Trade-Off Study of Heat Rejection Systems for Two-Stage Stirling Cooler

A combined two-stage Stirling and Joule-Thomson cycle refrigerator has been developed for advanced celestial observation missions. The specified cooling capacity is 40 mW at the final 4 K stage and 200 mW at the second 20 K stage. The precooler compressor power amounts to about 100 W, most of which is thermally dissipated. This study has therefore been made to propose a most suitable method of the waste heat rejection. Proposed are a loop heat pipe (LHP) and a mechanical pump loop (MPL). Computational sizing procedures of the two are presented with mass estimate models. Also presented is an empirically obtained relation between the cooling efficiency and the ambient temperature. This algebraic expression is then used to find the required electrical power for the 20 K stage cooling. Numerical results of the LHP/MPL design calculations are displayed in the figures as a function of the ambient temperature ranging from 150 K to 310 K. Considered as working fluids are ammonia, propane, propylene, and ethane, for each of which the LHP/MPL sizing has been done. A graphical comparison of the required radiation surface areas and that of the estimated weight values are made for a trade-off between the LHP and the MPL and also for the coolant selection. An all heat pipe system is taken as reference when compared those two. A result of the trade-off is that both are acceptable but rather MPL than LHP would be preferable in our case. This somewhat unexpected conclusion mainly results from possibilities of the weight reduction. A brief description of fabricated ammonia LHPs and developed trochoid/scroll gear pumps is given to discuss the trade-off from another viewpoint. Measured thermal conductances and measured pump efficiencies are plotted in the figures to show a present state of the LHP/MPL development. It has thus become clear that a considerable effort is still necessary in improving the performances.