Seasonal borehole thermal energy storage for zero-carbon city heating: Mechanisms, optimization, and application challenges

Cold-climate city heating sector faces increasing pressure to reduce carbon emissions, highlighting the need for a zero-carbon, efficient, and economical solution. Borehole thermal energy storage (BTES) is one of the most promising thermal energy storage technologies to meet the demands. However, it faces challenges such as extended storage cycles, significant thermal inertia, and low energy density. This review provides an overview of recent advances in BTES across multiple scales, covering heat transfer mechanisms, design optimization, system integration, and engineering applications. First, the development of heat transfer models is summarized, with a focus on computational cost, simulation accuracy, and reliable performance. The heat loss of BTES systems has a decisive impact on storage performance, influenced primarily by system scale, soil thermal conductivity, and operating temperature difference, and can be mitigated through optimized system design and geological assessment. Furthermore, heating systems integrating BTES should consider the characteristics of heat sources and application scenarios to define suitable operation modes. Reported large-scale engineering applications demonstrate the strong climatic adaptability and potential of BTES, with storage efficiencies ranging from 19% to 63%. Finally, we propose a high-efficiency BTES design framework and identify research priorities for system-level integration into district heating networks.