Investigation of the horizontally-butted borehole heat exchanger based on a semi-analytical method considering groundwater seepage and geothermal gradient. (June 2021) - Jia, G.S.
Abstract: The horizontally-butted borehole heat exchanger (BHE) can be used to extract heat efficiently from medium-deep strata, where the groundwater seepage and undisturbed geothermal gradient are critical influence factors. In the application, it is an important and fundamental work to predict the BHE performance for high efficiency geothermal energy utilization. In order to solve its outlet temperature and heat exchange rate, this study develops a semi-analytical solution based on an adjustable multi-layer model associated with the moving finite line source (MFLS) method. The predicted results of a 2505 m horizontally-butted BHE show that the insulation layer and groundwater seepage are beneficial to a better performance. When the insulation depth increases from 0 to 900 m, the outlet temperature can be elevated from 40.5 °C to 48.7 °C, with the fluid volumetric flow rate given at 15 m 3 ·h -1 . With a 240 m thick aquifer layer, the groundwater seepage leads to 0.34 °C and 5.73 kW improvements in the outlet temperature and heat exchange rate, respectively. The proposed method successfully solves the problem of predicting the large depth horizontally-butted BHE drilled through various geological formations and aquifers. It can serve as a reference for the rational design of BHE systems for utilizing effectively the geothermal energy. Graphical abstract: Image 1 Highlights: The developed semi-analytical solution reliably predicts the performance of horizontally-butted BHE. BHE insulation near the outlet section contributes to higher outlet temperature. Larger fluid flow rate is beneficial to saving the cost of insulation layer. Groundwater seepage leads to 19.0% maximum increment of local heat exchange rate.