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Efficient long-term heat storage
Projektinfo 14/2014
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Vacuum tank stores heat

An efficient long-term heat storage tank boosts the shares of fraction of solar thermal energy systems in buildings. Staff at Bavarian Center for Applied Energy Research and at steel and metal engineering company Hummelsberger have developed a storage system that minimises thermal losses and optimises temperature stratification. A vacuum between the inner and outer tank combined with a perlite powder filling reduces the thermal conductivity of the storage tank shell. The long-term heat storage tank is already being used for heat supply in single-family homes and apartment buildings.

The vacuum super-insulated (VSI) tank can store supplied heat over several weeks or even months. The technology allows high solar shares of fraction in existing small and medium-sized buildings. To achieve such returns, buildings heated by solar power should possess low-energy house standard as a minimum requirement. The intermediate storage of district heat or industrial process heat at 100 °C to 200 °C represents a further area of application for VSI tanks. The technology was initially developed through three progressively enhanced prototypes at Bavarian Center for Applied Energy Research (ZAE Bayern). The storage tanks were constructed and installed by company Hummelsberger. For this type of storage, volumetric capacities of 5 to 50 cubic metres are currently possible. For future applications, the researchers are planning to develop modularly structured storage tanks that can hold in excess of 100 cubic metres.

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The aim of the project was to minimise heat losses by the storage tank. To achieve this objective, the project’s members resorted to a principle that has been employed for many years in cryogenics for the storage of cold temperatures, including by Hummelsberger. The company manufacturers outer shells for double-walled cryotanks and places the perlite-filled cavity under vacuum. Containers of this nature are used to store liquefied gases at temperatures down to – 200 °C and thermal conductivities of less than 0.003 W/mK are achieved. The challenge lay in carrying this insulation principle over to thermal energy storage at higher temperatures.

Vacuum minimises air heat transfer

The hot water storage tank consists of an inner and outer steel tank. Concentric cylindrical containers connected one below the other by a special fixture with low thermal bridging properties proved the most suitable. The aim was to reduce heat transfer within the annular cavity between the inner and outer tanks. Three mechanisms of heat transfer need to be reduced in this regard: Convection and thermal conduction through air molecules, thermal conduction along the solid powder body and thermal radiation. In order to inhibit air heat transfer, the researchers generated a moderate vacuum of 0.1 mbar between the inner and outer tank, achieved with simple pump technology. This is entirely sufficient as the perlite’s micropores limit the mean free path of the air molecules.

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