Development of large water volume, low-cost hot water stores with highly efficient thermal insulation for outdoor application

Brief description

Thermal solar systems for the domestic hot water preparation and for the support of space heating have been established in Germany for quite some time and show increasing sales figures. In order to further increase the percentage of thermal solar technology within the regenerative heat supply sector, it is necessary to considerably increase the solar fraction of the total heat demand of new as well as of existing buildings. The long-term goal is the „Solar-Aktiv-Haus“ (solar active house) which covers its heat and cold demand completely by solar thermal energy. In order to achieve this, large water volume and highly efficient and at the same time low-cost hot water stores are essential. Especially the low-loss storage of heat over a period of several month is a promising approach in this context. However, the set-up of a large-volume hot water stores inside the building envelope includes various difficulties and disadvantages.

Therefore, the main objective of the project was to develop a series of hot water stores with a volume of approx. 5...100 m³ for outdoor application. The specific costs for such a heat store (incl. thermal insulation) should result in approx. 750...900 €/m³ water volume. Additionally, the thermal performance of the stores should be significantly increased compared to commonly available products by combining several innovative concepts.

Vacuum-insulated hot water store of the Sirch company in the solar house in Münster (c)
Vacuum-insulated hot water store of the Sirch company in the solar house in Münster

The main development focus was an extensive reduction of external and internal losses of the thermal energy store. External losses are heat losses through the envelope of the thermal energy store (walls, cover, bottom). By using highly efficient thermal insulation materials and techniques, these heat losses were supposed to be reduced by at least factor 5 compared to conventionally insulated hot water stores: the "vacuum thermal insulation" and "transparent thermal insulation" technologies were analyzed and adapted. Internal losses are losses of effectively usable heat (known as exergy losses) when mixing water of different temperatures inside the store. By flow visualization as well as by the selection of suitable devices for thermally stratified charging and discharging, the internal losses of the developed thermal energy store were also reduced as far as possible.

A further important development goal was the accessibility of the thermal energy store´s interior, even in case of double-wall store constructions with vacuum thermal insulation. This enables potential repairs as well as the adaption to potential changes of the system´s boundary conditions.

The research project included the conceptual design, the build-up and the detailed measuring of a pilot hot water store at an outdoor ITGE laboratory.


Project duration

01/2013 - 12/2015


Project partner

Firma Sirch Tankbau - Tankservice Speicherbau GmbH



The "StoEx" project was funded by the German Federal Ministry for Economic Affairs and Energy (BMWi), based on a decision of the German Bundestag by Projektträger Jülich (PTJ) under grant number 0325992B. The authors gratefully thank for the support of the project and take the full responsibility for the content of this publication.


Dipl.-Ing. Dominik Bestenlehner

Dipl.-Ing. Dominik Bestenlehner

Academic employee

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