Thermal design method for multiple precast energy piles

•3D numerical analyses are used to derive the semi-empirical pile g-functions.•Fluid temperatures obtained in multiple pile patterns are analysed.•Semi-empirical g-functions give fluid temperatures close to the 3D modelled.•Semi-empirical g-functions are faster and allow easier analysis. This paper investigates the applicability of numerical and semi-empirical heat flow models for calculating average fluid temperatures in groups of quadratic, precast pile heat exchangers. A 3D finite element model (FEM), verified with experimental data, is extended to account for multiple pile heat exchangers. Semi-empirical dimensionless temperature g-functions for multiple piles are developed by utilising single pile 3D FEM heat transport simulations with temporal and spatial superposition techniques to account for the thermal interaction between piles. The multiple pile g-functions, yet less accurate for short times, yield fluid temperatures similar to those obtained with full 3D modelling (with 7% of largest deviation), at minimal computational cost.