## 6 Results and Discussion
### Exergoeconomic and Environmental Results
It can be seen that although the new regenerative Brayton cycle has better thermal and energetic performances than the original one and remarkably increases energy efficiency compared to the basic Brayton cycle and the regenerative Brayton cycle [9]; however, its integration with steam Rankine cycle has lowest exergy efficiency among the other possible configurations. Table 8 indicates that the first configuration has the highest exergy efficiency (48.76%), the second configuration has lowest exergy unit cost of electricity (22.93 $/GJ), and the third configuration has the lowest total cost rate (25,450 $/h). Therefore, it depends on the preferences and criteria of each decision maker to select the best configuration among the three proposed configurations as the final configuration based on energy analysis, exergy analysis or exergoeconomic analysis. Of course, if this new regenerative gas turbine combined cycle operates at this particular operational condition, it consists of smaller compressor and turbines. Hence, low occupation space, more controllability, quicker starting process, and safer dynamical operation may be encountered as other advantages for the new configuration of this combined cycle. In addition, in order to better understand the performance of the combined cycle systems, the output values related to the energy and exergy rates of the various components as well as the exergy efficiency of the components of all three configurations shown in Figures 1-3 are given in Tables 9-11. As can be seen in these tables, the highest amount of exergy destruction occurs in the combustion chamber for all three configurations due to the chemical reaction and in which the first configuration is higher than the other configurations. Another notable point is the higher exergy destruction rate in the steam cycle in the first configuration than the other configurations which is due to the higher mass flow rates in the first configuration. In this case, despite addition of a regenerator in the second configuration, and a regenerator and a second gas turbine in the third configuration, the higher exergy destruction in the steam cycle of the first configuration results in a higher overall exergy destruction than the other configurations. The highest exergy efficiency is related to the gas turbine and the lowest exergy efficiency is in the steam condenser among the components of all three configurations. \begin{table}
\begin{tabular}{l c c c} \hline \hline  
For exergy improvement of turbines, efforts should be focused on their isentropic efficiencies, however operation optimization is also needed. For exergy improving of heat exchangers such as condenser and steam generator, if the quantity and quality of heat and mass transfer at the lowest possible temperature difference could be maintained between the two fluids, the exergy destruction can be greatly reduced. Increasing the surface area of heat transfer may help to cut down the exergy destruction, but it is a design consideration and relates to the economic and spatial limits. In practice, a more reliable way is adopting thermal insulation for this equipment. The exergy destruction in combustion chamber occurs is caused due to various reasons such as unburnt fuel, incomplete combustion, and heat loss to the surrounding area through to the combustion process. Overall, it should be noted that some improvement for one component might result in larger exergy destructions in other components in the system, so the feasibility of improvement measures should be considered from the perspective of the whole system. For this reason, an optimization process is needed to reduce total exergy destruction of systems. Tables 12-14 show the exergy-economic results for all configurations. As it can be seen from these tables, the combustion chamber should be considered more than the other components economically since in each of the three configurations, the maximum value of \(\dot{Z}_{k}+\dot{C}_{D,k}\) belongs to this component.