Performance Analysis and Multi-Objective Optimization of Two Organic Rankine Cycles with Different Fluids for Low Grade Waste Heat Recovery

The basic organic Rankine cycle (BORC) and ORC with an internal heat exchanger (IHORC) are studied with different working fluids under a given heat source condition to analyse the thermodynamic performances and net power output. The results demonstrate that the external exergy efficiency of IHORC is lower than that of BORC while the internal exergy efficiency is on the opposite with the same overall exergy efficiencies. A multi-objective optimization model with inlet pressure and temperature of expander as independent parameters and exergy and heat recovery efficiencies as objective functions is solved by NSGA-II (the second non-dominated sorting genetic algorithm). The Pareto optimal solutions are obtained by the optimization models. By calculation with the optimum conditions, it is determined that R236ea has the best comprehensive performance with exergy efficiencies being 40.69% and 41.38%, and heat recovery efficiencies being 83.2% and 75.6% in IHORC and BORC, respectively. The evaporators occupy the maximum exergy destruction, which can be reduced by decreasing pinch point temperatures and increasing evaporation pressures.