Characterizing lithological effects on large scale borehole heat exchangers during cyclic heating of the subsurface

This paper presents the results of a study where subsurface temperatures were monitored adjacent to the u-tube of a central borehole of a large scale 144 borehole heat exchanger (BHE) at multiple depths. This was an effort to develop a method to characterize the variability of thermal properties of the various lithological units surrounding the BHEs of large scale ground coupled heat pump systems during the operation of the system. Saturation of the units and lithology was shown to affect the performance of the BHE resulting variable temperature fluctuations and heat flux at different depths within the borehole. Median heat flux tended to increase with decreasing temperature ranges measured by the sensors. The lowest median heat flux was in the location nearest to the surface in the least saturated zone. In the more saturated zone, the highest median heat flux were at locations where the lithological units were composed of 75 percent or greater dolomite while the lowest median heat flux where in lithological units composed of 30 percent or greater chert.