Abstract
An Ocean thermal energy conversion (OTEC) system uses a plate heat exchanger (PHE) as an evaporator and a condenser, and it requires only the thermal energy created by the temperature gradient between surface and deep seawater to generate electricity. Titanium is usually used as the material of plates in the PHE because ammonia is used as the working fluid that is vaporized by hot seawater, which serves as the heat source, inside the PHE. However, the thermal conductivity of titanium is lower than that of the standard material of heat exchanger, and another material is thus required to improve the efficiency of the PHE. Since there are few materials that have high tolerance for ammonia, the authors attempted to use specially coated aluminum as the plate material. In this study, we employed a resin, poly-ether-etherketone (PEEK) and a WIN KOTE® film, both of which have high tolerance for acids and alkalis, as coating materials for aluminum. Because the heat transfer characteristics of aluminum coated by these materials are unclear, especially the saturated boiling heat transfer coefficients of ammonia in pool boiling were measured on the specially coated aluminum surfaces. In addition, since the coated aluminum’s tolerance for ammonia is also unclear, long-term (two months) tolerance tests of the coated materials were performed in ammonia and an ammonia/ water under pool boiling. The results of these tests showed that the heat transfer coefficient of the PEEK-coated aluminum surface was lower than that of a uncoated aluminum surface, in contrast that of the surface coated with the WIN KOTE film was higher than pure aluminum. The heat transfer coefficients and surface condition of these surfaces did not changed during the two months test period. Therefore, it is obvious that proposed coatings should also be used in ammonia pool boiling environments.
