## 1 Introduction
[18] proposed a self-optimizing control scheme using the ESC (extremum seeking control) strategy which could search and even track both fixed and slowly varying optimum COP. [PERSON] et al. [19] numerically studied a hybrid solar assisted loop heat pipe/heat pump water heater system, and the results showed that, on typical sunny days in spring or autumn, the proposed system could save 40.6% power consumption compared to heat pump mode. The influence of wet compression on a heat pump system was experimentally investigated by [PERSON] et al. [20], who found that the heating capacity and power input of wet compression increased more than that of dry compression, with a superheat of 10 \({}^{\circ}\)C.  
In addition to the heat pump cycle and components of ASHP, studies on refrigerant alternatives have been carried out for years because refrigerant is important for both energy performance and environment protection. The refrigerant R22 which is commonly used in China will be phased out in 2030. Ozone-friendly refrigerants such as R134a, R410A, R417A, R404A and R507 used in air conditioners were analyzed in several previous studies [21; 22; 23; 24; 25]. [PERSON] [21] compared the performance of residential air conditioners using R410A and R22 and concluded that the adoption of R410A could be helpful for air conditioner to decrease their heat exchanger size or improve their energy efficiency. [PERSON] [22] investigated the performance of R22 and its ozone-friendly alternative refrigerants (R404A and R507) in a window air-conditioner and showed the average COP of R507 increased by 10.6%, while that of R404A reduced by 16.0% compared to that of R22. [PERSON] et al. [23] proposed a numerical model on the performance of a novel frost-free ASHP using three different refrigerants and indicated that R134a has higher COP than R22 and R407C at given ambient temperature of \(-10\)\({}^{\circ}\)C and RH of 85%. [PERSON] et al. [24] presented a comparative analysis of a direct expansion geothermal evaporator using R410A, R407C and R22 as refrigerants. The results showed R410A would be a better choice to minimize pressure drop. [PERSON] et al. [25] evaluated the performance of a vapor compression plant using R134a, R407C and R22 as working fluids and indicated that the power consumption with R22 tends to decrease more slowly with increasing compression ratios than others. The studies above proposed and compared ozone friendly refrigerants. However, refrigerants with high GWP are also unfriendly to the environment, which pushed researchers to investigate and propose mixing refrigerant alternatives with low GWP. [PERSON] et al. [26] showed that a new ternary non-azeotropic mixture of HFC-161/125/143a's thermodynamic properties are similar to those of R404A, but its global warming potential (GWP) is much lower than those of R507A and R404A. [PERSON] et al. [27] found that the SEER and SCOP of R32 and mixing refrigerant R446A were higher than R410A by 8% and 1% in a residential heat pump system. A comparison of R404A with six low-GWP mixing refrigerants including mid-term alternatives R407A and R407F and long-term alternatives L40, DR-7, N40 and DR-33 was conducted by [PERSON] et al. [28] in terms of cooling capacity, volumetric flow rate and COP, and conclusions were given that the most efficient alternatives were the low-flammable refrigerants L40 and DR-7 considering two-stage and diverse operating conditions. [PERSON] [29] evaluated seasonal efficiency of four new low-GWP mixing refrigerants including R446A, R447A, R452B and R454B whose GWP values were lower than 750 and concluded that R452B has the most suitable SCOP among the alternative refrigerants. Feasibility studies on R32 refrigerant mixtures, such as R32/R290, R32/L41A and R32/CO\({}_{2}\) replacing R410A in heat pumps or household air conditioners were conducted in several studies [30; 31; 32], and the results demonstrated that, compared with R410A, higher cooling/heating capacities and similar COP could be achieved by R32 refrigerant mixtures. [PERSON] et al.