Characterization and lifecycle testing of hydride compressor elements for the Planck sorption cryocooler

Abstract

Continuous 20 K sorption-based coolers are being developed for the European Space Agency (ESA) Planck mission. Sorbent beds containing the hydrogen absorbing Lanthanum-Nickel-Tin alloy LaNi4.78Sn0.22 are being tested to evaluate their performance as compressor elements for unprecedented two years of flight operation thus demonstrating the basic performances of the cooler: namely cold end temperature, input power and cooling power. To provide basic characterization and life cycling data, a test facility was developed to test three prototype compressor elements under all conditions expected for the flight cooler operation. Each compressor element is continuously cycled in concentration and temperature to follow the absorption and desorption process assumed for the flight cooler. We present data on the hydrogen mass flow rates and thermal characteristics for all phases of the compressor cycle, with an emphasis on the controlled absorption of hydrogen. This function of the compressor element is the major driver of the cold end temperature and its stability. We also present data obtained over thousands of temperature cycles from these beds to determine the extent of degradation of the compressor element performance from changes in the hydride properties or other sources.