The effective thermal conductivity of fine-grained and coarse-grained expanded perlites, fumed silica and a mixture of coarse-grained expanded perlite and fumed silica at different vacuum pressures is experimentally investigated in this paper. For the investigations two different test rigs - a guarded cylinder apparatus and a guarded hot plate apparatus - were used. The lowest effective thermal conductivities were performed with fumed silica at ambient pressure, with the mixture at pressures between 1 and 10 mbar and with fine-grained expanded perlite at 0.1 mbar and lower. Mixtures of fumed silica and expanded perlites appear to be a great chance to achieve low effective thermal conductivities and lower prices compared to pure fumed silica at vacuum pressures that can be easily achieved for large-volume thermal energy stores.
Lang, S., Gerschitzka, M., Bauer, D., & Drück, H. (2016). Thermal Conductivity of Vacuum Insulation Materials for Thermal Energy Stores in Solar Thermal Systems. In Energy Procedia (Vol. 91, pp. 172–181). Elsevier Ltd. https://doi.org/10.1016/j.egypro.2016.06.196