Salt systems are widely used as storage materials and heat transfer fluids for different applications, e.g., for concentrated solar power plants. When selecting a suitable salt mixture, various requirements should be considered, e.g., high energy density, high thermal conductivity, low viscosity, thermal stability, low corrosion, and low price. To be able to vary these parameters a consistent and verified thermodynamic database containing multicomponent salt and metal systems should be developed. Multicomponent systems can be represented as a combination of three, four or even more components.

 Our current projects “PCM-Screening 2” and “SaltMe” are focusing on the development of thermodynamic database with inexpensive and available salts based on the following cations and anions Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺ / NO₃^-, Cl^-, CO₃^2-, SO₄^2- and including liquid solutions with metals (e.g., Li, Na, K, Mg, Ca) for enhancement of thermal conductivity (Fig. 1). To achieve this aim, we are combining CALPHAD assessment of thermodynamic and thermophysical properties with experimental measurements using a combination of simultaneous thermal analysis (STA), differential scanning calorimetry (DSC), dilatometry (DIL), high temperature X-ray diffraction (HTXRD) and Knudsen effusion mass spectrometry (KEMS). In this work, a methodology for validation and screening of multicomponent systems will be presented. Beyond equilibrium calculations, kinetic effects as well as reactions between samples and crucible materials and creeping behaviour of the samples should be taken into account by the experimental studies. The first results on the study of quasi-binary and reciprocal metal-salts (e.g., Mg-KCl and Ca-KCl) systems will be discussed. Furthermore, one multicomponent mixture in the Na⁺, K⁺// Cl^-, CO₃^2-, SO₄^2- system with the lowest melting temperature of 512 °C was selected for experimental validation as a potential candidate for thermal energy storage by phase change materials. The obtained experimental results show large deviations from the values calculated using existing databases. To improve these calculations, further validation of binary, ternary and reciprocal systems is necessary.

Acknowledgments

This work was supported by the Federal Ministry for Economic Affairs and Climate Action on the basis of a decision by the German Bundestag within the project “PCM-Screening 2” (FKZ 03EN6005D) and by the German Research Foundation within the project “SaltMe” (DFG 497349462).