Super-Insulated Long-Term Storage
Abstract
Today, solar installations with collector areas from 10 to 20 m² and storage volumes from 0.5 to 1 m³ provide only a small solar fraction (maximally 30 %) of the total domestic heat demand. Simultaneously, especially the field of conventional domestic heat supply generates a big fraction of the world wide CO2 emission. In order to reach higher solar fractions (50 % or more) in solar supported buildings, larger and better insulated storage tanks are necessary to achieve seasonal heat storage in the ideal case. But also for the intermediate storage of district heat (80°C - 130°C) or industrial process heat at even higher temperatures, storage tanks with improved thermal insulation are necessary to use energy more efficiently. For many years, the technique of vacuum super insulation (VSI) with expanded perlite has been used for cryogenic applications, especially for the storage of liquid gases at temperatures between 20 K and 90 K. Expanded perlite is an amorphous, highly porous, granular material of volcanic origin, see figure 1. Its two main components are SiO2 (70 %) and Al2O3 (15 %). Due to the small pore diameters and the small distances between the grains, gas conduction in the pores and in the intergranular spaces is suppressed already in the regime of fine vacuum (p = 0.01 mbar). As a consequence of the filigree structure of the material, solid conduction is inhibited to a great extent. Furthermore, thermal radiation is efficiently blocked by multiple absorption and re-emission in the opaque solid. Due to these properties and as a consequence of the temperature dependency of the radiative heat transport, which scales with T4, effective thermal conductivities Ȝeff as low as 3 to 5 mW/mK have been realized at cryogenic temperatures. Compared to conventional insulation materials like polyurethane or rock wool at ambient temperature, the thermal conductivity is lowered by a factor of 6 to 10. Moreover, perlite super insulations are more space-efficient compared to conventional insulation materials, and they have no problems regarding humidity or shrinking during their lifetime. In real cryogenic tanks, the perlite is homogenously filled into the annular gap between two concentric steel cylinders and subsequently evacuated to 0.01 mbar or lower, see figure 2. In a current federally granted research project at ZAE Bayern (grant number 0325964A, German ministry of environment), the approach is pursued to apply perlite-based VSI also at higher temperatures. Primarily, the long-term and seasonal storage of hot water up to T = 100 °C in solar thermal heating systems is considered, although other fields of application like the storage of industrial process or district heat could benefit even more from VSI because of the higher temperatures.