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Workers carry a storage module through a narrow cellar entrance.
© Consolar Solare Energiesysteme GmbH
Thermal storage
Projektinfo 03/2017

Compared with storage cascades, the heat loss currents do not increase as much proportionally to the volume in the new storage system. Unlike storage cascades, UniSto storage tanks have lower heat losses the greater the overall volume is. (VIP = Vacuum Insulation Panel)
© ITW Stuttgart

This UniSto storage tank consists of four modules. The elements in detail: (1) vessel shell, (2) thin steel plate, (3) rails, (4) pressure resistant end modules, (5) charging and discharging pipes, (6) connecting rods, (7) seal system.
© Consolar Solare Energiesysteme GmbH
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Large storage tank developed for small spaces

To heat an apartment building highly efficiently, using a solar power system or combined heat and power plant for example, you need large thermal storage tanks. They often do not fit through standard door openings in existing buildings. Scientists developed a compact storage tank consisting of multiple modules, which is not assembled fully until it reaches the boiler room. Heat losses are lower than with standard cascade models. Individual tanks no longer have to be connected.

The new storage system can be combined with fossil fuel and renewable energy-based heating systems. It is pressure-resistant (3 bar operating pressure) and can be hydraulically integrated directly into the heating system. The pipe connections between the individual modules are in the vessel. This saves space, simplifies installation and reduces heat losses compared with the storage cascades used to date. The individual tanks are insulated separately and connected to one another. The new UniSto storage tank has the same volume but a smaller and fully insulated surface, which reduces heat losses. By way of comparison: The heat loss current of cascades with a volume of 5,300 litres is 430 W, excluding heat losses in the piping. In the new storage model, the loss current for the same volume is 200 W. These values apply at a temperature difference of 45 K between the storage tank content and ambient air.

The oval individual steel modules of the storage system have a volume of 1,350 litres each. The vessel shell consists of steel plates that absorb the radial pressure. Thin face plates welded to this seal the modules but do not absorb any pressure, instead passing it on to the neighbouring vessel. The individual modules are mounted close to each other on rails during installation and are then pushed together. The charging and discharging pipes run inside the tank. Four connecting rods at both sides link the end modules. These elements, which are externally pressure stable, ensure that longitudinal pressure is absorbed. The storage tank is insulated with EPS (expanded polystyrene) hard shells connected via tongue and groove joints. Vacuum insulation panels (VIP) can be fitted in the corresponding gaps within the hard shells if necessary. All thermal insulation can be retrofitted on installation.

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Evenly stratified heat transfer medium

What makes the new concept unusual is the horizontal alignment of the heat tank. “As in conventional storage tanks, the heat is withdrawn from a central point. At 213 cm, the UniSto model is the same height as standard models. Achieving stratification is simple, and there are virtually no temperature differences in this regard between the modules,” says scientific project manager Dr Stephan Fischer from the Institute for Thermodynamics and Thermal Engineering at the University of Stuttgart. The interior charging and discharging pipes run the entire length of the vessel and facilitate even thermal stratification. To prevent significant mixing of individual temperature levels during charging or discharging, the researchers located the inlet and outlet slots of the pipes close to the ducts between the modules. The five charging and discharging pipes run through stainless steel sleeves here, which act like baffle plates for the outflowing water. “The flow is deflected and enters the storage tank in a longitudinal direction from the slot gap between the sleeve and pipe. This allows a small slot cross section to be used without the water flowing into the storage tank at high speed and with a lot of turbulence,” explains Fischer.

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