## 2 Methodology  
A single-stage heat pump is selected as the reference to investigate the influence of alternative refrigerants to R134a and R245 fa on the warming impact. The heat pump system comprises a compressor with an efficiency of 70%, a condenser, an evaporator and an expansion valve. The superheating and subcooling temperatures are assumed to be 5 K. Due to the high GWP of R134a and R245 fa, these HFC refrigerants should be replaced, and they are set as the reference refrigerants in this study. The analysis of the heat pump system is divided into two parts. First, the comparison between R134a and its five selected alternative refrigerants was made in a medium temperature heat pump (MTHP) when supplying a condensation temperature of 40 \({}^{\circ}\)C. Second, R245 fa and its five selected alternative refrigerants are employed in an HTHP to supply a condensation temperature of 100 \({}^{\circ}\)C.  
The selected refrigerants and their properties are listed in Table 1, and the pressure-enthalpy diagrams of these refrigerants are shown in Figure 1. The selected low-GWP refrigerants have comparable thermophysical properties to those of the reference refrigerants. The alternative refrigerants have lower toxicity than R245 fa, as this refrigerant belongs to the ASHRAE safety class B1. It is also worth highlighting that the selected hydrocarbons (R290 and R600) are included in group A3 due to their higher flammability. R1234 yf, R1234 ze(E) and R1234 ze(Z) are considered to be lower flammability refrigerants and belong to the A2L group. Other selected refrigerants are non-flammable according to the ASHRAE safety class. Concerning the ozone depletion potential (ODP), all selected refrigerants have a zero ODP except R1224 yd(Z) and R1233 zd(E), which are known as hydrochlorofluoroolefin (HCFO) refrigerants. It is noted that the ODP of these two refrigerants is very small (almost zero), which meets the requirements laid out by current national environmental regulations. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & **Molecular Mass** & **NBP** & **T\({}_{\text{crit}}\)** & **P\({}_{\text{crit}}\)** & **h\({}_{\text{fg}}\)** & **ODP** & **ASHRAE** \\  & **(kg kmol\({}^{-1}\))** & **(\({}^{\circ}\)C)** & **(\({}^{\circ}\)C)** & **(MPa)** & **(k) kg\({}^{-1}\))** & **GWP** & **ODP** & **ASHRAE** \\ \hline R134a & 102.03 & \(-\)26.08 & 101.06 & 4.06 & 151.81 & 1430 & 0 & A1 \\ R290 & 44.10 & \(-\)42.12 & 96.74 & 4.25 & 284.86 & 3 & 0 & A3 \\ R450A & 108.67 & \(-\)22.75 & 104.47 & 3.82 & 146.57 & 605 & 0 & A1 \\ R513A & 108.43 & \(-\)29.48 & 94.91 & 3.65 & 131.29 & 631 & 0 & A1 \\ R1234 yf & 114.04 & \(-\)29.49 & 94.70 & 3.38 & 122.13 & \textless{}1 & 0 & A2L \\ R1234 ze(E) & 114.04 & \(-\)18.98 & 109.36 & 3.63 & 145.69 & \textless{}1 & 0 & A2L \\ R245 fa & 134.05 & 15.04 & 153.86 & 3.65 & 135.59 & 858 & 0 & B1 \\ R1224 yd(Z) & 148.49 & 14.61 & 155.54 & 3.34 & 117.12 & \textless{}1 & 0.00012 & A1 \\ R600 & 58.12 & \(-\)0.50 & 151.98 & 3.80 & 258.26 & 4 & 0 & A3 \\ R1336 mzz(Z) & 164.06 & 33.45 & 171.35 & 2.90 & 127.08 & 2 & 0 & A1 \\ R1233 zd(E) & 130.50 & 18.26 & 166.45 & 3.62 & 142.25 & 1 & 0.00034 & A1 \\ R1234 ze(Z) & 114.04 & 9.72 & 150.12 & 3.53 & 144.44 & \textless{}1 & 0 & A2L \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Thermophysical properties of the low-GWP refrigerants and references (R134a and R245 fa) [27]. Most of the alternative refrigerants to R134a and R245 fa have similar two-phase regions to the reference refrigerants apart from the hydrocarbons (R290 and R600). In R134a and its alternative refrigerants, the critical temperature of R290 is much higher than R134a, but in other selected refrigerants, it is almost equal to or less than that of R134a. In contrast, the alternative refrigerants to R245 fa, except R1224 yd(\(Z\)), have almost equal or even higher critical temperatures than R245 fa, demonstrating a potential for performance improvement using alternative refrigerants compared to R245 fa.