Improving LEED-certified building loads on borehole heat exchangers by coupling subsurface variables

To evaluate the performance of a ground source heat pump (GSHP) system, it is necessary to assess the subsurface thermal regimen around the borehole heat exchanger (BHE). This paper presented the preliminary results of an analysis of a newly constructed GSHP system for the Campus Instructional Facility at the University of Illinois at Urbana-Champaign, which will be a Platinum-level Leadership in Energy and Environmental Design (LEED)-certified building. Energy modeling of the entire building was performed in DesignBuilder software by considering all the high-efficiency thermal-insulated building subsystems. A numerical model for a single BHE constructed in a heterogeneous interlayered geology was first validated with data from a distributed thermal response test. The numerical model was then used to simulate the ground thermal response over both the short term (1 to 3 months) and long term (1 year). The geological and hydrological impacts and the benefits of the LEED-certified building load were evaluated. The results showed that under the fixed length of the BHE, the modest heating and cooling demand of the Campus Instructional Facility would result in reduced temperature fluctuations in the U-bend pipe at the outflow, by a maximum 8.91 °C, and would raise the coefficient of performance of the heat pump by 26.8 % when operating in extreme weather conditions and 7.4 % during July. The presence of an external heat source underground—a segment of the campus steam tunnel—raised the local ground temperature by 1.99 °C, providing slightly improvement in the performance of the BHE during the heating season and a similar drop in performance during the cooling season. This study validated the insignificance of the local groundwater velocity (2.07 × 10^-7 m/s) on the thermal behavior of the BHE. The proposed model setup and simplifications could be applied to a large-scale borefield analysis at any site within a similar geological context. The proven enhanced efficiency of the BHE may also facilitate future implementation of GSHPs in green buildings.