Abstract
Borehole thermal resistance in Ground Heat Exchanger (GHE) installations is affected by several parameters such as geometrical attributes of heat exchanger in the borehole, pipes' characteristics and grout’s thermal conductivity. A study is carried out to compare the values computed by Ground Loop Design (GLD) Software, GLD 2009, with three ana-lytical solutions for U-shaped tubes. The analysis is focused on dimensionless ratios of borehole geometrical parameters (borehole diameter to outside pipe diameter and shank spacing to borehole diameter) and pipes according to Standard Di-mension Ratio (SDR) and on eight common grouts. Finally, the effect of heat conduction in the borehole is examined by means of finite element analysis by Heat Transfer Module of COMSOL Multiphysics. A two-dimensional (2-D) steady-state simulation is done assuming working fluid temperatures for winter and summer conditions and typical Greek undis-turbed ground temperature in a field of four ground vertical U-tube heat exchangers surrounded by infinite ground. The temperature profile is presented and the total conductive heat flux from the pipe to the borehole wall per meter of length of ground heat exchanger is computed for pipes SDR11 (the outside diameter of the pipe is 11 times the thickness of its wall), SDR9 and SDR17 for summer working conditions and three different configurations. It is attempted to reach to comparative results for borehole thermal resistance value through different types of analysis, having considered the major factors that affect it and giving trends for the influence of each factor to the magnitude of its value.
Highlights
Geothermal energy [1, 2] is conceived as a clean and cost effective form of energy with various applications for space heating and cooling
The objective of this study is to evaluate the influence of the factors which affect Ground Heat Exchanger (GHE) heat conduction in the borehole by steady-state analytical simulations and a 2-D steady-state numerical one
It is worth mentioning that the borehole resistance calculated by Eq (2) does not depend on the shank spacing between GHE pipes
Summary
Geothermal energy [1, 2] is conceived as a clean and cost effective form of energy with various applications for space heating and cooling. Ground Source Heat Pump (GSHP) systems, which are established to exploit the undisturbed ground temperature, consist of several parts with the Ground Heat Exchanger (GHE) to be the most important of them. Several attempts [3, 4], which utilize analytical and numerical models, steady-state and transient analysis, onedimensional (1-D), two-dimensional (2-D) and threedimensional (3-D) simulation, have been done to model the operation and efficiency of GHE in a GSHP system. The use of different numerical techniques in engineering environmental applications is becoming more and more popular [5, 6]. Numerical models [11] calculate finite differences to simulate the temperature distribution profile around the GHE. Steady-state analysis [12] is used to
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