Solarthermal Power Plants: Degradation of Materials in Molten Salts

© Photo Fraunhofer IWM

The increasing use of fluctuating renewable energy sources requires methods to store energy for a certain time. In concentrated solar power plants (CSP) molten salts are used as a storage and heat transfer medium, which require the use of tailored materials, even the development of new materials and the assessment of life time concepts for the components and its materials in use. Furthermore, molten salts are used in many branches of industry, for example as baths made of molten chloride-salt mixtures for the production of surface alloys or as fluoride salts to clean metal surfaces. In the meantime, molten salt from nitrates and nitrites have become attractive for thermal energy storage (TES) as they comprise high temperature stability, low freezing temperatures and good heat conductivity

In order for high temperature reservoirs and heat exchangers to work reliably and be used economically, comprehensive qualification measures are required for the utilized materials. Complex and time-dependent combinations of mechanical, thermal and chemical demands appear in TES, heat exchanges and pipes that may lead to a critical degradation of the materials.

 

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© Photo Fraunhofer IWM

Services

At the Fraunhofer IWM a team of experienced physicists, engineers and chemists are working on researching fundamental degradation mechanisms, selecting and optimizing raw materials, defining protection concepts, and predicting life expectancies. We have test stands at our disposal in our high-temperature corrosion laboratory that can simulate the important degradation mechanisms of power plant components that come into contact with molten salt.

Our services in detail:

Investigations into the chemical stability of molten salt at high temperatures

Statistical corrosion tests to determine corrosion rates and identify corrosion mechanisms

Corrosion investigations in flowing molten salt

Slow strain rate tests (SSRT - CERT) i.e. mechanical loading with low strain rate in molten salt environments to separate and quantify the amount of corrosion and stress corrosion cracking occurring in the raw material

Mechanical fatigue tests with superimposed corrosive stress due to molten salt

Cyclical thermal corrosive stress

Comprehensive microstructure and surface analyses using optical and scanning electron microscopy, element analyses (EDA), electron backscatter diffraction (EBSD), and X-ray diffraction analyses (phase and residual stress analyses) to reveal the underlying corrosion mechanisms

Development and testing protective layers

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Literature

Platzer, W.; Boie, I.; Ragwitz, M.; Kost, C.; Thoma, J.; Vogel, A.; Fluri, T.; Pfeiffer, W.; Burmeister, F.; Tham, N.; Pudlik, M.; Bohn, S.; Agsten, M.; Bretschneider, P.; Westermann, D.; Kranzer, D.; Schlegl, T.; Fraunhofer Zukunftsthemen "SUPERGRID". Supergrid - Ansatz für die Integration von Erneuerbaren Energien in Europa und Nordafrika; Fraunhofer ISE, Freiburg (2016) 37 Seiten Link

Preußner, J.; Peiffer, W.; Piedra, E.; Oeser, S.; Tandler, M.; von Hartrott, P.; Maier, G.; Long-term material tests in liquid molten salts; in Proc. of 8th International Conference on Advances in Materials Technology for Fossil Power Plants 2016; Parker, J.; Shingledecker, J.; Siefert, J. (Eds.); ASM International, Materials Park, OH, USA (2016) 1128-1139 Link

Gurr, M.; Bau, S.; Burmeister, F.; Wirth, M.; Piedra-Gonzales, E.; Krebser, K.; Preußner, J.; Pfeiffer, W., Investigation of the corrosion behavior of NiVAl multilayer coatings in hot salt melts; Surface and Coatings Technology 279 (2015) 101-111 Link

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