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In the interview, Dr Alexandra Lieb reports on the basic research carried out by her junior research group NEOTHERM.
© OvGU Magdeburg, NEOTHERM
BINE Interview with Dr Alexandra Lieb on sorptive heat storage
04.07.2017

"You cannot put a football field in a cellar"

Can new storage materials store unused solar thermal energy or industrial waste heat? What role could they play in cooling? To explore this, Dr Alexandra Lieb, head of the interdisciplinary junior research group NEOTHERM at the Otto von Guericke University Magdeburg, reports on her basic research on metal-organic frameworks and zeolites in this BINE interview.

BINE Information Service: You use the adsorptive capacity of microporous materials to store heat. How would you explain this to a layperson?
Dr Alexandra Lieb: Sorptive heat storage systems use the movement energy of water molecules in humid air. When a molecule settles on a surface, its movement is stopped and this energy is released in the form of heat. To store heat, you simply have to reverse the process. So you add energy to a material in the form of heat and mobilise the adhering water molecules. This empties the surface and stores the heat energy.
If you preserve this state using a hermetically sealed container, you then have a heat storage system that saves energy for any length of time and without losses.
So storing means putting heat into the system and removing moisture. When you need heat once again, then add back the moisture and release the heat.

What characteristics should the materials used for sorption storage systems have?
Lieb: As large a surface as possible that can be covered with water molecules, but then again you cannot put a football field in a cellar. That means, you need microporous materials that have a very, very large surface area given a small volume.

Which materials do you work with?
Lieb: We work with two groups: zeolites and metal-organic frameworks. Both of these microporous material classes cover different temperature ranges and can be used to control different applications.
The first are zeolites, which in turn are comprised of substances with various chemical compositions. Conventional zeolites like silicon-aluminium-oxygen compounds are suitable for heat storage systems operating at higher temperatures, for example, for waste-heat utilisation in waste incinerators. Then there are the modified compounds, so-called AlPOs or aluminium phosphorus oxides. These, and especially SAPOs with doped silicon, are more suitable for adsorption chillers. The second material group would be metal-organic frameworks, abbreviated as MOF.

Zeolites can be found in air-conditioning systems and heaters, or in the heat storage systems inside dishwashers. But thermal engineering applications of metal-organic compounds are still relatively unknown, right?
Lieb: Metal-organic frameworks have been known for quite some time, they have simply been ignored for a while. It was only in the last 15 to 20 years that they have been researched extensively. Now there are a large number of metal-organic frameworks, almost 70,000.

Why have MOFs been ignored for so long?

Lieb: The known and relatively inexpensive zeolites can be used very well in numerous applications, and application-oriented research was focussed in this direction. A major point of criticism levelled at MOFs was centred on the previously insufficient stability against higher temperatures or oxygen.

The full interview with the researcher Dr Alexandra Lieb is available at forschung-energiespeicher.info/en/. In the interview, the researcher explains the properties that make metal-organic frameworks more viable for cooling and low-temperature storage systems compared to zeolites.

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