Experimental study on the concentration shift characteristics in a single-stage recuperative heat pump with large temperature lift

Heat pumps have always attracted wide interest due to their ability to harvest energy from the environment, thereby alleviating the energy consumption and environmental pollution issues. The recuperative heat pump with refrigerant mixture can realize large temperature lift (over 50 K) to meet such demand and has several advantages over trans-critical CO2 and sorption heat pumps such as low discharge pressure and simple structure. However, the concentration shift results in the recuperative heat pump performance hardly operating under optimal conditions, since working fluid with large temperature glide is used to realize large temperature lift. In this study, a test facility has been built to experimentally examine the influence of charging concentration, throttling degree and water flow on the concentration shift. Besides, modeling studies are conducted to investigate the influence of the phase hold-up on the concentration shift. The results show that the concentration shift is remarkable in practical application. The maximum relative deviation is nearly 70% and 85% when the R1270/Hexane and R32/R4310mee refrigerants are utilized, respectively. Besides, the temperature has a significant impact on the concentration shift under most working conditions. In addition, a very high designed concentration of component with a high boiling point may not be realized in real application. The circulating concentration of the component with a high boiling point can be automatically optimized when the charging concentration is higher than the designed value. The modeling results show that the deviation between the charging concentration and the calculated concentration caused by the phase hold-up can reach 50%, indicating the influence of the different oil dissolving properties of refrigerants and other factors is remarkable.

Automated summary

Heat pumps have attracted wide interest due to their ability to harvest energy from the environment. The recuperative heat pump with refrigerant mixture can meet demand for large temperature lift and has advantages over other types of heat pumps. However, the concentration shift in the recuperative heat pump affects its performance. Experimental and modeling studies have shown that the concentration shift is significant in practical applications and is influenced by factors such as temperature and oil dissolving properties.