THERMODYNAMIC, Economic and Environmental Multi-objective Optimization of ORC under Varying Weight

orc2019 Tracking Number 80

Organic Rankine cycle (ORC) is an effective, simple and environmental friendly technology to make use of waste heat or renewable energy such as solar energy, geothermal energy and biomass. With the research on ORC develops, the optimization method gradually turns from single-objective optimization to multi-objective optimization (MOO). Thermodynamic, economic and environmental performance are key indicators to evaluate the ORC system. Recently there are mainly two ways to comprehensively optimize an ORC system such as (1) transform multiple objectives into single objective based on linear weighting method. (2) optimize several objectives simultaneously using multi-objective optimization algorithm. However, most researches optimize the ORC system under a fixed weight of objective and pays little attention to the effects of weight on parameter design and fluid selection. Actually objective weight directly affects the final optimal solution in decision-making progress and thus affects the system parameters and selection of working fluid. This study focus on the effects of various weights on the system design and optimization. Three indexes as levelized energy cost (LEC), exergy efficiency, and carbon dioxide emission reduction (CER) are selected as system evaluation indicators with a heat source of 100 ℃, in which CER is calculated by the Life Cycle Climate Performance (LCCP) method. Three working fluids as R1234yf, R290, and R134a are calculated and compared using Non-dominated Sorting Genetic Algorithm II (NSGA-II) algorithm. A multi-criteria decision making (MCDM) method applies in selecting from the Pareto frontier when weights of three objectives vary from 0 to 1 increasing by 0.05. The selection of working fluid and system parameters is discussed under all weights. The results show that R134a exhibits the best thermodynamic performance but the worst environmental performance. R1234yf is the optimum fluid when weight of CER is over 0.25 while R134a is optimal when weight of CER is under 0.2. In terms of system parameter, evaporation temperature and superheat degree are the most two sensitive parameters while pinch point temperature difference and heat sink outlet temperature are least sensitive to weight of CER and LEC since they are the minimum under all weights. This study firstly analyses the effects of objective weight on the system parameters and working fluid selection quantitatively using MOO and MCDM method. Which facilitates the comprehensive optimization of ORC system in future.