Refrigerants evaluation and energy performance optimization for a high-temperature carbon dioxide plus hydrofluoroolefin / hydrochlorofluoroolefin cascade heat pump dryer with two air heating circuits

The carbon dioxide heat pump is promising for energy savings and emission reductions in the drying industry due to its good performance. However, the efficiency of carbon dioxide gas cooler is low under drying working condition, due to significant temperature glide of supercritical state. Moreover, for many drying materials, the drying temperature provided by current heat pumps is still lower than the optimal drying temperature. To resolve these issues, a new low-carbon refrigerant + carbon dioxide transcritical cascade heat pump drying system has been proposed. In this new system, the discharged carbon dioxide from the compressor of lowtemperature stage is cooled by both fresh air and the low-carbon refrigerant through two distinct heat exchangers. System simulation and multi-objective optimization are conducted based on experimentally validated model. The results show that when R1234ze(Z) is used at high temperature stage, the system performs the best, with a maximum coefficient of performance of 1.78, heating capacity of 20.2 kW, condensing temperature of 110 degrees C, and low-temperature heat source of 10 degrees C. The best individual in Pareto front is selected, with 3.55 x 104 CNY/year and 3.47 x 104 kg/year. The optimal geometric parameters of components are determined as follows, 3.39 m2 gas-cooler heat transfer area, 48 m2 condensing heat transfer area and 9.68 MPa parameters of heat rejection pressure. Additionally, the impact of critical factors on system performance was investigated. The new system and the results of this paper are expected to provide new ideas for high-temperature heat pump drying system and system energy saving.