Multilevel comparison between magnetite and quartzite as thermocline energy storage materials

Highlights: • Experimental characterization of magnetite under thermal cycling for thermocline energy storage. • Numerical modeling of thermocline energy storage system. • TES performances comparison between magnetite and quartzite for medium temperature range. • Magnetite and quartzite are suitable filler material for thermocline energy storage. -- Abstract: This paper presents a multilevel comparison between two thermal energy storage materials: quartzite as the most known thermocline energy storage material and magnetite as a new potential candidate. This comparison involves: thermal and thermophysical properties, cycling effect and thermocline thermal energy storage performances. In first, an experimental characterization of magnetite under cycling has been performed, and then magnetite was compared to quartzite from this viewpoint. It has been demonstrated that for the medium temperature range (i.e. from 100 °C to 500 °C), thermal cycling has a positive impact on magnetite characteristics and performances. Thereafter, a numerical model for thermocline storage has been presented and validated. Then, the thermocline behavior and the thermal energy storage performances of both materials during charging and discharging processes have been investigated and tested for various heat transfer fluidscommonly used, including natural oil, synthetic oils and molten salts. This study shows that, for the different HTF tested, no significant difference between the thermocline zone thicknesses can be noted between the two TESM. It has been concluded that for the same storage tank size and the same discharge time, magnetite can store and restore more energy and requires less storage volume although quartzite presents higher efficiencies. For magnetite, this can represent an advantage from an economic and technical standpoint. While for the same storage tank size and the same HTF speed, the magnetite charges and discharges more slowly. On the other hand, the different combinations HTF/TESM tested show that thermocline performances are driven by not only the filler material but also the nature of the heat transfer fluid. In that sense, the molten salt fluids arelargely more efficient for both TESMs than other fluids.