Long-Term Oceanographic Survey for Social Implementation of OTEC Plant in Okinotorishima (Seawater Temperature, Salinity, Dissolved Oxygen, Inorganic Nutrients and Density)

In order to implement ocean thermal energy conversion (OTEC), it is necessary to maintain a stable temperature difference between surface seawater and deep seawater. Accordingly, numerous oceanographic surveys have been conducted to select sea areas suitable for OTEC deployment, not only around Japan but also in other countries around the world. However, long-term oceanographic survey data for the Okinotorishima area, a promising candidate for OTEC, are not yet available. While many studies have addressed OTEC design under generic conditions, designs based on specific areas have remained limited. However, the availability of oceanographic data necessary for conducting such studies is still limited. Therefore, a long-term oceanographic survey was conducted, starting in 2006. This study presents a systematic compilation of oceanographic survey results to lay the groundwork for evaluations for the installation of an OTEC plant. The survey items included the vertical profiles of seawater temperature, salinity, dissolved oxygen, and nutrients at each observation station. The results indicate that the temperature difference between surface seawater and deep seawater, which is the most important factor for OTEC, almost exceeded the threshold of 20 °C required for the realization of an OTEC plant. This study provides a foundational dataset for evaluating the feasibility of OTEC deployment, performing economic assessments, and organizing comprehensive long-term oceanographic data in Okinotorishima from an engineering perspective.