Secure energy storage

Underground heat and gas storage facilities could become very important as a result of the energy transition. Researchers from Kiel University are using their dedicated TestUM test site to investigate how well the effects on groundwater of such systems can be forecast.

Use of the geological substratum as a seasonal thermal storage facility, for example to provide on-demand heating and cooling for the above-ground infrastructure: based on estimates by Professor Andreas Dahmke and his working group at the Institute of Geosciences, this is likely to be a key building block within the scope of the energy transition. It allows heat supply and demand to be reconciled, as well as environmentally and socially acceptable active thermal management to be implemented, particularly in urban spaces. However, there are still only very few test sites and very little reliable field data on the environmental effects of such heat stores available worldwide. These are required in order to assess how well temperature-induced changes, for example in the groundwater chemistry, can be predicted using existing methods. Working in parallel with real laboratories, such as in Hamburg-Wilhelmsburg, the goal here is for field tests at a test site near Wittstock in Brandenburg to close this gap in knowledge. To this end, Kiel University got together with the Helmholtz Centre for Environmental Research UFZ in Leipzig three years ago to launch the TestUM-Aquifer joint project, for which a test site measuring several hectares is being used. The project is being funded by the Federal Ministry of Education and Research (BMBF) and will run until the end of October 2020. A follow-up project on the test site has also just been approved, while others are currently still in the application stage. The test site is also integrated into the Competence Centre Geo-Energy at Kiel University with the objective of securing close links between science and business.

“We are using various experiments to investigate what is happening in aquifers close to the surface on a geophysical, geochemical, hydraulic and microbiological level, for example when we input heat in the high-temperature range up to 80°C or when gases such as hydrogen or methane propagate there, potentially as a result of improper operation of underground material stores,” explained Dr Götz Hornbruch from the Institute of Geosciences, who is head of the test site. Among other things, release of trace elements, heavy metals and semimetals from the sediments, as well as changes in the redox medium are already known from laboratory tests on thermal input. After all, the heat introduced initiates processes that lead to at least partial release of components which have already been determined. Hornbruch said “We are using the site experiment to investigate the processes that we have already observed in the laboratory and in some cases have also forecast using numerical approaches. Questions here include: what happens in real world conditions in the field? How effective are our laboratory test-based monitoring methods and forecasts? These questions cannot be answered adequately with lab tests alone.”

For the heat input test, groundwater was removed, heated up to around 75°C and then introduced into an aquifer via a borehole. The temperature distribution around this was then recorded, while groundwater samples were regularly taken and examined to detect any changes. Temperature-induced effects are demonstrable, as Hornbruch confirmed, although the final evaluation of the results is not yet available. How the emissions change over time or with cyclical input of heat in the high-temperature range, as well as what environmental effects are caused by aquifers freezing and then thawing again is also to be researched in further experiments.

Alongside thermal storage facilities, material stores (for example for methane and hydrogen) also have an important part to play in the energy transition. Hydrogen is considered the energy source of the future, as it can be used in both industrial and transport applications without any harmful emissions - provided it is produced using renewable energies, such as solar or wind power. “We are investigating what happens when hydrogen or methane unintentionally enter aquifers near the surface, for example due to a leak, whether the gas phase propagates in line with numeric models and to what extent the secondary reactions derived from laboratory experiments can also be predicted for the field scale here. To simulate gas entries of this kind, we injected gas locally at a depth of around 18 metres over a few days,” explained Hornbruch. “Despite the urgency, nobody has yet researched this in a controlled field test for gaseous hydrogen anywhere in the world for implementation in practice. We are therefore delighted that the Federal Ministry of Education and Research (BMBF), in its Geo:N initiative, and Kiel University facilitated establishment of a test site for such cutting-edge research questions,” added Professor Andreas Dahmke, Head of the Aquatic Geochemistry and Hydrogeology workgroup.

www.testum-aquifer.de/de

Author: Kerstin Nees