Thermodynamic analysis of gas turbine with air bottoming cycle

• For certain pressure ratios, basic GT-ABC has higher efficiency than basic GT.
• Modified GT-ABC has lower efficiency but higher power output than modified GT.
• The performance of GT-ABC cycle is lower than conventional combined cycle.
• Increase the number of ABC cycles render the combined cycle impractical.

In this study, thermodynamic analysis of a gas turbine cycle with air bottoming cycle is presented to compare the efficiency ratios with modified gas turbine cycle as well as with conventional combined cycle. Both the modified gas turbine and modified gas turbine with air bottoming cycles consist of an intercooler and reheat exchangers. First and second laws of thermodynamics are employed in the analysis of each cycle. Codes are written in MATLAB to solve the equations at different operating conditions and efficiencies are calculated. The effects of pressure ratio of the topping gas turbine cycle (3≤prg1≤15)(3≤prg1≤15) and the pressure ratio of the bottoming gas turbine cycle (3≤prg2≤12)(3≤prg2≤12) on the thermal efficiency ratio and the work output are explored. It is observed that the basic gas turbine cycle with air bottoming cycle has a 4.78% higher efficiency than the basic gas turbine at their maximum efficiencies. Comparisons of thermal efficiency ratio and work output are presented for various combinations of each cycle. It is found that modified gas turbine with air bottoming cycle produces 1.27-fold higher work output than the modified gas turbine cycle at their maximum efficiencies. But it has less thermal efficiency and less work output than the combined cycle at all operating conditions. The exergy analysis shows that the use of the gas turbine bottoming cycle reduces the total exergy destruction of simple gas turbine by 6%. The exergy loss with the exhaust gas of simple gas turbine constitutes a large portion of 47% of the total exergy destruction. This is reduced to 31% for gas turbine with air bottoming cycle.