Single-stage or multi-stage compression: Which is better for sorption compressors?

With the advantages of no vibration, no electromagnetic interference, high reliability and long lifetime, sorption cryocoolers are attractive for cooling optical sensors like infrared detectors and X-ray detectors. Compared with mechanical cryocoolers, sorption cryocoolers suffer from relatively low efficiency. In order to improve the performance of sorption cryocoolers, multi-stage compression is an alternative. For mechanical compressors, multi-stage compression is always more efficient than single-stage compression from the thermodynamic perspective, whereas there is no such consensus for sorption compressors. In this study, the performance of single-stage and two-stage sorption compressors with the same pressure ratio is compared and analyzed based on a thermodynamic model to discuss when and why two-stage sorption compressors have better performance. It is concluded that there exist optimum cooling temperatures corresponding to the maximum compressor efficiencies for both single-stage and two-stage compression. The efficiency of a two-stage sorption compressor using activated carbon as the adsorbent and nitrogen as the working fluid is larger only when the cooling temperature is larger than 250 K, which is mainly due to less heat consumed by the adsorbent (normalized to the delivery amount of gas) in two-stage compression. The analysis method can be extended to different working conditions and the conclusions are helpful for the baseline design of sorption compressors.