Study on the thermal performance of several types of energy pile ground heat exchangers: U-shaped, W-shaped and spiral-shaped

Energy pile ground heat exchangers are an attractive and cost-effective solution for ground-coupled heat pump systems to partly take the place of borehole ground heat exchangers. The heat exchange tubes, arranged as U-shaped, W-shaped and spiral-shaped, could be buried in piles and coupled with a heat pump. This paper focuses on a comparative analysis of the transient thermal processes as occurring in the different types of ground heat exchangers in piles. Firstly, a transient 3D heat transfer model for the interactions between the ground and the circulating fluid was established, and solved by using the finite element method. Secondly, the changes in temperature and the thermal resistances as well as the heat transfer on the surface of energy piles were analyzed to study on the thermal performance of the different types of PGHEs. The spiral-shaped PGHE has been estimated to have the better thermal performance than the other two heat exchangers in terms of long-term and short-term thermal loads. Finally, as a practical case study, an in-situ temperature response test was carried out on a spiral-shaped PGHE and a three-year thermal process within the spiral-shaped energy pile carrying seasonal thermal loads was simulated to develop a scheme design of the PGHE system that has been used for an office building.