Distributed temperature sensing cable for geothermal monitoring, Italy

Smartec SA in cooperation with Tecnopenta s.r.l. provided a customized fiber optic monitoring system for a geothermal project in Italy.

The University of Bologna (dep. Fisica Tecnica) owner of the plant, built in the framework of the HEGOS European research project (new heat pumps for Harvesting EnerGeticO in Smart buildings) contacted us to measure the temperature inside a 26mm tube. The innovative plant adopts a dual-source (air and ground) heat pump designed to exchange heat with the ground or with external air. The use of external air, as an auxiliary heat source, enables to obtain a more stable energy performance even in the presence of an undersized (30% to 50%) borehole exchanger and/or by unbalanced heating and cooling loads. Using a control unit, it is possible to continuously select the best heat source and activate the corresponding heat exchanger (air or ground).

Figure 1, Layout of the Double-Source Heat Pump prototype. Courtesy of University of Bologna.

The distributed optical fiber sensors are used to monitor the temperature along the two ground boreholes. The following top view shows the position of the probes (S1 to S4), the connecting cables (purple lines) and the data-logger’s location.

Figure 2, The f.o. cables are installed in boreholes S3 (100m depth) and S2 (60m depth)

Smartec and Tecnopenta provided four distributed multi-mode fiber optic cables for the 2x100m and 2x60m polyethylene probes. So two cables have been installed respectively in the inlet and return flow of one 60m loop circuit (U-tube), while the other two have been placed, by the same logic, in one of the two 100m circuits. The cables are connected to a DiTemp Light Readout Unit. The measured temperature values have an accuracy of 0.1°C and a spatial resolution of 2m, so along the 320m of installed cable, the system provides 160 values per acquisition (every minute or less). The user can easily adjust the acquisition frequency and the spatial resolution.

Figure 3, 3.8mm diameter of fiber optic cable exiting through a special protective cap

In each pit, we installed a hermetic splice enclosure to create a physical connection between the fibers of the two temperature-sensing cables and the fibers of the interconnecting cable, see Figure 4.

Figure 4, splice enclosure: 2 IN 1 OUT

The purpose of the installation is twofold. On one hand, it continuously measures the temperature of the water in the probes over time, in both directions (inlet and return) during plant operation. This information is used to validate the design calculations. On the other hand, when the plant is stopped the measurements are used to calculate the temperature at different depths of the surrounding soil. That allows checking if the ground temperature drifts year by year, compromising the performance of the system both in heating and cooling.

Literature:

1. Grossi, M. Dongellini, A. Piazzi, G.L. Morini, Dynamic modeling and energy performance analysis of an innovative dual-source heat pump system, Applied Thermal Engineering, Volume 142, 2018, page. 745-759