Heat Transport in Evacuated Perlite Powders for Super-Insulated Long-Term Storages up to 300 °C

Vacuum super insulation (VSI) with expanded perlite powder is commonly used at cryogenic temperatures, but principally can also be adapted to applications at higher temperatures, such as the long-term storage of hot water in solar thermal systems. Due to the lack of experimental data in the respective temperature range, especially without external load, thermal conductivity measurements have been performed with commercial perlite powder up to 150°C mean sample temperature, corresponding to storage temperatures of around 300°C. Two different experimental geometries have been used: a guarded hot plate (GHP) setup and a cut-off concentric cylinder (CCC) apparatus. Furthermore, the radiative heat transport has been determined separately by extinction measurements using Fourier transform infrared (FTIR) spectroscopy. In addition to the laboratory experiments, a real-size prototype of a solar VSI-storage tank with 16.4 m3 water storage volume has been constructed, and the effective thermal conductivity of the perlite insulation has been determined from a heat loss measurement. The heat transport in evacuated perlite has also been treated theoretically using common models and approaches for gas heat conduction, solid-body conduction and heat transfer by thermal radiation. For the coupling between solid-body and gas conduction which occurs in the intergranular spaces of a powder material, a simple model has been developed. The total effective thermal conductivity λeff of a vacuum super insulation with dry, evacuated perlite powder (p≤0.01 mbar,ρ≈60 kg/m3) amounts to 0.007–0.016 W/mK for mean sample temperatures between 50°C and 150°C, compared to 0.003–0.005 W/mK at cryogenic temperatures. For the real-size storage prototype, the value λeff=0.009 W/mK has been obtained at T=90°C (storage temperature), p = 0.08 mbar and ρ=92.4 kg/m3, which compares to 0.03–0.06 W/mK for dry conventional storage insulations. With the applied theoretical models and approaches, the effective thermal conductivity of evacuated perlite and its individual contributions can successfully be described at different densities (55-95 kg/m3), compression methods, vacuum pressures (10-3-1000 mbar) and filling gases (air, Ar, Kr) up to mean sample temperatures of T=150°C. With regard to practical purposes, it has shown that vacuum super insulation with perlite is a suitable and economic method to achieve low thermal conductivities also at medium storage temperatures.