Experimental investigation on the impact of the asymmetrical heat exchange and operation modes on the thermal performance of a bored energy pile in unsaturated soil: A case study in Brazil

Energy piles are a type of geothermal exchanger, used in ground source heat pump systems, to provide heating or cooling energy for thermal comfort purposes, due to their cost advantages compared to deep borehole heat exchangers. This type of technology is still not used in Brazil, where the hot dominating weather and space-cooling demand predominate, and soils in unsaturated condition cover a significant part of the land surface. In this case of unbalanced heat transfer to the ground, this system may cause the increase of pile and ground temperatures over time, reducing its efficiency. Additionally, in unsaturated soils the ground thermal conductivity is normally inferior compared to saturated soils, and for this reason the increase of pile and ground temperatures with time may be more significant. Moreover, the positive impact of a groundwater flow on the heat transfer performance does not occur. Therefore, the current work investigates the thermal performance of energy piles with three different heat exchanger pipe configurations, in a typical Brazilian unsaturated tropical soil, with the aim of providing fundamental guidance for future application of this sustainable technology for the challenging combination of predominant cooling demand and unsaturated soil condition. For this study, asymmetrical and spiral pipe configurations and different operation modes were evaluated by field experiments to verify if these variables can influence the energy pile performance. Thermal performance tests were carried out on bored energy piles installed at a site of unsaturated soil in São Carlos city, Brazil. The energy pile of asymmetrical W-pipe configuration under intermittent operation mode presented a better heat transfer performance, and a lower increase of the average pile temperature compared to the spiral configuration case. These aspects can be advantageous for energy pile groups and also for the heat transfer performance and thermo-mechanical behaviour of a single energy pile.