The VSI tank in Bergen is disguised by a storage facility for wood.
© Hummelsberger Schlosserei GmbH
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Use in single-family homes

The VSI tank has already seen use in heat supply applications in existing and newly constructed buildings. One example is a single-family home in Bergen with 230 m² of living space. In this property, an eleven cubic metre storage tank was installed with half of the system below ground level. A mobile crane was used to lift the storage tank into the excavation. The hydraulic connection to the heating system was established in the boiler room. A solar energy system with a collector surface area of 55 m² feeds into the storage tank. The solar share of fraction is between 55 and 60 %. The remaining heat requirement is covered by a firewood plant and an oil-fired heating system.
Hummelsberger is currently constructing a VSI tank with a volumetric capacity of 37 cubic metres for a private building owner. The storage tank is being installed in a newly constructed building with 368 m² of living space and a total surface area of 600 m², which includes the basement, and will be adequate for the entire building structure from the basement to the roof. It was therefore particularly important to the building owner for the storage tank to be extremely well insulated with no thermal radiation into the building itself. Owing to the low thermal conductivity of its enclosure, this requirement is very well addressed by the VSI tank. A 97-m² solar energy system with evacuated tubes supplies heat to the storage tank. The remaining heat requirement is covered by a wood-fired heating system with water flow. This feeds heat initially to a small buffer storage tank via a heat exchanger. When this is filled, the heat is rerouted to the VSI tank. All instances of loading and extraction occur via this working storage tank in order to keep the movement of water as slight as possible in the long-term heat storage tank.

Huge potential for industrial process heat

To date, the VSI storage tank has predominantly been used for solar heat storage in residential buildings. Even greater potential, however, has been identified in the storage of industrial process heat. As the storage tank’s losses are virtually zero, its benefits increase with the application temperature. It can be theoretically operated up to 750 °C as perlite is resistant up to this temperature. Modifications to the tank base material and the heat transfer medium, however, become necessary.

Integrating storage tank in existing rooms

Energy storage is a key issue of energy transition. In addition to the storage of electricity, the holding of thermal energy plays an essential role. Heat storage systems provide the core element for almost all larger-scale heat generation systems based on solar thermal energy, biomass and heat pump technology. They also play an important role with CHP plants. To ensure the ongoing wide-spread use of these technologies, existing storage technologies need to be adapted to current requirements.
As in the research project outlined, an essential objective in this regard is to increase the thermal storage capacity and storage duration while keeping temperature losses to an absolute minimum. Scrutiny of other research projects reveals practical considerations around, for instance, the optimal utilisation of existing options for storage tank installation, to play an important role alongside the pure efficiency of the storage systems.
In the now concluded “Innovative concept for cost-efficient ground buried hot water storage tanks” research project, researchers of the Institute for Solar Energy Research (ISFH) in Hameln developed a simple and building-independent storage system. The below-ground system has three to thirty cubic metres of volumetric capacity and is buried in the earth in immediate proximity to the building. The system consists of a concrete cistern, an XPS-based insulating material (extruded polystyrene hard foam) and a film lining for absorbing the storage tank water. The system constitutes a viable option for buildings without a basement or a suitable location for storage tank installation.
A rectangular storage tank constructed from fibreglass-reinforced plastic (GFRP) is currently in development. The aim of the project is to develop a storage tank that makes optimal use of available space and which can be carried into buildings via access routes with limited room to manoeuvre. The project involved the development of a storage tank consisting of individual modules intended for on-site assembly by the Ilmenau University of Technology and ed energie.depot GmbH in Radeberg. Further research activities are being supported by the German government as part of the Energy Storage Funding Initiative. Recipients of funding include projects in the field of thermal storage exploring options for increasing storage density and cycle durability and reducing manufacturing costs. Alongside thermal storage, funding has also been concentrated around electrical and material storage systems. The German Federal Ministry of Education and Research and the German Federal Ministry for Economic Affairs and Energy made around 200 million euros of funding available for research projects in an initial phase up to 2014.

Projektinfo 14/2014:
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