| Understanding the heat transfer process between ground loop exchanger and surrounding ground that Ground Coupled Heat Pump (GCHP) systems use and the sustainable utilization of a specific type of ground formation are crucial to any efforts made to optimize the efficiency of heat extraction and/or injection from and/to the ground. Historically, studies of GCHPs have focused on the mechanical aspects of the system, but few have systematically accounted for the geological conditions with respect to questions of thermal sustainability and efficiency. The ground temperature can decrease or increase depending on the respective lengths and energy balance of the heating and cooling cycles. Over a period of many years, this fluctuation can cause thermal imbalances in the ground, such as overheating due to a high amount of heat injected into the ground during a period when the system is being used to cool the interior of a building or home. Such an imbalance is especially likely to develop when the system relies on a large number of boreholes to cool sizeable commercial buildings. In this study, a series of numerical models have been developed to evaluate the effects of a GCHP system on the thermal conditions of the ground into which the system's borehole(s) have been drilled. The long-term ground thermal response is simulated for the design of district-scale borefield and the sustainability of the GCHP's performance over time. To analyze the sustainability of a borefield under operation in Wisconsin, 2D, non-linear, district-scale borefield was simulated using pre-designed heating and cooling loads (cooling-dominate). This model considers hydrogeological conditions (e.g., groundwater flow, porosity) to assess the advective heat transfer process, as well as the heat conduction occurring in their transmissive aquifer. This model was coupled with enhanced 2D model to simulate potential future mitigation strategies to resolve overheating problems, which often occur in cooling-dominant commercial operations. Finally, to predict the CO2 emissions from the GCHP systems, a comprehensive "cradle-to-grave" life cycle analysis (LCA) was conducted. |
