PNNL

Abstract

The DEMO-FTES project investigated the feasibility of Fracture Thermal Energy Storage (FTES) as a seasonal energy storage solution in crystalline rock formations. FTES leverages hydraulically induced fractures to exchange heat between circulating fluids and the surrounding rock mass, enabling long-term thermal energy retention due to the high specific heat and low thermal conductivity of rock. This approach has the potential to reduce heating and cooling energy demands and enhance building energy resilience. The project combined dimensional analysis, numerical modeling, laboratory experiments, and meso-scale field tests to evaluate FTES performance and advance its technology readiness level from 3 to 5. Scaling analysis identified key dimensionless parameters governing heat transfer and fluid flow, ensuring laboratory and field tests were representative of larger-scale systems. Numerical simulations using TOUGH and iTOUGH2 frameworks supported experiment design and interpretation, modeling fracture geometry, thermal-hydraulic behavior, and thermo-mechanical coupling. Laboratory tests at EPFL involved creating single and multiple fractures in 25 cm cubic samples of Gabbro and Granite under true triaxial stress. Thermal cycles were performed by injecting hot water (up to 150?°C) at controlled flow rates, followed by cooling phases. Results demonstrated fundamental FTES processes—heat injection, storage, and recovery—while revealing key operational insights. Heat transfer efficiency was strongly influenced by injection rate and fracture transmissibility. High flow rates produced rapid heating but reduced storage efficiency due to advective losses, whereas lower flow rates favored energy retention. Injection pressure increased with temperature, indicating fracture aperture reduction from thermal expansion, a phenomenon also observed in field tests. Multi-fracture experiments showed flow concentrated in the most transmissive fracture, underscoring the need for flow-control strategies. Thermal Recovery Factor (TRF) values were low in the lab due to radiative heat losses, but corrected estimates suggest improved performance at field scale. Repeated cycles revealed cumulative heat retention and stabilized efficiency after several iterations. The meso-scale field test at the Sanford Underground Research Facility implemented a five-spot borehole pattern in amphibolite rock at 1,250?m depth. Hot water injection (~53?°C) over 73 days gradually increased production temperatures, confirming thermal charging. However, recovery during the cooling phase was limited, primarily due to stress heterogeneity and thermoelastic effects that reduced fracture permeability. Injection impedance correlated strongly with stress gradients between wells, indicating that geomechanical evolution critically affects hydraulic connectivity. These findings highlight the importance of accounting for coupled thermal-hydraulic-mechanical processes in FTES design. Overall, DEMO-FTES validated the core concept of FTES and identified critical challenges: managing fracture transmissibility, mitigating stress-induced closure, and controlling flow distribution. Future work should incorporate proppants to maintain fracture openness, improve boundary conditions to reduce heat loss, and develop advanced numerical models for upscaling and efficiency prediction. The dataset generated—including fracture properties, thermal response, and energy budgets—provides a foundation for optimizing FTES systems and advancing toward commercial deployment.