Study on intermittent drying process of lemon slices employing a closed-loop transcritical CO2 heat pump system

To address the decline in energy efficiency during the mid-to-late stages of the drying process in heat pump drying (HPD), this study introduces intermittent drying based on a closed-loop transcritical CO₂ HPD system, and with lemon slices as the research object, experimentally investigates the impact of intermittent ratio in closed-loop heat pump intermittent drying on the wet-basis moisture content (M_t), drying rate, coefficient of performance of the system (COP_sys), specific moisture extraction rate (SMER), specific energy consumption (SEC), mass of drying per unit volume per unit time (MDVT), rehydration ratio, brightness of dried material and vitamin C content. The findings suggest that when the intermittent ratio rises, the average drying rate, average SMER, SEC, MDVT, rehydration ratio, brightness of dried material and vitamin C content all increase, while the average COP_sys remains essentially unchanged. The average drying rate, average SMER, SEC, MDVT, rehydration ratio, brightness of dried material and vitamin C content have maximum values at an optimal intermittent ratio of 4/6. Accordingly, a combined drying process of “continuous drying-intermittent drying-continuous drying” is proposed: when the M_t reaches 70 %, the drying mode should transition from continuous drying to intermittent drying; when the M_t reaches 40 %, the mode should shift back from intermittent drying to continuous drying. The findings provide important references for the industrial drying of lemon.