Progress in underground thermal energy storage: research contents, hotspots, and development trends

Existing reviews on underground thermal energy storage (UTES) are often fragmented and lack analysis of the spatial-temporal evolution of research hotspots. This study aims to provide an objective and comprehensive analysis of the developmental trajectory and research trends in the global UTSES field. This study utilizes 7705 documents published over the past 30 years as its data source to conduct bibliometric and content analysis using knowledge graph techniques. It focuses on three core issues: research frontiers and technology maturity, core bottlenecks, and future trends and cost characteristics. The aim is to overcome the limitations of traditional qualitative reviews and establish a data-driven, multi-dimensional analytical framework. The results indicate that the UTES field has undergone three stages of development: embryonic, stable growth, and rapid expansion, with large-scale commercialization expected by 2065. Aquifer thermal energy storage (ATES) focuses on heat-fluid-solid coupling optimization. Borehole thermal energy storage (BTES) emphasizes the integration of phase-change materials (PCMs) with renewable energy. Energy piles (EPs) serve as a critical key link between underground structures and energy systems. Rock thermal energy storage (RTES) is shifting toward high-temperature material innovations, with EP demonstrating significantly higher research intensity. The cost ranking is as follows: EP < (BTES, abandoned mine type; ATES, with existing well reuse) < (BTES, without existing wells) < (ATES, without existing wells) < RTES. Through quantitative analysis and prediction models, this study provides a scientific basis for UTES technological innovation, collaboration establishment, and policy formulation. Moreover, it significantly enhances the scientific rigor of thermal storage system design and engineering translation efficiency.