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Modular storage system made of individual modules with a steel structure: The thermal insulation (1) surrounds the entire storage unit. The storage system is screwed together with charging and discharging pipes (2) via sandwich composite end plates (3).
© Consolar Solare Energiesysteme GmbH
Modular hot water storage system
11.08.2014

When assembling the individual modules, these must be precisely aligned with one another using their mounting frames.
© Consolar Solare Energiesysteme GmbH

Flexible deployment of heat storage systems

Those retrofitting heat storage systems in buildings often come up against their limits. Large-volume storage tanks often do not pass through standard doors. Together with Consolar, which manufactures solar thermal systems, scientists at the Institute for Thermodynamics and Thermal Engineering at the University of Stuttgart are therefore developing a storage system that consists of several modules and can be assembled on site. The modular storage system will be installed in an apartment building and tested in practice in autumn.

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. The main areas of application for these storage systems are apartment buildings. However, there is currently a lack of models that can be flexibly installed in existing premises and which are also space-saving, efficient and cost-effective. The aim of the UniSto project is to develop such a storage system and to transfer it to a rational, mass-producible production process.

The developed hot water storage system consists of individual oval-shaped modules that are made of steel. These are connected via charging and discharging pipes as well as jacket connection profiles and are sealed with sleeves (see graphic above). The individual modules are mounted close to each other during the installation and are then pushed together. Each individual module can hold 1,300 litres. “Even storage systems greater than ten cubic metres can be installed in existing buildings,” explains Dr Ulrich Leibfried, who is in charge of research and development at Consolar.
The pipe work is situated in the containers, which save space on site. This also helps to reduce the heat losses by up to 35 per cent. The fact that a single thermal insulation layer covers the entire storage system and the surface area is less than with cascade storage systems also reduces the heat losses. This is concerned with separately insulated individual storage tanks that are connected to one another.

Steel replaces plastic containers

It was originally intended to construct the storage system like a sandwich. Insulating polyurethane foam was inserted between the steel plates located at the ends of the slotted-together plastic containers. Experiments showed, however, that an insulation density of just 42 kilograms per cubic metre was achieved with standard machines. This meant that the necessary strength-increasing effect of the foam in combination with the plastic containers was unable to be achieved. Alternative production methods proved to be too expensive. In order to nevertheless achieve the desired operating pressure of four bars in the storage system, the scientists decided to produce the storage modules completely from steel. Only the end plates (upper graphic, number 3) are insulated with polyurethane. The necessary mechanical pressure stability is now achieved by the fact that either another steel storage module is located on the end face of each module or a sandwich-composite end plate.
With steel storage tanks, the container itself is able to absorb the pressure forces. The thermal insulation can be added later. “This makes the modules less bulky than the previous model. The market representatives involved in the project also believe that the steel container concept will be better accepted by heating engineers and installers than the plastic storage system,” says Leibfried.

Researchers are testing the storage system in an apartment building

The scientists believe the greatest potential for using the storage system is in apartment buildings. Therefore, in a next step, the storage system will be installed and tested in a building with 75 separate residential units. “Heat is produced here together with electricity using a CHP plant in the building,” explains Leibfried and adds: “The decentralised power generation is primarily geared to the needs of the residents. The cogeneration plant is buffered with a storage system consisting of three modules each with a 1,300-litre capacity.”

The research project, which is entitled “Development of a universal heat storage system and its testing in apartment buildings” (UniSto) is being funded by the German Federal Ministry for Economic Affairs and Energy. The work was conducted in close collaboration with companies, including from the storage production, solar thermal, heating systems and energy supply sectors. These companies incorporated their needs and experiences into the product specification and supported the development work.

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