Synthetic investigation of thermal storage capacities in crystalline bedrock through a regular fracture network as heat exchanger

Crystalline rocks are in general considered of poor interest for geothermal applications at shallow depths (<100 m), in particular because of their low permeability. In some cases, fractures may enhance permeability, but thermal energy storage at these shallow depths still remains challenging because of the complexity of fractured media to efficiently manage water circulation. To assess the feasibility of efficient thermal energy storage in shallow fractured crystalline bedrocks and to determine the fracture network geometric parameters that control heat storage in fractured media, we investigate abilities of a synthetic model especially dedicated to an innovative semi open-loop heat exchanger. The representative heat exchanger system consists of a network of star-shaped grooves radially arranged around a central borehole hydraulically and thermally insulated from the grooves. Fluid is injected in the downward direction into the grooves and withdrawn upward in the central insulated pipe. Heat is exchanged with the rock only through the grooves. The three geometrical parameters controlling the thermal storage are the width, the height and the number of grooves. With a dimensional analysis on heat transport equations and a sensitivity analysis on geometric parameters, we show that the conduction of temperature in rocks presents two well-separated short-term and long-term regimes. At short-term, maximum of exchanges is reached at the beginning of the heated water injection into the grooves. It is controlled by the thermal exchange surface depending on the number and width of the grooves. During the long-term regime, the heat exchanger capacity is mainly controlled by the accessible rock to thermal storage which depends mainly on the width of the grooves. We finally discuss about the possibility of extending the results obtained from the star-shaped geometric sensitivity study to natural fracture systems.