A soft spot for high temperatures

More than 50% of final energy in Germany is used as heat. However, the use of electricity from renewable energies is not yet making much headway in the heating sector. While heat pumps for private buildings are currently a big seller in view of the drastic increase in oil and gas prices, in industry, the electrification of the heat supply is a considerable challenge. “This is because of the high temperatures and power levels required in industry and large-scale plants,” says Prof. Annelies Vandersickel, who in January took over as head of the Department of Thermal Process Technology at the German Aerospace Center (DLR) and at the same time as professor of Thermal Energy Storage at the Institute for Building Energetics, Thermotechnology and Energy Storage at the University of Stuttgart. While classic water storage tanks and pressure tanks reach their limits at around 130°C, storage temperatures of up to 1,000°C and higher are required in industry. “We want to further develop high-temperature storage in this direction and thus strengthen the energy revolution in the broader sense,” says Vandersickel.

Different technological approaches

There are several technological approaches to this. In the temperature range between 130 and 300°C, latent storage processes based on phase transformation (i.e., the change from solid to liquid and back again) are used. Sensitive heat accumulators such as molten salts can handle up to 600°C. If energy is stored in rock (e.g., packed beds), temperatures of up to 1,000°C are possible depending on the filling and material. Thermochemical storage (e.g., through the chemical conversion of lime into calcium oxide and steam) is characterized by the possibility of a particularly long storage period. “The nice thing is that at the DLR, we cover all these technological approaches and thus all the options for heat storage,” says Vandersickel. The areas of application are also quite broad. They range from process heat for industry and power plants (the output of which is to be flexibly adapted) to the fluctuating supply of renewable energies to Carnot batteries (which store electricity in the form of heat as large-scale location-independent storage for when wind and sun are not available). The storage systems also play a role in the transport sector. For example, in the thermal management of the cabins as well as in the drives. 

Vandersickel’s personal research focus is heat storage in lime. Until she moved to Stuttgart, she headed a research group at the Technical University of Munich (TUM). “The problem with storing heat in lime is getting it into the material. That’s because lime has relatively poor thermal conductivity,” she explains. “In order to overcome this hurdle, we have been researching a fluidized bed process in Munich.” In the fluidized bed, steam flows through the lime particles from below until they behave like a liquid. The constant contact between the moving particles and the immersed heat exchanger tubes allows the heat to be efficiently stored and removed. The fluidized bed process also allows for the separation of storage performance and capacity. In simple terms, cold particles can be continuously fed, while hot particles can be discharged from the fluidized bed and stored at high temperatures. “In the end, it is enough to simply increase the size of the lime silos if more heat is to be stored.” At the DLR, similar concepts for building heat applications are being developed. Vandersickel is looking forward to contributing her expertise in order to implement the topic in Stuttgart as well. 

Enjoyment of management tasks

Vandersickel’s responsibilities at the DLR are less research-based and more strategic. The actual research is carried out by four working groups, which she heads and for which she provides the guidelines. “I am somewhat saddened about not being able to be directly involved in research anymore,” admits Vandersickel. Nevertheless, the Belgian-born scientist, who has also conducted research at the Massachusetts Institute of Technology (MIT) in the USA and at ETH Zürich, didn’t have to think twice when the offer in Stuttgart came up. “I enjoy management tasks, I’m always curious, and the position here is a great challenge. I am also excited to be able to expand my own field of research and venture into new areas of technology.” It is important for her to develop thermal storage systems not in isolation but rather always in connection with their role in the overall system. “My research strategy focuses on the application-oriented (further) development of the various storage technologies.”

Strengthening cooperation between the DLR and the university

As a professor at the University of Stuttgart, she would also like to strengthen the cooperation between the DLR and the university, especially in the field of intelligent systems, and thus support the university’s vision of intelligent systems for a sustainable society. In concrete terms, this can be achieved through the transfer of knowledge in the teaching and the training of doctoral students. Specific research collaborations are also on the agenda. For example, Vandersickel’s DLR department is involved in the Collaborative Research Center “Adaptive skins and structures for the built environment of tomorrow” (SFB 1244, spokesperson Prof. Oliver Sawodny), where it is working with Prof. André Thess on building energetics for the world’s first adaptive high-rise building. Vandersickel also has her eye on a Research Training Group and DFG projects: “There are synergies to be had in this as well, and my expertise is in demand.”

About Prof. Annelies Vandersickel

Annelies Vandersickel was born in Belgium in 1984 and studied mechanical engineering in Leuven; her area of specialization was energy technology. After completing doctoral degree studies at the ETH Zürich in 2011, she was a postdoc at the Massachusetts Institute of Technology (MIT), Boston, USA, from 2012 to 2013. From 2014 until her move to Stuttgart, she headed the Thermal Storage and Integrated Energy Concepts group at the Institute for Energy Systems at the TUM. Since 2019, Vandersickel has been a mentor in the FeelScience program at the University of Stuttgart. This is a support program for qualified young female researchers that specifically targets female doctoral students.