Thermal Response of Energy Screw Piles Connected in Series

Energy piles are a consolidated underground heat exchanger alternative to traditional boreholes in ground source heat pump (GSHP) systems. Previous works focused on assessing the differences between piles and boreholes, but few assessed small piles in operational conditions. Moreover, most of these studies centered around cylindrical concrete piles, overlooking short screw piles. Using in-situ testing, established analytical methods, and advanced three dimensional (3D) finite element model simulations, this work assesses three thermal response tests (TRT) executed in different energy pile structures, one being a unique group of eight short energy screw piles connected in series, located in the same site in Melbourne, Australia. Detailed numerical analysis provided reliable soil and structure thermal parameter predictions and detailed computations allowed the study of thermal effects for the energy screw piles steel components. The results show limited impact of the steel components on effective thermal conductivity, but a reduction in thermal resistivity that may provide a speedier thermal exchange in short term GSHP operation. In addition, the more traditional TRT rigs and analytical interpretation provided reasonable results for the pile group in series, and show a similar performance to a borehole heat exchanger of similar pipe length; however, the short piles engage only the upper soil layers, with potentially lower thermal conductivity. TRT in single short screw piles require careful consideration, because common rigs may be unable to cater for the required low fluid flow rates and heating power. Thus, for the cases assessed herein, the pile group TRT proved to be more reliable than individual pile testing, due to their short length.