ZeoMet project optimises tiny grains that store summer’s heat for winter

Tiny grains known as zeolite granules can store the summer’s heat to warm homes in winter, but the material’s impressive chemical storage is held back by poor conductivity.

Now, a team of researchers from the Fraunhofer Institute in Dresden, Germany, say they have doubled zeolite’s thermal conductivity with just one step, and they aim to take it much further to help reduce energy consumption from heating and cooling.  

In Germany, the Fraunhofer research announcement said, 55% of final energy consumption goes towards heating and cooling – but a lot of heat dissipates because it is not generated as-and-when-required. Roof-mounted solar collectors can be used to heat water, but large volumes are required, and heat is lost over time despite good insulation.  

In contrast, thermochemical storage can store thermal energy generated in summer, until winter. Unlike water, zeolites do not store heat directly – instead, the heat removes the water that is stored within the material. In their energetic state, zeolites are completely dry, but they release heat when water vapour is passed through them. Because the thermal energy is stored through chemical means, it is not lost during long-term storage.  

Poor thermal conductivity makes it difficult to transfer the heat from the heat exchanger to the material and back again, however. The team from the Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP tackled this problem through the ZeoMet project.  

“We coated the zeolite pellets with aluminium,” said project manager Dr Heidrun Klostermann. “This doubled thermal conductivity after just the first attempt, without negatively impacting water adsorption and desorption. We are currently aiming to increase this by five- to ten-times through adjusting the coatings.” 

This poses ‘considerable challenges’, the research announcement said. For a litre of granules with a 5mm diameter, about 10,000 must be evenly coated with aluminium. For a grain size of 1mm, this equates to 1m pellets with an overall surface area of 3.6m².  

‘The smaller the grain, the more challenging the process,’ the announcement said. Smaller grains increase the specific power density of thermal storage systems, however. To achieve sufficient thermal conductivity, the coat must be dozens of microns thick – for vacuum coating processes, this is a lot thicker than normal.  

To tackle the challenges, the researchers looked to thermal evaporation, in which aluminium wire is continuously fed onto a heated ceramic plate in a vacuum. The aluminium then evaporates, and is deposited onto the granules. The pellets must be continuously circulated in a barrel so they are all covered evenly.  

“The main difficulty lay in coating the granules while they roll around, as well as ensuring that the coating was applied evenly to a sufficient degree,” said Klostermann. “The excellent collaboration of our engineers, physicists and precision mechanics was the principal asset in helping us to achieve this.” 

Zeolites could also provide cooling for domestic use, the researchers said, working alongside solar collectors or in mobile applications. In commercial vehicles, for example, heat loss from the powertrain could be used for air conditioning as part of a thermochemical cycle.  

The team now hopes to strengthen connections with material developers and systems engineers from research and industry, to develop solutions for flexible heating and cooling. 

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