Experimentelle Untersuchung von Ausfällungsprozessen in Hochtemperatur-Aquifer-Wärmespeichersystemen im Zuge des Projekts „ATES Vienna“

This work was conducted as part of the project "ATES Vienna". The aim of this project is the development of high-temperature aquifer heat storage systems (ATES), which shall be integrated into the Vienna district heating network in the future.
The principle of the ATES system is based on storing excess heat from the district heating network in a deep water-bearing layer (aquifer) in summer and using this warm thermal water to supply heat in winter by extracting the thermal energy from the water and then returning the water to the same aquifer. This principle can be used for seasonal heat energy storage.
The aquifer used is in equilibrium at around 60 °C, into which water at higher temperatures (90 to 120 °C) is injected. Therefore, a relevant aspect of this project is to assess the chemical-mineralogical effects of high-temperature underground storage on the thermal water and its utilisation in the planned ATES plant in order to identify adverse effects on the operating processes of the plant or on the aquifer.
To this purpose, laboratory experiments were carried out as part of the project in order to investigate the dissolution and precipitation processes. Autoclave experiments were conducted with water and rock samples from boreholes at three locations at different temperatures (90 and 120 °C) and reaction times (one and four months). The starting materials and the reacted samples were analysed using suitable test methods (IC, ICP-OES, XRD, SEM).
The results of the precipitation experiments show that precipitates were formed at all three locations at the different temperatures and test durations, essentially illite - a chemical weathering product of muscovite -, albite - a sodium aluminium silicate -, and magnesium calcite. However, these do not constitute an obstacle to the construction of an ATES plant.
In addition, the results at the three locations differed only slightly from one another, from which it can be concluded that the results can also be transferred to other locations with comparable geological and hydrochemical conditions.
In summary, it can be concluded from the present work that it is possible to construct the planned ATES pilot plant at locations that fulfil the above-mentioned conditions. Based on the results of the precipitation experiments, it is recommended to monitor the hydrochemistry and the resulting precipitations during the operation and thus to ensure a continuous risk assessment. If the pilot plant can be operated successfully, the ATES system represents a promising technology that is also capable of advancing the decarbonisation of the heating
sector.