Ocean thermal energy conversion systems: The heat losses effect of the cold-water pipe

Ocean Thermal Energy Conversion (OTEC) systems are categorized as Renewable Energy Systems (RES), as the natural sea temperature difference (ΔΤ), between the surface water and the deep seawater, can be exploited either to produce electricity or to deliver a by-product. The sea water temperature varies according to location/country, also depending on sea depth and distance from shore. Therefore, the minimum distance from shore can vary in order to meet the required ΔΤ for the system to perform in a beneficial manner. The present study investigates the effect of such cases, by examining the magnitude of the heat transfer loss of the cold-water pipes (CWPs), in terms of the outcome in the theoretical efficiency of the system. CWPs are computationally investigated using the COMSOL Multiphysics software, to assess a more accurate ΔΤ. Contrary to this study, in most literature cases the ΔΤ used in the performance estimations, is the one found on location and without any computational process. The CWP pump was also assessed in terms of required power and case. A parametric analysis of the size of the CWP, mass flowrate, pumping power, pipe material, distance from shore, and a comparison between onshore and offshore positioning of the OTEC systems is presented. Offshore systems seem to exhibit a desired lower temperature difference by up to 86 % from that of long-CWP onshore systems. Overall results indicate, even considering the non-ideal case of heat loss, OTEC systems - onshore or offshore - may become viable, but site specific ΔT estimations are required per case.