Study on the Coupled Heat Transfer Model Based on Groundwater Advection and Axial Heat Conduction for the Double U-Tube Vertical Borehole Heat Exchanger

Abstract

In this paper, a dynamic heat transfer model for the vertical double U-tube borehole heat exchanger (BHE) was developed to comprehensively address the coupled heat transfer between the in-tube fluid and the soil with groundwater advection. A new concept of the heat transfer effectiveness was also proposed to evaluate the BHE heat exchange performance together with the index of the heat transfer rate. The moving finite line heat source model was selected for heat transfer outside the borehole and the steady-state model for inside the borehole. The data obtained in an on-site thermal response test were used to validate the physical model of the BHE. Then, the effects of soil type, groundwater advection velocity, inlet water flow rate, and temperature on the outlet water temperature of BHE were explored. Results show that ignoring the effects of groundwater advection in sand gravel may lead to deviation in the heat transfer rate of up to 38.9% of the ground loop design. The groundwater advection fosters the heat transfer of BHE. An increase in advection velocity may also help to shorten the time which takes the surrounding soil to reach a stable temperature. The mass flow rate of the inlet water to the BHE should be more than 0.5 kg·s−1 but should not exceed a certain upper limit under the practical engineering applications with common scale BHE. The efficiency of the heat transfer of the double U-tube BHE was determined jointly by factors such as the soil’s physical properties and the groundwater advection velocity.

Evaluating Groundwater Flow Effects for Enhancement of Ground-Source Heat Pipes in the Case of the Toyohira River Alluvial Fan, Japan

“…To reduce the capital cost of ground-source heat pipe systems, the authors expected that another heat transfer process by groundwater-advection heat flux could enhance the thermal performance of heat pipes in a typical site where groundwater flows actively toward the borehole. The advection effect has been validated in many theoretical studies [8,9] and reported in field studies [7,10]. However, few studies have demonstrated field evidence of enhancement of ground-source heat pipe systems with this method.…”

“…The geothermal energy is extracted through the evaporation of the working fluid and transported upward to the condenser due to the temperature difference between the condenser and the evaporator [3]. At the condenser, the energy is dissipated to the heat sink (for example, snow on road surface which is then melted) and then the condensate returns to the evaporator by gravity to be heated again by the ground heat energy, consequently creating a cyclic process of evaporation and condensation [7].…”

Enhanced Steady-State Solution of the Infinite Moving Line Source Model for the Thermal Design of Grouted Borehole Heat Exchangers with Groundwater Advection

“…Several studies investigated analytical models, which consider both conductive and advective heat transport in the subsurface for configuring BHEs [7,16,[24][25][26][27][28][29][30][31][32][33][34][35]. For instance, Claesson et al [27] introduced a groundwater g-function, which is used to reduce the values computed by a g-function for heat conduction.…”

“…Therefore, a combination of the infinite cylinder source for short times and the finite MLS for larger times and a significant flow velocity is used. Zhang et al [35] combined the multipole method according to Hellström [37] with the finite MLS solution of Molina-Giraldo et al [32]. The models for the heat transfer inside (multipole method) and outside the borehole (finite MLS solution) are coupled by an iterative procedure [35].…”

“…Zhang et al [35] combined the multipole method according to Hellström [37] with the finite MLS solution of Molina-Giraldo et al [32]. The models for the heat transfer inside (multipole method) and outside the borehole (finite MLS solution) are coupled by an iterative procedure [35]. The outlet temperature is iteratively defined for a known inlet temperature and the mass flow rate of the U-tube.…”