Performance evaluation of a novel heat pump system for drying with EVI-compressor driven precooling and reheating

Heat pump drying is widely used in the agricultural and industrial sectors. This study proposes a novel heat pump system for closed-loop drying, which employs an enhanced vapor injection (EVI) compressor (HPDEVI). The system precools the return air to a closer-to-saturation state to enhance dehumidification and reheats the dehumidified cold air while increasing the refrigerant subcooling. Meanwhile, EVI is fulfilled to modify the heat pump cycle. To demonstrate its superiority, the HPDEVI's energetic, exergetic and economic performances are compared to those of the basic configuration (HPDbasic) and the system with a stand-alone heat pipe (HPDHP). The results show that HPDEVI outperforms the other two systems by increasing latent heat ratio of evaporator, compressor isentropic efficiency and refrigerant subcooling. Under the design condition (Tdb = 45 degrees C, RH = 40%), HPDEVI reaches a specific moisture extraction rate (SMER) of 3.07 kg/kWh and a moisture extraction rate (MER) of 43.8 kg/h. These values are increased by 16.7% and 12.9% compared to HPDbasic, and 8.9% and 4.5% relative to HPDHP, respectively. Moreover, HPDEVI generates the lowest exergy destruction per latent heat of water removal at 0.367 W/W, and costs only 0.267 CNY per litre of water extracted after ten years of operation. Finally, the performance improvements of HPDEVI under off-design conditions are also revealed.