## 6 Results and Discussion  
### Exergoeconomic and Environmental Results  
All equations related to the simulation of the combined cycle systems are simulated by EES software. Initially, in order to validate the programming code and simulation results, the irreversibility values of the various components of the simulated Brayton cycle in the present study are compared with the Brayton cycle of [28] with completely identical inputs according to Table 5. Table 6 reveals that there is a good agreement between the results of present program with the results of [28]. Input values used to simulate the combined cycle systems shown in Figures 1-3 are listed in Table 7[12; 28]. It should be noted that due to the high temperature of the exhaust gas at point 4 in the first configuration, the inlet steam temperature to the steam turbine is considered 550 \({}^{\circ}\)C and this is the highest recommended value in the literature [11; 13; 20]. In the parametric analysis it is tried to keep the temperature difference of at least 20 \({}^{\circ}\)C in the hot side of steam generator. However, in the second and third configurations, considering that some of the exhaust gas energy from the gas turbine output is consumed in the regenerator, the maximum temperature of 550 \({}^{\circ}\)C is not achievable for the inlet of the steam turbine. Therefore, the temperature difference of the hot side of the steam generator is assumed to be 20 \({}^{\circ}\)C in the first and second configurations. Moreover, to move closer to the real plant case, the inter-cooler multistage centrifugal compressor with pressure ratio 10 is selected. It is true that the exergy efficiency calculations for inter-cooler multiple stage compressors is a little different, however, as this research aims to conduct a comparative study over three configurations of combined cycles, the overall conclusions remain valid due to considering similar conditions (without inter-cooler) for all the examined configurations. For the steam Rankine cycle, the condenser pressure is specified. In a typical steam turbine, the exhausted steam condenses in the condenser, and it is at a pressure well below atmospheric. Therefore, 10 kPa is selected for condenser pressure so that maximizes the energy extracted from the steam, resulting in a significant increase in net work and thermal efficiency. However, this parameter (condenser pressure) has also its engineering limits. \begin{table}
\begin{tabular}{l c c} \hline \hline \multirow{2}{*}{**Components**} & **Irreversibility (MW)** & **Irreversibility (MW)** \\  & **(Present Programming Code)** & [28] \\ \hline Compressor & 2.09 & 2.12 \\ Regenerator & 2.55 & 2.63 \\ Combustion chamber & 25.34 & 25.48 \\ Gas turbine & 2.99 & 3.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparative results obtained between present programming code and Brayton cycle of the system studied in [28]. \begin{table}
\begin{tabular}{l c} \hline \hline \multicolumn{1}{c}{**Parameter**} & **Value** \\ \hline Ambient temperature (\({}^{\circ}\)C) & 25 \\ Ambient pressure (kPa) & 101.3 \\ Overall produced power of gas turbine cycle (MW) & 30 \\ Compressor pressure ratio & 10 \\ Gas turbine inlet temperature (\({}^{\circ}\)C) & 1246.85 \\ Regenerator outlet air temperature (\({}^{\circ}\)C) & 576.85 \\ Gas turbine isentropic efficiency & 0.86 \\ Compressor isentropic efficiency & 0.86 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Input values for Brayton cycle of the system according to data reported in [28]. Using the input data according to Table 7 and the equations described in Tables 2 and 3, the output results related to the energy, exergy, exergoeconomic and environmental analysis of the first to third configurations are calculated in Table 8. As seen in this table, the use of the regenerator in the second configuration, increase the combustion chamber inlet air temperature which reduces the fuel consumption in the combustion chamber compared to the first configuration.