Thermal stability of a mixed working fluid (R513A) for organic Rankine cycle

Organic Rankine cycle (ORC) is an effective way to utilize low-grade energy, with R1234yf being considered a promising working fluid for ORC for its low global warming potential (GWP). Nevertheless, the weak thermal stability has limited the application R1234yf in ORC, emphasizing the necessity to find a way preventing the its pyrolysis in ORC. In this connection, a systematic study on the mechanism of R134a affecting the thermal stability of R1234yf in ORC was conducted in this work. Our experimental results indicate that the pyrolysis temperature of R513A (R1234yf/R134a) is in the range of 190–210 °C. Further, ReaxFF simulations and density functional theory (DFT) calculations were performed on the pyrolysis process of R1234yf, R134a and R513A. It is revealed that the main pyrolysis reaction pathways of working fluids can be divided into three types: the reactions of working fluid molecules collision with F radical (FCR) or H radical (HCR), and the self-decomposition reactions of working fluid molecules (SDR). In R513A, F and H radicals are the origin for the affected thermal stability of R1234yf. The results are of great significance to guide the selection of additive species to enhance the thermal stability of HFO working fluid.