Optimization of the thermal design of a cryogenically-cooled computer

The performance and costs of three types of cryogenic cooling systems are analyzed; a once-through system in which liquid nitrogen is allowed to boil on the CPU boards and is then vented to the surroundings; a saturated pool boiling refrigerated system in which gaseous nitrogen generated from boiling on the CPU boards in condensed in the vapor space above the liquid nitrogen pool; and a subcooled pool boiling refrigerated system in which the condenser is completely submerged in a subcooled liquid nitrogen pool surrounding the CPU boards. Thermal/fluid numerical models used in conjunction with an optimization code GRG2, were used to predict the performance and cost of the cryogenic cooling systems. The uniform annual cost of the once-through system was calculated to be $28.8 K. This compares with $34.6 K for the optimal saturated pool boiling system and $34.7 K for the optimal subcooled pool boiling system. Due to the extremely high initial cost of the refrigerated systems, the once-through cryogenic cooling system is recommended if the local heat flux on the CPU board remains below 15 W/cm²