Thermal Response Test of Geothermal Boreholes Using Distributed Fiber Optic Sensing

Shallow geothermal is a renewable green energy that can provide heating and cooling for buildings in a safe, non-emitting, and affordable way, thus reducing the dependence on natural gas. The ground source heat pump (GSHP), also known as a geothermal heat pump, is the most efficient technology to utilize shallow geothermal by transferring heat between the shallow ground and buildings. It primarily consists of borehole heat exchangers with a U-pipe installed inside a borehole where water is continuously circulated in a closed (supply-return) loop using a heat pump. A thermal response test (TRT) is performed to assess the performance of a borehole heat exchanger. A constant input heat (Q) is applied to the circulating water, and its temperature on the supply and return side of the closed loop is continuously measured. Conventional instrumentation (i.e., the thermocouples installed at the end of the supply and return pipes) and methods for analyzing TRT results (e.g., using the line source method) evaluate only the average thermal properties without considering any variations along the depth. Using distributed fiber optic sensors (DFOS) in TRT can provide continuous temperature measurement throughout the depth with a good spatial resolution. The temperature profile data can provide a complete understanding of the thermal mechanism, eventually leading to a better and more efficient design. This paper describes a field-scale installation, testing, and sensing of three 600-feet geothermal boreholes. DFOS were installed inside and outside the U-pipe to measure the circulating water and the nearby soil temperature. A Brillouin-based interrogator was used to monitor the temperature changes during TRT with an accuracy of 1°C and a spatial resolution of 1 m. The temperature profile data were analyzed to understand the thermal response and estimate the variation of thermal properties with depth. The study also shows that DFOS can be effectively deployed in a borehole heat exchanger to provide real-time and long-term monitoring capabilities resulting in a better-informed and more efficient design.