Modelica-based modelling of heat pump-assisted apple drying for varied drying temperatures and bypass ratios

•CO2 heat pump systems are used to replace fossil fuel-based burners in food drying processes.•A Modelica-based drying cabinet model was developed and successfully validated.•Decreased energy demand by 77–84% when heat pump-assisted drying is applied.•The SMER is 3–4 times higher compared to open loop systems.•Increased drying time by 29–69% due to increased humidity in the closed loop air cycle. Drying is an energy and time intensive process in which thermal energy demand is mostly provided by fossil resources. It is important to increase the energy efficiency of drying processes especially in the food processing industry in terms of organic products and sustainability. The potential for using a heat pump with CO2 as a working media to provide heating and cooling in the drying cabinet was investigated for typical food drying temperatures of 50–70 °C and for various ratios of moist air being bypassed. A dynamic heat pump-assisted dryer model was developed and experimentally validated. The model was created with respect to heat transfer, pressure loss and flow requirements. The simulation results show that a closed loop heat pump-assisted drying process reduces the energy demand by up to 84% compared to open loop drying processes with fossil resources as the energy source. The specific moisture extraction rate for a heat pump dryer is up to four times higher than that of an open loop dryer. However, in the heat pump dryer case the drying time is increased by up to 69%.