Abstract
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.
Highlights
For heating and cooling, an increasing number of industrial and multifunctional buildings, e.g., hotels, communal buildings, greenhouses or construction halls, and even city quarters, are being equipped with borehole heat exchanger (BHE) fields (e.g., Fütterer et al 2011; Omer 2008)
We present a successful test of the novel temperature sensor module (TSM) for measuring groundwater flow
At a depth of 82 m, we found groundwater flowing in a NW direction with a velocity of approximately
Summary
An increasing number of industrial and multifunctional buildings, e.g., hotels, communal buildings, greenhouses or construction halls, and even city quarters, are being equipped with borehole heat exchanger (BHE) fields (e.g., Fütterer et al 2011; Omer 2008). The efficiency of BHE fields, especially their long-term efficiency, is affected by the distance between neighboring BHEs (Hellström 1983) and by the overall BHE field geometry (Claesson and Eskilson 1988; de Palya et al 2012). The influence of these factors can significantly increase with groundwater flow. Due to its Michalski and Klitzsch Geotherm Energy (2019) 7:37 advective heat transport, groundwater flow influences the efficiency of a single BHE. Especially for dense BHE fields, the influence of groundwater flow cannot be underestimated (Diao et al 2004; Hecht-Mendez et al 2013). Recent research shows that considering groundwater flow for BHE field design and operation can lead to effective long-term behavior, i.e., to sustainable use of the BHE field (Riveraa et al 2015)
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