Modelling of long-term hygro-thermal behaviour of vacuum insulation panels

The low thermal conductivity of Vacuum Insulation Panels (VIPs) degrades with time due to gas permeation through VIPs barriers. There is additional ageing of their envelope and core material. Accelerated experiments are carried out in order to better understand this ageing process but they are not sufficient to predict the long term performance of panels. Models have to be used to connect the short term evaluation and the long term behaviours in order to improve the prediction of the thermal conductivity evolution over 50 years. This paper describes the development of a VIP model in the Dymola® software. This model takes into account the envelope and core material hydro-thermal characteristics and behaviours, and integrates the actual solicitations of the panels during accelerated ageing tests. The thermo-activation of the envelope permeance is integrated. Many properties of the core material are modelized: type of core material, sorption isotherm, hygro-thermal ageing, pore size distribution, etc. Simulations in constant conditions in temperature and humidity have been carried out. The results show that the real behaviour of the VIPs conductivity can't be simply evaluated through a linear extension of its short-term evolution. Given the important impact of the core material detailed characteristics, it is absolutely necessary to get an accurate determination of the core material sorption curve and of the ageing of this curve.