A transient quasi-3D entire time scale line source model for the fluid and ground temperature prediction of vertical ground heat exchangers (GHEs)

Abstract

The energy performance of Ground-Coupled Heat Pump systems (GCHPs) depends heavily on the fluid temperature over the entire time scale inside the ground heat exchangers (GHEs) and the ground temperature profile outside the borehole. These temperatures define not only the coefficient of performance (COP) of GCHPs but also the ground receptivity of the energy demand of buildings. Although GCHPs have been studied for many years, it is still a great challenge to develop a model that can accurately predict both short-term and long-term responses due to the complex borehole configuration and the thermal capacity of grout. A new transient quasi-3D entire time scale line source model is proposed in this paper, which introduces the concept of transient borehole thermal resistance and considers the heat flux profile along the U-pipe as a variable. The proposed model is firstly compared with several existent models, including several traditional line source models and a full scale response model, using the data collected from a reported Sandbox experiment. The comparison study shows that the proposed model is able to predict the temperature with a relative error less than 5%. Then, the outside ground temperature profile that defines the borehole distance is analyzed and compared with the Sandbox experiment result, which shows that the proposed model leads to a maximum relative error being less than 3.85%. Finally, the impact of the heat flux profile along the U-pipe on the ground temperature profile prediction is investigated, which shows that when the heat flux profile along the U-pipe is considered as a variable, the determination of borehole distance will be much more accurate. Therefore, the transient quasi-3D entire time scale line source model is an effective method for the fluid and ground temperature prediction and may offer the theoretical basis for the system control and the borehole distance determination.