[in Japanese]

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In order to implement Ocean Thermal Energy Conversion (OTEC), it is necessary to secure the stable temperature dierence between surface seawater and deep ocean water, and many oceanographic surveys have been conducted to select ocean areas suitable for OTEC introduction, not only in the seas around Japan, but also in other countries around the world. However, long-term oceanographic survey data for the Okinotorishima area, a strong candidate for OTEC, are not yet available. In this study, a long-term oceanographic survey was conducted to lay the groundwork for evaluation whether the Okinotorishima area is suitable or not for the installation of an OTEC plant. The survey items were the vertical proles of seawater temperature, salinity, and dissolved oxygen at each station, as well as their changes over time. The results indicated that the temperature dierence between surface seawater and deep ocean water, which is the most important item for OTEC, almost exceeded 20℃ that would allow an OTEC to realize except January and February. It became the groundwork to organize salinity graph etc. which are sources of calculating the heat transfer coecient and pump powers.

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This study investigates the current status and challenges in the multi-stage utilization of deep seawater (DSW), which involves making full use of its three major characteristics̶low temperature, purity, and nutrient richness̶before eventual discharge. First, an ideal model of multi-stage utilization is examined, in which the coldness of DSW is used as a thermal energy source, followed by its application in food and drinking water production by utilizing its purity. The water is then used for the cultivation of cold-water species such as salmonids and abalone, and nally, its nutrient- rich content is employed for the cultivation of feed microalgae and macroalgae. In parallel, a eld survey was conducted in three prefectures̶Toyama, Kochi, and Okinawa̶where relatively large volumes of DSW have been utilized over extended periods. The results revealed that, with a few exceptions, multi-stage utilization is currently not being implemented in these regions. The identied challenges include the limited space of current intake facilities and the pipeline systems, which were not designed with multi-stage use in mind, as well as the technical diculty of managing water quality across multiple stages. Nevertheless, considering the cost of seawater intake and the economic benets derived from its use̶factors that have received little attention to date̶the ndings suggest that multi-stage utilization should be seriously considered in the future.

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