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Learnings from Borefield Temperature Monitoring in an Operating Geothermal Energy Network Using Distributed Fiber Optic Sensing
Jiahui YANG, Eric JUMA, Eric BOSWORTH, Isabel Varela GUTIERREZ, Kecheng CHEN, Nikki BRUNO, Kenichi SOGA
[UC Berkeley, USA]
The first utility-led retrofit geothermal energy network (GEN) system in the US was built in Framingham, MA by Eversource in 2024. The system was designed to provide heating and cooling energy to 36 nearby buildings through a single ambient loop connecting three geothermal borefields. Although GEN systems are estimated to have a lifetime of 50 years, these systems may suffer from efficiency reductions due to subsurface thermal drifts after multiple years of operation, especially when the heating and cooling loads are not balanced. To ensure that the GEN system is operating in an optimal and sustainable manner, a ground temperature monitoring system based on distributed fiber optic sensing (DFOS) technique was designed and installed at the Framingham GEN system. Fiber optic cables were inserted in 14 (out of 90) boreholes among the three borefields to measure both the temperature distribution along the depth and its variation with time. This study focuses on ground temperature data collected from the Normandy Lot borefield of the GEN system during its first 16 months of operation. Measured borehole temperatures ranged from a summer maximum of 21 °C to a winter minimum of 9 °C, relative to an average initial ground temperature of 12 °C, indicating a substantially greater net heat injection during the summer period. Difference in borehole temperature responses were observed and can be attributed to multiple factors, including borehole position, pipe flow rate, drilling deviation, and grouting quality. Thermal interaction occurred within the top 50 meters of the subsurface and turns significant when temperature reaches extrema during the ground heating and cooling phases. Below 50 meters, thermal interactions were found negligible. As system continues to operate under imbalanced heating and cooling loads, the thermal interaction zone is expected to expend to greater depths, highlighting the importance of long-term subsurface temperature monitoring for maintaining GEN system performance and sustainability. Eventually, the recorded ground temperature data will be used to improve understanding of subsurface thermal response to building energy loads, validate the GEN system design, and optimize the system operation strategy.
Topic: Field Studies