Abstract
Seawater desalination is an effective way to reduce water scarcity affecting many world areas. However, desalination processes require considerable power that might not be available in remote areas, especially in insular zones. Ocean Thermal Energy Conversion (OTEC) is a promising technology to be coupled with reverse osmosis (RO) in remote tropical areas thanks to the constant electricity production, the high number of equivalent hours, and the zero carbon emissions.
In this study, the thermodynamic and economic feasibility of a system made of OTEC and RO for freshwater production is assessed to determine the optimal design parameters of the plant. A numerical model was created in Aspen Hysys to simulate all the main OTEC parts, including heat exchangers, pumps, turbine, and seawater pipes. For the RO unit, average performance parameters from the literature were considered. A sensitivity analysis was carried out as a function of evaporation and condensation temperatures of the cycle and riser pipe length. Furthermore, two different fluids, ammonia and R1234yf, and two different materials for the heat exchangers, stainless and polyvinylidene difluoride (PVDF), were compared. The maximum second law efficiency of 26.1% was reached with ammonia at condensation and evaporation temperatures of 13.5 °C and 23 °C, respectively, and a cold seawater depth of 800 m. Despite its higher specific cost, the adoption of stainless steel led to lower water production costs compared to PVDF because of the lower heat transfer areas required. The minimum cost of water of 2.56 $/m3 was obtained with ammonia at a cold seawater depth of 650 m for condensation and evaporation temperatures of 11 °C and 25 °C respectively.