Numerical simulations of ecosystem change due to discharged water from ocean thermal energy conversion plant

Ocean Thermal Energy Conversion (OTEC) is one of ocean renewable energy production technologies which exploits temperature difference between surface ocean water and deep ocean water to generate electric power. In Japan, demonstration and commercial scales of 1MW-100MW OTEC plants are being planned. Since the OTEC plant necessitates large flows of ocean water, it is important to investigate environmental impacts due to the discharged water which has lower temperature and higher nutrient concentration than the surrounding seawater. The present study focuses on the influence of the discharge water from OTEC plant of demonstration scales by numerical simulations. MEC-NEST is employed for calculating hydrodynamic processes, and a nested grid system is used to calculate the phenomena near the OTEC plant with finer grid. A lower trophic ecosystem model is embedded on MEC-NEST to analyze behaviors of nutrients and planktons. The simulations were carried out for 100MW offshore plant and 1MW onshore plant. For the offshore floating-type plant case, the discharged water from OTEC plant descends as being mixed with surrounding seawater and horizontally spread at the depth where the density of mixed water becomes equal to that of surrounding seawater. Changes of water temperature and water qualities such as nutrients and planktons concentrations due to the discharged water are not significant because it is diffused rapidly in the ocean current. For the onshore plant case, the impacted area is rather limited in the vicinity of discharged point because of the smaller discharge rate from the plant. It is also predicted the changes of water qualities becomes smaller when the discharge point is set at deeper location.