## 3 Results
### Heat Pump \(COP_{h}\)  
The \(COP_{h}\) simulation results of ASHP using different refrigerants as working fluids for space heating under the designed cases are shown in Figures 6-9. Because the critical temperature of all refrigerant candidates are high enough to condense at 58 \({}^{\circ}\)C, all the refrigerants are available choices to supply heating water at 50 \({}^{\circ}\)C, but the \(COP_{h}\) are quite different from each other. Figure 6 shows that, to supply heating water with 50 \({}^{\circ}\)C under varied outdoor air temperature, the \(COP_{h}\) of ASHP using RHR-1, R134a, R410A, R407C and R22 are in the ranges of 2.43-3.59, 2.37-3.56, 2.24-3.28, 2.25-3.33 and 2.43-3.57, respectively. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Class No.** & **Outdoor Air Temperature Range (\({}^{\circ}\)C)** & \
\begin{tabular}{c} **Representative Outdoor** \\ **Air Temperature (\({}^{\circ}\)C)** \\ \end{tabular} &
\begin{tabular}{c} **Number of Hours (h)** \\ \end{tabular} \\ \hline Class 1 & \(-15\) to \(-10\) & \(-12.5\) & 38 \\ Class 2 & \(-10\) to \(-5\) & \(-7.5\) & 504 \\ Class 3 & \(-5\) & \(-2.5\) & 1020 \\ Class 4 & 0-5 & \(2.5\) & 871 \\ Class 5 & 5-10 & \(7.5\) & 356 \\ Class 6 & 10-15 & \(12.5\) & 98 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Outdoor air temperature categories and its number of hours in Tianjin. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Outdoor Air Temperature (\({}^{\circ}\)C)** & \multicolumn{3}{c}{**Supply Water Temperature (\({}^{\circ}\)C)**} \\ \hline \(-12.5\) & 35 & 40 & 45 & 50 \\ \(-7.5\) & 35 & 40 & 45 & 50 \\ \(-2.5\) & 35 & 40 & 45 & 50 \\ \(2.5\) & 35 & 40 & 45 & 50 \\ \(7.5\) & 35 & 40 & 45 & 50 \\ \(12.5\) & 35 & 40 & 45 & 50 \\ \hline \hlineFigure 8: The _COP\({}_{h}\)_ of ASHP with different refrigerants to supply water at 40 \({}^{\circ}\)C under varied thermal environments. Figure 6: The _COP\({}_{h}\)_ of ASHP with different refrigerants to supply water at 50 \({}^{\circ}\)C under varied thermal environments. Figure 7: The _COP\({}_{h}\)_ of ASHP with different refrigerants to supply water at 45 \({}^{\circ}\)C under varied thermal environments. To supply heating water at 45 \({}^{\circ}\)C under varied outdoor thermal environment, the \(COP_{h}\) of ASHP with RHR-1, R134a, R410A, R407C and R22 are in the ranges of 2.59-3.95, 2.54-3.94, 2.41-3.66, 2.42-3.69 and 2.59-3.94, respectively (Figure 7). To supply heating water at 40 \({}^{\circ}\)C under varied outdoor thermal environment, the \(COP_{h}\) of ASHP with RHR-1, R134a, R410A, R407C and R22 are in the ranges of 2.76-4.38, 2.73-4.41, 2.61-4.13, 2.60-4.13 and 2.77-4.39, respectively (Figure 8). To supply heating water at 35 \({}^{\circ}\)C under varied outdoor thermal environment, the \(COP_{h}\) of ASHP with RHR-1, R134a, R410A, R407C and R22 are in the ranges of 2.96-4.93, 2.94-4.98, 2.82-4.70, 2.80-4.68 and 2.97-4.95, respectively (Figure 9). Theoretically, all the refrigerant candidates in this study could be used for ASHP to supply heating water from 35 \({}^{\circ}\)C to 50 \({}^{\circ}\)C, but the \(COP_{h}\) vary largely from one to another. Under the designed operation cases, the \(COP_{h}\) of ASHP using RHR-1, R134a and R22 are higher than that with R410A and R407C. Among RHR-1, R134a and R22, the \(COP_{h}\) comparisons show different characteristics in different cases. RHR-1 shows highest \(COP_{h}\) when the supply water temperatures are 50 \({}^{\circ}\)C and 45 \({}^{\circ}\)C, which are normally adopted in space heating systems.