Editorial: Thermal energy storage technology and applications

Thermal energy storage technologies mainly include three types: sensible heat storage, latent heat storage, and thermochemical storage. These technologies are constantly improving in terms of materials, structures, and system design. The review by Quasi-Efahah and Okopako systematically summarizes the latest progress of various energy storage mechanisms, particularly emphasizing the cutting-edge developments in nano-enhanced phase change materials, hybrid energy storage systems, and intelligent integration strategies. The article also points out that the energy storage density of thermochemical storage is 300-600 kWh/m³, significantly higher than that of latent heat storage (100-150 kWh/m³) and sensible heat storage (25-80 kWh/m³); adding 1.0 wt% carbon nanotubes can increase the thermal conductivity of paraffin by 210%, machine learning optimization scheduling can reduce operating costs by 12%-18%; the "sensible heat + latent heat" hybrid configuration increases the storage density by more than 35%; digital twin applications can reduce the failure rate of energy storage units by 22%. This review provides a clear performance benchmark and evolution path for the next-generation TES technologies.Addressing the seasonal mismatch between renewable energy supply and building heat demand is one of the core challenges of current low-carbon heating systems. Schmidt et al. reported the first-of-its-kind pilot application of a thermal chemical energy storage system based on the calcium oxide/water reaction in a real building environment. This system provides stable thermal energy at 60°C, with a theoretical energy storage density of 450 kWh/m³. It can be directly connected to the existing building heating infrastructure and successfully replaces fossil fuel boilers. The technical maturity has been enhanced to TRL 5 level and a continuous operation zero-emission seasonal heating demonstration has been achieved.In areas lacking stable power supply, simple and reliable thermal energy storage systems are crucial for promoting clean cooking. Nydal et al. proposed a passive temperature control method based on natural oil circulation for a cooking thermal storage system. This system automatically breaks through the liquid seal barrier and starts the circulation when the oil temperature reaches 180°C, without the need for external control. At a heating power of 2.5 kW, the cooking platform temperature stabilizes at 200-220°C. A single charge can store 2.6 kWh of thermal energy, meeting the needs of two meals for a household. After completing 150 charge-discharge cycles, the temperature control deviation is less than ±5°C. It provides a highly robust and clean cooking solution for resource-constrained areas.Combining heat pumps with thermal energy storage systems can significantly enhance the system's energy efficiency and operational flexibility. Agalave and Kulkarni conducted experimental research on an integrated system that combines a heat pump using the environmentally friendly refrigerant R290 with a phase change material storage unit. The waste heat from condensation is recovered through shell-and-tube heat exchangers. The average storage power of the system is 3,481 W, the outlet temperature of the hot fluid is 63.3°C, and the total storage capacity is 28.6 MJ.The theoretical COP of the heat pump is 4.0, while the measured average COP is 2.6. This is the first time that the feasibility of the coordinated operation of low-GWP refrigerants and phase change energy storage has been verified at the system level.The research included in this special issue showcases the vitality and diversity of thermal energy storage technology at multiple levels, ranging from basic materials and system innovation to practical applications. Future research should continue to focus on addressing several key challenges: including further reducing the cost of storage materials and systems, enhancing cycle stability and service life, developing standardized and modular designs to facilitate large-scale deployment, and deepening the integration research of intelligent control strategies and multi-energy complementary systems. We look forward to promoting the significant role of thermal energy storage technology in building a sustainable, resilient, and inclusive global energy system through continuous technological innovation and interdisciplinary collaboration.The successful publication of this special issue would not have been possible without the outstanding contributions of all the authors, reviewers and the editorial team. We sincerely hope that these research results will stimulate greater attention from the academic community and the industrial sector towards thermal energy storage technology, and jointly accelerate its progress from the laboratory to large-scale application.