Investigation On Heat Transfer Behavior Of Vertical Ground Heat Exchangers In A Layered Subsurface In The Presence Of Groundwater Advection

The underground heat exchangers are used to exchange heat between the buildings and underground medium during the operation of ground source heat pump(GSHP)systems.The economics and reliability of the GSHP systems strongly depend on the accurate designing of the underground heat exchanger,which is the key point to ensure the safety and efficiency of the operation of the GSHP system.Therefore,in order to fully understand the heat transfer process of the underground heat exchanger,developing an accurate model for the underground heat exchanger to predict its heat behavior is a prerequisite for better sizing of the underground heat exchanger to ensure optimum performance of the GSHP systems with minimum initial costs.The in-situ underground geological structures are not homogeneous,which are generally stratified with different thermal properties,and even the aquifers are usually expected.The underground heat exchanger experience heat transfer with the layered ground at the presence of groundwater advection,which is essentially a transient heat transfer process in complex conditions.A detailed understanding of thermal behavior of underground heat exchanger in heterogeneous medium with layered ground and groundwater advection seepage plays an important and practical role for designing and optimizing of the underground heat exchanger.In this work,with the methodology of analytical solutions,numerical simulation,laboratory experiment and field testing,heat transfer mechanism of vertical heat exchanger in a layered subsuface is investigated in details.By taking ground stratification and groundwater advection into consideration,modeling of the vertical heat exchanger analytically and numerically is proposed to evaluate the temperature response in the subsurface inside and around the heat exchanger.The proposed numerical model for vertical heat exchanger is coupled with heat pump unit and the building load model to carry out the long-term dynamic performance simulation of GHSP system in heterogeneous ground condition,which will provide theoretical basis and technical guidance for the optimal design and long-term efficient operation of the practical GSHP system.The main contents are as follows:(1)First of all,based on the temperature response to the single instantaneous ring-coil heat source,the generic form of analytical solutions to temperature field in a layered subsurface using cylindrical surface model for borehole heat exchagers and multiple ring-coil heat source model for pile heat exchangers are obtained based on heat source theory,the methods of separation of variables and Green's function with considering the underground as transverse isotropic medium.The heat transfer characteristics around a single vertical heat exchanger and arrays in a double-layered ground are both studied in great details.The effects of different impact factors(order of ground layers,layer thickness,thermal conductivity ratio of the layers,borehole depth and interaction of multiple boreholes)on the temperature response around the vertical heat exchangers are examined,and the discrepancies of temperature response under the layered and homogeneity conditions are provided,which indicates the necessity and scientificity of using the layered model to analyze the heat transfer around the vertical heat exchanger.(2)In order to more accurately describe the heat transfer process between the vertical heat exchanger and the surrounding ground at the presence of groundwater seepage,an analytical model for cylindrical surface source considering the convective ground surface boundary,groundwater seepage and the transversely isotropic features in the ground is developed by using the theory of moving heat source and Green's function method.The analytical solutions for the theoretical model with consideration of groundwater seepage are derived.The proposed analytical model is more practical and general with better accuracy by using mixed boundary conditions in place of Dirichlet boundary conditions at the ground surface.The effects of groundwater seepage,heat source radial size and ground surface convection on the temperature response around the vertical heat exchanger are investigated.The results can be used to guide the designing of GSHP engineering in complex geological conditions with ground groundwater seepage.(3)Based on the modified pipe flow module,a three-dimensional numerical model is developed incorporating heat conduction,fluid flow in pipes and groundwater advection.Combing the 1-D linear elements for the U-tubes and 3-D elements for the other components leads to significantly improved computational efficiency with satisfied computational accuracy.The heat behavior inside borehole for a single BHE and BHE arrays are investigated in a layered subsurface in the presence of groundwater advection.As a result,the flow temperature and heat flow profile are presented along the borehole surface and inside the U-tubes.(4)A full-scale BHEs experimental system is designed and constructed for studying the heat transfer of BHEs with focus on the temperature response of the flowing fluid in U-tubes.By using the p-linear distribution approximation to fit the temperature distribution measured in the experiment,the optimal p value and the effective thermal physical parameters of the layered ground are obtained.In addition,the applicability of the 3-D coupled heat conduction and groundwater advection numerical mode on the engineering scale is verified by using fluid temperature data gained in the in-situ thermal response measurement.The experimental data and the simulation results are in good match,indicating the effectiveness of the 3-D numerical model on the engineering scale,which lays a foundation for the long-term performance simulation of GSHP system under complicated geological conditions.(5)A coupled model for long-term dynamic performance simulation of GSHP system is developed,which combines building load model,3-D coupled heat conduction and groundwater advection numerical mode for BHEs and heat pump unit model.The improved long-term dynamic performance simulation method of GHSP system under complex conditions is put forward.The long-term dynamic performance of GSHP system in a layered subsurface in the presence of groundwater advection with cooling-dominated building load are evaluated.The effects of building load,shallow ground seasonal temperature variation,groundwater advection and borehole spacing in BHEs array on the long-term efficiency of the GSHP system are examined.The proposed coupled simulation model provides a better method for the optimal designing of the GSHP system and utilizing of the underground thermal energy storage(UTES)system.