Performance investigation of a novel heating tower heat pump system with integrated air gap membrane distillation regenerator

Abstract

Solution regeneration plays an important role in maintaining the frost-free operation of heating tower heat pump (HTHP). The traditional packed regenerator has limitation on thermal energy utilization due to its low regeneration efficiency. This paper suggests a novel HTHP system with integrated air gap membrane distillation regenerator (AGMD), using the sensible heat of high-temperature refrigerant gas at the compressor discharge to drive the regeneration process, realizing the cascade utilization of thermal energy and reducing the deterioration of COP (DCOP). Thermodynamic model of the novel system is developed and model simulations are carried out to study the system performance, meanwhile the comparison between the proposed system and conventional reverse-defrosting air source heat pump (ASHP) is performed. The results show that the DCOP of the novel system decreases from 4.4% to 2.6% as the evaporating temperature decreases from −15 °C to −25 °C under typical working condition. Decrease of evaporating temperature and increase of condensing temperature will greatly enhance the regeneration efficiency and reduce DCOP. The influence of solution concentration on DCOP is also distinct, which becomes greater as the water removal rate increases. The maximum coefficient of separation performance (COSP) increases from 0.0042 to 0.0052 kg/kJ as the ambient temperature decreases from 0 °C to −20 °C, while it decreases from 0.0031 kg/kJ to 0.0016 kg/kJ with a dramatic decline of 48% for the traditional reverse-defrosting ASHP, indicating that the novel system performs better than the traditional one and is more suitable to find applications at low ambient temperature conditions.