In this research paper, a complete thermodynamic modeling of one of the gas turbine power plants in Iran is performed based on thermodynamic relations. Moreover, a complete computer code is developed for simulation purposes using Matlab software. To assess system performance, exergy and exergo-economic analyses are conducted to determine the exergy destruction of each component and cost of each flow line of the system. A complete parametric study is also carried out to study the effect of certain design parameters such as exergy efficiency and total cost of exergy destruction on system performance variation. The exergy analysis results revealed that the combustion chamber (CC) is the most exergy destructive component compared to other cycle components. Also, its exergy efficiency is less than other components, which is due to the high temperature difference between working fluid and burner temperature. In addition, it was found that by increasing the TIT (gas turbine inlet temperature), the exergy destruction of this component can be reduced. On the other hand, the cost of exergy destruction, which is a direct function of exergy destruction, is high for the combustion chamber. The effects of design parameters on exergy efficiency showed that an increase in the air compressor pressure ratio and TIT increases the total exergy efficiency of the cycle. Furthermore, the results revealed that an increase in the TIT of about 350 K can lead to a reduction of about 22% in the cost of exergy destruction. Therefore, TIT is the best option to improve cycle losses.
Energy systems involve a large number and various types of interactions with the world outside their physical boundaries. Therefore, the designer must face many issues, primarily related to energy, economy and the environment, in short “3E”. Gas turbines are a good candidate for power generation units because they are widely used in both gas cycles and combined cycles. Hence, thermodynamic modeling and performance assessment of gas turbines form a significant subject of interest for thermal system designers. Combined cycle power plants (CCPP) utilize the exhaust heat from the gas turbine engine to increase power plant output and boost overall efficiency up to 50%. Recently, exergy analysis, which is based on the second law of thermodynamics, has been found to be a potential tool for enhancing the understanding of system performance by determining the amount of irreversibilities for each component and providing better insight into system design.
The exergy analysis approach is based on the simultaneous application of the first and the second laws of thermodynamics .The energy crisis of the 1970s and the continuing emphasis on efficiency (conservation of fuel resources) have led to a complete overhaul of the way in which power systems are analyzed and improved thermodynamically .
Today, many electrical generation utilities are striving to improve efficiency and the heat rate at their existing thermal electric generating stations, many of which are over 25 years old. Often, a heat rate improvement of only a few percent appears desirable, as it is thought that the costs and complexity of such measures may be more manageable than more expensive options. Thus, a better understanding is attained when a more complete thermodynamic view is taken, which uses the second law of thermodynamics in conjunction with energy analysis, via exergy methods. One of the most commonlyused methods for evaluating the efficiency of an energy-conversion process is first-law analysis although it cannot determine the location of devices in which exergy destruction would occur.
It is well-known that exergy can be used to determine the location, type and true magnitude of exergy loss (or destruction). Thus, it can play an important role in developing strategies and in providing guidelines for more effective use of energy in the existing power plants . Moreover, another important issue to improve the existing system is the origin of the exergy loss and components in which the most exergy destruction take place. Hence, a clear picture, instead of only the magnitude of exergy loss in each section, is required. There are numerous research papers in the literature, which have presented exergy and exergo-economic analysis. However, they do not usually pay much attention to the effect of key parameters on the cycle components, especially the cost of exergy destruction.
According to literature, exergy analysis is a methodology for the evaluation of the performance of devices and processes, and involves examining the exergy at different points in a series of energyconversion steps [2–5]. Exergy analysis results can aid efforts to improve the efficiency, and possibly the economic and environmental performance of gas turbine power plants. In parallel to exergy analysis, thermo-economics can also help the designers to enhance the understanding of the system performance by consideration of the system costs. Thermoeconomics combines exergy analysis with economic principles and incorporates the associated costs of the thermodynamic inefficiencies in the total product cost of an energy system. These costs may lead designers to understand the cost formation process in an energy system and that can be utilized to optimize thermodynamic systems, in which the task is usually focused on minimizing the unit cost of the system product . Several researchers carried out exergy analysis of and applied exergo-economics to systems in which a gas turbine played a significant role. Sahin and Ali  carried out an optimal performance analysis of a combined Carnot cycle (two single Carnot cycles in cascade), including internal irreversibilities for steady-state operation. Ameri et al.  performed an exergy analysis of supplementary firing in a heat recovery steam generator in a combined cycle power plant. Their results revealed that if a duct burner is added to the heat recovery steam generator (HRSG), the first and second law efficiencies are reduced. Also, Ameri et al.  performed an energy, exergy and exergo-economic analysis for one of the largest steam power plants in Iran. It was determined that the boiler has the highest exergy destruction rate. Therefore, this device should be considered for further improvements. The reason for the greatest exergy destruction in this device is due to the combustion and heat transfer processes, which take place across large temperature differences between burner temperature and working fluid. The same results were obtained in other research performed by Ameri et al. . It was found that in combined cycle plants, the combustion chamber destroys the inflow exergy due to the high temperature difference. However, that paper did not pay much attention to the key parameters. Ahmadi et al.  performed thermodynamic and exergo-environmental analyses, and multi-objective optimization of a gas turbine power plant.They applied the multi-objective based optimization to an actual power plant in Iran and determined the optimal design parameters. The results showed that by selecting the optimized parameters, a 50% reduction in environmental impacts is obtained. Ehyaei et al.  carried out exergy, economic and environmental analyses of absorption chiller inlet air cooler used in gas turbine power plants. They conducted the analyses for two different regions in Iran, (i.e. hot-dry and hot-humid climate conditions). The results showed that using this system in the hot months of a year is economical. They also concluded that application of an absorption chiller increases the output power by 11.5% for the hot-dry climate and 10.3% for the hot-humid climate. The present study, which is an extended version of earlier research carried out by the authors [1, 3] mainly focuses on the followings items which are the specific contribution of the current paper in this subject:
• Complete thermodynamic modeling of a major gas turbine power plant in Iran is performed.
• A simulation computer code is developed using Matlab software to mode all parts of the power plant and this code is validated with actual data from the power plant.
• Exergy analysis of a gas turbine power plant is performed.
• Exergo-economic analysis of a gas turbine power plant is conducted.
• The effects of some major key parameters on both exergy and the exergo-economic performance of the cycle are investigated.
2. Exergy analysis
Exergy is composed of two important parts. The first one is the physical exergy and the second one is the chemical exergy. In this study, the kinetic and potential parts of exergy are negligible . The physical exergy is defined as the maximum theoretical useful work obtained as a system interacting with an equilibrium state. The chemical exergy is associated with the departure of the chemical composition of a system from its chemical equilibrium. The chemical exergy is an important part of exergy in the combustion process. It is important to observe that, unlike energy, exergy is exempt from the law of conservation . Irreversibility associated with actual processes causes exergy destruction.
In order to perform the exergy analysis, mass and energy balances of the system are required to be determined. If one combines the first and second laws of thermodynamics, the exergy balance equation is formed as :
Continuity equation: X m˙ i = X m˙ e (1)
Energy equation: Q˙ − W˙ = X m˙ ehe − X m˙ ihi (2)
Exergy balance equation: Ex˙ Q + X m˙ iei = X m˙ eee + Ex˙ D + Ex˙ W (3)