : for sustainable energy and water resource On-line Number 5029 Yasuyuki Ikegami 1 1 Institute of Ocean Energy, Saga University, 1 Honjo-machi, Saga-shi, Saga 840-8502 Japan E-mail :ikegami@ioes.saga-u.ac.jp ABSTRACT At the beginning of the 21 st century, we are confronting to new problems as urgent issues. They are global warming of the earth, energy crises, population issue, foodstuffs problem and water problem. Under the circumstances, Ocean Thermal Energy Conversion (OTEC) System is expected as one of effective solution. OTEC system has the characteristic of suitable for large scale, stability, multipurpose, etc. The technology of OTEC has come a long way in the last ten years. In this paper, the principles of OTEC, new development and present state of OTEC technology for sustainable energy and water resource are shown. KEYWORDS OTEC, sustainable energy resource, ammonia/water mixtures, deep seawater, desalination INTRODUCTION At present, the global society has five important problems such as population, energy water, food and environment as urgent issue. Energy, water, air and food are the most important things for human surviving. In this day, the energy resources are mainly from fossil fuels, such as oil, coal, gas, etc. Because of the emission of the gas, which causes the green effect, the utility of fossil fuels results a big problem in the worldwide. According to the world convention on the decrease of the gas emission, it is necessary to develop the fossil furl replacements and the technology on energy saving. Now various kinds of fossil fuel replacements are being developed, including solar energy, geothermal energy, wave energy, wind energy, biomass, etc. However, the development of those kinds of energy is still far from enough for the current energy demands. Thus, we must develop alternative energy resources to oil, gas, coal and uranium. Solar power, wind power, geothermal power are using in the world as alternative energy resources but it is insufficient to supply energy resources that are required in the world. Under the circumstances, Ocean Thermal Energy Conversion (OTEC) System is expected as one of effective solution for sustainable energy and water resource. OTEC system has the characteristic of suitable for large scale, stability, multipurpose, etc. On the one hand, the demand for water increases along with the rapid industry development, population increase and the improvement of the living standard; on the other hand, it is also difficult to find the water resources. Furthermore, the distribution of the rainfall is very different in the different area owing to the recent abnormal weather, and in some regions, the freshwater is mixed with seawater owing to the warming of earth weather and the ascending of the sea level. Therefore, water economizing and purification of river are being implemented in the regions where the water supply is less than the water demand. More unluckily, some countries or regions are lack for not only the energy but also the water resource. Therefore, for those countries and regions, both the energy replacement system and seawater desalination are necessary. Saga university have developed a way to solve those two problems simultaneously, in which the energy is obtained using OTEC and the water resource is obtained by Spray Flash Evaporator Desalination. In the recent decade, the technology of OTEC has come a long way(ikegami and Uehara 2003). Some big demonstrate project have been tried using new technology. In the Institute of Ocean Energy, 1
Saga University (IOES), the new Integrated OTEC system for demonstrate and basic research has been developed in 2003. The IOES has investigated on a highly efficient OTEC system using ammonia/water mixtures, known as the Uehara Cycle. In this paper, the principles of OTEC, present state of OTEC development for sustainable energy and marine products are shown. And the new integrated hybrid ocean thermal energy conversion system of IOES is introduced briefly. OCEAN THERMAL ENERGY CHARACTERISTICS Ocean thermal energy has the following characteristics. 1) Ocean thermal energy just like solar energy is a natural energy and is renewable. 2) Ocean thermal energy is a clean energy which does not pollute the natural environment 3) The volume of ocean thermal energy in the world is large but energy density in low 4) Ocean thermal energy does not fluctuates by the hour, month and day. 5) We can product flesh water, hydrogen, lithium, uranium, seaweed and fish etc. from the sea water that is used in OTEC Plant. OTEC is a system of converting ocean thermal energy into electricity by using the temperature difference between warm sea water at the ocean s surface layer and the cold sea water of the depths. OTEC PRINCIPLES Figure 1 shows the example of ocean sea water profile measured along a vertical depth in the tropics and subtropics. Sea water temperature in the surface layer is a warm 20-30 while sea water temperature at a depth 500 to 1000m in a cold 2-7. Figure 2 shows the principle of OTEC using the Rankine cycle, which is one cycle for OTEC. Open cycle that is shown in Figure 3 can also use for OTEC. In an open cycle warm sea water from the surface layer is sprayed inside a vacuum vessel to create steam. Electrical power is generated as the steam