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Modélisation du comportement thermique à long terme des Panneaux Isolants sous Vide (PIV)

Abstract : Two types of thermal insulation materials exist for building application: the conventional insulation and the super-insulation materials which is characterized by an insulating performance higher than that of a simple layer of still air (25 mW/m/K). Vacuum Insulation Panels (VIP) belong to the second category. VIP is not a homogeneous material, but a product consisting of a core material maintained under vacuum by an envelope. The thermal performance of VIP is based on the nanoporous property of the core material and on the vacuum maintained by the envelope which has a very high gas barrier properties. While conventional insulation material has a thermal conductivity values from 21 mW/m/K for polyurethane foams to 50 mW/m/K for the worst wools, that of new VIPs is around 4 mW/m/K. Nevertheless, like every insulation materials, their performance degrades over time. This increase of thermal conductivity is even more detrimental for VIPs because of their very high initial performance and of their high cost. It is therefore important to study their thermal performance evolution over all their service-life in building, over 50 years. In order to manage this, modelling has been chosen, because experiments cannot be realised over such long periods. Studying the thermal performance of VIPs is going through different research topics which take place at different scales. The first one concerns the gas transfer mechanisms through the VIPs’ envelope, also called barrier complexes. The challenge is to improve our understanding of the relationship between the barrier complexes morphological properties and the water vapour and dry air diffusion phenomena through the different layers of materials which compose these barrier complexes. The results do not allow to provide a correct model at this scale, but put forward some trends and physical mechanisms that open up new avenues of exploration. The second research topic is focused on the hygro-thermal behaviour at panels’ scale. A numerical model of VIP has been developed in order to take into account its geometric, thermal and hygric properties in the global thermal performance calculation of the panel. The model integrates the ageing process of the core material by moving its water vapour sorption isotherm. VIPs made with different types of core material has been studied in different constant conditions of temperature and humidity. Simulation results allow to better understand the thermal conductivity evolution of VIPs, to analyse their global behaviour and to determine the main characteristics which are relevant to improve their performance. Then, the third part of the research studies is dedicated to the development of a method which allows to analyse the VIPs’ performance in real conditions of installation in building, in different French climate conditions and several insulation applications. The aim is first to determine the real solicitations imposed on VIP, and then to simulate their long-term thermal performance in order to predict their mean performance. Results show a large dispersion of solicitations submitted to VIPs according to the climate conditions and insulation systems. Temperatures and humidities are highly variable according to the seasons, but finally remain relatively moderate. It is turns out that the mean thermal performance of VIPs over 50 years differs little from applications, but more from climate conditions and even more from the type of silica used for the core material. Contrary to what the short term tests would suggest, hydrophobic silicas are most favourable. The mean thermal conductivity of VIPs can varies between 4.7 and 7.3 mW/m/K depending on the applications and the climates.
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Submitted on : Friday, December 8, 2017 - 6:27:10 PM
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Antoine Batard. Modélisation du comportement thermique à long terme des Panneaux Isolants sous Vide (PIV). Thermique [physics.class-ph]. Université Savoie Mont Blanc, 2017. Français. ⟨tel-01659806⟩

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