Condensers for Open-Cycle Systems
After steam passes through the turbines, it can be condensed in direct-contact condensers or surface condensers. Direct-contact condensers do not have the intermediate solid wall of a surface condenser and, therefore, they provide more effective condensation (Bharathan, Parsons, and Althof 1988). In one design—a two-stage condenser developed at SERI—cold seawater is distributed through two open-ended vessels filled with a commercially available structured packing material. About 80% of the steam is condensed as it flows through the first vessel in the same direction as the cold seawater. The remaining steam is routed into the bottom of the second vessel and flows through it in the opposite direction to the seawater. At the top of the second vessel, a vacuum system pumps out the noncondensable (inert) gases along with any uncondensed steam.
Two-Stage Condenser
Surface condensers keep the cooling seawater separate from the spent steam during condensation. By using indirect contact, the condensers produce desalinated water that is relatively free of seawater impurities. The surface condensers considered for use in OTEC systems are similar to those used in conventional power plants; however, these surface condensers must operate under lower pressures and with higher amounts of noncondensable gases in the steam.
Steam in the open-cycle system contains noncondensable gases that can interfere with power production. These gases—oxygen, nitrogen, and carbon dioxide—are released from the seawater when it is exposed to low pressures under vacuum. Air also enters the open-cycle vacuum vessel through leaks, although good construction techniques can reduce the rate of air leakage to very low levels. Unless these gases are removed from the vacuum chamber, they can interfere with condensation, particularly with surface condensers, by blanketing the condensing surfaces; they can even build up enough pressure to stop evaporation. An exhaust compressor can remove these noncondensable gases. The maximum power required to run the compressor is estimated to be about 10% of the gross power generated by the system (Parsons, Bharathan, and Althof 1985).