passes through a steam turbine. Steam coming out of the steam turbine enters a condenser and is cooled by cold sea water from a deep layer, to become pure liquid water. Several pilot plants were made since 1930 when G.Claude conducted first experiment for OTEC. An OTEC system using the closed cycle shown in Figure 2 is essentially the same cycle as that used for thermal power generation and nuclear power generation. However the temperature difference (10-20 ) between turbine inlet and outlet in OTEC is fairly smaller than that (120-550 ) in thermal and nuclear power generation. Since the thermal efficiency of cycle in OTEC is smaller than that in the thermal and nuclear power generation, the sea water flow rate is large and the surface area of evaporators and condensers is large. A large amount of research has been carried out into OTEC. In 1994, we invented the new cycle (named Uehara cycle) to enhance the thermal efficiency of cycle which used a mixture of ammonia and water as working fluid. This is explained further in a later section of this paper. HISTORY OF OTEC DEVELOPMENT The history of OTEC development is surprisingly long. It was 122 years ago, in 1881, that the Frenchman, J.D Arsonval, first conceived it. Later another Frenchman, G.Claude, attempted to put OTEC to practical use. From 1926 to 1950 various experiments were tried but they ended in failure. The reason for Claude s failure was that he employed an open cycle. At that time techniques for power generation were somewhat primitive. In 1964 J.H.Anderson and J.H.Anderson Jr. conceived of a new OTEC plant which overcame the weak points of Claude s system and a patent (USA Pat. No.3312054) was obtained. This invention of Anderson s attracted considerable attention and created the opportunity for rekindling research 2
concerning OTEC. Later, the first energy crisis in 1973 provided the motivation for Japan and America to perform fundamental research. The principle research outcomes which followed are listed in Table 1. Recently actual OTEC plants have been constructed in rapid succession thus enabling experiments to be performed. In 1981, Dr. Kalina introduced his new invention of Kalina cycle, in which ammonia / water mixture was used as the working fluid. Uehara et al(1994) verified the effectiveness of utilizing Kalina cycle for an OTEC plant, where they analyzed the thermal efficiency of cycle, the concentration of ammonia / water mixture, the heat transfer performance of the regenerator, the effect of the temperature and pressure at the inlet of the turbine. As a result, under the condition that the temperatures of the warm and cold seawater were 28 and 5 C, respectively, the thermal efficiency of cycle can reach 5%. Uehara et al. (1994) developed a new cycle (Uehara Cycle) with absorption and extraction processes, and demonstrated the cycle performance. In this cycle, ammonia / water mixture was used as the working fluid. Compared with Kalina cycle, the thermal efficiency of Uehara cycle is about 1-2 % higher in theoretical consideration. After operating the cycle using the 30 kw experimental equipments, IOES has got the thermal power out of turbine of 30kW. For Study on the spray flash evaporation, Miyatake et al.(1981) proposed the spray flash evaporation method that can evaporate the seawater under relatively low temperature and efficiency for more than 40. Uehara et al. (1988) optimized the hybrid cycle, which combines OTEC with seawater desalination method in order to utilize the ocean thermal energy effectively. Then, they employed with the SFED as a seawater desalination method and developed the optimization method for combining OTEC cycle with SFED. Further, they (1889) studied experimentally to develop the spray flush device in order to improve the performance of hybrid cycle. The study indicated that it is possible to evaporate the seawater well using the spray flush evaporation process if the liquid temperature is 30 C. Uehara et al. (19996) implemented the performance analysis for Integrated-Hybrid OTEC (I-H OTEC) cycle, which consist of OTEC and SFED, and it was also compared with Joint-Hybrid OTEC (J-H OTEC) They reported that the I-H OTEC cycle can obtain 33-80% higher desalination ratio than the J-H OTEC. Figure 1. Temperature distribution of seas water 3
Figure 2. The principle of OTEC (closed cycle) Figure 3. Open cycle OTEC Table 1. History of OTEC 1881 French scientist J. D Arsonval presented concept of OTEC 1926 Initial study for commercialization by French scientist 1933 Cluade built a floating 1200 kw OTEC Plant, but it failed 1970 Research Association of New Power Generation System started the experiment as one of the theme. 1973 Saga university started experiment OTEC was a priority subject for Sunshine Project. 1974 USA launched their OTEC study under ERDA project First International OTEC Conference in USA 1977 Saga University succeeded in 1kW OTEC experiment 1979 Mini-OTEC by USA succeeded in generating 50kW OTEC 1980 Saga University conducted off-coast experiment off 1981 Tokyo Electric Co. and others succeeded in 120kW OTEC test in Nauru Republic. 1982 Kyushu Electric Co. succeeded in generating 75kW in 1985 Saga University built 75kW OTEC pilot plant.. 