Optimization method for mixed refrigerants in Joule–Thomson refrigerators with fixed-temperature heat loads

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Mixed-refrigerant Joule–Thomson refrigeration is a widely used and effective method for refrigeration in the temperature range of 80–230 K. The demand for cooling heat loads at fixed temperatures, such as biological storage and cryogenic treatment of materials, is significant. The mixed refrigerant determines the refrigeration temperature and capacity in many cases; therefore, optimizing the mixed refrigerant composition is essential. The current mathematical approach for optimizing mixed refrigerants is not well-defined, owing to the challenge of determining how to adjust the concentrations of the components in the optimization process. Therefore, the current approach may exhibit poor robustness or a longer optimization time. This study proposes an alternative optimization method based on the effective refrigeration effect. The method aims to overcome the challenges associated with complex algorithms typically used in optimizing mixed refrigerants. The maximum refrigeration capacity was achieved by optimizing the isothermal throttling effect in the full-temperature zone without cycle simulations. Only one component with a significant refrigeration effect was changed at each optimization step. The proposed method was applied to a cryogenic refrigerator at 120 K, achieving a higher coefficient of performance than a genetic algorithm and other references. The study determined that varying the initial concentrations of the refrigerant components changed the performance coefficient to less than 0.002, indicating good robustness of the proposed optimization approach. Thus, our proposal is reliable for optimizing the composition of mixed refrigerants and can be extended to all mixed-refrigerant Joule–Thomson refrigeration systems with fixed-temperature heat loads.