The effect of seasonal groundwater saturation on the effectiveness of large scale borehole heat exchangers in a karstic aquifer

A study was completed comparing the effectiveness of the borehole heat exchanger (BHE) of a large scale, closed-loop geothermal system to seasonal fluctuations in the saturation of the surrounding bedrock. The BHE consisted of 144 production wells drilled to a depth of 122 m into karstic bedrock. Water levels in a nearby groundwater monitoring well were used to characterize the changes in the saturation of the bedrock. The study showed a significant increase in the effectiveness of the BHE as the saturation of the aquifer increased. The effectiveness of the BHE was typically near 0.8 when saturation was highest. The effectiveness decreased to 0.4 when saturation lowered past a threshold, but tended to remain stable as saturation continued to decrease. Our study assembled evidence that a single borehole thermal response tests in karstic environments may not capture the seasonal changes in the saturation of the surrounding bedrock. Complete characterization of the site's subsurface geology, hydrology, and use of multiple borehole thermal response tests during periods of high, low, and median saturation of the surrounding bedrock will better help stakeholders plan and design large scale geothermal systems in karstic environments.