Report on solids as sensible heat storage materials

Extensive experimental study of dual media thermocline thermal energy storage using rock and sand as solid matrix and thermal oil as heat transfer fluid is presented. The work is devoted to the establishing of repeatable behaviours observed with multiple charge/discharge cycles and to the influences of operating parameters. Experimental results show that established behaviours are independent of most of operating parameters, notably tank initial state, mass flow rate and temperature ranges, and mainly depend on thermocline control strategy, i.e. part of the thermal gradient which is pulled out of the tank in charge and discharge. Experimental performances have been determined for a large range of control strategies and show tank utilisation rate ranging from about 10% to more than 80%. Thermocline response to perturbations has been studied and shows that partial charge perturbations lead to performances reduction with rapid return to established behaviours. These results may strongly enhance dual media thermocline understanding and may help design and integration of such TES in a given process.

Combined sensible/latent heat storage allows the heat-transfer fluid outflow temperature during discharging to be stabilized. A lab-scale combined storage consisting of a packed bed of rocks and steel-encapsulated AlSi₁₂ was investigated experimentally and numerically. Due to the small tank-to-particle diameter ratio of the lab-scale storage, void-fraction variations were not negligible, leading to channeling effects that cannot be resolved in 1D heat-transfer models. The void-fraction variations and channeling effects can be resolved in 2D models of the flow and heat transfer in the storage. The resulting so-called bypass fraction extracted from the 2D model was used in the 1D model and led to good agreement with experimental measurements.