Thermodynamic Comparison of Gas Turbine and ORC Combined Cycle with Pure and Mixture Working Fluids

This paper proposed a gas turbine and organic Rankine cycle (GT-ORC) combined cycle to further improve the energy efficiency of gas turbines. Alkanes, linear siloxanes, and aromatics were selected as working fluids for the bottoming organic Rankine cycle. Based on the mathematical model and solution procedure proposed, a thermodynamic comparison of GT-ORC combined cycle was conducted with pure and mixture working fluids. Simulation results showed that the mixtures made the combined cycle achieve higher efficiency than pure fluids. Compared with conventional steam Rankine cycles, the organic Rankine cycle had larger potential in recovering exhaust heat from gas turbines. The thermodynamic analysis showed that the thermal efficiency of the bottoming cycle increased with the rise of turbine inlet pressure. Besides, the ORC net power was maximum at the optimum turbine inlet pressure. Four commercial gas turbines with different exhaust temperatures (553-778 K) were also examined, and results indicated that the Trent 60 combined cycle achieved the highest thermal efficiency of 56.48%. For gas turbine of different power levels, the toluene/benzene mixture was more suitable in recovering waste heat from small and medium size gas turbines, while the cyclopentane was more applicable for microgas turbines.