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Ground Thermal Properties Test and Analysis for Ground Source Heat Pump

Author: ChangGuiQin
Tutor: CuiWenZhi
School: Chongqing University
Course: Power Engineering and Engineering Thermophysics
Keywords: Ground source heat pump Ground Thermal properties Thermal response test Line source model Vertical heat exchanger
CLC: P642.11
Type: Master's thesis
Year: 2011
Downloads: 212
Quote: 0
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Abstract

Ground source heat pumps utilizes soil as heat source or sink via the circulating fluid, which exchanges heat with surrounding soil in a closed loop. Ground conductivity, volume heat capacity, borehole resistance and initial ground temperature are key factors in ground source heat pump design, installation and operation. Therefore, in this thesis an experiment apparatus is established and thermal response tests are carried on to obtain ground thermal properties surrounding the borehole. This thesis then analyzes the interaction among these parameters.A long term ground temperature monitoring system of 100m depth range of vertical borehole is established. Ground temperature distribution rule in the test area is obtained. Specifically, the depth of thermocline layer is from 0m to 15m; the depth of constant temperature layer extends from 15m to 30m and the geothermal gradient is about 0.00486℃/m and the increasing temperature layer, whose geothermal gradient is about 0.01645℃/m, locates between 30m and 100m. Moreover, the integral average temperature within the range of borehole depth is from 19.81℃to 20.21℃, which are calculated from fitting and integrating the ground temperature data. However, the initial ground temperature is from 20.0℃to 20.4℃by water cycling method. The integral average temperature is lower than the temperature measured by cycling water method in same month. Initial ground temperature in vertical heat exchanger length design should be based on ground temperature in site. The soil temperature data in experiment compared with the temperature calculated by empirical formula, semi-empirical formula and they are in good agreement.Using thermal response test method, Experiments are carried on single-U and double-U heat exchanger for several times. Directly fitting method and parameters estimation method are both used to analyze the experiment data. The results show that the value of equivalent thermal conductivity is 2.50W/mK to 2.97W/mK. In addition, the borehole thermal resistance is 0.066 mK/W to 0.0792mK/W for double-U heat exchanger and that of single-U heat exchanger is 0.11 mK/W to 0.12 mK/W. The thermal conductivity is much higher than the Northeast and North China. This will be favorable for the ground source heat pump system application and promotion in sandstone and mudstone typical bedrock geological conditions.At present, there is no agreement to deal with the measurement duration of thermal response test and the range within which the experiment data should be abandoned when linear source model is used. This study shows that the test time length should be greater than 60 hours and the first-10-hour data should be cast off in thermal response test.Overall, the results obtained in this study lay an experimental and theoretical basis for the ground source heat pump application and promotion.

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