Numerical investigation of vertical borehole heat exchanger heat transfer under coupled conditions of groundwater seepage and layered subsurface

Vertical borehole heat exchangers (VBHEs) have been considered to be one of the best sustainable geotechnical technologies for building heating or cooling. In most of the current numerical models for the analysis of VBHE heat transfer, the assumption of subsurface homogeneity is considered. However, the actual subsurface presents a layered structure, and some aquifer groups have groundwater seepage. An improved numerical model is proposed that takes into account not only the coupled condition of groundwater seepage and the layered subsurface but also the heat transfer of VBHEs at different locations, including inside the borehole, borehole wall and outside the borehole. The heat transfer characteristic at these different locations under groundwater seepage and layered subsurface coupled condition is investigated. Moreover, the model is applied to investigate the thermal behaviors of VBHE arrays in two typical arrangements (matrix and staggered arrangement). Furthermore, the influence of groundwater seepage in different layers on the heat transfer ability of the VBHE is discussed. The results demonstrate that the seepage can effectively improve the average heat transfer performance of VBHEs array, and the relative improvement rate is about 62.0%, Compared with the matrix arrangement, the average heat flow of staggered arrangement is about 0.09 kW higher, and the relative increase rate is about 3.3%.