## 3 Results and Discussion
### Exergy Analysis  
Figure 6 indicates that the exergy efficiency (\(\eta\)) corresponding to the optimal COP increased with rise in NRC. In addition, at sufficient charge conditions, \(\eta\) was equal to 29.79%, 28.32% and 26.15% at 35 \({}^{\circ}\)C, 38 \({}^{\circ}\)C and 40 \({}^{\circ}\)C, respectively. As the ambient temperature rose from 35 \({}^{\circ}\)C to 40 \({}^{\circ}\)C, \(\eta\) decreased by 12.22%. It is worth noting that although the system was undercharged (i.e., NRC between 0.100 and 0.248), the exergy efficiency almost remained the same. In contrast, the value went up sharply in excessive charge conditions. However, although the compressor power input was small as the NRC was 0.10, the power input of the indoor/outdoor fans was relatively high, which led to the reduction of \(\eta\). Furthermore, in the excessive charge condition, the total product exergy rate and compressor power consumption increased with the rapid rise of the CO\({}_{2}\) mass flow rate. Therefore, the exergy efficiency increased, since the fan power input accounted for a small proportion of the total power input. It could be concluded that the value of the \(\eta\) was not sensitive to slight insufficient charge, whereas overcharging the system resulted in even greater exergy efficiency under the condition of ensuring the maximum COP. Figure 7 shows the effect of the discharge pressure (P\({}_{\rm dis}\)) on the exergy efficiency (\(\eta\)), the total product exergy rate (\(\dot{E}_{p,tot}\)), the total power input (\(\dot{W}_{tot}\)) and the total exergy destruction rate (\(\dot{E}_{D,tot}\)) at 35 \({}^{\circ}\)C. It can be seen from Figure 7(a) that the value of \(\eta\) first increased and then decreased with rise in P\({}_{\rm dis}\) under different charging conditions. Total power input went up almost linearly with the increase in P\({}_{\rm dis}\), whereas the growth rate of \(\dot{E}_{D,tot}\) continues to rise (shown in Figure 7(b)), resulting in a peak value of \(\eta\). However, the P\({}_{\rm dis}\) corresponding to the maximum \(\eta\) of each charging condition was smaller than that of the maximum COP. The maximum value of \(\eta\) increased first and then decreased with the NRC increase and achieved the maximum value of 30.74% in the sufficient charging condition. In addition, similar to cooling capacity, the total product exergy rate (\(\dot{E}_{p,tot}\)) first  
Figure 5: **(a)** Variation of COP and cooling capacity with discharge pressure and **(b)** P-h diagrams of the transcritical CO\({}_{2}\) system at the insufficient charge, sufficient charge and excessive charge at different EEV opening degrees (OD). increased and then decreased with the rise in \(\mathrm{P_{dis}}\) in the insufficient and sufficient charge conditions, whereas no peak value in the excessive charge condition was observed. Figure 7b shows that \(\dot{E}_{D,tot}\) increased significantly in insufficient charge conditions and it rose even more sharply as \(\mathrm{P_{dis}}\) was greater than 10.9 MPa (\(\mathrm{P_{dis,opt}}\)) in sufficient charge conditions. The main reason is that the temperature difference between air and refrigerant in the evaporator increased due to the low evaporator pressure. Furthermore, the refrigerant drawn by the compressor was at the state of superheated vapor under these conditions, causing an increase of \(\mathrm{T_{dis}}\) and thus higher exergy destruction within the gas cooler. The variation of exergy destruction percentages of the compressor, gas cooler, EEV and evaporator with NRC corresponding to the optimal COP at ambient temperature of 35 \({}^{\circ}\)C is shown in Figure 8. It was found that the exergy destruction percentage of the compressor varies more significantly, increasing from 24.69% to 44.27% as NRC increase from 0.100 to 0.373. This is related to the slight increase of the compressor isentropic efficiency.