Simulation and performance evaluation of closed and semi-closed heat pump dryers

Heat pump drying technology has been drawing increasing attention for improving energy efficiency and enhancing drying product quality. In this study, simulation models for closed-loop heat pump drying (CHPD) and semi-closed-loop heat pump drying (SCHPD) systems were established and verified to investigate the effects of some influencing parameters on both systems performance, i.e., supply air temperature, circulating air volume, outdoor temperature, and outdoor circulation air volume ratio. The results show that increasing the supply air temperature increases the Moisture Extraction Ratio (MER) of the CHPD and SCHPD systems but decreases the Coefficient of Performance (COP) of the CHPD and SCHPD systems. The energy performance and the Specific Moisture Extraction Ratio (SMER) of the SCHPD system are not as good as those of the CHPD system. The maximum difference in SMER between the CHPD system and the SCHPD system is 3.31, which occurs at a supply air temperature of 65°C. Increased circulating air volume increases the MER, SMER and COP of CHPD and SCHPD systems, reduces compressor power consumption, but resulted in a significant decrease in supply air temperature. As the outdoor temperature increases, the performance parameters of the SCHPD system all improve. The compressor power consumption is reduced by 60% from 0°C to 35°C outdoor temperature, but it is still higher than that of the CHPD system in most cases. An increase in the outdoor circulating air volume ratio means more energy input, resulting in a significant increase in the power consumption of the compressor of the SCHPD system. Therefore, the outdoor circulating air volume of the SCHPD system should be reasonably designed according to the actual drying demand.