## 3 Results and Discussion
### The Effect of Different Factors on the System Performance of Two Heat Pumps
For the R515B-1 subcritical heat pump, the power consumption variation  
pattern is consistent with the latter half of the power consumption variation pattern of the R744/R1234 yf (90/10) subcritical heat pump, mainly due to its lower optimal charge amount. With respect to the changes in heating capacity of the two systems, an increase in superheat increases the discharge temperature of the compressor, thereby increasing the heating capacity of the system. At the same time, an increase in the charge amount also increases the heating capacity of the system, so the heating capacity of both systems shows an increasing trend. In addition, compared to the R744/R1234 yf (90/10) transcritical heat pump, the R515B-1 subcritical heat pump has a greater range of changes in power consumption and heating capacity. Figure 12(b) shows the variation of COP with changes in the charge amount for the two heat pump systems. It can be observed from the figure that as the charge amount increases and under the combined influence of system heating capacity and power consumption, the two systems exhibit the maximum COP, at which point the corresponding charge amount optimal. Meanwhile, even though the COP of the R515B-1 subcritical heat pump at the optimal charge amount is still much lower than that of the R744/R1234 yf (90/10) transcritical heat pump, the optimal charge amount for the R515B-1 subcritical heat pump is 81.8% of that of the R744/R1234 yf (90/10) transcritical heat pump. Figure 12(c) shows the variation of outlet temperature of heating water with changes in the charge amount of the two heat pump systems. From the figure, it can be observed that as the charge amount increases, the change in the outlet temperature of heating water of the R515B-1 subcritical heat pump is greater, exceeding the outlet temperature of the heating water of the R744/R1234 yf (90/10) subcritical heat pump. Figure 12: The effect of T\({}_{\text{h-w-i}}\) on the system performance of different high-temperature heat pumps (T\({}_{\text{e}}\) is 293 K, T\({}_{\text{e-w-i}}\) is 313 K, T\({}_{\text{co}}\) is 365 K, and the charge amount is 4 kg). Figure 13d 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 charge amount increases, the cost recovery period shows a trend of first increasing, then decreasing. When the charge amount is optimal for the R515B-1 subcritical heat pump, the cost recovery period is the longest--up to 17 years. At the same time, within the given range of charge amount, the lowest cost recovery period is also around 12 years. Within the given range of charge amount, 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 97.13%, the power consumption decreased by an average of 48.41%, and the average cost recovery period is 14.67 years. Meanwhile, with the change in charge amount, in the R515B-1 subcritical heat pump system, the heating capacity increased by 26.28%, and the outlet temperature of heating water increased by 10.36 \({}^{\circ}\)C. In the R744/R1234 yf (90/10) transcritical heat pump system, the heating capacity increased by 8.97%, and the outlet temperature of heating water increased by 3.96 \({}^{\circ}\)C.  
Finally, it is necessary to point out that the specific values for the optimal charge amount are limited to the experimental platform built in this paper, and the values are only for reference, mainly to reveal the relationship between the optimal charge amounts of the two systems and their impact on system performance. For different test benches or heat pump equipment, the charge amount will vary according to the specific operating conditions.