# Parametric Analysis of a Polygeneration System with CO\({}_{2}\) Working Fluid
## 3 Results and Discussion
### Preliminary Analysis
Figure 10: Useful energy products for different values of the heat exchanger effectiveness. Figure 9: Energy and exergy efficiencies for different values of the medium pressure. Figure 11: Energy and exergy efficiencies for different values of the heat exchanger effectiveness. Figure 12: Useful energy products for different values of the refrigeration temperature. Figure 3:0 Useful energy products for different values of the heat rejection temperature. Figure 3:1 Energy and exergy efficiencies for different values of the refrigeration temperature. Figure 3:2 Useful energy products for different values of the high heating temperature. Figure 3:3 Energy and exergy efficiencies for different values of the heat rejection temperature. Figures 4 and 5 show the system performance variation for the different values of the turbine inlet temperature (TIT). This parameter is a critical one for Brayton cycles and so it is also important for the present system. Figure 4 indicates that higher TIT increases power production and high heating, while it reduces the low heating and refrigeration production. Figure 5 proves that higher values of TIT increase both the energy and the exergy efficiency of the system. So, it is proved that higher TIT values are beneficial for the system performance, such as in the Brayton cycle where higher TIT increases the power production. Figures 6 and 7 illustrate the impact of the high-pressure level (or turbine inlet pressure) on the system performance. This is also a critical parameter for the system analysis and it is also used in Brayton cycle studies. Figure 6 shows that an increase in the high-pressure level only increases electricity production and the other useful outputs show a slight decrease. However, the increase in power production is relatively strong enough and both the energy and exergy efficiency show an increasing trend with the high-pressure increase according to Figure 3:4 The obtained results are reasonable because according to the general thermodynamic theory of the Brayton cycle, higher turbine inlet pressure leads to higher work production. Figures 8 and 9 exhibit the impact of the medium pressure on the results. This pressure level has to be over the CO\({}_{2}\) critical pressure, which is 73.8 bar, and so the minimum examined value was 75 bar. Generally, values very close to the critical point were avoided because of stability problems in this area. The increase of medium pressure leads to a higher low-heating temperature level and to lower net electricity production. The refrigeration production increases at a great pace up to 85 bar and after this limit, at a slower pace. The high heating production reduces up to 81 bar at a great pace, and then at a slower pace, it was minimized approximately at 95 bar and after this limit, it presents a rough increase. The above-described behavior of the Figure 8 curves can be explained by the great deviation of the CO\({}_{2}\) thermal properties with its pressure, in the cases with pressure levels close to the critical point (75-85 bar). Figure 9 indicates that the increase of the medium pressure leads to higher energy efficiency but to lower exergy efficiency. So,  
Figure 3:5 Energy and exergy efficiencies for different values of the high heating temperature. there is no global optimum value for this parameter and an intermediate value is maybe the optimum one. Moreover, different values of this parameter can be selected in order to adjust the useful output values (or fraction) according to the energy rate demands in every application. Figures 10 and 11 illustrate the system behavior with the increase of the heat exchanger effectiveness. According to Figure 10, this parameter leads to higher electricity, refrigeration and low heating production but to lower high heating production. Practically, the higher effectiveness of the heat exchanger gives a lower margin for high heating production after the heat exchanger, and thus, this parameter has different behavior than the other parameters.