Study on the performance of heat pump drying system under the synergistic effect of humidity enthalpy enhancement and solar heat storage under low temperature working conditions

•The energy utilization efficiency of different energy supply modes at the average temperature of 0 °C, the heating change of condenser and the heating rate of drying chamber are given quantitatively.•The combined operation mode of dehumidification and enthalpy increase and solar energy storage and replenishment is proposed. The comprehensive heat utilization efficiency is increased to 86.60 %, and the heating capacity is 26.84 % higher than that of HPD at normal temperature.•At 0 °C to −20 °C, the drying time and energy consumption were saved by 31.10 % and 37.99 %, respectively, compared with the conventional drying time and energy consumption. The heat pump drying system in cold regions could be prone to frost and resulted in a low heating capacity, high energy consumption, and unstable heat supply. Therefore, this paper, studied four drying modes in terms of energy supply and energy demand, drying time, and the coefficient of performance COP. A synergistic drying modes along with the heat pump drying (HPD) mode were proposed to improve the stability of energy supply and prevent the frost formation, such as, dehumidification and increasing air enthalpy (DEHPD) mode, solar assisted heat pump drying (SAHPD) mode, and a combination of SAHPD and DEHPD mode. The experiments were conducted in Shangri-La, China, which has a low winter temperature and abundant solar radiation to dry 2000 kg of Tibetan medicine “Aucklandiae radix” at a low ambient temperature of 0 °C to −20 °C. Results showed that the DEHPD mode increased the drying chamber temperatures by 25 % and increased the refrigerant enthalpy by 53.79 kJ/kg as well as recovered 78.25 % of the wasted heat compared with the heat pump drying mode HPD. While, the SAHPD mode maintained the heating capacity of heat pump under low ambient temperature similar to that under normal temperatures with a little reduction of 3.27 %. Moreover, the combination of energy supply by DEHPD and SAHPD improved the heat utilization efficiency by 86.60 % and shortened the drying time and energy consumption by 18.55 % and 26.24 %, respectively comparing with the HPD mode. The lowest frost accumulation rate was under the combination mode (SAHPD&DEHPD) and the fastest was under HPD mode. After comparison, the synergistic mode of DEHPD and SAHPD (Mode1) was the optimal drying mode for a continuous full load drying under low temperatures of 0 °C to −20 °C; Mode1 could decrease the drying time by 31.10 % and the energy consumption by