Degradation of a rock bed thermal energy storage system

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Large-scale energy storage is inevitable for the increasing integration of renewable energy and decarbonization of the electricity sector. The potential degradation of a rock bed thermal energy storage system is investigated systematically from both material- as well as system-level perspectives. The performance changes of a 1 MWhth rock bed pilot plant which has been operated up to 675 °C for 249 cycles (3458 h) is evaluated. We have assessed the potential chemical, structural and thermo-physical changes of the storage material by comparing the properties of rocks as received and rocks retrieved during a post-operational inspection using optical and scanning electron microscopy, energy-dispersive x-ray spectroscopy, dilatometry, densitometry, a vibrating-sample magnetometer, x-ray diffraction and differential scanning calorimetry. However, system-level changes are identified as the main reason for the decreased storage performance even though a 13% decrease in the rock’s heat capacity due to cycling is measured. This is the first study showing the long-term behavior of a rock bed thermal energy storage and no significant changes of the storage material were found. Therefore, the paper demonstrates that thermal storage based on rocks offers promising calendar as well as cycle lifetime but storage material and design should be selected holistically in order to minimize the avoidable performance losses stemming from system-level aspects such as rock rearrangement and thermal ratcheting.