Energy Efficiency and Life Cycle Assessment of a District-Scale Geothermal Exchange Field

Abstract

District-scale Geothermal Heat Exchange (GHX) systems are increasingly portrayed to have significant environmental and economic benefits. However, independent and comprehensive life cycle analysis (LCA) and sustainable energy science are typically not implemented to quantify the full and accurate environmental benefits. A GHX system uses steady subsurface temperature for space conditioning and domestic hot water by circulating mostly water to exchange heat with the subsurface. This clean, quiet, and sustainable heat exchange mechanism is advertised to have convincing cost efficiency in energy consumption. However, long-term feasibility and energy balance assessments of GHX systems are lacking, especially for district-scale systems. To begin filling this gap, we evaluate the environmental 'costs' and embodied energy (e.g., energy payback time) for constructing a district-scale GHX field with 2,596 152-m-deep GHX borings located in the upper Midwest of the US. As this field was fully instrumented during construction and monitored throughout operations, we can calculate that 645.9 TJ of thermal energy has been exchanged since this field came online from early 2015 through September 2022. The energy exchanged varies from a low of 62.4 TJ in 2015 to a high of 94.4 TJ in 2019. With an average household in Wisconsin consuming approximately 8,500 kW h of electricity each year, the yearly average of the district-scale field's exchange of 81 TJ is equivalent to the annual electrical use of 2,650 Wisconsin households. LCA methodology was then used to perform a quantitative, comparative analysis and rating of the material procurement and construction of this district-scale GHX field, including the embodied energy required for material production, manufacturing, transportation, and construction. This embodied energy for field construction equates to 100 TJ, which is "paid back" in 14.5 months by the thermal energy exchanged in the field. The results of the LCA show that top contributors to embodied energy consumption in borefield procurement and construction are related to borehole drilling and well completion; thus, GHX systems that efficiently minimize the number of GHX wells and cumulative vertical length are not only more economically viable, but also more energy sustainable.