## 3 Results and Discussion
### The Effect of Different Factors on the System Performance of Two Heat Pumps
It is worth noting that in the R744/R1234 yf (90/10) transcritical heat pump, the decrease in heating capacity is smaller than the decrease in power consumption, so its COP shows an increasing trend. Figure 11c shows the variation in the outlet temperature of heating water with evaporation temperature for the two heat pump systems. It can be observed from the figure that compared to the R515B-1 subcritical heat pump, the R744/R1234 yf (90/10) transcritical heat pump has the ability to produce higher-temperature hot water. Meanwhile, due to the fact that the outlet temperature of heating water is mainly determined by the heating capacity of the system, under higher-temperature evaporation conditions, the heating capacity of the R744/R1234 yf (90/10) transcritical heat pump is lower than that of the R515B-1 subcritical heat pump, resulting in a decrease in the outlet temperature of heating water. Figure 11d shows the years of cost recovery for the R744/R1234 yf (90/10) transcritical heat pump under a target heating capacity of 1000 kW. As mentioned earlier, the cost of a transcritical heat pump is more than three times that of a subcritical heat pump, but its COP is higher, and it is more energy-efficient, which makes it possible to recover costs within the limited 20-year lifespan. From the figure, it can be observed that as the evaporation temperature increases, the overall cost recovery period shows an increasing trend, and the  
shortest cost recovery period is 9 years. Therefore, for users considering long-term use of heat pumps, it is recommended to use a transcritical heat pump system, while for users only considering short-term use, it is recommended to use a subcritical heat pump system. Within the given evaporation temperature range, compared to the R515B-1 subcritical heat pump system, the COP and heating capacity of the R744/R1234 yf (90/10) transcritical heat pump system increased by an average of 80.25% and 26.25%, respectively. The outlet temperature of heating water increased by an average of 5.79 \({}^{\circ}\)C, the power consumption decreased by an average of 37.23%, and the average cost recovery period is 14.06 years. Meanwhile, with the change in evaporation temperature, in the R515B-1 subcritical heat pump system, the COP increased by 61.11%, the heating capacity increased by 11.3%, the power consumption decreased by 30.94%, and the outlet temperature of heating water increased by 4.94 \({}^{\circ}\)C. In the R744/R1234 yf (90/10) transcritical heat pump system, the COP increased by 65.91%, the heating capacity decreased by 13.59%, the power consumption decreased by 49.97%, and the outlet temperature of heating water decreased by 7.55 \({}^{\circ}\)C.  
3.2 The Effect of T\({}_{\text{h-w-i}}\) on the System Performance of Different High-Temperature Heat Pumps  
Figure 12 shows the effect of T\({}_{\text{h-w-i}}\) (inlet temperature of heating water) on the system performance of R515B-1 and R744/R1234 yf (90/10) applied to the subcritical heat pump and transcritical heat pump, respectively, 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  
Figure 11: The effect of evaporation temperature on the system performance of different high-temperature heat pumps (T\({}_{\text{e-w-i}}\) is 313 K, T\({}_{\text{co}}\) is 365 K, T\({}_{\text{h-w-i}}\) is 313 K, and the charge amount is 4 kg). pump. Figure 12a shows the changes in power consumption and heating capacity of the two heat pump systems with T\({}_{\text{h-w-i}}\). It can be observed from the figure that as the T\({}_{\text{h-w-i}}\) increases, the power consumption and heating capacity of both heat pump systems increase, but the magnitude of the power consumption increase is higher. This is mainly because in the R515B-1 subcritical heat pump and R744/R1234 yf (90/10) transcritical heat pump, with the increase in T\({}_{\text{h-w-i}}\), the refrigerant temperature at the condenser outlet and gas cooler outlet increases, that is, subcooling decreases.