This study investigates the long-term performance of a cooling-dominated, district-scale geothermal heat exchange (GHX) field in the Midwest of the United States. The district-scale system includes multiple borefields, heat sink ponds, and central energy plants that provide heating and cooling to a 13,000-plus-employee campus. We work to isolate and analyze the performance of the largest borefield (2596 152-m deep exchange wells in 0.1-km2 area). Conventionally, the coefficient of performance (COP) is the metric used to analyze GHX system performance. However, district-scale GHX systems can be too complex to simply characterize using COP alone; in part, due to the centralized heat exchange between building systems and external exchanges between multiple earth-exchange systems. In this study, the largest borefield (out of four) has been isolated, and we discretize the performance of this borefield using parameters including field power, energy exchanged with the ground, change in temperature of exchanger fluid, fluid flow rate, distributed ground temperature data, and others. By analyzing combinations of these parameters, relationships and trends are compiled to understand the long-term response of the field. These relationships can then be used to inform the standard-of-practice for field management. After an initial conditioning of the field, the average field temperature increased from approximately 14 C to 17 C over the past seven years. This increase has led to a decrease in field efficiency for heat dissipation and a corresponding enhancement in efficiency for heat extraction. Both the power exchange in the field and temperature difference between supply and return fluid have experienced decreases over time under heat dissipation conditions. The reduction in cooling efficiency is of concern as the campus is cooling-dominated. Many factors influence the performance of GHX systems; therefore, we must understand how long-term temperature changes in the ground and operation of the centralized energy plants affect the performance and energy efficiency of a district-scale GHX network, such as the one evaluated in this study.

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