LONG-TERM MONITORING OF CAMPUS-SCALE GEOTHERMAL HEAT PUMP SYSTEM USING DISTRIBUTED TEMPERATURE SENSING

Geothermal heat pumps are an increasingly common sustainability strategy to accelerate the clean energy transition in response to climate change. However, the relationship between heat pumps and the encompassing geologic and hydrogeologic systems is often difficult to predict during the project design phase. Two major unanswered questions are: 1) What is the impact of subsurface heterogeneity (rock type, fractures, and groundwater) on system efficiency? 2) What is the risk of thermal pollution in groundwater? Long-term monitoring of subsurface temperature across space and time is one strategy to address these questions.

Carleton College (Northfield, MN) installed three geothermal borefields from 2016 to 2018. Two fields have vertical bores that extend 260 meters through the local Paleozoic stratigraphy. Caliper logs show that some wells intersect large fractures in the Ordovician Prairie du Chien group. Although these features are not traceable between wells, these types of bedding-parallel fractures are known to facilitate groundwater flow in southeast Minnesota. The two borefields were grouted differently, creating a natural experiment: one has thermal grout in each hole while the other uses pea gravel in the Prairie du Chien interval. In the pea-gravel field, we installed high-resolution fiber-optic distributed temperature sensing cables in five boreholes. These sensors provide insight into the daily, seasonal, and yearly temperature patterns in the field.

Groundwater flow is detectable in certain stratigraphic intervals based on temperature anomalies. These patterns persist across all timescales, indicating the importance of bedding-parallel groundwater flow. Data on total energy exchange for each borefield suggests that the pea-gravel field is more efficient than the fully grouted field. However, it is not yet clear whether the subsurface reservoir acts like a battery (storing thermal energy in summer to be released in winter) or like a sink (absorbing and transmitting away thermal energy so that the temperature oscillates around a constant mean). Both the influence of groundwater and the long-term response of the subsurface have significant implications for the design of new geothermal heat pump systems. Including a monitoring plan should therefore be a high priority for new installations.