## 7 Conclusions  
In the present study, three different configurations of the combined cycle system are analyzed from the perspective of energy, exergy, exergy-economic and environmental. The results of the comprehensive analysis on these three configurations reveals that the first,  
Figure 16: Pareto optimal solutions for the configuration shown in Figure 3.  
Figure 14: Pareto optimal solutions for the configuration shown in Figure 1.  
Figure 15: Pareto optimal solutions for the configuration shown in Figure 2.  
the second and the third configuration are the best among the other systems if exergy efficiency, exergy unit cost of generated electricity, and overall cost rate are the objective functions, respectively. 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. For the purpose of parametric study, the effect of changing in the Brayton designed parameters in the operating range such as the compressor pressure ratio and the turbine inlet temperature on the performance of the combined cycle systems are investigated from the perspective of energy, exergy, exergy-economic and environmental. Moreover, the application of exergy-economic multi-objective optimization shows that the total performance of combined cycle systems improved significantly in a way that:  
1. The exergy efficiency is increased from 48.76% to 49.6% for the first configuration, from 47.92% to 48.28% for the second configuration, and from 39.29% to 43% for the third configuration.
2. Exergy unit cost of generated electricity is reduced from 23.66 $/GJ to 22.36 $/GJ for the first configuration, from 22.93 $/GJ to 21.13 $/GJ for the second configuration, and from 24.89 $/GJ to 23.28 $/GJ for the third configuration.
3. Exergoeconomic factor for the studied systems is increased from 7.53 to 14.47 for the first configuration, from 9.59 to 14.78 for the second configuration, and from 10.63 to 17.96 for the third configuration.
4. However, these improvements achieve with 38.87%, 16.38% and 57.58% increase in the capital investment cost of the first, the second and the third configurations, respectively from studied combined cycle systems.  
For the future works, it is very good idea to do more research on these configurations of gas turbine combined cycle systems from a purely economic point of view, taking into account the costs of return, and the reliability, etc. This is because the focus of the present study was mainly on the investigation of thermodynamics, exegoeconomics and environmental aspects of these integrated systems.  
Conceptualization, A.B.; methodology, A.B.; software, A.B.; investigation, A.A.-M.; writing--original draft preparation, A.B.; writing--review and editing, A.A.-M.; supervision, A.B. All authors have read and agreed to the published version of the manuscript.  
[PERSON] acknowledges the support of \"HeatReFlex-Green and Flexible Heating/Cooling\" project funded by Danila Fellowship Centre and the Ministry of Foreign Affairs of Denmark under the grant no. 18-M06-AAU.  
Not applicable.  
The authors declare no conflict of interest.