# A Theoretical Comparative Study of CO\({}_{2}\) Cascade Refrigeration Systems
## 3 Results and Discussion
### Yearly Energetic and Environmental Analysis  
The yearly analysis of the system is performed using the weather data for Athens (Greece) which is a warm location. The distribution of the ambient temperature level is given in Figure 3 with a step of 1 K. The model is applied for all the temperatures and the mean yearly COP is calculated according to equation 19. The results for the typical evaporator temperature level of \(-35\)\({}^{\circ}\)C are given in Figure 8. It is obvious that the CO\({}_{2}\)/CO\({}_{2}\) system has the lowest efficiency among the examined systems with a value of 1.901. The most efficient is the R152a/CO\({}_{2}\) system with 2.381, while R1270/CO\({}_{2}\) is the second one with 2.377. Also, highly efficient systems are R290/CO\({}_{2}\) with 2.372, R600a/CO\({}_{2}\) with 2.364, as well as R600/CO\({}_{2}\) and NH\({}_{3}\)/CO\({}_{2}\) with 2.362. The less efficient choices are R407C, R227 ea, R404a, R507A and R448A. Generally, the refrigerants with the greater GWP present decreased efficiently. This is an important result which makes these harmful refrigerants to be not efficient and so they have no chances for application in the future refrigeration cascade configurations. Figure 9 gives a detailed comparison for four evaporating temperatures by presenting results about CO\({}_{2}\)/CO\({}_{2}\), R1270/CO\({}_{2}\), R152a/CO\({}_{2}\) and NH\({}_{3}\)/CO\({}_{2}\). It is obvious that in all the evaporating temperatures, the mean yearly COP is lower for the configurations with CO\({}_{2}\) in the high-temperature circuit. The three cases (R1270, R152a and NH\({}_{3}\)) have similar mean COP with R152a to present a roughly increase which is not so important. However, the high efficiency of R152a in combination with the low GWP of 124, are two factors which indicate it as a possible choice for refrigeration systems. Moreover, it is no toxic and less flammable than other natural refrigerants. So, the investigation and the application of R152a have a great interest. Lastly, it can be said that Table 3 includes all the data about the mean yearly COP of all the cases. Figure 8: Mean yearly COP of the examined systems for \(T_{e}=-35\)\({}^{\circ}\)C.  
The next step is the environmental evaluation of the different refrigerants using the TEWI. This criterion needs the yearly electricity consumption and it regards a system lifetime of 10 years for this work. The results for all the systems are included in Table 4, while Figures 10 and 11 give important results. More specifically, Figure 10 shows the total TEWI for all the refrigerants with (\(T_{e}=-35\)\({}^{\circ}\)C). Moreover, Figure 10 depicts the separation of the TEWI into the direct and indirect part. The most harmful fluids, which have the highest TEWI are the R507A, R404a, R227 ea, R407c, R134a and R448A. The next category includes R32, R513A, R450A, while the lowest GWPI is found for the natural refrigerants, the R152a and the R1234 yf, R1234 ze(E). The system with CO\({}_{2}\)/CO\({}_{2}\) is not the most environmentally friendly choice because of the huge indirect CO\({}_{2}\) emissions. The low COP of this system makes the indirect TEWI be huge, the fact that makes this system to be among the most