Enhancing exergy and economical performance of a waste heat power generation system: Multi-objective optimization and comparative analysis

Organic Rankine cycle (ORC) technology with attractive advantages can be effectively used for low temperature heat recovery. Due to the current research deficiencies and ORC system characteristics in ammonia synthesis manufacturing plant, the developed therm-economic optimization is established, considering the consumption of circulating water subsystem. According to the equilibrium of pay and gain exergy, the coefficient of exergy performance (COEP) of total system is proposed. The behaviors and detailed discussions of the electricity production cost (EPC), which consists of direct energy cost, primary non-energy cost, circulating water subsystem cost, and organic fluid cost, are examined. The exergy and economic comparison is conducted, and optimization is implemented for maximum COEP and minimum EPC. Furthermore, according to evaluation results, there are contradictions between equipment optimization and system optimization in certain operating conditions. Better system performance is achieved in specific evaporation temperature, lower condensation temperature and larger circulating water temperature rise. Under the optimal operation condition, the exergy efficiency of condenser and system is improved by 35.53% and 5.00%, the COEP increases by 1.65, and the EPC is reduced by 0.043 Yuan/kWh, respectively. The annual carbon dioxide emission reduction is developed to 3736.30 tons according to the international specification and the latest carbon dioxide emission factor of power generation in China. The improved therm-economic optimization method is of significance for the selection of optimization methods in practical projects.