Simulation study of a heat pump drying system using a solar assisted flash tank vapor injection cycle

Heat pumps are widely used in many fields due to their high efficiency and energy saving potential. However, the application of single-stage vapor-compression heat pump dryer is limited due to low drying temperature and the decreasing reliability for high temperature drying. In order to apply heat pump to high temperature drying, a heat pump drying system using a solar assisted flash tank vapor injection cycle (HPDS-SFVIC) is presented. This system allows the solar collector to absorb solar energy at a lower temperature and achieves significant improvement of heating capacity and drying temperature. The effects of component configuration and various operating conditions on the drying system with open, bypass, and closed air circulation loop are evaluated, respectively. The results show that under a typical operating condition, the proposed cycle achieves great performance improvement of 21.8% and 75.8% in both the heating coefficient of performance (COP_h) and heating capacity, compared with the conventional cycle. Utilization of solar energy is advantageous to provide higher air temperature for drying. With increased solar radiation intensity and collector area, heating capacity and drying temperature can be improved, while COP_h remains constant as 5.0–5.2. When the drying system provides dry air at 65 °C stably, it saves 27.8%–41.7% of power consumption, and COP_h is improved by 41.2%–62.1% on average.