"We're achieving high energy densities"
In the SOLIDSTORE project, scientists from the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT are developing new thermochemical heat storage materials for an application range between 150 and 300 degrees Celsius. The focus is on storing heat in the smallest possible volume. In the interview, project leader Dr Barbara Zeidler-Fandrich talks about her basic research.
forschung-energiespeicher.info: You're researching acid-base reactions that could potentially be used to realise thermochemical heat storage systems. The focus here is on guanidines. Can you tell us a little bit more about these substances?
Dr Barbara Zeidler-Fandrich: Guanidines are nitrogenous substances that are also partly found in nature, for example in sugar beet juice, and which because of their molecular structure are among the strongest organic bases – the organic "super-bases". Guanidine is a base according to the Lewis acid-base concept. It provides an electron pair that is absorbed by the acid. Reaction partners can, in principle, be all organic acids that have an electron gap.
So you're using the energy conversion of this acid-base reaction for storing heat?
Zeidler-Fandrich: Yes, that's the aim. However, our work is still very much focussed on the basic research. We have begun combining various organic acids with the guanidines to see how they react and what reaction heat is released.
Which substance pairs look promising?
Zeidler-Fandrich: We've come up with a combination of tetramethylguanidine and pyruvic acid. Pyruvic acid is a very strong acid. Therefore it's not surprising that the reaction releases relatively large amounts of energy. In the course of our research, however, we've found that the pyruvic acid decomposes in the reaction. If several cycles are run consecutively – and this happens of course with thermal energy storage – then the carbon dioxide is the actual reaction partner. We investigated and characterised this basic reaction, tetramethylguanidine with CO2, in further experiments.
Stable measurement data for more than 30 cycles
How can I imagine this in practical terms? Carbon dioxide is a gas. In which form does the guanidine occur?
Zeidler-Fandrich: It depends on which derivative you have. Our tetramethylguanidine is liquid at room temperature. We simply had the CO2 flow through it in order to synthesize the first product – in other words a very simple experiment.
We obtained a crystalline solid as the reaction product. This has provided the starting product for our further investigations. We repeatedly heated and cooled it down again to see if we can reverse the reaction and whether the product can be repeatedly split into the starting material and CO2 and then reunited.
Were you successful?
Zeidler-Fandrich: It works in a sealed container. We have shown this at the milligram scale. We repeatedly heated the high pressure crucible and cooled it down again. We obtained pleasing and stable measurement data in over 30 cycles. It works.
When heat is added, the starting material therefore decomposes into tetramethylguanidine and CO2?
Zeidler-Fandrich: Precisely. The CO2 is a gas at room temperature and tetramethylguanidine is liquid. That's why the separation is accordingly easy. The substances can be stored in different containers and recombined if necessary. So the theory. However, we have not yet tested it.
This initially sounds pretty straightforward. Do you foresee any problems in practice, such as toxicity, availability or price?
Zeidler-Fandrich: All strong bases are very corrosive. There could be a material problem, particularly in terms of the sealing. At higher temperatures the tetramethylguanidine becomes vaporous. That makes it more difficult to handle and separate the materials.
The future lies in the polymer structures
Where could the solution lie?
Zeidler-Fandrich: We worked very hard on the monomeric structures as a way of initially approaching the subject matter, but the future is more likely to lie, however, in the polymer structures because they are simply easier to handle.
What's the difference?
Zeidler-Fandrich: A monomeric structure is a relatively small molecule. Polymeric structures have a higher molecular weight. They are therefore more stable and not so sensitive. They are also often not present as liquids but as solids. In our investigations, we have specifically used substances that are commercially easily available as intermediates in the chemical industry.
What are the temperature levels with these substances?
Zeidler-Fandrich: In the case of the monomeric substances, we traversed a temperature range from 40 to 220 degrees Celsius. In order to regenerate the storage system, temperatures above 220 degrees Celsius are required. In the case of the polymeric substances, we were not quite so high. There we had a maximum of 120 to 130 degrees Celsius.
What's the advantage relative to established chemical or sorptive storage systems?
Zeidler-Fandrich: We achieve relatively high energy densities. However, I ought to qualify this by saying that we're in a correspondingly wide temperature range. You always have to look at what you're comparing.
Where do you see application possibilities?
Zeidler-Fandrich: I can only provide a very rough look into the future. In particular, industrial processes are conceivable that run discontinuously, have to temporarily store heat and fit the temperature level. Drying processes are particularly conceivable. However, that's something we would need to verify when we've made further progress with the research. Since we've only worked at a laboratory scale so far, it's very difficult to talk about technical applications.