Heat Transfer Analysis And Application Of Deep Borehole Heat Exchanger In Ground Source Heat Pump Systems

The ground-coupled heat pump(GSHP)technologies have great appeal in offering higher levels of efficiency than traditional HVAC technologies.However,penetration of the GCHP technology into the market has been hindered by its limitations such as higher capital cost,requirement of a certain land plot for installation of the ground loop and concern over the possible heat or cold build-up in the ground heat exchanger.The vertical Borehole Heat Exchangers(BHE)are recognized as the most-widely-used ground heat exchanger for the GCHP system.The concept of Deep Borehole Heat Exchanger(DBHE)has aroused growing interests in China recently.The DBHE may go down to depths of 1000-2500m;and temperature at the borehole bottom may reach 40-90℃.Therefore,they constitutea desirable alternative to traditional shallow BHE in the GCHP systems with advantages of much less land demand and higher efficiency of the heat pumps.The DBHE may also be employed for the purpose of seasonal heat storage owing to its flexibility,higher temperature available and enormous storage capacity.It is crucial to develop adequate and convenient means for thermal analysis of the DBHE.This study focuses on the heat transfer analysis of the DBHEs.Two models are presented for DBHE thermal analysis.One follows the traditional approach based on analytical solutions in GCHP studies.Governing equations have been established describing the heat transfer amongthe borehole wall and the fluid in the inner/outer pipes of the coaxial tubes.Analytical expressions of temperature distributions of the fluid in the inner/outer pipes are obtained for both the flow configurations,i.e.fluid entering the borehole in the annular channel and returning from the inner tube or flow in an opposite circulation.The effective borehole thermal resistance is then derived for the boreholes with coaxial tubes.The other approach is a numerical simulation scheme based on the Finite Difference Method(FDM)which takes the geothermal gradient into account and incorporates the coaxial borehole with surrounding soil..The numerical model and algorithm,which is specifically developed for the DBHEs,features much higher efficiency in DBHE simulations than most commerciallyavailable software toolkits based on Finite Element Method(FEM).To achieve efficient computation the model takes advantages of the specific features of the problem,these are1.The subsurface surrounding the coaxial borehole can be treated as a regular domain in the cylindrical coordinates,and then the FDM can readily be used in discretizing the domain concerned.2.It is appropriate and acceptable to treat the flow and convective heat transfer in the long pipes as one-dimensional so that the complex simulation of transient fluid dynamics and convective heat transfer inside the pipes can be greatly simplified.3.A coordinate transformation is introduced to realize the variable step sizes in the radial direction.4.Large spatial step sizes in the axial direction are possible in FDM scheme owing to the relatively minor temperature gradient in this direction.5.An algorithm based on the chasing method is adopted to achieve direct solution of the derived algebraic equation set for the transient two-dimensional heat transfer problem without turning to time-consuming iterations.The results of the two models are compared accordingly.Performance of DBHEs is then assessed with parameter analyses.Nominal DBHE capacities are evaluated according to their key factors such as borehole depth,subsurface conductivity and geothermal heat flux.Finally,an experimental facility has been constructed for study of heat transfer in the ground heat exchangers.The facility features the largest size in the world among similar experimental with its dimensions of 2.55 m x 2.55 m x 21 m.Tests have been carried out on the entire process of heat transfer between a borehole and a homogeneous medium surrounding it.Thermal energy balance and self-recovery ability of the subsurface were investigated in the tests.Errors were assessed on the heat flux and temperature measurements,and their influence on the test results have also evaluated.The results and conclusions of the theoretical and experimental studies will contribute to research and development of the deep-borehole heat exchanger technology.A pilot DBHE project is being constructed with technical assistance of this study.