Coupled Utilization of Geothermal Energy and Thermal Energy Storage: Current Technologies and Future Prospects

Temporal mismatches between geothermal output and user demand limit resource utilization, whereas thermal energy storage (TES) can decouple heat production from consumption. This review organizes geothermal–TES coupling by resource depth and answers four engineering questions. First, temperature, duration, geology, and end-use requirements determine compatibility: water tanks and low-temperature phase-change materials are preferred for short-term regulation; BTES and ATES provide seasonal capacity where geology permits; and pressurized-water, molten-salt, or thermochemical systems are reserved for higher-temperature resources. Second, short-term TES primarily suppresses cycling and peak demand, whereas seasonal TES restores annual subsurface balance and raises geothermal-well utilization. Third, selection must jointly consider recovery efficiency, usable discharge temperature, auxiliary electricity, land and drilling requirements, technology readiness level, and cost rather than storage density alone. Fourth, coupling introduces constraints that are absent from stand-alone components, including groundwater advection, thermal breakthrough, corrosion and scaling, well-flow limits, and temperature matching. Nordic experience demonstrates the value of integrating underground TES with district heating and cooling, but economic feasibility remains site specific. Indicative storage-capacity costs are approximately 0.1–1.3 EUR/kWhth for ATES, 0.6–3 EUR/kWhth for BTES, and 0.5–1.7 EUR/kWhth for PTES, with strong scale and boundary dependence. A compatibility matrix, maturity assessment, and staged development roadmap are provided to support system selection and research prioritization.