Hot transfer – In the heat transfer fluid laboratory of the Institute of Future Fuels at the DLR site in Cologne-Porz, researchers are investigating innovative materials, components and processes that are suitable for transferring thermal energy at high temperatures. They are needed, for example, for the production of energy carriers and their base materials based on renewable energies and transfer thermal energy to the points of consumption in energy systems.
Investigation of thermal stability and ageing
Gas-tight sealable pressurised containers such as ampoules made of glass or steel are used for thermal stability testing, as well as steel autoclaves for larger tests. Molten salts are tested in open crucibles made of inert, i.e. chemically uninvolved materials such as aluminium oxide.
Thermal ageing experiments can be carried out in the heat transfer fluid laboratory for operating temperatures of up to 600 degrees Celsius and even higher temperatures for molten salts.
The ageing experiments are based on established standards such as DIN 51528 but go beyond the scope of these standards. In contrast to standardised investigations, the ageing studies are evaluated over time in order to determine the ageing behaviour and the change in properties under thermal stress over time at several temperatures. The researchers can therefore evaluate the data in terms of reaction kinetics and calculate the ageing behaviour or the formation of certain decomposition products on this basis.
Samples of a biphenyl diphenyl ether mixture
In a laboratory experiment, the mixtures were subjected to thermal stress at 410 degrees Celsius for between 100 and 2,000 hours.
In addition to state-of-the-art analytics for standard tests such as gas chromatography (GC-FID and GC-MS), measurement of density, viscosity and heat capacity at saturation vapour pressure, special experiments are also available. With their help, the application-relevant properties can be analysed up to the intended operating temperature range at practice-relevant pressures and gas contents as well as ageing at the relevant material contacts. This includes vapour pressure measurements, isobaric density measurements, isobaric heat transfer and the investigation of gas solubility or the solubility of water and other substances in heat transfer media.
Applications in solar thermal power plants
Together with colleagues from the DLR Institute of Materials Research and the Institute of Solar Research, we are also investigating the corrosion behaviour of construction materials, which is particularly relevant for the use of molten salts in solar thermal power plants. Ageing at defined material and atmospheric contacts can be investigated in the heat transfer fluid laboratory, as can the physical properties up to at least 600 degrees Celsius. Ion chromatography is primarily used to analyse the salt composition, whereby the unwanted chromate can also be quantified. This is formed when oxidising salts come into contact with chromium-rich stainless steels.
We determine the release of gases from molten salts using gas chromatography and mass spectrometry. Thermogravimetric and calorimetric methods are used to investigate thermal properties such as phase transformation processes (glass points, melting and solidification behaviour) or heat capacity.
In the heat transfer fluid laboratory, experiments can also be carried out to analyse components and develop fluid maintenance processes. We also investigate the permeation of hydrogen through steel pipes and receiver elements of solar thermal receivers as well as the hydrogen absorption capacity of so-called getter materials. Further experiments serve to qualify sensors and selective treatment processes.
The heat transfer fluid laboratory and its testing facilities are used both for the development of new fluids, processes and components in research projects and for orders from industry, as well as for analysing used fluids and components. Used samples come from ageing tests in the heat transfer fluid laboratory, pilot plants such as the MOPUW at the DLR site in Cologne, test facilities at CIEMAT's Plataforma Solar de Almería in Spain and from solar thermal power plants worldwide.
