Analysis and experimental study on optimal working fluid mixture ratio of auto-cascade heat pump systems based on reducing intermediate heat exchanger exergy loss

Starting from the exergy loss of the auto-cascade heat pump system cycle, this paper analyses and finds that the intermediate heat exchanger has the largest exergy loss. Then, a theoretical analysis is conducted on the methods to reduce the exergy loss. Taking two binary mixed working fluid combinations of R290/R170 and R600a/R1150 with different boiling point differences as examples, the influence of the mixture ratio of the mixed working fluid on the exergy loss of the intermediate heat exchanger is studied in depth. Through experimental testing, the changes in the overall performance under different mixture ratios are verified. The results show that the exergy loss of the intermediate heat exchanger depends on the temperature matching degree of the high-pressure side and the low-pressure side during the heat exchange process, and can be reflected by the thermal equivalent ratio. The closer the thermal equivalent ratio is to 1, the smaller the exergy loss is. For different mixed working fluid combinations, there is an optimal mixture ratio that makes the overall performance the best. The optimal mixture ratio is close to the mixture ratio with the smallest exergy loss in the intermediate heat exchanger as analysed theoretically.