Experimental and Simulation Analysis of Siloxane Mixtures Used in Organic Rankine Cycle with Thermal Stability Limits

Abstract

The thermal stability of siloxanes has significant influence on the selection of working fluid and the performance of organic Rankine cycle systems. In this study, a thermal decomposition experimental apparatus was designed to measure the thermal stability of hexamethyldisiloxane (MM), octamethyltrisiloxane (MDM), and their mixtures; a reaction kinetics model based on first order reaction theory was built to analyze the thermal stability of siloxane mixture fluids in a long operation period. And the influence of the mass fraction and evaporation temperature on the net power and thermal efficiency of the system was analyzed under the constraints of thermal stability. The results showed that the thermal stability of MDM was worse than that of MM, and the mixture of MM and MDM had significant inhibition effects on the de-composition of pure fluids. The activation energy of decomposition reaction was 50.50 kJ/mol, and the pre-exponential factor was 5.80 × 10−3 s−1. With the evaporation temperature limit, the net power and thermal efficiency were both lower than those without the evaporation temperature limit. Comparing the obvious decrease in the thermal efficiency, the change of the net power was limited. Siloxane mixtures emerged as a superior choice for ORC systems in the conditions of this paper. MM/MDM (0.6/0.4) improved the net power and heat efficiency of the system by 8.1% and 1.7%, respectively, comparing with that of the pure working fluids.