1988 OTEC Association, Japan was organized. Member was 25 private companies of electric power, engineering, 1989 First Deep ocean water utilization facility was built off Toyama bay by Science & Technology Agency. 1990 IOA (International OTEC Association) was organized by Taiwan, USA, Japan and others. 1993 USA built 210kW pilot plant, that is open cycle on Kona 1994 Saga University started construction of experimental plant to study Uehara Cycle. 1995 Saga University started their experiment using Uehara 1997 National Institute of Ocean Technology, India & Saga University signed memorandum on technical 1999 International OTEC/DOWA Conference was held in Imari city, near Saga University 2001 1MW floating OTEC plant was completed by NIOT 2002 Experiment of NIOT OTEC plant will start. Forum on Desalination using Renewable Energy in Palau (FDE in Palau) was held in October New Pilot Hybrid OTEC Plant will be constructed in 2003 FDE in Saga and 3rd WWF in Kyoto was held in March OTEC USING AMMONIA/WATER MIXTURES AS WORKING FLUID We, Saga University research team have carried out the research of OTEC using the Rankine cycle since 1973 and reported the results of many theoretical and experimental researches. In 1981 Kalina invented various thermal power generation system using a new cycle (named Kalina cycle) which ammonia and water mixtures is used as the working fluid. In1994, Uehara and Ikegami(1996) invented a new cycle (named Uehara cycle) for the vapor power generation. A block diagram of Uehara cycle is shown in 4
Figure 4. An Ammonia/water mixture is used as the working fluid in Uehara cycle. The concept presented in the figure can be explained as follows. 1) The ammonia/water mixtures are sent to evaporator by working pump2 and heated by warm sea water to evaporate. 2) The vapor and liquid mixture of ammonia/water mixtures enters the separator and is separated the liquid and vapor of ammonia/water mixtures. 3) Ammonia/water mixtures vapor enters ammonia/water turbine1, and the ammonia/water turbine is rotated as mixture vapor flows out. At that time, electrical energy is generated by a power generator linked to turbine1. Meanwhile, the ammonia/water mixtures vapor passing through the turbine1 enters the turbine2. At that time, some of mixtures vapor is extracted and enter heater and is condensed by mixtures condensate passed from working fluid tank1. 4) The tarbine2 is also rotated by mixtures vapor, and electrical energy is generated by power generator linked to turbine2. Meanwhile, the ammonia/water mixtures vapor passing through the turbine2 enters the absorber. 5) Meanwhile, ammonia/water mixtures liquid separated in separator passed through regenerator and decompression valve enters absorber. 6) Ammonia/water mixtures liquid entered absorber absorb some of the ammonia/water mixtures vapor entered from turbine2, and flow down into tank1. The ammonia/water mixtures vapor that mixtures liquid can not absorb enters the condenser. The condensate enter tank1. The condensate pump up by working fluid pump1 enter heater, and condense the mixtures vapor extracted from tubine1 outlet. 7) The condensate passed through heater mix with condensate of heater, and enters tank2. The condensate is pumped up the regenerator by working fluid pump2, and heated the mixture liquid entered from separator and enter the evaporator. By this cycle power can be generated using only sea water, without using oil, coal or uranium. Figure 4. OTEC system using a new Figure 4. OTEC using Ammonia/Water Mixtures (Uehara Cycle) 5
INTEGRATED HYBRID OTEC SYSTEM OF IOES OTEC has OTEC system has the characteristic of suitable for large scale, stability, multipurpose, etc. The use of multipurpose is for Electricity, Fresh Water, Hydrogen Generation, Lithium Production, and Aquaculture etc. The Integrated Hybrid OTEC System for Multipurpose is shown in Figure 5. In 2002, Saga University established Institute of Ocean Energy, Saga University (IOES) for study on integrated hybrid OTEC system. In 2003, experimental facilities of integrated hybrid OTEC system are constructed in IOES. Then integrated hybrid OTEC system of IOES is shown in Figure 5. The integrated hybrid OTEC system consists of 30kW OTEC plant using Uehara cycle, spray flash desalination plant, hydrogen generator, hydrogen storage plant, fuel cell using hydrogen, lithium collector and experimental equipment for observing the plume of the discharged sea water. The building and experimental facilities of integrated hybrid OTEC system are contracted by support of Ministry of Education, Culture, Sports, Science and Technology (MIXT). In this chapter, these systems are introduced briefly. Figure 5. Products of Hybrid Figure 5. Integrated Hybrid OTEC System for Multipurpose OTEC plant and desalination plant Diagram of OTEC plant and desalination are shown in Figure 6. Figure 7 shows the turbine of OTEC plant. Figure 8 shows the flash chamber of Desalination plant. The out put power of OTEC plant using Uehara cycle is 30kW for design conditions that inlet warm sea water temperature is 28, inlet cold sea water temperature is 8. In the OTEC plant plate type evaporator, condenser, regenerator, heater are used. The plate type condenser is also for condenser of desalination. The desalination plant can produce the fresh water of 10 tons per day. 6
