## 6 Results and Discussion
### Parametric Study  
In this section, the effect of changing the Brayton compressor pressure ratio and the Brayton turbine inlet temperature on the performance of the combined cycle systems are investigated from the perspective of energy, exergy, exergy-economic and environmental factors. Note that for the purpose of parametric analysis, only the desired parameter is changed in the operating range and the rest of the inputs in the systems remain constant according to Table 7. The impact of changes in the compressor pressure ratio on the overall output power and the exergy efficiency of all configurations are examined in Figures 4 and 5. As shown in these figures, the first to third configurations have the highest output power and exergy efficiency respectively. Increasing the compressor pressure ratio in the operation range reduces the overall output of the first configuration and increases the total power of the second and third configurations. The effect of changes in the compressor pressure ratio on the overall cost rate and environmental cost rate of the configurations examined is shown in Figures 6 and 7. As shown in Figure 6, the highest overall cost rates are related to the first and the third configurations, respectively. As mentioned, increasing the compressor pressure ratio reduces the required fuel flow in the first configuration, which significantly reduces the environmental cost rate and the required fuel cost as shown in Figure 7. In addition, in the first configuration, an increase in the compressor pressure ratio, reduces overall exergy destruction cost and increases overall initial cost which has a greater impact on the reduction of overall system cost. The performances of the second and the third configuration are similar in this case. In both the first and the second configurations, despite lower environmental cost rates due to reduced carbon monoxide and nitrogen monoxide emissions, the fuel cost rate, the exergy destruction cost rate, and the initial cost rate have increased, which increases the overall cost rate. In addition, the effect of changing the compressor pressure ratio on the exergy unit cost of electricity generated shown in Figure 8. As shown in this figure, despite the lower overall cost rate, the third configuration has the highest cost per unit of exergy generated. Further, according to this figure, the second configuration for the pressure ratio in the range of 8 to about 17.3 and the first configuration for the pressure ratio from 17.3 to 20 has the lowest exergy unit cost of electricity that makes it difficult to choose the best cycle from the exergoeconomic and environmental perspective. In this case, although the third configuration has the lowest overall cost rate, it results in the highest exergy unit cost of electricity generated during the operating range. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Energy Unit Cost** & **Config. 1** & **Config. 2** & **Config. 3** \\ \hline \hline \begin{tabular}{c} Exergy unit cost of produced electricity by the first gas turbine, \(c_{el,gt,1}\left(\frac{\mathrm{g}}{\mathrm{G}}\right)\) \\ \end{tabular} & 20.332 & 20.929 & 22.46 \\ \
\begin{tabular}{c} Exergy unit cost of produced electricity by the sec ond gas turbine, \(c_{el,gt,2}\left(\frac{\mathrm{g}}{\mathrm{G}}\right)\) \\ \end{tabular} & - & - & 23.429 \\
\begin{tabular}{c} Exergy unit cost of produced electricity by the steam turbine, \(c_{el,st}\left(\frac{\mathrm{g}}{\mathrm{G}}\right)\) \\ \end{tabular} & 28.332 & 30.165 & 36.61 \\ \hline \hline \end{tabular}
\end{table} 
Table 15: Unit cost of exergy for the produced electricity by turbines in all configurations. Figure 4: Effect of compressor pressure ratio change on the overall output power. Figure 5: Effect of compressor pressure ratio change on the overall exergy efficiency. Figure 6: Effect of compressor pressure ratio change on the overall cost rate.