Natural core material based Vacuum Insulation Panels for application at up to 70 ℃

Abstract

Energy costs are rising and environmental concerns are gaining prominence in today’s society, where the significance of effective insulation in the pursuit of energy efficiency cannot be overstated. The need to reduce heat transmission has emerged as a driving force for technical advancements across several domains including buildings, transportation, industrial processes, cold chain logistics, and energy storage. Vacuum Insulation Panels (VIPs) have generated a huge interest in recent years due to their ability to achieve a thermal conductivity of less than 7 mW.m-1.K-1. Fumed Silica (FS) and Glass Fibres (GF) are often used as core materials in VIPs owing to their inherent characteristics that enhance the panels’ insulation performance. Primary drawbacks associated with these core materials are their significant impact on the environment and the high costs involved in their preparation. The aim of this study was to develop an alternative core material from tree-based waste to partially or completely replace fumed silica and to thoroughly characterise it for its suitability in vacuum insulation panels. Three materials developed from tree-based waste, namely Tree-based Natural Fibres of varying sizes (TNFm and TNFs) and Tree-based Natural Ash (TNA). Subsequently, ten composites with FS were produced, followed by laboratory experimental characterisations. The materials microstructure was examined using a Scanning Electron Microscope (SEM) and image analysis, revealing that TNFm and TNFs had an average fibre length of 0.82 mm and 0.22 respectively, and TNA had an average particle size of 9.37 μm. SEM revealed that small agglomerates of FS particles infiltrated the interior pores of both TNFm and TNFs particles, thereby reducing the pore size. The pore size and porosity of materials and composites were analysed using Mercury Intrusion Porosimetry (MIP). Only fumed silica exhibited pores smaller than 0.1 μm, attributable to the diminutive size of primary particles and their aggregation behaviour, whereas the tree-based samples displayed an average pore size between 0.7 and 2.43 μm, with porosity ranging from 77.34% to 84.93%. The average pore diameter of all FS with tree-based natural fibre composite samples was determined to be smaller than 0.26 μm, which significantly contributes to achieving low gaseous conductivity, even under high VIP pressure. The infrared (IR) transmission spectra of the materials were acquired by Fourier Transform Infrared (FTIR) spectroscopy. Both TNF and TNA had a better spectrum extinction coefficient than FS within the 2.5 to 7.5 μm range, indicating decreased radiative leakage over the analysed wavelength spectrum. FS-TNFm1 (30 wt.% TNFm) showed the lowest thermal conductivity of 8.28 mW.m-1.K-1at 70 ℃ among all FS-TNFm composites. FS-TNFs1 (30 wt.% TNFs) showed the lowest thermal conductivity of 8.25 mWm-1K-1 at 70 ℃ among all FS-TNFs composites. FS-TNA1 (30 wt.% TNA) showed the lowest thermal conductivity among all composites studied with a value of 7.41 mW.m-1.K-1at 70 ℃. The results were utilised to create a COMSOL Multiphysics model of a Car Painting Booth (CPB) operating at 70 ℃. The energy consumption over a decade was analysed using three optimal composites along with FS VIP, considering the ageing effects of VIPs with various core materials, and was compared to EPS, the most common insulation employed in CPBs. Car painting booth insulated with FS-TNA1 exhibited the lowest energy consumption at 42.5 GJ over 252 days, succeeded by FS-TNFm1 at 42.7 GJ, FS-TNFs1 at 42.8 GJ, FS at 43.3 GJ, and EPS at 55.1 GJ over one year. After ten years of operation, CPBs insulated with FS VIP exhibited a reduction of 14 GJ and 11 GJ compared to FS-TNFm1 and FS-TNFs1, respectively. Following two years of operation, FS-TNFs1 and FS-TNFm1 exhibited superior thermal conductivity compared to FS, resulting in this reduction. The minimum energy consumption was 434 GJ for FS-TNA1, while the maximum was 551 GJ for EPS throughout a ten-year period. The CPB results were used to evaluate a cradle to cradle Life Cycle Assessment (LCA) of VIPs, comprising primary material extraction, production, usage phase, and transportation of VIPs. LCA approaches Cut-off and APOS were used alongside two environmental impact assessment methods: Cumulative Energy Demand (CED) and EN15804 + A2. Impact categories were adjusted according to electricity consumption in the UK. FS-TNFm1 and FS-TNFs1 had the highest non-renewable impact at 848 GJ and 846 GJ, respectively, whereas FS had 837 GJ and FS-TNA1 had 829 GJ. The FS-TNA-insulated CPB used the least energy over a decade. The pyrolysis of TNA during early material processing prevented TNA composites from reducing their climate change impact below TNFm composites. When assessing acidification, climate change, and renewable and non-renewable resource impacts, FS was the "Green core" for substitution. Reusing core materials like FS and FS-TNA reduced environmental impacts by 74%, whereas FS-TNFm1 and FS-TNFs1 reduced impacts by 57%-59%, albeit increased energy requirements. Additionally, the cost per R-value of all manufactured VIPs at 20 ℃ and 70 ℃. FS yielded the greatest cost value of £5.13 kg⁻¹ per R-value, surpassing FS-TNFm1, FS-TNFs1, and FS-TNA1, which had values of £2.70, £3.07, and £2.39 kg⁻¹ per R-value, respectively. These results demonstrate that tree-based waste materials can be transformed into sustainable, cost-effective alternatives to conventional VIP cores, reducing energy consumption by up to 7 GJ over 10 years of operation in a car painting booth and core cost by up to 29% when replacing up to 30 wt.% of tree-based waste with fumed silica, while also lowering environmental impact in both high/ low-temperature applications such as car painting booths, buildings, and cold storage.