Effect of Heat Rejection Conditions on Cryocooler Operational Stability

Abstract

It is well known that cryocooler thermal efficiency is a strong function of heat rejection temperature, roughly following the dependency described by Carnot. An equally important and generally overlooked implication of cryocooler heat-rejection thermodynamics is the effect of the heat rejection temperature control mode on cryocooler performance and operational stability. Example heat rejection temperature control modes include constant reject temperature (generally maintained via closed-loop temperature control), heat rejection temperature rising linearly with power dissipation (typical of conduction/convection to a constant temperature heat sink), and heat rejection dependent on the fourth power of reject temperature (typical of radiation to deep space). This paper presents a useful algorithm for computing the effect of changing heatsink temperature on cryocooler performance and uses the algorithm to examine the implications of various heat rejection temperature control modes on cryocooler operation. A useful system-level thermal performance map is developed to display the stability boundaries and available stable operating space for coolers of interest for various typical heat rejection control modes.