Experimental comparison of tailored and natural solid fillers for high-temperature air-packed-bed thermal energy storage

This study presents a comparative assessment of two kinds of materials intended as random fillers in high-temperature air-solid packed bed thermal energy storage (TES) systems. One material is a specially tailored waste-recycled ceramic product (STWR), while the other consists of natural river pebbles (NRP). Both materials were experimentally tested at device scale under identical working conditions (700 °C air inlet, 50 kg/h airflow), using a dedicated packed-bed facility for standardized thermal performance evaluation. Key performance indicators (KPIs), as defined in the Horizon 2020 SFERA-III project, were applied. These include storage capacity, mean thermal power, utilization rate, energy and exergy efficiencies and thermal losses. Initial results showed that NRP slightly outperformed STWR in terms of energy and exergy efficiencies (73.3 % vs 63 % and 71.2 % vs 66.7 % respectively), utilization rate (79.2 % vs 73.3 %), and total storage capacity (36.9 kWh vs 28.6 kWh) during the first two charge-discharge cycles. However, repeated cycling at 700 °C caused significant mechanical degradation in NRP, particularly in the upper bed region directly impacted by the hot air jet. This led to fragmentation, increased pressure drops, and altered thermal profiles, ultimately compromising performance. Notably, such degradation was not observed in prior small-scale thermal cycling tests or at lower temperatures. In contrast, STWR remained structurally and thermally stable over repeated cycles, delivering consistent performance throughout the experimental campaign. Although NRP is significantly cheaper (68 €/ton vs. 800 €/ton) and more widely available, only STWR met the mechanical stability requirements for reliable operation in TES systems at 700 °C. These findings underline the importance of assessing both thermophysical and mechanical properties under realistic operating conditions when selecting filler materials for high-temperature TES applications.