Pool Boiler Heat Transport System For A 25 Kwe Advanced Stirling Conversion System

The overall operating temperature and efficiency of solar-owered Stirling engines can be improved by adding a heat transport system to more uniformly supply heat to the heater head tubes. One heat transport system with favorable characteristics is an alkali liquid metal pool boiler. One potential issue with liquid metal pool boilers is unstable boiling. To obtain stable boiling, enhanced boiling surfaces were used with nucleation sites that promote continuous boiling. This paper reports on experiments to determine alkali metal/enhanced surface combinations that have stable boiling. Two enhanced surfaces and two alkali metal working fluids were evaluated. The enhanced surfaces were an EDM hole covered surface and a sintered-powder-metal porous layer surface. The working fluids tested were potassium, and eutectic wdium-potassium alloy (NaK) - both with and without undissolved non-condensible gas. Non-condensible gas and Xe) was added to the system to provide gas in the nucleation sites, preventing quenching of the situ. The experiments demonstrated the potential of an alkali metal pool boiler for use in a solar-powered Stirling engine. The most favorable fluid/surface combination tested was NaK boiling on a -1 00 + 140 mesh 304L stainless. steel sintered porous layer, with no undissolved non-condensible gas. This combination provided stable, high-performance boiling at the operating temperature of 700'C. Heat fluxes into the system ranged from 10 to 50 W/cm/sup 2/. The transition from free convection to nucleate boiling occurred at temperatures near 540'C. Based on these experiments, a pool boiler was designed for a fall-scale 25 kWe Stirling system.