Design and Thermal Modeling of a Sealed Autoclave for Compatibility Testing of Mineral Oil–Rock Systems in Thermal Energy Storage Applications

Abstract

This paper presents the design, numerical modeling, and experimental implementation of a sealed autoclave developed for compatibility testing of mineral thermal oil with natural rocks in packed-bed thermal energy storage (TES) systems. Delcoterm Solar E15 was selected as the heat transfer fluid (HTF) based on its potential thermo-physical properties, low vapor pressure (<0.3 bar at 300°C), high specific heat (2180-2900 J/kg. K), and economic viability compared to synthetic oils. The structural material of the autoclave was chosen as AISI 304 stainless steel due to its corrosion resistance and thermal conductivity (16.2 W/m. K). A finite element model was developed using COMSOL Multiphysics to simulate transient heating and cooling cycles between 100°C and 300°C. Results show uniform internal temperature distribution after 432 s during charging and a discharge time of approximately 1000 s. Based on simulation results, heating and cooling rates were optimized at 5°C/min and 1-2°C/min, respectively. The proposed system enables controlled isothermal and cyclic compatibility testing under inert atmosphere conditions up to 2 bars. The preliminary compatibility tests showed a good compatibility with the selected AISI304 and trondhjemite rock.