Study and optimization of the influence of the water tank temperature on the performance of a solar assisted multi-source heat pump drying system

Solar assisted air source heat pump drying (SASHPD) system has been widely studied due to its excellent energy saving and high dried quality of product. To improve system performance, the control logic of a solar assisted multi-source heat pump drying (SMSHPD) system was investigated, and the simulation model and the experimental platform were established in this paper. The seasonal operating characteristics of the air source heat pump drying (ASHPD), the SASHPD and the SMSHPD modes were studied. The energy consumption, coefficient of performance (COP) and specific moisture extraction rate (SMER) of the three drying systems were calculated and analyzed. Additionally, the water tank temperature were also measured to determine the seasonal control logic of the drying system. It was found that compared to the ASHPD system, in summer, the energy consumption of the SASHPD system was reduced by 33.06 %, and COP and SMER was increased by 49.4 % and 49.38 %, respectively. Moreover, because of the high ambient temperature and water tank temperature, the experimental results indicated that only the SASHPD control logic should be implemented in summer. The maximum error of the simulation results was 9.1 % and the accuracy of the simulation model was confirmed. In autumn, the lower water tank temperature after drying illustrated that the solar energy can be more fully utilized, which explained why the performance of the SMSHPD mode was increased by 6.5 % compared to the SASHPD mode. Consequently, it is necessary to implement the SMSHPD control logic in autumn. Moreover, the operating temperature of the water source heat pump drying mode in autumn was optimized and the optimal temperature range was 28–54 °C. The average COP in one week after optimization was 6.21 % higher than that of the original operating temperature. Based on the validated simulation model, the optimal operating temperatures in spring and winter were also calculated and the control logic of the SMSHPD system operating in the four seasons were all obtained. This paper has important guiding significance for the reasonable choice of the control logic of a solar combined air source heat pump drying system.