.

The thermal conductivity of the evacuated tank shell consists of radiation (λs) and solid-state thermal conductivity (λr) and is dependent on the density and average temperature (Tr) in the perlite filling.
© ZAE Bayern

Comparison of thermal conductivity of conventional insulating systems with innovative VSI insulation at 20 °C and 120 °C average temperature.
© ZAE Bayern

Schematic view of a VSI storage tank
© ZAE Bayern

Rough grained crude perlite (left) and technically expanded perlite.
© ZAE Bayern
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Perlite reduces thermal radiation

Unlike convection and thermal conduction, thermal radiation also occurs in the vacuum. All bodies radiate electromagnetic waves with a spectrum that is characteristic for their temperature and therefore exchange energy with their environment. In order to minimise this mechanism, perlite, which is a porous and consequently poorly thermally conducting volcanic rock, is poured into the annular cavity. The radiation is absorbed in the powder filling and re-emitted homogeneously in all directions. Diffusion also occurs at the powder grains. This drastically reduces the thermal radiation to the outer wall of the container.
The perlite-based vacuum super insulation delivers around five-times the level of insulation of dry conventional heat insulation at 100 °C when perfectly installed. The lowest level of thermal conductivity of the perlite achieved at storage temperatures of approx. 100 °C under laboratory conditions was around 0.008 W/mK. At a storage temperature of 200 °C, around 0.011 W/mK is achieved. A 16.5-m³ prototype installed at company Hummelsberger achieved 0.009 W/mK at 90 K above ambient temperature in winter over a period of measurement of two months, corresponding to just 0.2 K of cooling per day. This value already includes connection losses. Thermal bridges on the VSI tank are the connection points for the solar and heating systems and the fixture for the inner tank inside the outer tank. As the wiring and storage tank fixtures are clad in evacuated perlite over large sections or are made from a poorly-conducting material, the proportion of the calculated total losses that these account for is just 0.3 %. A further reason for this low value is the large storage tank surface in comparison to the cross-sectional area of the conduit wiring.

Properties of perlite

Perlite is a naturally occurring mineral of volcanic origin. The hard base material obsidian (“volcanic glass”) is transformed into “crude perlite” through ageing and moisture absorption. This material has a relatively high water content. The open-cast mined crude perlite exhibits a density of 900 to 1,000 kg per cubic metre and can be described as an almost inexhaustible raw material. For further use, the crude perlite is heated to between 850 and 1,000 °C. This causes the water held in the material to be vaporised and creates a microporous structure. The crude perlite is then cooled and its mechanical stability is consequently restored. Treated in this way, the “expanding perlite” has a much lower density of 30 to 240 kg/m³ compared to the raw material. The low density and the high porosity make the expanding perlite a suitable material for vacuum super insulation. Further positive properties of perlite include the low price of around 50 euros per cubic metre and temperature resistance up to 800 °C. The material is also non-poisonous and incombustible. Perlite has a high porosity and therefore relatively favourable insulating properties including without evacuation. For this reason, alongside vacuum super insulation, the material is also used in large quantities as a loose fill insulating material in building construction and as a material for soil bulking in the agricultural sector.

Stratified charger with enhanced flap mechanism

In addition to the storage tank shell’s insulation, thermal stratification within the storage tank affects its efficiency too. Mechanisms that inhibit the mixing of hot and cold water in the storage tank are ideal. The researchers developed a stratified charger with an optimised flap mechanism. Outlet ducts branch off from the main vertical conduit of the stratified charger. A reversal mechanism was attached at each of these outlets. A plastic disc lies flat on each outlet opening, providing a seal. The flaps are opened only by the upward force of the hot fluid. If colder fluid flows past the flaps, these remain closed. The streaming of hot fluid in a conventional stratified charger by contrast would lead to a „depression“ in the stratified charger, which would result in colder fluid reaching the stratified charger via the charging conduits. This is prevented by the flap mechanism and no mixing of the temperature layers occurs in the storage tank.
For the flap mechanism itself, a plastic is used with a density close to that of water. If a heavier material were to be used for the flaps, the density difference wouldn’t be sufficient to raise the flaps or allow for stratification. The plastic that the stratified charger is made from demonstrates low thermal conductivity. This works to further thwart the vertical destruction of layering in the storage tank. The existing „flap principle“ was optimised in the research project and a patent registered for the improvements made.

Projektinfo 14/2014:
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