Multi-objective Optimization of Cogeneration of Power and Heat in a Combined Gas Turbine and Organic Rankine Cycle

A multi-objective optimization method of cogeneration of power and heat in a combined gas turbine and organic Rankine cycle (ORC) is conducted to achieve the best system design parameters from both thermodynamic and economic aspects by utilizing nondominated sorting genetic algorithm-II (NSGA-II). Exergy efficiency and total cost rate of the system have been considered as objective functions. The cogeneration system consists of a gas turbine (GT) and an organic Rankine cycle (ORC) in which the two cycles are connected through a single-pressure heat recovery steam generator (HRSG). In order to optimize the system, air compressor pressure ratio, air compressor isentropic efficiency, air preheater outlet temperature, turbine inlet temperature, isentropic efficiency of the gas turbine, pinch point temperature of HRSG, pinch point temperature of evaporator, evaporator temperature, and condenser temperature have been selected as decision variables. Optimization results indicate that exergy efficiency of the cycle increases from 51.41% at base case to 55.6% while more than 9.15% reduction is achieved in the total cost rate of the cycle. Also by applying multi-objective optimization, the exergo-economic factor has reached from 10.68 to 27.40.