A comprehensive review of different thermal energy storage materials for concentrated solar power has been conducted. Fifteen candidates were selected due to their nature, thermophysical properties, and economic impact. Three key energy performance indicators were defined in order to evaluate the performance of the different molten salts, using Solar Salt as a reference for low and high temperatures. The analysis provided evidence that nitrate-based materials are the best choice for the former and chloride-based materials are best for the latter instead of fluoride and carbonate-based candidates, mainly due to their low cost.

Renewable energies are one of the most important actions in the fight against climate change. The development of thermal energy storage materials that increase the operating temperature range and energy density is a line of research with enormous potential. The development of chloride-based molten salts is a relevant option due to their lower energy cost and higher operating temperature. Those based on sodium and potassium chlorides, and to a lesser extent lithium chloride, have been the subject of research. However, there are no studies of this type of salts containing strontium chloride and therefore it is essential to carry out research in this direction, especially when this compound is a strategic mineral for the European Union and of which Spain represents a third of the world production. The study of the LiCl-NaCl-KCl-SrCl2 system, in fact the pseudoternary system, (LiNa)ClssKCl-SrCl2, by complete solid solution in the LiCl-NaCl system, aimed to define compositions where molten salts can be formulated. The well-established binary systems were critically reviewed, except for the KCl-SrCl2 system which presents discrepancies in its definition, which determined its experimental evaluation. The ternary systems where SrCl2 is present showed erroneous and inconsistent data and were experimentally evaluated to build a reliable database for thermodynamic modeling. The pseudobinary system (LiNa)Clss-SrCl2 and the LiCl-KCl-SrCl2 system were redefined and certain inconsistencies in the NaCl-KCl-SrCl2 system were corrected. The configuration of the system (LiNa)Clss-KCl-SrCl2 was established, being divided into two pseudoternary subsystems (LiNa)Clss-KSr2Cl5-SrCl2 and (LiNa)Clss-KSr2Cl5-KCl and their experimental evaluation allowed to define the areas where the liquid phase is formed at the lowest temperature. The thermodynamic description of the system was carried out using the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) methodology and Thermo-Calc software. The adjustment of the interaction parameters of the re-evaluated systems allowed to build a consistent database, named LiNaKSr-Cl, to reliably model the pseudoternary system and the subsystems that comprise it. A regular or subregular sublattice model and the Muggianu geometric model were applied, obtaining the first thermodynamic model of the (LiNa)Clss-KCl-SrCl2 system. The establishment of isoplethal sections allowed to determine, in the (LiNa)Clss-KCl-KSr2Cl5 subsystem, the minimum global temperature of liquid phase formation, 325 C at a mass composition of: 46.1 % LiCl, 11.1 % NaCl, 35.2 % KCl and 7.6 % SrCl2. Two new compositions were defined as a compromise between maximizing their energy density, using the General Solution Model (GSM), specific heat, obtained by the thermodynamic model used for the phase diagram, and minimizing their energy cost. The optimization yielded two new compositions (in mass), 32.3 % LiCl 13.1 % NaCl 49.6 % KCl and 5.0 % SrCl2 and 24.8 % LiCl 9.5 % NaCl 53.4 % KCl and 12.3 % SrCl2, which melted at 350 C and incorporated maximum and minimum values of SrCl2. The three compositions obtained presented properties equal to or improved from those of their chloride-based commercial competitors at a substantially lower specific cost. These salts have an operating temperature of around 150 C higher, at a cost between 2.5 and 4 times lower than the reference salt based on nitrates, although in exchange they have an energy density 50 % lower. Finally, from the point of view of industrial application of these salts, a wide range of compositions has been defined, which not only take into account technical considerations but also economic and opportunity factors at each moment. RESUMEN Las energías renovables suponen una de las acciones de mayor relevancia en la lucha contra el cambio climático. El desarrollo de materiales de almacenamiento térmico de energía por calor sensible que aumenten el rango de temperaturas de operación y la densidad energética es una línea de investigación de enorme potencial. El desarrollo de sales fundidas basadas en cloruros es una opción relevante por su menor coste energético y su mayor temperatura de operación. Las basadas en cloruros de sodio y de potasio, y en menor grado las de cloruro de litio, han sido objeto de investigaciones. Sin embargo, no hay estudios de este tipo de sales que contengan cloruro de estroncio y por tanto es esencial la realización de investigaciones en esta dirección, maxime cuando este compuesto, es un mineral estratégico para la Unión Europea y del que España representa un tercio de la producción mundial. El estudio del sistema LiCl-NaCl-KCl-SrCl2 de hecho el sistema pseudoternario, (LiNa)ClssKCl-SrCl2, por la solución sólida completa en el sistema LiCl-NaCl, tuvo como objetivo definir composiciones donde se puedan formular sales fundidas. Se revisaron críticamente los sistemas binarios, bien establecidos, a excepción del sistema KCl-SrCl2 que presenta discrepancias en su definición, lo que determinó su evaluación experimental. Los sistemas ternarios donde el SrCl2 está presente mostraban datos erróneos e inconsistentes y se evaluaron experimentalmente para construir una base de datos fiable para el modelado termodinámico. El sistema pseudobinario (LiNa)Clss-SrCl2 y el sistema LiCl-KCl-SrCl2, fueron redefinidos y en el sistema NaCl-KCl-SrCl2 se corrigieron ciertas inconsistencias. Se estableció la configuración del sistema (LiNa)Clss-KCl-SrCl2, quedando dividido en dos subsistemas pseudoternarios (LiNa)Clss-KSr2Cl5-SrCl2 y (LiNa)Clss-KSr2Cl5-KCl y su evaluación experimental permitió definir las áreas donde se forma fase líquida a la menor temperatura. La descripción termodinámica del sistema se realizó usando la metodología CALPHAD (Computer Coupling...