The hidden weakness of heat storage
Ruben D'Rose defended his PhD thesis at the Department of Mechanical Engineering on April 9.
As renewable energy becomes a dominant force in powering our buildings, one major challenge remains: energy supply doesn鈥檛 always match demand. Wind and solar produce energy when conditions allow, not necessarily when we need it. While heat batteries offer a promising and sustainable solution, this research of Ruben D鈥橰ose shows a key limitation: potassium carbonate is not suitable for long-term seasonal heat storage in open systems.
Renewable energy sources such as wind and solar are inherently intermittent. On sunny or windy days, energy production peaks, but that energy often goes unused if demand is low. During calm or cloudy periods, supply drops. This mismatch makes energy storage essential for a reliable energy system.
Heat batteries as an alternative
Heat batteries store energy directly as heat instead of converting it into electricity. A promising method uses thermochemical materials (TCM), which absorb and release heat through chemical reactions. In this case, dry salt absorbs water and releases heat, while heating the material removes the water and resets the system. This allows heat to be stored efficiently over longer periods, using materials that are often abundant and relatively inexpensive.
What happens inside the reactor
The research of examines a reactor filled with potassium carbonate tablets. Inside the system, water and heat do not distribute evenly. Some areas receive more water than others, leading to uneven reaction zones known as hydration fronts. These fronts appear both within individual tablets and across the reactor as a whole. Advanced imaging techniques such as neutron CT and X-ray CT make it possible to observe these patterns and understand how they influence performance.
The role of structure
The structure of the packed bed strongly affects how the system performs. When tablets are packed more densely, heat transfer improves, but this can also increase heat loss and reduce efficiency in other ways. Increasing the empty space in the reactor can improve efficiency, but at the cost of storing less energy. Designing an effective system therefore requires balancing these competing effects.
Material degradation over time
The material itself also changes during operation. As the tablets absorb water, they swell, and when they dry, they shrink again. Repeated cycles of swelling and shrinking can lead to cracking and structural damage. Over time, this reduces the reliability and lifespan of the system.
Chemical stability issues
A major limitation is the reaction of potassium carbonate with carbon dioxide in the air. This unwanted reaction gradually reduces the material鈥檚 ability to store and release heat. For long-term applications, especially in open systems, this makes the material unsuitable.
Need for alternative materials
Although heat batteries remain a promising technology, this research shows that potassium carbonate does not meet the requirements for seasonal heat storage. The findings highlight the need for alternative materials and improved reactor designs to make thermochemical heat storage a viable solution for the future.
Title of PhD thesis: . Supervisors: Prof. David Smeulders, Dr. Bart Erich, and Dr. Leo Pel.