Field Test and Thermal Performance Comparison of a Novel Underground Thermal Battery with a Single U-tube Borehole Heat Exchanger for Geothermal Heat Pump Application

Abstract

Geothermal Heat Pumps (GHP) are a proven technology to efficiently provide space heating and cooling by utilizing the thermally stable subsurface of the ground as a heat sink or heat source. However, the high drilling costs of conventional borehole heat exchangers (BHEs) associated with GHP systems and subsurface heterogeneity have been two barriers hindering widespread adoption of GHPs. In this paper, we present results from a field test of an Underground Thermal Battery (UTB) as a potential alternative to a conventional BHE. To the UTB performance, a side-by-side field test was conducted to compare the UTB with a conventional BHE installed at the same site. The UTB was installed in a 7.01-meter-deep borehole having a diameter of 0.9 meter, whereas the conventional BHE (with single U-tube loop) is in a borehole with 36.6-meter depth and 0.15-meter diameter. Both are integrated into a GHP system to meet the thermal demands of an office space at the Illinois Energy Farm on the University of Illinois Urbana-Champaign campus. The system’s performance is compared in terms of heat transfer rate, outlet temperatures, and contributions to the overall thermal power of the ground heat exchanger under varying thermal demands and in different heat pump operation modes: heating only, cooling only, and hybrid (alternative, diurnal heating and cooling). Results to date indicate that the UTB delivers a more consistent outlet temperature during a period of varying thermal demands. Its performance is better than the conventional BHE during short-term with higher thermal demands, while the conventional BHE performs better under continuous, lower thermal demands.