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Influence of Ground Surface Form on Heat Transfer Performance of Ground Heat Exchangers

Author: TangZuo
Tutor: WangYong
School: Chongqing University
Course:
Keywords: ground source heat pump ground surface form heat transfer performance of ground heat exchangers
CLC: TU831
Type: Master's thesis
Year: 2011
Downloads: 37
Quote: 1
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Abstract


With the rapid economic development, global energy shortage is gradually reflected in each industry. Large building energy consumption and global warming phenomenon caused by non-renewable energy also attract people’s attention, development of renewable energy resources becomes the trend inevitably, and geothermal energy as a kind of clean and renewable energy resource gradually gets the favour of people, therefore, development of ground source heat pump technology also gets the attention. In recent years, ground-coupled heat pump system has a wide application in our country, most of the researches focus on the heat transfer model and heat transfer performance of ground heat exchanger.There are many influencing factors on heat exchange performance of ground heat exchangers. The ground surface is the upper boundary of underground rock-soil, but nowadays, researches about the influence of ground surface on the heat exchange performance of ground heat exchangers have not been involved. In the present heat transfer model calculation, because of the complexity of calculation, the ground surface boundary condition has been simplified as a constant boundary condition. However, the ground surface as the interface between underground rock-soil and outside environment, by which heat can transfer through to realize heat exchange of the two sides. Therefore, it is necessary to carry on a detailed analysis of the upper boundary conditions.This paper starts from heat transfer process analysis of nature situation (no ground heat exchangers) and after ground heat exchangers running situation, establishing thermal equilibrium equation on the ground surface boundary, and comparing the thermal parameters and characteristics of typical ground surface forms, then, carrying a test in Chongqing area, outdoor meteorological data and ground surface temperature of three typical forms are obtained. Using test data and calculating results of thermal equilibrium equation to calculate the net heat gains on the ground surface boundary, consequently, ground surface boundary conditions of different forms are obtained.In this paper, establishing two dimensional transient heat transfer model of soil around horizontal heat exchangers, discretizing differential equation and boundary conditions by finite difference method, then, using MATLAB software programming to solve them. According to different input conditions,respectively calculate continuous operation and intermittent operation continuous, different underground rock-soil temperature distribution, heat affected distance and load trace under different ground surface forms are analyzed. Under intermittent operation condition, temperature recovery at night and load trace at the end of running period of three ground surface forms are compared. Difference of load trace between the two kinds of operation conditions are also put forward, the result of comparison indicates that intermittent operation mode has great significance for keeping high efficiency of ground heat exchangers. In addition, the relationship between buried depth of ground heat exchangers and influence of ground surface form is discussed. Besides that, respectively for short-term operation and long-term operation condition, the fitting relationship between unit heat exchange capacity and heat affected distance of three ground surface forms are got, the variation law of heat affected distance is also analyzed.For the vertical U-tube ground heat exchangers, heat transfer model theory and the main solution methods of vertical U-tube are expounded. Using CFD software to establish and calculate three dimensional transient heat transfer model of vertical U-tube ground heat exchanger, from the calculating results, for the continuous operating condition, analyzing difference of average outlet water temperature, unit heat exchange capacity and rock-soil temperature distribution under different ground surface boundary conditions. According to the layer heat transfer theory, the different depth of saturated heat transfer layer of three ground surface forms are compared, and the load trace values and heat affected distance of different depth are listed. In addition, intermittent operation condition of vertical U-tube ground heat exchangers is also calculated, the underground rock-soil temperature recovery at night under this operation mode of three surface forms are analyzed.

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CLC: > Industrial Technology > Building Science > Housing construction equipment > Air-conditioning, heating, ventilation,and its equipment > Air-conditioning
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