How zeolites can help store summer heat for use in winter

In recent times, ‘energy storage’ has become synonymous with storing electrical energy, in some other form, typically in electrochemical batteries.

But how would you store heat energy? Hot water? Molten salt? Well, with these methods, you can store heat for a few hours or a few days, at the most. Would we be able to take up heat in summer and store it till winter?

The answer seems to be ‘yes’, going by the research at Germany’s Fraunhofer Institute. Their solution: zeolites.

Zeolites are the minerals used in oxygen concentrators, where their molecular sieves prevent (the larger molecule) nitrogen from passing through, to give out air that is highly rich in oxygen. Researchers at Fraunhofer Institute are putting zeolite to another use — storing heat.

A paper from the institute says that zeolites are a “thermochemical” storage material. “Unlike water, zeolites do not store the heat directly — instead, the heat removes the water that is stored within the material. In the energetic state, zeolites are therefore completely dry; conversely, when water vapour is passed through the pellets, heat is released.”

The advantage here is that the energy is not stored in the form of increased heat but as a chemical state. This means no heat is lost during long-term storage.

However, there is a flip side. Zeolites have poor thermal conductivity, which makes it harder to transfer heat from the heat exchanger to the material and back.

A team of researchers at the Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP have solved this problem through their ZeoMet project. “We coated the zeolite pellets with aluminium — this doubled thermal conductivity after just the first attempt, without negatively impacting water adsorption and desorption.”

The researchers are currently trying to increase this five to ten fold by adjusting the coatings, according to Heidrun Klostermann, Project Manager at Fraunhofer FEP.

While this sounds relatively simple, it actually poses considerable challenges. For a litre of granules of five millimetre diameter, around 10,000 of these tiny pellets must be evenly coated with aluminium. For a grain size of one millimetre, this amounts to one million pellets with an overall surface area of 3.6 square metres.

The smaller the grain, the more challenging the process. However, smaller grains also increase the specific power density of thermal storage systems. In order to achieve sufficient thermal conductivity, the coating must be tens of micrometres thick — for vacuum-coating processes, this is a lot thicker than the usual.

Nonetheless, the researchers overcame these challenges. They looked to thermal evaporation, whereby aluminium wire is continuously fed onto a heated ceramic plate in a vacuum, so that the aluminium is evaporated and deposited onto the granules as a coating.

The pellets must be continually circulated in a barrel to coat them evenly. “The main difficulty lay in coating the granules while they rolled around, as well as ensuring that the coating was even to a sufficient degree,” says Klostermann. “The excellent collaboration of our engineers, physicists and precision mechanics was the principal asset in helping us achieve this.”

Zeolites are good not only for thermal storage but also to provide cooling for domestic use alongside solar collectors, as well as for mobile applications. For example, in commercial vehicles, heat lost from the power unit could be used for air conditioning as part of a thermochemical cycle. According to the Fraunhofer FEP researchers, the hybrid materials used present new challenges.

As a result, they are looking to strengthen their connections with materials developers and systems engineers to find solutions for the flexible supply of heating and cooling.

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