Hybrid analytical model for composite heat transfer in a spiral pile ground heat exchanger

A spiral pile ground heat exchanger (PGHE) is a recently developed type of ground heat exchanger (GHE) used in ground-coupled heat pump system. Existing analytical models are not applicable to a spiral PGHE when the pile has thermal properties that are different from those of the soil. A novel hybrid analytical model to represent this situation was developed in the present study. The proposed model utilizes a decomposition algorithm to solve the problem of a coil line source embedded in a composite cylinder. The decomposition algorithm uses a composite cylindrical surface source solution to deal with the composite heat conduction along the radius and a coil line source solution to deal with the heat conduction between tube loops in depth direction. An assumption of homogeneity enables the solution of the hybrid model with only a slight error. The error of the hybrid model was investigated with respect to thermal properties and geometries of a spiral PGHE and compared with those of existing models. The error under normal cases was found to be acceptable for engineering applications. The proposed hybrid model is superior to existing analytical models, enabling simultaneous and reasonably accurate evaluations of the effects of the thermal properties of the pile and the coil pitch.