Development of sustainable core systems for vacuum insulation panels in building applications

Abstract

Vacuum Insulation Panels (VIPs) are a remarkable state-of-the-art thermal insulation solution with promising building applications. However, current market products still pose several significant challenges, including the usage of highly embodied carbon footprint and costly materials, i.e. fumed silica (FS), mostly necessary to cope with long-term performance. The ultimate goal of this study has been to assess the possibility of entirely or, at least, partly replacing the conventional FS VIP cores with cheaper and more environmentally friendly alternatives, such as, among others, a fine perlite powder (PEa) and industrial residues in the likes of inorganic aluminium anodising sludge (ALW) and renewable organic cork powders (CP). The fundamental study includes understanding the primary heat transfer mechanisms involved throughout comprehensive experimental and modelling campaigns. The designs comprise sole and hybrid core solutions with a straightforward mechanical blending technique, wherein the proposed mixed materials are combined with different weight ratios of fumed silica (25%, 50% and 75%). The study also encompasses a thorough raw material characterisation and comprehensive investigation of novel core designs involving the structure, density, porosity and extinction coefficient determination. Thermal conductivity is measured by both steady-state and transient methods, and its correlation with density is explored, while monitoring the effect of internal gas pressure changes within the VIP cores served as a key indicator for both pristine and long-term proficiency considerations. Accelerated ageing tests conducted in controlled harsh temperature and moisture conditions facilitate an insightful evaluation of the performance of different core designs over time. Numerical models are introduced to evaluate and predict the thermal efficiency of novel solutions and quantify the contribution of each mechanism participating in the heat transfer process of the different cores. Effective centre-of-panel thermal conductivities of the pure alternative solutions are close to 10 mW·m-1·K-1, or even below, at fully evacuated pristine conditions. However, within the 1 to 10 mbar range, the solid-gas coupling effects start to become significant and soon dominant over gas conduction, as the inner pressure rises in these cores due to moisture and air permeation through the envelope. A likely rapid decline in performance over time, foreseen by their estimated extremely low 𝛲/ values, is corroborated by accelerated ageing tests indicating that, without using dedicated getters and desiccants or superior barrier envelopes, neither the pure PE or the CP alternative core systems envisaged is particularly suitable for standard long-term building insulation applications following the current established practices, and the pure ALW core design may struggle to technically compete with a single FS core. However, all pure core systems may find possible utilisations in less demanding markets. On the other hand, while presenting lambda values within the range of 5.4·mW·m-1·K-1 to 7.0 mW·m-1·K-1 when fully evacuated, several PEa/FS, ALW/FS and CP/FS hybrid compositions are revealed to be far more promising towards the construction industry adoption due to their ease of production, low density, excellent superinsulation thermal performance and acceptable service lifetime likelihood. Moreover, utilising either cheaper materials or, even better, recycled ones driven from both industrial inorganic wastes and renewable organic residues, as the hybrid core designs in this study, would undoubtedly enhance the potential benefits of these alternative solutions.