Sand Batteries for Thermal Energy Storage: A Review of Operating Principles, Technologies, Challenges and Future Perspectives

Sand-based thermal energy storage (TES) has attracted increasing attention as a potential solution for storing renewable electricity and supplying heat over extended periods. This review examines operating principles, storage configurations, materials, thermal performance, economic aspects, environmental considerations, safety, reliability, optimisation, and the potential integration of sand-based TES with renewable energy and wider energy systems. Particular attention is given to the relationships and trade-offs among these aspects and to current technological limitations. The reviewed studies indicate that sand-based TES can provide high-temperature heat storage using abundant, relatively low-cost storage media, while its performance depends strongly on system configuration, operating temperature, charging and discharging conditions, heat losses, and integration strategy. Reported economic and environmental results vary considerably according to electricity prices, utilisation, system lifetime, efficiency, system boundaries, and other methodological assumptions. Although sand-based TES shows potential for large-scale, long-duration thermal storage, comprehensive life-cycle assessments, long-term operational data, and system-level reliability and safety evaluations remain limited. Hybrid configurations, advanced optimisation, artificial intelligence, and integration with renewable energy systems represent promising areas for further investigation. Future research should therefore focus on long-term experimental validation, consistent techno-economic and environmental assessment, system-specific safety and reliability analysis, and the development of robust control and optimisation strategies to support practical deployment.