The StoEx2 project, funded by the German Federal Ministry of Economic Affairs and Energy (BMWi) under the call 03ET1667A+B, was carried out as a consortium between Sirch Tankbau‑Tankservice Speicherbau GmbH and the Institute for Building Energy Engineering, Thermotechnology and Energy Storage (IGTE) at the University of Stuttgart. Sirch coordinated the project, while IGTE contributed the scientific and technical expertise. The work followed the earlier StoEx1 programme (2013‑2015) and aimed to advance the technology of large‑volume, externally mounted water‑storage tanks with ultra‑efficient thermal insulation from a Technology Readiness Level (TRL) of roughly 4‑5 to a level of 7‑9, thereby preparing the system for commercial deployment.
A central technical objective was the optimisation of the manufacturing process. The insulation filling of the vacuum‑insulated tank was simplified by developing a new filling technique that reduces material handling and labour. The vacuum evacuation time of the insulation cavity was cut by a combination of pre‑heating the insulation material, performing a pre‑vacuum step, enlarging the evacuation cross‑section, and employing a turbo‑molecular pump. Comparative tests showed that the evacuation time for a freshly produced, stored, untreated, and hydrophobicised perlite sample could be reduced by up to 40 % when using the turbo‑molecular pump, compared with conventional rotary pumps. The use of zeolite as a drying agent further shortened the process by 15 %. These measures lowered the overall energy consumption of the production line by approximately 20 % and reduced the cycle time by 25 %.
The project also addressed quality assurance by investigating the formation of voids and settling in the insulation material. Imaging techniques and acoustic sensors were used to detect voids in the insulation layer, and a set of design rules was derived to minimise their occurrence. The resulting guidelines were incorporated into the manufacturing protocol, ensuring a more uniform insulation layer and improving the long‑term thermal performance of the tanks.
Thermal modelling was performed using both a node‑based approach and a neural‑network model. The node model captured the heat transfer through the transparent thermal insulation (TWD) and the tank walls, while the neural network was trained on experimental data to predict temperature profiles under varying operating conditions. The models were validated against measurements from the pilot tanks and achieved a prediction error of less than 3 % for the core temperature and less than 5 % for the surface temperature.
Two pilot tanks were fabricated and installed in a real‑world setting. Pilot 1 was operated for 12 months, during which the heat loss was measured at 0.8 % of the stored energy per year, a value that is 30 % lower than the target set in the design phase. The storage efficiency, defined as the ratio of useful energy stored to the total energy input, reached 92 %. Thermal stratification was observed, with a temperature gradient of 4 °C between the top and bottom of the tank, confirming the effectiveness of the TWD in maintaining a stable temperature profile. Pilot 2 was used to test the system as a buffer for combined heat and power (CHP) units and to evaluate operation at temperatures above 110 °C. The tests demonstrated that the tank could safely operate at 120 °C with only a 2 % increase in heat loss compared to the nominal 100 °C operation.
The project also produced a catalogue of architectural integration options, enabling the external tanks to be blended seamlessly into building façades. This catalogue, together with the technical data sheets, was disseminated through conferences, journal publications, and a dedicated project website, ensuring that the findings reach both the scientific community and potential industry adopters.
In summary, StoEx2 achieved significant advances in manufacturing efficiency, thermal performance, and system integration, while elevating the technology’s readiness for market entry. The collaboration between Sirch and IGTE, supported by BMWi funding, has laid a solid foundation for the commercial deployment of high‑efficiency, externally mounted water‑storage tanks.