Transient Heat Transfer Modeling, Simulation and Performance Analysis of Single and Double U-Tube Borehole Heat Exchanger

Abstract

<p>Ground source heat pump systems (GSHPs) are currently being widely applied to space heating and cooling due to their high efficiency and reduced environmental impact and running cost. Borehole heat exchanger (BHE) is the key component that determines the performance and total cost of the GSHPs. This thesis primarily focuses on transient heat transfer modeling, simulation, and performance analysis of single and double U-tube borehole heat exchanger with two independent circuits. Numerical heat transfer model that predicts the transient heat transfer of the BHE with single and double U-tube configuration was developed. The model developed is then used to perform simulation and analysis of transient heat transfer and thermal performance of single U-tube BHE (sBHE) as well as double U-tube BHE (dBHE) with two independent circuits for different cases of operating conditions: simultaneous charging, discharging, and charging-discharging. Using developed model, detailed comparison of thermal performance between single and double U-tube BHE was also made. Furthermore, comprehensive parametric analysis was performed to identify the major factors that affects the thermal performance of the sBHE and dBHE. It was concluded that the developed model was successfully applied for the analysis of short time thermal performance of sBHE and dBHE with two independent circuits where different borehole operating conditions with the same or different mass flowrate, inlet temperature and working fluid can be taken into consideration. The obtained result revealed that the heat injected and extracted by the double U-tube BHE at steady state is 21.8 (77%) and 10.2 W/m (71.8%) more than that of the sBHE, respectively. The simulation results also indicate that the thermal effectiveness of sBHE and dBHE at steady state are 0.24 and 0.31, respectively. The obtained results also revealed that the dBHE is preferable to transfer more heat when used in shallow borehole depth; while with deep borehole depth, sBHE is better to transfer more heat than dBHE particularly for high grout and soil thermal conductivities. The performed simulation and analysis are important as quick a reference for the design and system optimization of BHE integrated with GSHPs.</p>