Study on insulation characteristics of core materials in vacuum insulation panels

Abstract

Heat transfer in core materials of vacuum insulation panels (VIPs) is investigated for the insulation performance. Existing porous core materials are studied first and a new core structure is proposed and analyzed. In the first part, typical porous materials such as phenolic foam, glass wool and fumed silica are examined. While the phenolic foam is friable, glass wool and fumed silica are flexible and their porosities change when pressed. Relations between pressing load and porosity are found from measurements. Heat transfer through them occurs by solid conduction, gas conduction and radiation. Solid conductivities of the sample materials are theoretically derived using idealized models. Gas conduction strongly depends on the residual gas pressure and the pore size of material. Radiation is estimated using the diffusion approximation. For experimental validation, effective thermal conductivities of specimen are measured at different vacuum levels and pressing loads using vacuum guarded hot plate (VGHP) which has 5 to 8% of uncertainty. Radiative conductivity is estimated from the transmissivity data which are obtained using Fourier transform infrared spectroscopy (FTIR). Solid conductivity is found by subtracting the radiative conductivity from the measured effective thermal conductivity. As the result, theoretical solid conductivity of phenolic foam shows 6% of relative error with the measurement. On the other hand, both glass wool and fumed silica show large discrepancy between theoretical and measured values. Thus, empirical relations are suggested to estimate solid conductivities of glass wool and fumed silica. From the analysis, effective thermal conductivities of glass wool and fumed silica turn out to decrease when the pressing load is small. However, when they are used as core of VIPs, they are usually pressed by 1 atm of pressure. For this reason, an artificial core called pillar type VIP is proposed, which is the second part of this dissertation. I...