A simplified model of energy pile for ground-source heat pump systems

• A simplified model was developed for a cylindrical energy pile.
• The material surrounding the energy pile affected the fluid temperature more.
• The thermal resistance depended more on the material inside the energy pile.
• All pipes should be equalled-spaced and connected consecutively for optimal design.

A simplified three-dimensional finite difference model for a single (cylindrical energy pile) CEP was developed. The present model was first verified with an analytical solution based on the special case that the material inside and outside the CEP had the same thermal properties. For the case of different thermal properties in the various ground regions, the calculated thermal resistance of the CEP (Rep) under a constant applied load was compared with the one determined with the assumption of a steady state inside the CEP. In both situations, the differences were found to be small. The effect of the thermal properties of various regions on the performance of the CEP was investigated. It was found the material underneath the CEP had negligible impact on the CEP performance. The soil material outside the CEP had stronger effect on the fluid temperature leaving the CEP (Tf, out) while Rep depended more on the material inside the CEP. Meanwhile, Rep reached a higher value at the beginning of each load cycle. The influence became significant for intermittent loading of the CEP. Furthermore, it was found that for optimal design involving a single pipe circuit inside the CEP, all pipes should be equally-spaced and connected consecutively.