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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


THE ADVANTAGE OF OTEC AS THE ENERGY SOURCE FOR THE OCEAN NUTRIENT ENHANCER
 
Takayuki Watanabe1, Kazuyuki Ouchi2, Toshio Yamatogi3 and Sadayuki Jitsuhara4
 
1Engineering Department, Xenesys, Inc.
Tokyo, JAPAN
watanabe@xenesys.com
 
2Ouchi Ocean Consultant, Inc.
Nagano, JAPAN
 
3Nakashima Propeller Co. Ltd.
Okayama, JAPAN
 
4Engineering Department, Xenesys, Inc.
Tokyo, JAPAN
watanabe@xenesys.com
 
ABSTRACT
 
For fertilizing the ocean by increasing primary production, the ocean nutrient enhancer (ONE) which pumps up the nutrient-rich deep ocean water (DOW) and discharges it into the euphotic zone has been studied. For establishing the entire concept of ONE, various requisite technologies have been evaluated because it is a completely new type of system that we have never had before. This paper focuses on the energy source for the ONE, which is one of the essentials for establishing the ONE.
 
According to feasibility studies, ocean thermal energy conversion (OTEC) has been chosen as the most suitable energy source for the ONE. The use of OTEC enables us not only to use low temperature of DOW, but also to adopt submerged floating structure which could stand for the rough sea conditions. Then actual possible field operation of the ONE was tested by simulating the conditions off the north of Okinawa in Japan, and has come out conclusion producing much more than enough electricity for pumping up the DOW for the ONE.
 
INTRODUCTION
 
Upwelling of deep ocean water (DOW), which contains rich nutrients such as nitrogen, phosphorus, etc. can fertilize the sea, because the poor nutrient conditions in the euphotic zone of the stratified sea can be improved and phytoplankton production can be enhanced. Actual fishing ground naturally developed in waters off Peru is one of the examples of the DOW upwelling area. Many experts are pointing out that sea can be fertilized by upwelling DOW artificially (Ryther, 1969), and this is one of possible solutions for overcoming food shortage in the future.
 
Marino-Forum 21, the subsidiary of the Fisheries Agency of Japan, awaited a research fund and organized the research and development project to design and test an ocean nutrient enhancer (ONE) which pumps up DOW, discharges it into the euphotic zone for the purpose to enhance primary production of the sea. The project has been started in April 2000 for five years.
 
The following four requisite technologies have been focused for establishing the ONE.
(1) How to pump up DOW and to discharge it into the euphotic zone.
(2) TO find out a suitable energy source for pumping up and discharging DOW.
(3) To design configuration of floating structure and riser pipe to stand against rough sea conditions in open ocean.
(4) How to set up the system in the sea.
 
Regarding for the subject (1), density current generator was applied for discharging and plunging DOW horizontally into a certain layer of stratified water as density current (Ouchi et al., 1998). The concept and outline of the ONE operated by density current generator is shown schematically in Figure 1.
 
This concept has such advantages that intentional mixing of DOW with surface water makes density current which could stay in a certain layer in the euphotic zone (Ouchi, 1999). A prototype of density current generator is now operating in Gokasyo Bay, Mie Prefecture, 150 km of the south of Nagoya in Japan.
 
(Enlarge: 82KB)
Figure 1. Schematic Design of the ocean nutrient enhancer
 
For establishing the ONE, it is essential to be reliable, low cost, easy maintenance, and highly efficient, and feasibility studies focusing on the subject (2) and (3) are particularly essential. Choice of an energy source has a great impact on the entire design because it affects the configuration of the floating structure.
 
In this paper, an advantage of ocean thermal energy conversion (OTEC) for the energy source of the ONE is shown based on feasibility studies under considerations of various energy sources and a combined system of the ONE and OTEC is proposed. Finally, the application of combined ONE operated by OTEC system was tested by simulation for the case waters off the north of Okinawa main island, and has come out a conclusion that this system can generate much more than sufficient electricity for pumping up DOW throughout the year.
 
FEASIBILITY STUDIES ON VARIOUS ENERGY SOURCES FOR THE ONE
 
The ONE is set in the open ocean far from the shore because DOW is pumped up from depth more than 300m. Therefore, renewable energy available at site seems more competitive rather than conventional energy sources such as shore-based electric power, diesel engine, etc. Feasibility of various energy sources was evaluated under the following; for the next three cases, DOW is pumped up from 300 m depths and the location of the ONE is ten miles from the shore.
 
Case 1. Pumping DOW of 104m3/day with riser pipe of 0.32m diameter, impeller diameter of 0.9 m shaft power of 5 kw
Case 2. Pumping DOW of 105m3/day with riser pipe of 1.0m diameter, impeller diameter of 2.8 m, shaft power of 45 kw
Case 3. Pumping DOW of 106m3/day with riser pipe of 3.2m diameter, impeller diameter of 8.8 m. shaft power of 400 kw
 
Photovoltaics, wind, wave. OTEC, diesel, and shore-based electric power were evaluated as possible energy source (Ouchi, 2001). The following points were particularly concerned; maximum output power of system, system size, initial cost, running cost, total cost for ten years operation, practicality level, suitable floating structure, and cost of floating structure. Table 1 shows the summary, which includes analyses of two OTEC systems such as Rankine cycle and Uehara cycle.
 
From this study, in terms of the total cost, which includes initial ten years running cost, wind turbine. OTEC, and diesel engine were short-listed as the most feasible energy supply system for the ONE.
 
Table 1. Summary of various energy supply systems for the ocean nutrient enhancer
(Enlarge: 104KB)
 
EVALUATION OF THE ENERGY SUPPLY SYSTEM
 
The ONE is planned to be operated in Sagami Bay, Kanagawa, Japan, where there was a counterclockwise surface voltex current often observed (Iwata et al., 1989). The voltex current could help not to dilute nutrients supplied by DOW so quickly before enhancing primary productivity in the euphotic zone, and this is the main reason to choose this site. Pumping capacity of DOW is planned to be 500,000m3/day from the depth of 450m.
 
Three energy systems such as wind turbine, OTEC, diesel engine, were then further evaluated from the point of practicality in the ocean, initial cost, total life cost, float and mooring cost, influence on environment, use of DOW property, adaptability in Sagami Bay, and scalability to a large system (Ouchi, 2001). Table 2 shows the comparison of the evaluation of three technologies. From this study, it is concluded that OTEC is the most feasible for the engine of the ONE. The output of OTEC engine is affected by the vertical temperature distribution, and the required power for pumping up DOW is affected by the vertical density distribution, so that the engine system of ONE should be designed according to these water quality data.
 
Table 2. Evaluation of energy system for the ocean nutrient enhancer
◎:Good, ○:Fair, △:Poor
Items Energy sources
Wind OTEC Diesel
Practicality in the ocean
Initial cost advantage
Total life cost advantage
Float and mooring cost advantage
Influence on environment
Utilization of DOW property
Adaptability in Sagami Bay
Scalability to large system
Overall evaluation







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更新日: 2022年11月26日

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