Mathematical modeling and performance analysis of direct expansion heat pump assisted solar drying system

Abstract In order to solve the problems of instability and poor controllability of only using solar energy in the drying process, a novel direct expansion heat pump assisted solar drying system was presented and constructed in this study. Theoretical modeling of collector/evaporator with glass were developed to obtain the outlet air temperature, the operation performance of system and drying characteristics of lemon slices were investigated by experiment and theory. The results shown that for the outlet air temperature and heat collection efficiency, the maximum deviation of the simulated and experimental values was 1.7 °C, 0.04 respectively; the R2 were 0.98, 0.96 respectively. The average drying rate of direct expansion heat pump assisted solar drying system was higher 0.23 g water/g dry matter than that of opening sun drying due to the higher drying air temperature. The time range of the air temperature of drying chamber from 40 °C to 50 °C accounts for 60.7% of the total drying time. In addition, two term and modified Page were found to be best model for descripting lemon slices drying characteristics; the system specific moisture extraction ratio was obtained as 0.85 kg/(kW·h). This study has proved the feasibility of the system operation and provided the basis for further research.