Potential environmental impact of open-cycle thermal energy conversion

Abstract

Ocean Thermal Energy Conversion (OTEC) is a power-generating system that uses the temperature difference between warm surface water in the tropical ocean and the cooler water at depth to run a Rankine-cycle heat engine. For the temperature difference typically available in the upper 1000 meters in tropical and subtropical waters, 20-25C, energy extraction efficiencies are low, two to four percent (Dugger et al. 1981), compared to conventional steam generation plants. Because of the low efficiencies involved, large flows of ocean water (the fuel in an OTEC system) are required: about 10 m3/sec per megawatt (DOE 1979a). Two OTEC operating cycles are currently under development in the United States at Argonne National Laboratory, Argonne, Illinois and at the Solar Energy Research Institute at Golden, Colorado: closedand open-cycle. Because the technology is highly experimental, most of the technical details have not yet appeared in the literature. In closed-cycle systems a low-boiling point working fluid (ammonia or Freon) is evaporated by warm surface waters. The vapor is expanded to drive a turbine. The expanded vapor is then condensed by cooler deep ocean water and returned to the warm side. In open-cycle operations warm surface water is used as the working fluid. Surface sea water is introduced into a evaporator under partial vacuum separating the sea water into steam and brine. The steam, after passing through a turbine, is condensed using cold ocean water. Several configurations for OTEC plants have been considered to date: freefloating plants (grazing and moored), bottom-resting facilities (at various distances from shore), and shore-based plants. The floating and far-offshore con-