Accelerated and long-time creep testing of extruded polystyrene using isothermal and stepped isothermal method

Seasonal Sensible Thermal Energy Storages based on existing inside structures are cost effective and require insulation materials with high performance. The insulation must endure high temperatures and high pressure for a long period. In this study, the creep effects of the insulation material XPS (extruded polystyrene foam) at high temperatures and pressure are investigated to determine whether XPS is a suitable insulation material to be applied on the inside of STES. Two studies were conducted: an isothermal long-term creep experiment and a SIM (Stepped Isothermal Method) experiment. The results of the first study were fitted using the Findley approach and extrapolated to 50 years. The SIM study is a method to accelerate creep effects using thermal effects and the Boltzmann superposition principle. Creep strain after 50 years is directly calculated by extrapolation. Both studies indicate that creep strain in XPS remains well below 6% with an applied load of 61.1 kPa at 60 °C after 50 years. Additionally, a material model that predicts creep strain at different or varying temperatures and stress was developed using the SIM data.