Ground-source heat pump systems provide low-cost, sustainable, long-term heating and cooling over a range from single residential homes to district-scale. As the scale of the geothermal system increases from a single ground heat exchange (GHX) boring for a home to several thousand GHXs for a district-scale facility, overheating the exchange field and depleting the geothermal resource becomes a concern. District-scale facilities generally cool their buildings more than heat them, resulting in more heat discharged to the exchange field than removed from it annually. In addition, because thousands of borings are often placed in a single exchange field, the temperature profiles of neighboring GHXs are superimposed, reducing their ability to transmit heat to the field. We are studying a district-scale exchange field in Verona, Wisconsin (USA), and evaluating how district-scale geothermal fields operate to improve their efficiencies. This field and heat pumps supply heating and cooling for a campus of approximately 10,000 employees. We collected energy usage and geologic data while monitoring temperature time series to determine the geothermal field's heating budget. That budget consists of (1) heat from the campus moved into and out of the field by the GHXs, (2) changes in heat stored in the field, and (3) heat flow into and out of the field into the surrounding rock and air. This budget allows us to understand how the field is behaving. If the field is a reservoir, then the heat flow from GHXs should equal the change in heat storage with little flow out of the field into the environment. If the field is a radiator, then the heat flow from the GHXs should equal the heat flow from the field into the surrounding rock and atmosphere with little change in the heat stored in the field. Results suggest that the field initially behaved as a reservoir while the temperature of the rock increased during the initial conditioning process. After this initial one- and one-half years, the field is shifting towards acting more like a radiator, likely due to a greater temperature difference between the field and the surrounding rock and air. The geometry of the field and BHXs and the imbalanced heating and cooling loads will require that the field be routinely reconditioned or cooled. It is receiving more heat than it can dissipate, as indicated by the increasing field temperatures, and will likely need to be routinely conditioned by water from surface water reservoirs to maintain cooler temperatures and efficiency over the long-term use of the field.

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