Report on protocols for standardized testing procedures and data analysis criteria

The present report is concerned with the description of possible standardized analytical methods to be applied for the characterizations of HTF (heat thermal fluids) and HSM (heat storage materials, both based on specific and latent heat: PCM, phase change materials). Evidently, given the wide variety of fluids that are or can be proposed as HTF/HSM, a precise protocol has to be specifically determined for each of them. The aim of this document is to provide general rules and practical, laboratory validated, examples, to be used as principles for the several materials characterizations; the reported results clearly demonstrated that most of the described procedures are absolutely feasible to be proposed as standard protocol for a large range of materials. The first part is focused on thermo-physical characterizations; the second on material compatibility (corrosion) tests procedures.
The reported techniques are mostly dedicated to nitrate and nitrite mixtures, regarding the other possible materials:

• Halides can be analyzed respect to their specific heat, viscosity, heat conductivity, density and chemical composition in exactly the same way and with the same procedures described for nitrates/nitrites. Concerning their thermal stability, their oxidation under air, with contemporary release of halogens, is the major issue; therefore, also in this case, the degradation onset can be defined as the temperature at which alkaline or heart alkaline oxides are detectable.
• Carbonates present characteristics similar to molten nitrates/nitrites but a quite higher freezing points; for this reason the experimental equipment necessary to investigate these materials in their molten state can be very difficult to get and, in case, pretty expensive. Carbonates (especially the alkaline ones) are expected to present far higher thermal stability limits respect to nitrates.
• Liquid metals are in general chosen with low melting features, they can be analyzed with practically the same methodologies of molten nitrates, provided the compatibility with the equipment materials is ensured. Chemical analysis can be performed by solubilization in water (by reacting them with acids, for example). As said, the main issue concerning these fluids is their compatibility with construction materials.
• Concerning solid fillers as HSM, the same below described techniques can be employed for the specific heat. More than thermal stability, the compatibility with the complementary HSM fluid needs to be investigate; this topic depends specifically on the type of filler under considerations, and won’t be addressed in this report. Apparent density and, especially, heat conductivity can be determined with simpler methods than the ones for molten salts, where, for instance, the effects of convective heat must be separated from heat conductivity.
• Gases eventually employed as HTF are species of large use (steam, CO2…) and in general quite well characterized.