District-Scale Geothermal System Performance Evaluation using Thermodynamic and Environmental Analysis

Abstract

The focus of the work presented in this thesis aims to improve geothermal heat system efficiency by investigating subsurface thermal properties that influence system performance over the lifetime of the system, as well as quantify the environmental impacts associated with geothermal systems. This work is done through the development of a heat budget and life cycle assessment tool that can assist with more efficient geothermal system design and operation that best benefit the end-user. The heat budget combines long-term temperature measurements of a district-scale geothermal heat exchange system in Verona, Wisconsin and the thermal properties of the borefield subsurface to calculate the change in borefield heat storage over time. This allows for a heat budget, or understanding of the heat being transported into and out of the borefield, to be developed. Given the cooling-dominated heat load that the borefield is carrying, quantifying the amount of heat independently exiting the borefield allows for a more in-depth understanding of how “leaky” the heat reservoir is. The heat budget investigation in this thesis determined that the borefield behaves as a leaky reservoir of heat. Because of the cooling-dominated heat load combined with the cool climate in Wisconsin, having a natural means of heat escape supports borefield longevity and prevents a reduction in system efficiency due to borefield overheating. This understanding of borefield behavior is useful in making better-informed operational decisions, especially on a district-scale, as there is less speculation regarding where heat travels once placed in the borefield. These improvements in district-scale geothermal systems allow for data-based, sustainable growth in the geothermal sector by providing information that allows for optimizing borefield design for more efficient geothermal systems. This thesis also investigates the feasibility of implementing a deep direct-use geothermal heat recovery system by investigating the environmental impacts of the system through a Life Cycle Assessment (LCA). The LCA was performed using a spreadsheet tool that was simultaneously developed to provide further insight into the cradle-to-grave environmental impacts. This analysis offers insight into the environmental and economic costs associated with the system, and allows a user to determine if a deep-direct use geothermal system is a feasible option for their site. While geothermal systems are often considered sustainable energy sources, further investigation into the environmental performance of these systems reveal significant impacts associated with various components of DDU systems throughout the lifecycle of the system. The results of the assessment of the proposed system show significant environmental impacts associated with the acquisition of raw materials and electricity required to operate the system. Even with these sizable environmental costs, the proposed geothermal system has potential to offset the emissions associated with the traditional fuel source alternatives that are currently being utilized on the existing campus in approximately 10 years.