Analyses on soil temperature responses to intermittent heat rejection from BHEs in soils with groundwater advection

• A new analytical model was presented for a multiple BHEs with groundwater advection.
• The model was validated by experimental data and dynamic numerical simulation.
• Soil temperature responses to continuous or intermittent heat rejection were studied.
• The influences of various factors on soil temperature variations were analyzed.

Multiple boreholes heat exchangers (BHEs) are widely used in ground source heat pump systems (GSHPS) and are operated in intermittent mode in general. The soil type and groundwater advection that possibly exist may affect their heat transfer performances. Thus, based on the moving finite line heat source model (MFLS) and the superposition principle, an analytical solution to soil temperature response to heat rejection from multiple BHEs was proposed. It is programmed by using MATLAB and the calculation results were validated by 3D transient numerical simulation results. Then the influence of groundwater advection velocity and soil type on the soil temperature responses to heat rejection from a single BHE in intermittent and continuous operational modes was compared and discussed. The dynamic temperature responses of the positions monitored for the multiple BHEs can be calculated by the new analytical model. Results indicate that thermal accumulation phenomenon in the soil can be mitigated by intermittence and the amplitude of soil temperature recovery tends to be large. Not only the advection velocity of groundwater, but also the soil properties play an important part in the soil temperature recovery characteristics when BHEs reject heat intermittently. In addition, soil temperature variation distributions around two kinds of multiple BHEs layouts were also illustrated. It is suggested that the consideration of both groundwater advection and boreholes layout be important in designing the BHEs configurations.