Development of spiral heat source model for novel pile ground heat exchangers

The extensive application of ground coupled heat pump system (GCHP) is restricted by the installation cost of conventional borehole ground heat exchangers (GHE), which is not only expensive but also requires additional ground area. The idea of combining the GHE with building foundation piles, i.e., the "energy pile," has arisen in recent years. The novel pile GHEs consisting of vertical spiral coils buried in the piles of a building, are proposed for their flexibility and convenience for applications. Evolved from existing models, a new spiral source model and its analytical solutions are established in this article for thermal analysis and design of the pile GHEs. The temperature field of the pile and its surrounding soil is simulated; in addition, the temperature responses of the coil pipe wall and the circulating water entering/effusing the pile GHE to the short time-step heat transfer loads are deduced based on the new spiral source model. The study indicates that the heat transfer features of the novel pile GHE can be described adequately by the proposed new spiral heat source model, which provides an appropriate tool for thermal analysis and design of the novel pile GHE or other potential similar engineering problems.