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Recent Advances in Marine Science and Technology, 2002

 事業名 海洋科学技術に関する太平洋会議の開催
 団体名 国際海洋科学技術協会 注目度注目度5


INITIAL DESIGN OF ENGINE SYSTEM FOR THE OCEAN NUTRIENT ENHANCER
 
The components and arrangement of OTEC engine system is schematically shown in Figure 2 (Ouchi, 1999). Evaporators and condensers of plate type heat exchanger made by titanium closed piping system of ammonia, as the working fluid, and turbine with electric generator are the main components of the OTEC engine part of the ONE.
 
(Enlarge: 44KB)
Figure 2. Schematic diagram of OTEC engine system
 
Figure 3 shows OTEC system in detail based on Uehara cycle, which is much more advanced system than Rankine cycle. The working fluid, which is the mixture of ammonia and water, is fed into evaporator to be warmed up by the warm surface seawater. After the working fluid evaporates, the vapor drives turbine 1 and triggers generator 1. One to five percent of the vapor in turbine 1 is diverted into heater, which warms up the working fluid arriving from tank 1 , while the rest generates turbine 2. Finally, the vapor is routed to condenser to be cool off and condensed by deep cold seawater. At this stage, OTEC cycle is repeated from the beginning.
 
Figure 3. Schematic diagram of Uehara Cycle
 
Generally, Uehara cycle can attain a 30 to 50% higher efficiency compared with Rankine cycle. Moreover, due to more efficient plate heat exchangers developed by Saga University, the power consumption of pumps for cold and warm seawater can decrease to 30 to 40% of the conventional case. As shown in Table 1, regarding the same gross electric power output planed, the net power of Uehara cycle increase to 75% compared with 65% of Rankine cycle in case of 20℃ temperature difference between warm seawater and cold one.
 
OTEC engine is probably ideal for the ONE. However, an auxiliary diesel engine, which is convenient and reliable, is needed in order to assist the OTEC engine for starting the OTEC engine and maintaining DOW pumping capacity when the OTEC output decreases due to insufficient temperature difference between DOW and surface water. Figure 2 also shows the schematic diagram of the entire engine system of the ONE.
 
Followed by the choice of OTEC and Diesel as a energy supply system, configuration of the floating structure was studied to be fitted with the engine, impeller pump, riser pipe, and discharge nozzle systems under considerations of severe sea condition at the center of Sagami Bay where there is 10 m significant wave height, 14 second average wave period, 50m/s wind velocity and 1.6 knot surface current. The ONE has then been designed with minimum small areas of water plane and the project above the water line, which resulted in little motion of the floating structure decreased riser pipe design scantling (Ogiwara et al., 2001). The principal characteristics of the ONE system designed for Sagami Bay is shown as Table 3 (Ouchi et al., 2001).
 
Table 3. The principal characteristics of the ONE system designed for Sagami Bay
Total height 470m Depth of discharge nozzle 30m
Maximum breadth 30m Diameter of impeller 6.1 m
Draft (During operation) 450m Motor capacity for impeller 80 kw
Draft (During maintenance) 420m Main engine (OTEC) 100kw
Displacement (During operation) 5,800t Auxiliary engine (Diesel) 100kw
Diameter of riser pipe 2.50m DOW rising capacity (m3/day) 500,000
Diameter of surface suction pipe 2.86m Surface suction capacity (m3/day) 750,000
Diameter of discharge ring nozzle * slit clearance 25m*0.7m Discharge capacity (m3/day) 1,250,000
 
APPLICATION OF COMBINED SYSTEM OF ONE OPERATED BY OTEC POWER IN CASE WATERS OFF THE NORTH OF OKINAWA MAIN ISLAND
 
The combined system of ONE operated by OTEC power has then been checked its operation in case waters off the north of Okinawa main island in Japan. Okinawa island locates in the subtropical area where is one of the suitable areas along Japan for OTEC, due to relatively higher temperature of the surface throughout the year. Figure 4 shows vertical temperature profile in waters off the north of Okinawa main island. Surface seawater temperature reached 28.5℃ in summer and 21.0 ℃ in winter, while deep seawater temperature is almost constant at 5℃ on average.
 
Figure 4. Temperature profile in waters off the north of Okinawa
 
It is assumed that DOW pumped up by the same ONE system as Sagami Bay is used for the present OTEC test simulation. In summer, which provides a temperature deference of 23℃ between the surface water and DOW, the OTEC system generates 5.1 MW electric power using 500,000 m3/day of DOW. In this temperature condition, net electricity production of OTEC increases up to 80%, so that this OTEC can provide 4.1 MW electric power for other uses than for the ONE. This surplus electric power can be used direct electricity use through the electric transmission by undersea cables, etc. It means that this combined system of the ONE operated by OTEC generates more electricity than needed for the ONE.
 
CONSIDERING SEASONAL CHANGES OF OTEC OUTPUTS
 
OTEC output is influenced by the temperature difference between surface and deep seawater, so that there are seasonal changes of OTEC outputs. Then it was evaluated in case waters off the north of Okinawa's main island (Fig.5). Gross output of OTEC varies from 5.1 MW in summer to 2.8 MW in winter which is provided by temperature deference of 16℃elsius.
 
Figure 5. The output of electricity generated by OTEC due to changes in sea surface water temperature
 
This seasonal change is due to the change of surface water temperatures, which occurs every year, so that this change is easily predictable. Consequently, with a solid plan for electricity use in an every season, the electric power from OTEC could be used in the best way.
 
SUMMARY
 
Ocean Nutrient Enhancer (ONE for short) for creating a fishing ground by pumping up deep ocean water (DOW) in the open ocean is proposed.
 
By feasibility studies concerning various kind of energy supply source for the main engine system of the ONE, OTEC is found to be the most feasible.
 
A design of ONE applied with an OTEC / Diesel hybrid system is proposed for the project to be operated in Sagami Bay.
 
A simulating case study for the combined ONE and OTEC system in waters off the north of Okinawa main island suggests for producing enough electricity power.
 
Possible influences of seasonal change of surface water temperature on the output of OTEC are evaluated.
 
REFERENCES
 
Iwata, S. and M. Matsuyama. 1989. Surface circulation in Sagami Bay -the response to variations of the Kuroshio axis-, Journal of the Oceanographical Society of Japan. Vol.45.
 
Kobayashi, H., S. Jitsuhara and H. Uehara. 2001. The present status and features of OTEC and recent aspects of thermal energy conversion technologies.
 
Ogiwara, S., Y. Awashima, H. Miyabe and K. Ouchi. 2001. Conceptual design of a deep ocean water upwelling structure for development of fisheries, Proc. of the ISOPE / OMS'01.
 
Ouchi, K. and H. Nakahara. 1999. A proposal of the deep ocean water upwelling machine using density current, Proc. of the ISOPE / OMS'99.
 
Ouchi, K., T. Yamatogi, K. Kobayashi and M. Nakamura. 1998. Density current generator -A new concept machine for agitating and upwelling a stratified water area-, Proc. of Ocean Community Conference '98 Baltimore USA, MTS.
 
Ouchi, K., T. Yamatogi and S. Jitsuhara. 2001. A feasibility study on the energy source for the ocean nutrient enhancer, Proc. of The Fourth (2001) Ocean Mining Symposium, Sept. 2001
 
Ryther, J.H., 1969. Photosynthesis and Fish Production in the Sea. Science Vol.166.







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