Theoretical analysis and Application of Borehole Heat Exchangers
Date Issued
2011
Date
2011
Author(s)
Liang, Nai-Wen
Abstract
The aim of this study is to explore the theoretical analysis and application of Borehole Heat Exchangers (BHE). The U-shaped Borehole Heat Exchangers (UBHE) is usually used for the main subject of research of BHE. There, this study also uses the UBHE as its main research subject. The four major parts of the theoretical analysis are: determination of UBHE heat transfer analysis model, calculation of temperature distribution and outlet temperature of UBHE, using the mapping method and the shape factor method to calculate thermal resistance of UBHE, dimensionless UBHE heat transfer analysis. In this study, the applications of UBHE include installation of adiabatic plate and design of eight-branch-pipes UBHE.
In addition, this research paper also compares the results with practical experimental data for verification, and further discusses the influences from related variables on outlet temperature of UBHE, so that can understand the general design criteria of UBHE. Moreover, this study investigated the influence variables of heat backflow for the optimal heat transfer performance of UBHE.
In this study, the heat transfer analysis includes measurements of both outside and inside of the borehole. Outside the borehole, the finite line-source theory is applied to calculate wall temperature of the borehole in the steady and unsteady state. Inside the borehole, the quasi-three-dimensional theory is applied to evaluate temperature distribution of the working fluid. The research results show that mapping method were more accurate than Hellstorm G. method in the case of heat dissipation. Furthermore, compare calculated results with four cases of experimental data, and the accuracy range is within 3.1% for single UBHE. The study proposes a new thermal resistances calculation method to solve the heat transfer of UBHE.
In this study, the effect of adiabatic plate in the middle of borehole was also considered. The major purpose of adiabatic plate was to prevent heat backflow which would cause raised of outlet temperature of UBHE and decrease the heat dissipation rate. The heat backflow would occur when the distance between two branch pipes (2D) was shorter and the depth of well (H) was deeper, and when the flow rate (Q) of working fluid was lower. For the condition of serious heat backflow, adiabatic plate can be added in the middle of borehole to increase the heat transfer rate. However, if the heat backflow is not so much, the adiabatic plate will block the heat transfer from one to the other side of borehole and result in temperature raise in outlet of UBHE. Hence, the increasing of D value is more efficient to raise the heat transfer of UBHE than using the adiabatic plate.
This study takes one step further to investigate heat transfer of 8-branch-pipes BHE. Our theoretical analysis showed that, the energy efficiency of 8-branch-pipes BHE raised from 23.5% (low total flow rate) to 44.7 % (high total flow rate) when comparing with the single U-tube in the same total flow rate; and the total heat transfer rate of 8-branch-pipes BHE raised from 23.63 % (low total flow rate) to 42.18% (high total flow rate). Hence the 8-branch-pipes BHE proved to be a good design for increasing heat transfer rate of UBHE. Similarly, for the design of 8-branch-pipes BHE, it should try to avoid the occurrence of heat backflow. When the depth of well is deeper, the distance of downward branch pipe and central upward branch pipe is smaller, and the flow rate is lower, the effect of heat backflow will become more obvious. In this condition, the use of central upward branch pipe coated with adiabatic material will increase the heat transfer rate.
Subjects
thermal resistance
U-shaped borehole heat exchangers
mapping method
shape factor
energy efficiency
SDGs
Type
thesis
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