Study on the thermal stability of a new siloxane working fluid modified by octamethyltrisiloxane and its application potential in organic Rankine cycle

Octamethyltrisiloxane (MDM) is widely utilized as the working fluid in medium-high temperature organic Rankine cycle (ORC) systems. Despite the presence of strong Si-O and Si-C bonds in MDM, its thermal stability often falls short in various practical applications and thermal stability tests. Enhancing this thermal stability is essential for expanding its use in medium-high temperature thermal applications. This work evaluates the creation of new compounds through intermediate methyl substitutions with -C2H5, -C6H5, -OSi(CH3)3, -F, and -Cl groups. The findings indicate that trimethylsiloxyalkyl substitutions significantly mitigates Si-Si bond polarization, resulting in an increase in the apparent activation energy for decomposition by approximately 47.9 kJ mol⁻1. Substitutions with -F and -Cl not only elevate bond dissociation free energy but also help suppress Si-Si bond polarization, potentially leading to substantial improvements in thermal stability. Thermal stability tests of MDM_OSi(CH3)3, formed through trimethylsiloxyl substitution, demonstrated its stability at 350 °C, almost 100 °C higher than MDM. In terms of safety, branched siloxanes are more suitable as working fluids in medium-high temperature ORCs compared to straight-chain siloxanes. Moreover, the thermodynamic performance evaluation of MDM_OSi(CH3)3 as a working fluid in the regenerative ORC shows that it has a net output power and thermal efficiency similar to MDM, and has the potential for use in medium-high temperature thermal utilization ORC systems. This study offers valuable insights for designing modified siloxane-based working fluids for medium-high temperature ORC applications.