Thermodynamic and thermoeconomic optimization of an integrated gas turbine and organic Rankine cycle

A multi objective optimization method is conducted for a cogeneration system to achieve the best system design parameters from both thermodynamic and economic aspects by utilizing NSGA-II (non-dominated sorting genetic algorithm-II). Exergy efficiency and total cost rate of the system have been considered as objective functions. The cogeneration system consists of a gas turbine and an ORC (organic Rankine cycle) in which the two cycles are connected through a single-pressure HRSG (heat recovery steam generator). In order to optimize the system, air compressor pressure ratio, isentropic efficiencies of air compressor and gas turbine, air preheater outlet temperature, turbine inlet temperature, Pinch point temperatures of HRSG and evaporator, condenser and evaporator temperatures are selected as decision variables. Optimization results indicate that exergy efficiency of the cycle increases from 51.4% at base case to 56.15% at the optimized condition while more than 12.98% reduction is achieved in the total cost rate of the system. Also, by applying multi-objective optimization method, the exergo-economic factor has reached from 10.68 to 28.54 suggesting that optimum system is achievable when the system costs are due to the investment costs. Furthermore, it is found that the gas turbine inlet temperature has important role on the trade-off between exergy efficiency and cost criteria.