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Here, we present the first application of a temperature sensor module (TSM) for deducing groundwater flow velocity and direction at borehole heat exchangers (BHEs). The TSM maps the horizontal temperature distribution around a BHE. As groundwater flow distorts this temperature distribution, flow velocity and direction can be inferred from the measured temperatures. As modular systems, TSMs can be attached to a BHE at any depth. For the studied BHE, the depths of interest are 82 m and at 94 m. We recorded TSM data for 2 weeks before and during the operation of the BHE. After simulating the working fluid temperature, we model the horizontal temperature distributions using the working fluid temperatures at the depths of interest as input. We use the latter simulations for inferring groundwater flow by minimizing the root mean square error between the measured and simulated temperatures. We obtain a groundwater flow of 0.4 m/day in the NW direction and groundwater flow below the detection limit of 0.01 m to 0.02 m/day at 82 m and 94 m depths, respectively. A flow meter measurement in a nearby groundwater well confirms the flow direction at 82 m but gives an order of magnitude higher velocity, which we attribute to the measurement principle. Moreover, long-term monitoring of a BHE equipped with multiple TSMs could provide information on seasonal variations in groundwater flow, changes in the thermal properties of the BHE filling or changes in the thermal resistance between BHE and ground.

First field application of temperature sensor modules for groundwater flow detection near borehole heat exchanger