OTEC Desalination Figure 6. Hybrid OTEC Plant with Desalination System Figure 7. Turbine for OTEC for 30kw Figure 8. Desalination Plant (Flash Chamber) Hydrogen generator, storage and fuel cell Hydrogen is expected to be the energy of the 21 st century. However, the hydrogen used for fuel cell and hydrogen engine in now is almost produced using fossil fuel. Therefore, it is necessary to product 7
quantities of cheap hydrogen without fossil fuel. We construct the hydrogen production plant with electrolyzation process using pure water produced by desalination plant and OTEC plant. This hydrogen production plant can produce the hydrogen of 1Nm 3 /h with purity more than 99.9999%. Two tanks of hydrogen storage are made of Metal hydride. The capacity of the larger tank is 10 Nm 3, that of the smaller tank is 1 Nm 3. Fuel cell of polymer electrolyte fuel cell(pefc) type has the out power of 900w. Figure 9 shows the picture of the hydrogen generator, hydrogen storage and fuel cell. Figure 9. Hydrogen Production System using OTEC Experimental equipment for observing plume of discharged sea water Experimental equipment for observing plume of discharged seawater is made. In this facility, the vertical temperature and salt density of seawater, the height of wave the speed of wave can be changed. The water tank provided in this equipment has length of 10m, width of 1m and depth of 1.6m while actual depth of water is 1.2m. Figure 10 shows the water tank of experimental equipment for observing plume of discharged seawater. Figure 10. Deep Ocean Water Simulation 8
Lithium recovery equipment Lithium is one of important energy resources for solutions of the 21 st century energy issues. The depth seawater is clean and includes many mineral such as lithium. The concentration of lithium in the depth seawater is very low with 0.1~0.2ppm. Therefore, lithium correction equipment is constructed to correct lithium included in the depth seawater with high efficiency. INTERNATIONAL TREND OF OTEC Recently, many countries in the world has interest on OTEC system, such as Palau, Philippine, India, Fiji, Kucks Island, etc. OTEC research group of Saga University and National Fishers University measured the temperature and density of sea water in ocean of Palau and Fiji and selected the suitable site to construct OTEC plant. Our group can be selected the site of OTEC for each country using database measure temperature and density, current velocity and topography. Since 1997, Saga University has been implementing a joint development of a 1 MW practical OTEC plant with Indian National Institute of Ocean Technology (NIOT). Under mutual collaboration, they had built in March 2001 a barge named Sagar Shakthi, shown in Figure.10 and Figure 11, on which the whole OTEC power plant was completely installed. But the connection between the barge and the intake pipe of 1000m has not yet been completely. The whole budget of this project was supported by India government. They are to resume test operation in the coming fall. After finishing the project of 1MW OTEC, Uehara Cycle will be implemented. After the evaluating performance of the 1 MW OTEC, a 25-50 MW commercial OTEC plant will be constructed. Figure. 10 Indian floating type OTEC power plant, 1,000kW Figure. 11 Final Configuration after Deployment 9
Figure. 12 The Plan of OTEC Project in Palau The Republic of Palau sent their first delegation to IOES in April 2000 to get the first hand information on OTEC power generation technology. Their concern is that OTEC technology would serve to vitalize their indigenous industry, securing fresh water source through desalination process. In compliance with the request by then former President Mr. K. Nakamura for technical collaboration on OTEC technology, IOES dispatched a study mission to survey possible sites for construction of OTEC power plant. In following up these step stones, incumbent President Mr. T. Remengesau Jr. and his delegation visited us in April 2001 and this has resulted in signing of agreements on scientific exchanges between Palau and Saga University. His government has tentatively selected a construction site near at the new capital city, which is located very close to the ocean, and is currently conducting various preparatory works for realization of OTEC power plant. IOES held a Forum on Water in Palau in the October 2002, under close cooperation by the government of Republic of Palau. Main theme is desalination of seawater through OTEC technology. This Forum is planned as a pre-forum of the 3 rd World Water Forum to be held in Kyoto. The Plan of OTEC Project in Palau is shown in Figure 12. CONCLUSION In this paper, the principles of OTEC, new development and present state of OTEC technology for sustainable energy and water resource were shown. And the new integrated hybrid ocean thermal energy conversion system of IOES was introduced briefly. I may say that OTEC technology is gaining the worldwide attention as one of the most reliable means to give a solution to the major problems in the 21 st century I mentioned in the beginning. I firmly believe OTEC technology is a good news particularly to countries who are suffering from lack of fresh water and energy. We are ready and happy at any time to exchange views on what we can possibly offer to the needed. 10
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