Enhancement of geothermal borehole heat exchangers performances by improvement of bentonite grouts conductivity

Conventional boreholes used as vertical ground heat exchangers are significantly limited in thermal performances by the bentonite grouts thermal conductivity. Bentonite-graphite composites have been elaborated using Graphite Flakes, Expanded Natural Graphite (ENG) and ground Compressed Expanded Natural Graphite CENGg particles. Using graphite flakes, the grouts thermal conductivity enhancements are similar to those already published in previous studies while using CENGg particles, the expected effective thermal conductivity of 5 W m−1 K−1 was obtained at reduced graphite content (5% wt). Partial drying of the grouts induces significant decrease in thermal conductivity in the range of 1 W m−1 K−1 for 10% wt of water content reduction. This inhibiting effect has to be taken into account for geothermal heat storage using borehole based systems.