## 2 System Description  
Three different combined cycle systems are considered in the present study. Figure 1 shows the combined simple Brayton and Rankine cycle (first configuration). The Brayton cycle of this configuration consisting of a compressor, a gas turbine, and a combustion chamber, and has no regenerator. Air enters to the compressor and is supplied to combustion chamber with highest cycle pressure. It absorbs heat from fuel within the combustion chamber and a high temperature gas enters the turbine and then is exhausted to the atmosphere at the gas turbine outlet after producing power. Figure 2 schematically shows a combined regenerative Brayton and Rankine cycle (second configuration). The Brayton cycle of this configuration contains a regenerator for preheating the compressed airflow at the compressor outlet by hot exhausting airflow after the gas turbine. Figure 3 shows a new combined regenerative Brayton and Rankine cycle (third configuration). The basic difference between the new combined cycle and the basic one is that the exhausted gases do not completely expand to the atmospheric pressure before entering the regenerator in the regenerative Brayton cycle of this configuration. The mixture of gases with high pressure and temperature enters the first gas turbine to generate power. At the output of the first gas turbine, the mixture of exhaust gases expands to certain pressure above the atmospheric pressure to meet the required power of compressor. Then the hot exhaust gas of the first gas turbine enters the regenerator to preheat the outlet compressor airflow. In the second gas turbine, the exhaust gases expand to atmospheric pressure and supply the cycle's output power. In the Rankine cycle of all three configurations, exhaust gases from the gas turbines expand to atmospheric pressure and with passing through the steam generator, the working fluid is converted to superheated steam. Superheated steam goes to the steam turbine and generates power. The steam turbine outlet fluid in the condenser discharge heat into the medium and the outlet saturated fluid from the condenser is pumped up by pump and returned to the steam generator. Figure 1: Schematic of the combined simple Brayton and Rankine cycle (Config. 1). In order to simulate and exergy analysis of the combined cycle systems (Figures 1-3) the following assumptions are considered [28; 29]:  
* The systems operate in the steady state condition;
* The kinetic and potential energies in the various components are ignored;
* The pressure loss in the pipelines is negligible;  
Figure 3: Schematic of the new combined regenerative Brayton and Rankine cycle (Config. 3). Figure 2: Schematic of the combined regenerative Brayton and Rankine cycle (Config. 2). * In order to calculate the air and gas mixture properties in the Brayton cycle, ideal gas mixture relationships are used;
* The combustion chamber fuel is assumed to be natural gas with 100% methane and is considered as ideal gas;
* Heat loss from the combustion chamber is assumed to be 2% of the lower heating value of the fuel and other components operate adiabatically;
* Combustion in the combustion chamber is assumed to be complete;
* Pressure losses in the gas side and air side of the regenerator are assumed 3% and in the combustion chamber and the gas side of steam generator are considered 5%.