ºÚÁϸ£ÀûÍø

It is a standard physics question for high school students: what is the temperature of a stable mixture of ice and water? That fact that such a mixture is, by definition, at the ice’s freezing point, forms the working principle of so called phase change materials to store heat: When they melt, transitioning from solid to a liquid, they absorb large amounts of heat at a nearly constant temperature. Reversibly, when they solidify, they release that heat at an almost constant temperature. So, by selecting a material that melts at the proper temperature, enhancing its conductivity and improving its stability and nucleation behaviour, heat can be stored and released as needed.

Combining expertise
At ºÚÁϸ£ÀûÍø, researchers from different departments have built broad, multidisciplinary expertise on various aspects of phase change materials, explains Silvia Gaastra-Nedea, principal scientist in the EIRES focus area Energy Generation & Storage. ‘Within the focus area, we have established a Core Team Heat. This team consists of some eight researchers from the departments of Applied Physics, Built Environment, Chemical Engineering & Chemistry, and Mechanical Engineering. We meet every four weeks to discuss our work on heat storage and possibilities to join forces in new proposals.’

Industrial heat is becoming an increasingly important research field, as decarbonizing energy-intensive processes requires reliable and economically viable solutions for generating and storing heat, particularly at high temperatures. ºÚÁϸ£ÀûÍø is developing its expertise in this area in collaboration with TNO and through several PhD projects on industrial heat, including two specifically focused on industrial heat storage. Recognizing the importance of this challenge, EIRES has decided to broaden the scope of its BEHeaT program from the built environment towards industrial applications. At the same time, the multidisciplinary cooperation within the Core Team Heat creates new opportunities to adapt technologies originally developed for buildings to the higher temperatures and more demanding operating conditions of industry.

One of the promising research directions the team recently identified had to do with the work of , postdoc in the BEHeaT program. He worked on salt hydrates, a type of phase change materials, for integration in the built environment. ‘One of the problems with salt hydrates is that they can suffer from supercooling: when cooled below their freezing point, the materials do not crystallize but remain fluid,’ explains Gaastra-Nedea. ‘One solution is to add nucleators to induce crystallization, but these additives can reduce the latent heat capacity or complicate the formulation and thus affect the efficiency of the heat storage.’

Inspiration for new solution 
The BEHeaT project combined simulations with experiments to better understand the supercooling process and study solidification triggering methods. â€˜The results of this project inspired us to propose another solution to induce crystallization without compromising the heat capacity,’ Gaastra-Nedea explains. ‘By using a smart topology design, changing the internal surface of the capsules that hold the phase change material, we think we should be able to promote crystallization earlier and more reliably while preserving the material’s heat storage capacity. Molecular dynamics simulations can help us design nano-textured internal surfaces that act as nucleation catalysts, allowing the capsule surface itself to trigger crystallization instead of relying on chemical additives.’  

In a follow up project with TU Delft, funded by 4TU, Oluah further developed this hypothesis and performed first simulations. ‘We 3D printed our designs, and Bart Erich from the Department of Applied Physics and Science Education used NMR, MRI and CT scans to follow the crystallization process and gain insights into the interactions between our 3D printed capsules and the salt hydrates. The first results are promising,’ Gaastra-Nedea says. 

Herbert Zondag: We decided that encapsulation of phase change materials could be a promising route toward an economically feasible, predictable and reliable carbon-neutral heat storage solution for industry.

Exploring further potential
During a recent BEHeaT EIRES Connect meeting, the Core Team Heat came to the conclusion that this research has great potential for industrial heat as well. ‘Inspired by an EnergyDays meeting recently organized by Herbert Zondag on industrial heat, we decided that encapsulation of phase change materials could be a promising route toward an economically feasible, predictable and reliable carbon neutral heat storage solution for industry.’

In order to establish a first proof of principle, EIRES decided to supply a small amount of funding to expand the 4TU work toward higher temperature ranges. ‘When you talk about industrial heat, temperatures can go as high as 800 degrees Celsius. In the field of phase changing materials, not many people are aiming for these high temperature ranges. But we regard industrial decarbonization to be an interesting new field to shift our focus to.’

Maturing idea
Being a new idea, 3D printing capsules for salt hydrates is still in its infancy, Gaastra-Nedea emphasizes. ‘There are some papers about promoting crystallization, but none of them are targeted at topology optimized phase change material capsules. This is a scientifically challenging topic, and at the same time potentially relevant also for industrial heat storage. There are a lot of unanswered fundamental questions though. Which capsule topologies best improve heat-transfer and crystallization performance? How can we accurately simulate their behavior while accounting for deviations between the digital design and the 3D-printed structure? And how does the topology influence the evolution of the solid and liquid phases during melting and solidification?

The 3D-printed capsules also enhance heat transfer, which is highly relevant for industrial applications, where rapid charging and discharging are essential. This complements the recent ºÚÁϸ£ÀûÍø PhD research of Lisette Wijkhuijs on overcoming the low thermal conductivity of PCMs. While her work focused on improving heat transport in paraffin-based PCMs for the built environment, the present research investigates how engineered capsule surfaces can promote crystallization and potentially create more effective heat-transfer paths. Together, these approaches illustrate how structural design can improve both the power and reliability of PCM-based heat storage.

‘Moreover, in a new starting Msc project, we add a complementary digital materials-design route: because identifying suitable PCMs, eutectic mixtures or composite formulations for industrial heat applications is experimentally demanding, AI-assisted screening could help narrow down promising candidates before they are tested in the lab,’ Gaastra-Nedea says.

Creating synergies
To the principal researcher, these types of projects are exactly what the focus area Energy Generation and Storage, and its underlying Core Team Heat are about. ‘We want to create synergies to generate new ideas and span our research into new directions. It is my firm belief that when you bring people together with some common interest and drives, they will organize collaborations themselves.’ The role of students in sparking new innovations is indispensable, she emphasizes. ‘A lot of the work that has led up to this new project has been done in bachelor and master end projects, and most of the follow up research will be conducted by PhD students. It is often students’ work that forms the nucleus for new collaborations and projects to grow from.’