The Honest Ceiling on Ocean Thermal Energy Conversion

Ocean thermal energy conversion (OTEC) uses the temperature difference between warm tropical surface water and cold deep water to run a heat engine. This paper computes, from stated assumptions and with all arithmetic reproducible from an accompanying script, three successive ceilings. First, the Carnot efficiency between surface water at 27 °C and deep water at 5 °C is 7.33%. Second, after heat-exchanger approach temperatures of 2.0 K on each side, a real-cycle factor of 0.55 relative to the reduced Carnot limit, and pumping parasitics, the net efficiency of a reference plant falls to 2.26%. That plant, using a cold-water flow of 100 m³/s and a warm-water flow of 160 m³/s, delivers 36.9 MW net from 1,634 MW of heat drawn from the warm stream. Third, a global ceiling is set by two independent bounds: reheating of the tropical surface by the sun permits at most about 71 TW of electricity before the surface reservoir is steadily cooled by 1 K, but renewal of the cold reservoir by deep-water formation restricts sustainable extraction to about 1.8 TW, with a sensitivity range of 1.1–3.7 TW. The honest global ceiling is therefore of order 2 TW — a substantial resource, comparable to a large fraction of present global electricity generation, but well below the 10–20 TW and effectively unlimited figures sometimes quoted in the popular literature. Every number in this paper is produced by the accompanying computation script; literature inputs are cited where they are used.