Development of a 77K Reverse-Brayton Cryocooler with Multiple Coldheads

RTI will design and optimize an 80 W, 77K cryocooler based on the reverse turbo Brayton cycle (RTBC) with four identical coldheads for distributed cooling. Based on preliminary analysis, the single centrifugal compressor with integrated motor will be 10 cm in diameter and 15 cm in length, and rotate at about 100,000 rpm. Each coldhead (consisting of a recuperative heat exchanger, turbo-expander and cold side heat exchanger) is expected to be about 6X6X15 cm in linear dimensions, and weighs about 0.2 kg. Key technical innovations are superior aerodynamics for a compact, reliable and efficient compressor, distributed RTBC cryocooling with a single compressor, and techniques to drastically minimize performance-degrading axial conduction in recuperative heat exchangers. Microfabrication is a key enabler for implementation of this concept. Phase I will involve system design, and design and fabrication of the compressor. The Phase II goal is to integrate the compressor with motor, electronics and gas foil bearings with a target isentropic efficiency of 80% at a pressure ratio of 1.75. It is expected that the coefficient of performance of the overall RTBC system when completed would be at least 0.12, with the minimum and maximum cycle temperature being 64 K and 440 K respectively.