Thermodynamic optimization of sorption-based Joule–Thomson coolers

A cooler consisting of a sorption compressor with a Joule–Thomson (JT) cold stage has several advantages. These coolers have no moving parts which is attractive for a variety of reasons. Unfortunately, the efficiency of sorption-based JT coolers is in many cases relatively small. This paper presents a thermodynamic description of the compressor and the cold stage separately. Their efficiencies are optimized individually. Besides, an optimization of the total cooler is performed. Furthermore, the thermodynamic impact of design changes to improve these efficiencies is discussed. The efficiency of the sorption compressor can be increased significantly by incorporating two-stage compression. The efficiency of the cold stage can largely be increased by precooling. Generally, a TE cooler is used for this purpose. Two-stage compression opens the opportunity to integrate a JT precooler into the cold stage that is driven by the pressure difference created by the second compression stage. Also an additional sorption cooler can be used for this purpose. A case study is performed for nitrogen as the working gas and both Saran and Maxsorb as the adsorbent. Four cooler configurations are thermodynamically compared. In this study, the cooler efficiency increases by a factor of 3 by the application of two-stage compression compared to single-stage compression. Another factor of 3 is obtained by using an internal JT precooler. Incorporating an external sorption cooler as a precooler gives a further improvement with nearly a factor of 2, resulting in a 17 times larger efficiency compared to single-stage compression. Depending on the configuration, a Saran-based cooler has a 1.5–2 times better efficiency than a cooler operating with Maxsorb.