Exergetic and heat load optimization of high temperature organic Rankine cycle

This paper presents an optimization of a high temperature organic Rankine cycle (ORC) system providing the basis for optimal fluid selection and subsequent design parameters based on the working fluid selected. The working fluids used are m-xylene, propylcyclohexane and decane having high critical temperatures. The proposed system deals with the application of biomass due to the high content heat available during its combustion. The system is optimized through non-dominated sorting genetic algorithm (NSGA-II) by taking the prime operators such as; exergetic efficiency (ηex) to extract maximum work and total heat transfer requirement (UA) to get a prediction of the heat transfer area and hence the cost of the system. The parameters subjected to constraints for optimization are evaporation pressure, degree of superheating and pinch point conditions at heat exchangers. The optimization results exhibit an increase of 22.9% for propylcyclohexane and 45.5% for decane in UA values, relative to m-xylene. Highest exergetic efficiency values for m-xylene among three working fluids further ensures its use in the system as the most viable option from both thermodynamic and economic aspect. Moreover, optimal evaporation pressure range is evaluated by taking the maximum and minimum of exergetic efficiency and UA value, respectively. Both objective functions show negative trend with increase in degree of superheating, with less significant drop. As the pinch point value increases, the UA value decreases showing significantly smaller areas of heat transfer and less cost, but with low exergetic efficiency, therefore, moderate pinch point condition of 8-10 °C is recommended.