Study on pillar-supported VIP structure and VIP edge conduction

Abstract

Vacuum insulation panel (VIP) is a promising thermal insulator and it is commonly composed of an evacuated filler material and a vacuum envelope. They act as heat transfer paths and affect the insulation performance significantly. In this study, a pillar-supported VIP and edge taping method are investigated to reduce the heat transfer through the filler material and the envelope, respectively. The pillar-supported VIP is composed of support structure (to sustain the atmospheric pressure) and filler layer. Since the filler layer is free from compression unlike the existing VIPs, its effective thermal conductivity is much lower. The mechanical stability and heat transfer of the pillar-supported VIP are investigated for optimal design, and a multi-pass support (MPS) and multi-layered filler are introduced to further reduce the solid conduction through pillars and radiation through the filler material. A test sample VIP is fabricated with stainless steel cover plates, MPS, and multi-layered filler material stacked with glass wool sheets and radiation shields. The thermal conductivity measured by vacuum guarded hot plate (VGHP) is $1.18 mW/m \cdot K$ , which is more than twice better than that of glass wool-based VIP. The heat transfer through the envelope is called edge conduction. From a numerical analysis, it is shown that the edge conduction is a serious heat transfer mechanism. It usually results from the aluminum layer of the envelope and shows an almost one-dimensional heat conduction behavior. Based on the results, a theoretical model for the edge conduction is obtained and the experiment is performed with a new type of samples. The error between the numerical model and the experiment result is within 1.6%. When a taping material is attached at the envelope edge, the edge conductivity is significantly reduced, and it is named as edge taping method. It is analyzed using various means, i.e., numerical, analytical and experimental tools. Finally, a dimensionless form using the theoretical model is proposed for practical performance evaluation.