Thermodynamic performance comparison of a two-stage cryocooler and cascade cryocoolers with thermal anchoring for precooling purpose

Abstract

This paper describes the prospective issue of actively incorporating a cryocooler for thermal anchoring purpose to block the heat ingress from room temperature and reduce the everlasting parasitic heat load. Two systems, one with a single two-stage cryocooler and the other with two independent cascade cryocoolers, are examined in this paper to elucidate the involved thermodynamic inefficiencies in terms of energy and entropy flows. The overall system efficiency is greatly affected by the percent Carnot efficiency of the actual cryocooler covering the large temperature span. The single two-stage cryocooler configuration is more efficient than the cascade cryocoolers in general because it can transfer the heat load from the primary cryogenic load temperature to room temperature directly by thermomechanical coupling. The cascade cryocoolers, however, have to handle the heat load at the intermediate temperature section when the lower temperature unit dumps heat to the cold end of the upper temperature stage, which often creates significant entropy burden to the upper one. On the other hand, the true cascade cryocoolers can also attain higher efficiency than the single two-stage cryocooler if the heat pumping process from the primary cryogenic load temperature to room temperature is too inefficient with excessively large temperature ratio. This efficiency reversal is possible by virtue of dividing the large temperature span into two small ones. In this case, the appropriate intermediate temperature must be selected and a highly effective external heat exchanger needs to be carefully incorporated in between. Parametric studies have been performed to discuss the impact of the intermediate temperature on thermal anchoring and different parasitic conduction or radiation heat leak.