## 3 Results and Discussion
### The Effect of Different Factors on the System Performance of Two Heat Pumps
This causes the refrigerant temperature to rise after being throttled by the throttle valve, further increasing the refrigerant temperature at both the inlet and outlet of the evaporator, ultimately leading to an increase in the suction temperature of the compressor, a decrease in the specific suction volume, and a decrease in the refrigerant flow rate. At the same time, the discharge temperature increases with the increase in the compressor suction temperature, resulting in an increase in both unit power consumption and unit heating capacity. The impact of both is greater than that of the refrigerant flow rate, leading to an increase in both heating capacity and power consumption of the two systems. In addition, the power consumption and heating capacity of the R744/R1234 yf (90/10) transcritical heat pump are generally lower than those of the R515B-1 subcritical heat pump, indicating that in higher-temperature evaporation conditions, the heating capacity of the R744/R1234 yf (90/10) transcritical heat pump still does not present an advantage. Figure 12b shows the variation of COP with T\({}_{\text{h-w-i}}\) for the two heat pump systems. It can be observed from the figure that the COP of the R744/R1234 yf (90/10) transcritical heat pump is generally higher than that of the R515B-1 subcritical heat pump. Figure 12c shows the variation of outlet temperature of heating water with T\({}_{\text{h-w-i}}\) for the two heat pump systems. It can be observed from the figure that due to the larger heating capacity of the R515B-1 subcritical heat pump, its outlet temperature of heating water is higher. However, the increase in the outlet temperature of heating water in the R744/R1234 yf (90/10) transcritical heat pump is greater. Figure 12d shows the years of cost recovery for the R744/R1234 yf (90/10) transcritical heat pump. From the figure, it can be observed that as the T\({}_{\text{h-w-i}}\) increases, its cost recovery period remains almost unchanged, corresponding to about 15 years. Within the given T\({}_{\text{h-w-i}}\) range, compared to the R515B-1 subcritical heat pump system, the COP of the R744/R1234 yf (90/10) transcritical heat pump system increased by an average of 98.31%, the heating capacity decreased by an average of 1.91%, the outlet temperature of heating water decreased by an average of 0.91 \({}^{\circ}\)C, the power consumption decreased by an average of 50.34%, and the average cost recovery period is 14.82 years. Meanwhile, with the change in T\({}_{\text{h-w-i}}\) in the R515B-1 subcritical heat pump system, the COP decreased by 6.74%, the heating capacity increased by 3.24%, the power consumption increased by 10.52%, and the outlet temperature of heating water increased by 1.57 \({}^{\circ}\)C. In the R744/R1234 yf (90/10) transcritical heat pump system, the COP decreased by 13.17%, the heating capacity increased by 5.19%, the power consumption increased by 21.17%, and the outlet temperature of heating water increased by 2.44 \({}^{\circ}\)C.  
3.3.3 The Effect of Charge Amount on the System Performance of Different High-Temperature Heat Pumps  
Figure 13 shows the impact of the charge amounts of R515B-1 and R744/R1234 yf (90/10) on the system performance of their respective heat pumps, as well as the expected cost recovery period of the R744/R1234 yf (90/10) transcritical heat pump compared to the R515B-1 subcritical heat pump. Figure 13a shows the changes in power consumption and heating capacity of the two heat pump systems with changes in the charge amount of refrigerant. It can be observed from the figure that as the charge amount increases, the power consumption of the two systems exhibits similar and complex patterns of change. As the charge amount increases, the power consumption of the R744/R1234 yf (90/10) transcritical heat pump shows a trend of first increasing, then decreasing and finally increasing again.