Boundary Element Analysis Of Heat Transfer Of U-type Ground Heat Exchangers

As a new technology of high efficiency, energy saving and green environmentprotection, ground source heat pump (GSHP) has been getting more and moreconcern in recent years. It is currently one of the frontier research topics in the airconditioning field, and also an important form of shallow geothermal energyutilization. Heat transfer performance of ground heat exchanger, which is animportant part of GSHP system, is a key factor influencing on the operating effectof the system. In the thesis, based on the analysis of the existing heat transfermodel, a steady-state heat transfer model about vertical ground heat exchanger inGSHP is established by analyzing the actual heat transfer process. The boundaryelement method (BEM) with the regularization of the nearly singular integrals,which is able to deal with the thin body problems, is used to analyze its heattransfer performance. The main results and contributions in the thesis are in thefollowing.1. The heat transfer performance of the vertical single and double U-type heatexchangers are respectively calculated by the BEM for cooling and heating cases.Consider the influence of the key parameters in the heat transfer process, such asthe material, diameter and wall thickness of the pipe as well as backfill material,branch pipes’ distance and radius of boreholes. And differences between the twoexchangers are discussed. The computed results show that, the heat transfer usingthe ordinary PVC pipe is more impeded, and high-density polyethylene pipe (HDPE)or steel pipe is recommended as the pipe material of the ground heat exchanger. Thethickness of the plastic pipe should be reduced if the pipe body is safe in the stressstrength and stability. The heat transfer capacity of the ground heat exchanger canbe effectively enhanced by improving and optimizing the thermal conductivity ofbackfill materials. The heat transfer capacity of single and double U-type heatexchangers is increased with the increase of the branch pipes’ distance. The heattransfer capacity of double U-type heat exchanger is10%higher than that of singleU-type heat exchanger when the radius of boreholes changes from0.055-0.075m.2. The heat transfer capacity, inlet and outlet temperature of the vertical singleburied pipe are respectively calculated by the BEM with the present heat transfermodel and FLUENT software according to the three-dimensional model. The results of the two models are in excellent agreement, which demonstrates that thepresent analytic model is reasonable and reliable. Then for given the velocity, theinlet and outlet temperature difference of the pipes, the pipe lengths of single anddouble U-type heat exchangers in two cases are respectively obtained by theboundary element analysis.3. The coupling of the thin pipe wall and thick soil body in the heat transfer ofthe ground heat exchanger is a difficult problem to be analyzed by the conventionalnumerical methods such as the finite volume method. The present boundaryelement method is an effective tool to solve the coupling problem of the heattransfer of the ground heat exchanger. The BEM is used by dividing elements onthe boundary of the different material regions. One of advantages of the BEM is touse the fewer computer resources and obtain the more accurate results. Accordingto the present results, some beneficial suggestions are proposed for the designparameters of the pipe distribution and construction of the ground heat exchanger.Moreover, the quantitatively analytic results can be used to instruct the selection ofsystem operation parameters in GSHP.