Limits of Thermal Insulations – Heat Transfer within Evacuated Porous High-Performance Insulations

Abstract

Abstract High-performance thermal insulations (HPI) have outstanding thermal properties to minimize heat transfer for a specific application. The term "high performance" is a relative rating: whether an insulating property, i.e. the thermal conductivity or thermal transmittance, is significantly lower than those of conventional insulating materials or systems depends on the use case and the related conditions. Applications fields for thermal insulations are the energy, building, industry and transport sector, but also the space and aviation industry. The total effective thermal conductivity of evacuated HPI depends on the contributions of the heat transfer via the solid skeleton of the porous insulation material and the radiative heat exchange. For a given operating temperature, the effective thermal conductivity yields a minimum depending on the density and the infrared-optically extinction properties of the insulation material. Examples for HPI with outstanding low thermal conductivity values will be presented and compared with the theoretical predictions based on a percolation model for the solid thermal conductivity of the skeleton and on a diffusion model for the radiative heat transfer.