Alkali-metal Rankine topping-cycle system design and development

The alkali metal vapor topping cycle with a steam cycle offers the potential of achieving a higher cycle efficiency than the steam cycle alone. An efficiency of about 50% is predicted for a cycle which employs either potassium or cesium with a vapor temperature at 1540/sup 0/F (837/sup 0/C) at the turbine inlet in a binary cycle where the metal vapor cycle is superimposed on a conventional steam cycle. A considerable amount of design and development work has been done by the Oak Ridge National Laboratory and other organizations on the alkali metal vapor Rankine cycle over the past several years. This effort was directed toward power systems for two applications: nuclear auxiliary power systems for space vehicles and central station electric generating plants. A potassium Rankine cycle nuclear space power system was designed and about 25,000 h of development testing was done on system components, including the boiler, turbine, pump and condenser. No component failures or problems nor materials problems were encountered during the course of the testing. Preliminary design studies have been carried out for central station electric power plants of 200 to 600 MW(e) output for potassium and cesium vapor topping cycles with both natural gas and fluidized bed coal firing. A full-scale potassium boiler tube bundle module of the gas-fired plant design was tested at 1540/sup 0/F with gas firing. The potassium boiler performed very well and no boiler leaks were encountered. A leak occurred in a weld at a tee in the liquid return line from the condenser, but this problem was solved by installing a thermal sleeve in the tee to avoid thermal cycling of the metal from liquid temperature variations. The alkali metal vapor power system designs are described and the results of the testing and status of the technology are presented in the paper