Optimization of the thermal design of a cryogenically cooled computer

In this paper, the performance and cost 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 is 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 $28.8K. This compares with $34.6K 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