## 2 The Wustenrot Pilot Site
### Geothermal Monitoring  
As mentioned above in Section 2.1, the existing agrothermal collector field excelled the assumed values for the heat supply of the DHC network. As a reminder, the primary specific heat capacity of the collector field was estimated at between 40 and 70 kWh/m\({}^{2}\) per year with the assumption that 40 kWh/m\({}^{2}\) per year could be extracted from the soil a total size of around 6500 m\({}^{2}\) would have been necessary to cover the low-temperature DHC network's total heat demand of 255 MWh. Thus, that a size of only 4400 m\({}^{2}\) of collector area actually can serve the complete heat demand of the connected 23 consumers had to be investigated in detail. To map the thermal behavior of the agrothermal collector field and the effects of the temperature and humidity conditions of the tethered soil was the one main objectives of the geothermal monitoring planned for Wustenrot. To balance the needs of accuracy versus technical and financial aspects of the monitoring, only one collector tube was equipped with several thermal and humidity sensors, as shown in Figure 7.  
Three horizons (A-A, B-B, and C-C) of temperature and humidity measurements along one collector pipe (red line) were installed (Figure 7). The metering point horizon A-A is approximately 25 m far from the hydraulic interlocking shaft of the collector, displayed as a red rectangle in the lower \begin{table}
\begin{tabular}{c c c} \hline
**Property** & **Value** & **Unit** \\ \hline Monoethylene glycol concentration & 20 & \% \\ Freezing point & \(-8\) & \({}^{\circ}\)C \\ Reference temperature, Tref & 10 & \({}^{\circ}\)C \\ Density at Tref & 1028 & kg/m\({}^{3}\) \\ Specific heat at Tref & 3.97 & kJ/(kg K) \\ Dynamic viscosity at Tref & 2 & \(10^{-3}\) Pa s \\ \hline \end{tabular}
\end{table} 
Table 2: Physical properties of the monoethylene glycol—water mixture circulating in the district heating (DH) loop in Wüstenrot [23]. Figure 7: Sensor installation along one collector line. left side of the scheme. The distance among the horizons A-A, B-B and B-B, C-C is equal: 20 m each. All sensors are connected to the data acquisition in the interlocking station by wires. For fast and easy installation of the measurement setup, all sensors were preinstalled on slotted filter pipes with a total length of 4 m each. One measurement horizon consists of two of these tubes buried vertical in the soil (top edge 0.5 m under ground level, see Figure 8). The distance between the collector tube and its metering points is about 0.5 m on the left- and right-hand sides of the tube. For the mounting of the sensors, slotted filter pipes were chosen because of their hydraulic behavior. These tubes are hydraulically \"transparent\", which means that there is almost no influence of the groundwater flow nearby the installation. The supposed groundwater flow through the collector field has been logged by the three metering points GWA-A, GWB-B, and GWC-C. At all three metering points, the level of groundwater can be measured down to a depth of 4 meters under ground level. All the metering points shown in Figure 7 were calibrated by GPS measurements and precise leveling of the geographical height. With the three sensors GWA-A to GWC-C, the level of the groundwater can be measured within the collector field. With their exact height above sea level and the pressure differences caused by the varying water pillars over the sensors at the bottom of the installation, the gradient of the groundwater level can be calculated. Combined with the properties of the soil, the velocity and the direction of the groundwater can be derived. In a first try to explain the significant increase of the thermal capacity compared to the design assumptions of the collector field, a simplified investigation was done. A one-dimensional groundwater gradient between metering point GWB-B and GWC-C was calculated for each week in the 2017-2018 heating season. In Figure 9, these gradients are shown as blue lines.