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

Wei Yu, Shukun Wang, Chao Liu, Qibin Li and Xiaoxiao Xu

Energy, 2025, vol. 320, issue C

Abstract: 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.

Keywords: Organic Rankine cycle; Siloxane; Improvement of thermal stability; Group substitution; ReaxFF; DFT (search for similar items in EconPapers)
Date: 2025
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Persistent link: https://EconPapers.repec.org/RePEc:eee:energy:v:320:y:2025:i:c:s0360544225006590

DOI: 10.1016/j.energy.2025.135017

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