Challenges in large-scale application of phase change materials with shallow geothermal energy system for building heating/cooling: A systematic review

Combining phase change materials (PCMs) with shallow geothermal energy (SGE) systems offers a promising solution to thermal imbalance and efficiency degradation in both heating and cooling applications, yet large-scale implementation remains limited. This review systematically examines the characteristics, application modes, and key challenges of different PCMs. Organic PCMs provide good stability but suffer from low thermal conductivity, while inorganic PCMs face issues of corrosion and phase separation. Modified PCMs can mitigate leakage and enhance performance, though standardized preparation methods are still lacking. In practice, PCMs are applied either as heat transfer media (e.g., borehole heat exchangers (BHEs), earth air heat exchangers (EAHEs)) or as energy storage modules (ESMs). Major challenges include cost reduction, environmental risk assessment, structural innovation to balance thermal conductivity and soil isolation, long-term performance prediction under cyclic loading, and alignment of phase change processes with dynamic energy demand. Future development is expected to focus on data-driven load optimization and integration with other renewable energy sources. This article outlines a research pathway emphasizing scalable manufacturing, environmental safety, and multi-scale modeling to accelerate the transition of PCM-SGE systems from experimental studies to practical applications.