Performance analysis of various U-tube ground heat exchanger with concrete pile foundation between U-legs

Ground heat exchanger, a fundamental component of ground source heat pump facilitates building heating and cooling through geothermal energy utilization. This study numerically evaluates the thermal performance enhancement of vertical U-tube ground heat exchangers by introducing spiral, double, multiple inlet/outlet configurations. Three-dimensional simulations were conducted in ANSYS FLUENT for a 40 m concrete pile with polyethylene U-tubes, water as the heat transfer fluid and a fixed inlet temperature of 300 K. The effects of pipe positioning and flow rate variation were also assessed. Results demonstrate that geometric modifications significantly improve outlet temperature reduction and average heat transfer rates compared to the traditional. Among the configurations, adjacent dual-inlet merging into one outlet has achieved 27.2% increase in average heat transfer compared to traditional design. Though the double U-tube design provided the highest absolute heat transfer 36.22%, higher than single U tube but increased frictional resistance reduces its overall efficiency. Pressure drop consequences were evaluated to identify designs offering an optimal balance between heat transfer efficiency and hydraulic performance. The introduction of the integrated evaluation factor (η) enabled a balanced assessment which confirms adjacent positioning dual inlet configuration as the optimal configuration with enhanced heat transfer gains achieved at a manageable hydraulic cost. Doubling the flow rate from 2 L/min to 4 L/min improved heat transfer by 47.02%, but substantially raises pumping power demand. High-density polyethylene as tube materials delivers the highest heat transfer rate among high-density polyethylene, polyvinyl chloride and polyethylene due to enhanced thermal conductivity. These findings will further facilitate the potential of U tube ground heat exchangers to provide cost-effective and energy-efficient solutions for compact ground source heat pump installations.