Page - 148 - in Water, Energy, and Environment - A Primer

temperature differential is small and this affects the economic feasibility of ocean thermal energy for electricity generation. 8.7.4.1 Barriers Using cooling water from depths where the water temperature is close to freezing, OTEC systems are then geographically limited to the tropics, where surface water temperatures are highest (85–90°F, 29–32°C). In such situations the maximum thermodynamic heat engine (Carnot) efficiencies are still in the low range of 6–7%. Actual efficiencies achieved to date are only in the range 2–3%. Nevertheless, it is important to point out that a small percentage of a very large number (the thermal energy stored in the oceans) is still a large number. It is estimated that OTEC’s resource potential is larger than that of anyother ocean energy technology. WhereOTECstruggles is that the lowconversion efficiencies require pumping large amounts of seawater and large heat-exchanger surfaces tomake the technology feasible. Large amounts of pumping impose requirements for large amounts of parasitic power on OTEC systems, and large heat exchangers built for reliability in ocean environments are expensive and hard tomaintain (e.g.,maintaininggood thermal conductivity of the heat exchanger surfaces in the presence of microbial biofouling). To date these factors have kept the technology fromcommercial application. 8.7.4.2 OTECtechnologies The technology comes in three types: closed cycle, open cycle, and hybrid. All three require that near-freezing deep seawater bebrought to the ocean surface.The closed-cycle systempumps warm seawater through a heat exchanger to vaporize a fluid with a low boiling point (e.g., ammonia) to power a turbine to generate electricity. Cold water is pumped through a second heat exchanger to condense the vapor to a recycled liquid. Water,Energy, andEnvironment–APrimer148