Laboratory Investigation of Fracture Thermal Energy Storage (FTES) in Crystaline Rocks

Fracture Thermal Energy Storage (FTES) offers a way to store energy (as heat) at large-scale in low-permeability soils. Laboratory experiments performed in our lab investigate how fracture transmissivity, geometry, and flow control govern system efficiency. Using 25 cm granite and gabbro blocks, we created controlled hydraulic fractures and performed repeated injection–recovery cycles through injection and production wells intersecting with the fractures. The cycles were done with an injection temperature ranging from 80 to 150 °C and an injected flow rates between 0.1 and 25 mL/min. Temperature monitoring was distributed both on the surface of the sample and at the intersection between the fractures and the outlet wells. The system exhibits stable and reproducible thermal cycles, enabling systematic analysis of how flow rate and transmissivity influence heat storage and recovery. Multi-fracture experiments show that, without active control of individual fracture flowrates, injection concentrates in the most transmissive fracture, reducing overall efficiency. When individual fracture transmissivities are known and flow is balanced, heat distribution and recovery improve significantly. These results identify the key parameters governing FTES performance and provide a basis for studying, optimizing and scaling this subsurface energy-storage method.