A Comparative Conceptual Analysis of CO₂ Heat Pump Dryers With Closed-loop and Open-loop Air Cycles

Abstract

This study has comprehensively compared and analysed CO₂ heat pump dryers operating under closed-loop and open-loop air cycles to evaluate their energy efficiency and drying performance. Unlike the conventional closed-loop air cycle that uses dry recirculated air as its inlet, the open-loop air cycle operates only with fresh ambient air. The physical models and working principles have been illustrated using psychrometric charts, and the influence of moisture variation has been considered in the fin-and-tube heat exchanger design for both the gas cooler and the evaporator. In the case study under typical hot and humid climate conditions (ambient temperature of 40 °C), the simulation compares three cycles over an air mass flow rate ranging from 0.5 kg/s to 1 kg/s. The open-loop air cycle with a wet air outlet achieves the largest heating capacity, i.e.: 17.44 kW at 1 kg/s, because the air is cooled in the evaporator first, allowing a greater temperature rise in the gas cooler. The open-loop air cycle with a dry air outlet produces the highest air temperature after the gas cooler, i.e.: 60.8 °C at 0.5 kg/s, which increases the air’s moisture absorption capacity. Compared with the closed-loop air cycle, the open-loop air cycle with dry air outlet proves more efficient for drying, delivering a shorter drying time (27.77 min at 0.5 kg/s) and a higher drying efficiency (0.8640 kg/kWh at 0.5 kg/s). Although the open-loop air cycle with a wet air outlet achieves the highest coefficient of performance of 2.31 at 1 kg/s, its drying performance declines obviously at higher mass flow rates, with specific moisture extraction rate dropping to 0.0767 kg/kWh. Overall, the configuration of open-loop air cycle with dry air outlet is the superior option, as it combines the shortest drying time and the highest specific moisture extraction rate, which are two critical metrics for heat pump dryers.