Thermal performance analysis of multiple borehole heat exchangers in the heat conduction and advection coupled layered soil

Safe, clean, and efficient geothermal energy has been widely popular in various countries, particularly for reducing carbon dioxide emissions and achieving the transition from fossil energy to renewable energy. A rational design of heat pump systems depends mainly on the accurate acquisition for the heat transfer of multiple Borehole Heat Exchangers (BHE) in complex soil. In the present study, a multiple-layered numerical model was established for three-dimensional multiple BHE considering the groundwater advection using Fluent software. The accuracy of the model was validated using a 48h thermal response test data. Two indexes were introduced to evaluate the performance, including the regional thermal efficiency (E) and Dynamic Performance Loss (DPL). Furthermore, an analysis was made to clarify the influence of the location and thickness of the advection layer on the heat transfer of multiple BHE in layered soil. The result indicated that there was a small difference of E between the layered and homogeneous models, when the advection velocity was 100 m/a, and the thickness of the advection layer was large. The heat transfer of multiple BHE in the layered soil was simplified as that in homogeneous soil under this condition. The E of multiple BHE was ignored, only to adjust the position and thickness of the soil layer with different thermal conductivities at a groundwater advection velocity of 100 m/a. It infers that there was a small effect of the variation in thermal conductivity in different depths of soil layers on the heat transfer of multiple BHE. Therefore, the backfill material with higher thermal conductivity should be balanced to use in the backfill layer of buried pipe in the multiple BHE, when the advection velocity of groundwater was 100 m/a. There was no influence of variable equivalent inlet velocity (0.115, 0.158, and 0.237 m/s) and inlet temperature (300, 310, and 320 K) on E the multiple BHE. The downstream DPL of the staggered arrangement multiple BHE with 3 m pipe spacing was only 1.27 percentage points higher than that of the aligned multiple BHE with 6 m pipe spacing, after 2 000 h of continuous operation under the groundwater advection velocity of 100 m/a. It was suggested that the borehole spacing of aligned and staggered arrangement should be 4 and 3 m, respectively, when advection velocity was 100m/a and the thickness of the advection layer was large. The average unit heat transfer rate of multiple BHE with 105 m advection layers increased by 24.4%, compared with that with 35 m advection layer. When the advection velocity was 100 m/a, the inlet temperature was 300 K, and the equivalent velocity in the pipe was 0.115 m/s, there was no effect of the advection layer location on the heat transfer of multiple BHE, whereas there was an obvious effect of the advection layer thickness on the heat transfer of multiple BHE. In addition, the E of multiple BHE was greater, while the DPL was lower, and the rate of stabilization was faster, as the advection velocity was larger. The findings can provide a sound reference for the rational design of multiple BHE in the layered soil with groundwater advection.