Cross-sectional thermo-mechanical responses of energy piles

Abstract

Despite the widespread research on energy piles, there remain critical knowledge gaps in the cross-sectional thermal responses of concrete energy piles. This paper implements a unique research approach by developing and validating a numerical model with cross-sectional temperatures and strains measured in a field-scale energy pile (diameter = 0.6 m and length = 10 m), strengthening the reliability of modelling for energy piles. The numerical model was further used to investigate the influences of inlet fluid temperature, soil thermal conductivity, soil elastic modulus, soil thermal expansion coefficient, and the presence of a nearby energy pile at a centre-to-centre distance of 3.5 m on the cross-sectional thermal responses of an energy pile. These investigations demonstrate the practical significance of the above parameters on the cross-sectional thermal responses of energy piles. The results show that the temperature and stresses were largest at the centre of the pile and reduced with increasing radial distance to the pile's edge, with differences up to 4°C and 2.2 MPa, respectively, between the centre and the edge. A comparison of the cross-sectional results with existing stress estimation methods,in the cross-section of the piles, commonly based on average cross-sectional temperature and temperature measured at a single spot, reveal that existing methods lead to an overdesign of 2 MPa. Therefore, the actual temperature and stress variations in the planar cross-section of energy piles should be accounted for in the design of energy piles.

Thermal resistance analysis of an energy pile and adjacent soil using radial temperature gradients

“…The pile was heated for 18 days, followed by 50 days of natural temperature recovery. The fluid and soil temperatures in this heating test were reported in previous studies conducted at the site (Faizal et al 2019a;Moradshahi et al 2021a). However, the pipe and interface temperatures and the thermal resistance of the pile and the soil were not reported in any of the previous studies conducted at this site.…”

“…The energy pile had a diameter of 0.6 m and a length of 10 m. The concrete used had a water to cement ratio of 0.42 and consisted of 7 mm aggregated cement, slag, fly ash and water. The thermal conductivity of the concrete was numerically back-analysed in previous studies conducted on this site and had a value of 1.5 W/mK (Moradshahi et al 2021a;2021b). The thermal conductivities of the soil from numerical back-analysis were 1.1, 1.7, 2, and 2.3 W/mK for depths of 0 -0.5 m, 0.5 -3.5 m, 3.5 -6 m, and 6 -12.5 m, respectively (Moradshahi et al 2021a;2021b).…”

“…The thermal conductivity of the concrete was numerically back-analysed in previous studies conducted on this site and had a value of 1.5 W/mK (Moradshahi et al 2021a;2021b). The thermal conductivities of the soil from numerical back-analysis were 1.1, 1.7, 2, and 2.3 W/mK for depths of 0 -0.5 m, 0.5 -3.5 m, 3.5 -6 m, and 6 -12.5 m, respectively (Moradshahi et al 2021a;2021b). The setup was heavily instrumented with vibrating wire strain gauges and thermocouples described in detail in Faizal et al (2019 a, b).…”

“…Heat exchanger pipes are commonly formed into Uloops and attached to the reinforcement cages of the piles, although spiral configurations have also been used (Brandl 2006;Zarrella et al 2013;Go et al 2014;Wu et al 2021). During GSHP operation, transient thermal gradients develop in the pile cross-section and the surrounding soil due to the thermal resistances of the pile material (often concrete) and the soil, respectively (Bourne-Webb et al 2016;Faizal et al 2016a;Ozudogru 2016a, 2016b;Caulk et al 2016;Han and Yu 2020;Moradshahi et al 2021a). The thermal resistances of energy piles and the surrounding soils are necessary parameters required in the thermal design and analysis of energy pile systems.…”

“…Several analytical and numerical modelling studies have characterized the internal heat transfer and thermal resistance of energy piles (e.g., Bozis et al 2011;Li and Lai 2012;Powrie 2013, 2014;Lee and Lam 2013;Zarrella et al 2013Zarrella et al , 2017Hu et al 2014;Ozudogru 2016a, 2016b;Caulk et al 2016;Park et al 2018;Maragna et al 2019;Han and Yu 2020;Moradshahi et al 2021a) but omitted transient internal temperature gradient measurements between the heat exchanger pipes and pile-soil interface typical of insitu conditions. The internal thermal resistances of borehole heat exchangers have been thoroughly studied (e.g., De Carli et al 2010;Lamarche et al 2010;Bauer et al 2011;Koenig 2015;Javed and Spitler 2017;Cui et al 2018) but the lessons learned cannot be applied to energy piles as different thermal behaviour of piles is expected due to differences in dimensions and number of heat exchanger pipes.…”