## 2 Materials and Methods
### Test Procedures  
To investigate the impacts of refrigerant charge on system performance in the transcritical CO2 refrigeration system for electric buses, a series of steady-state tests were conducted under different refrigerant charges, different EEV opening degrees and different ambient temperatures. The experiments were carried out in the upper and lower separated enthalpy difference chamber, both of which can independently control the environment temperature and humidity. The photos of the test rig in both parts of the enthalpy difference chamber are shown in Figure 2. The outdoor ambient temperatures were set to 35 \({}^{\circ}\)C, 38 \({}^{\circ}\)C and 40 \({}^{\circ}\)C in the experiments, while the indoor dry/wet bulb temperatures were set to 27 \({}^{\circ}\)C/19.5 \({}^{\circ}\)C. Furthermore, the air flow rates of the indoor and outdoor fans were kept constant at 3100 m\({}^{3}\).h\({}^{-1}\) and 8800 m\({}^{3}\).h\({}^{-1}\), respectively. The discharge pressure and temperature of the compressor were controlled to not exceed the aforementioned safety limits, while the compressor speed was set to 3600 RPM in the whole experimental campaign. In addition, the opening of the EEV is reduced by 30 steps per second in the beginning and 10 steps when the system is close to the optimal state. The optimal values of each parameter were obtained in the experiment.  
Furthermore, [PERSON] et al. [14] proposed the concept of a normalized refrigerant charge (NRC) to eliminate the effect of the system volume. The actual charge in the various unit scales can be normalized as indicated in Equation (1). The NRC varies from 0 to 1.  
\[\text{NRC}=\frac{M_{actual}-M_{vapor}}{M_{liquid}-M_{vapor}} \tag{1}\]  
in which \(M_{actual}\) is the actual refrigerant charge (kg) while \(M_{vapor}\) and \(M_{liquid}\) are the corresponding refrigerant charges (kg) at saturation conditions. The values of \(M_{vapor}\) and \(M_{liquid}\) can be obtained by multiplying the system internal volume with the refrigerant densities at the saturation conditions evaluated at the ambient temperature of 25 \({}^{\circ}\)C. Table 2 shows the normalized charge value.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Equipment Name** & **Quantity** & **Specification** \\ \hline \multirow{3}{*}{Compressor} & \multirow{3}{*}{1} & Type: Rotary compressor \\  & & Displacement: 5.4 m\({}^{3}\) h\({}^{-1}\) at 3600 RPM \\  & & Speed range: 1800–5400 RPM \\ \hline \multirow{6}{*}{Outdoor HX} & \multirow{3}{*}{1} & Type: Fin tube heat exchanger \\  & & Dimension: 1530 mm (length), 76.2 mm (width), \\  & & 210 mm (height) \\  & & Tube (mm): \(\Phi\gamma\) \(\times\) 0.7 copper tube \\  & & Fin material: Aluminum \\  & & Fins per inch: 15.875 \\  & & Fin thickness (mm): 0.2 \\ \hline \multirow{6}{*}{Indoor HX} & \multirow{3}{*}{2} & Type: Fin tube heat exchanger \\  & & Dimension: 1150 mm (length), 76.2 mm (width), \\  & & 162 mm (height) \\ \cline{1-1}  & & Tube (mm): \(\Phi\gamma\)\(\times\) 0.7 copper tube \\ \cline{1-1}  & & Fin material: Aluminum \\ \cline{1-1}  & & Fins per inch: 15.875 \\ \cline{1-1}  & & Fin thickness (mm): 0.2 \\ \hline \hline \multirow{2}{*}{EEV} & \multirow{2}{*}{1} & Range of 0–480 step \\ \cline{1-1}  & & Driven by a stepper motor \\ \hline Accumulator & 1 & 4.8 L \\ \hline \hline \end{tabular}
\end{table}
Table 1: Specifications on system components.