Heat Exchangers for Closed-Cycle Systems
Heat exchangers are a big part of the major performance and cost issues relating to closed-cycle systems. Surface heat exchangers must be large enough in area to transfer sufficient heat at OTEC's small temperature difference. Several have been designed. One form of conventional heat exchanger uses a shell-and-tube configuration in which seawater flows through the tubes, and the working fluid evaporates or condenses in a shell around them. This design can be enhanced by using fluted tubes: the working fluid flows into the grooves and over the crests, producing a thin film that evaporates more effectively. In an advanced plate-and-fin design, working fluid and seawater flow through alternating parallel plates; fins between the plates enhance the heat transfer.
Titanium was the original material chosen for closed-cycle heat exchangers because it resists corrosion. However, it is an expensive option for plants that use large heat exchangers. Corrosion-resistant copper-nickel alloys, which can be used to protect platforms and cold-water pipes, are not compatible with ammonia, the most common working fluid. A suitable alternative to these materials may be aluminum. In tests conducted by Argonne National Laboratory (ANL), brazed-aluminum tubes and channels similar to those used in full-size heat exchangers performed well under marine conditions. Results indicate that selected aluminum alloys may last 20 years in seawater (Thomas and Hillis 1989).
Layers of slime and marine organisms can grow quickly on surfaces exposed to warm seawater. In heat exchangers, this buildup—known as biofouling—reduces the heat-transfer efficiency. Laboratory experiments indicate that biofouling can be prevented by mixing chlorine in the pipes intermittently, for a total of about 1 hour a day, at a concentration of 70 ppb. This is well below the level of 200 ppb for 2 hours a day allowed by the U.S. Environmental Protection Agency (Panchal, Larsen-Basse, and Little 1984).