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This study presents a centrifuge modeling approach to characterize the transient thermo-mechanical response of energy foundations during heating-cooling cycles in order to provide data for calibration and validation of soil-structure interaction models. This study focuses on the response of a scale-model energy foundation installed in an unsaturated silt layer with end-bearing boundary conditions. The foundation response was assessed using embedded strain gages and thermocouples. Other variables monitored include foundation head displacements, soil surface displacements, and changes in temperature and volumetric water content in the unsaturated silt at different depths and radial locations. Measurements during the initial heating process indicate that the thermal axial stress is greater near the toe of the foundation due to the restraint associated with mobilization of side shear resistance along the length of the foundation. The thermal axial strains were close to the free-expansion thermal strain near the soil surface and decreased with depth. The thermal axial displacements calculated by integrating the thermal axial strains correspond well with the independently-measured head displacements. The mobilized side stresses calculated from the thermal axial stresses increased with height and were consistent with the shear strength of unsaturated silt. During successive heating-cooling cycles, slight decreases in upward thermal head displacement were observed due to changes in stiffness of the unsaturated soil due to thermally-induced water flow away from the foundation and potential down-drag effects. However, little change in the thermal axial stress was observed during the heating-cooling cycles.

Eight full-scale energy foundations were constructed for a new building at the U.S. Air Force Academy (USAFA). The foundations are being used to demonstrate this technology to the United States Department of Defense, and have several experimental features in order to study of their thermal-mechanical behavior. Three of the foundations are instrumented with strain gages and thermistors, and their thermo-mechanical response during a heating and cooling test were evaluated. For a temperature increase of 18°C, the maximum thermal axial stress ranged from 4.0 to 5.1 MPa, which is approximately 25% of the compressive strength of concrete (estimated at 21 MPa) and the maximum upward displacement ranged from 1.4 to 1.7 mm, which should not cause angular distortions sufficient enough to cause structural or aesthetic damage of the building. The end restraint provided by the building was observed to change depending on the location of the foundation. The heat flux per meter was measured by evaluating the temperatures and flow rates of a heat exchanger fluid entering and exiting the foundations. The heat flux values were consistent with those in the literature, and the foundation with the 3 continuous heat exchanger loops was found to have the greatest heat flux per meter. The transient thermal conductivity of the subsurface measured using the temperatures of the subsurface surrounding the foundation ranged from 2.0 to 2.3 W/mK, which is consistent with results from thermal response tests on energy foundations reported in the literature.

Borehole thermal energy storage (BTES) in soils combined with solar thermal energy harvesting is a renewable energy system for the heating of buildings. The first community-scale BTES system in North America was installed in 2007 at the Drake Landing Solar Community (DLSC) in Okotoks, AB, Canada, and has since supplied >90% of the thermal energy for heating 52 homes. A challenge facing BTES system technology is the relatively low efficiency of heat extraction. To better understand the fluid flow and heat transport processes in soils and to improve BTES efficiency of heat extraction for future applications, a three-dimensional transient coupled fluid flow and heat transfer model was established using TOUGH2. Measured timedependent injection temperatures and fluid circulation rates at DLSC were used as model inputs. The simulations were calibrated using measured soil temperature time series. The simulated and measured temperatures agreed well with a subsurface having an intrinsic permeability of 1.5 ´ 10 −14 m 2 , thermal conductivity of 2.0 W m −1 °C −1 , and a volumetric heat capacity of 2.3 MJ m −3 °C −1 . The calibrated model served as the basis for a sensitivity analysis of soil thermal and hydrological parameters on BTES system heat extraction efficiency. Sensitivity analysis results suggest that: (i) BTES heat extraction efficiency increases with decreasing soil thermal conductivity; (ii) BTES efficiency decreases with background groundwater flow; (iii) BTES heat extraction efficiency decreases with convective heat losses associated with high soil permeability values; and (iv) unsaturated soils show higher overall heat extraction efficiency due to convection onset at higher intrinsic permeability values.Abbreviations: BTES, borehole thermal energy storage; DLSC, Drake Landing Solar Community.Growing concerns about greenhouse gas emissions and fossil fuel consumption have motivated the increased development of renewable energy systems including solar thermal energy harvesting technologies for the heating and cooling of buildings. In recent decades, borehole thermal energy storage (BTES) systems with heat derived from solar technology are rapidly gaining attention and use worldwide (Claesson and Hellstrom 1981;Dalenbäck and Jilar, 1985;Nordell and Hellström, 2000;Sanner and Knoblich, 1999;Sanner et al., 2003;Morofsky, 2007;Sibbitt et al., 2007Sibbitt et al., , 2011Sibbitt et al., , 2012Wang et al., 2010;Dehkordi and Schincariol, 2014a;Acuna and Palm, 2013;Başer and McCartney, 2015). In BTES systems, a series of U-tube pipes placed in closely spaced (1.5-2.5 m) vertical boreholes are connected to form a closed-loop heat exchanger (Fig. 1). Heat exchange is achieved by circulation of a heat carrier fluid through the closed-loop U-tube pipes. Many BTES systems store heat collected from solar thermal panels in the summer months until it can be extracted for use during the winter months. These thermal energy storage systems present a potentially economical and environmentally sustainable alternative to traditional...

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