Numerical modelling of ground loop configurations for direct thermal applications

The rigorous design of direct geothermal heat pump systems that use concrete piles or boreholes as heat exchangers to extract or reject heat with the ground needs a model for the thermal process in the ground, the ground heat exchanger (GHE) and the carrier fluid circulating within. Thermal interference between pipes in the GHE is an important factor which may significantly affect system efficiency. Different pipe configurations including single U-Pipe, double U-Pipe and double cross U-Pipe, are modelled using finite element methods to investigate the thermal interference that occurs between the pipes within the GHEs. In this work, water is the carrier fluid circulating through the pipes and exchanging heat with the ground. Water inlet temperatures and ground far-field temperatures were chosen as being typical for Melbourne conditions. U-Pipes are located vertically in concrete piles or grouted boreholes surrounded by the ground. The efficiency of the GHEs is investigated in heating mode. The results presented confirm the importance of geometry in design and the significant variations in performance that can be obtained using different pipe configurations.