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Performance of Ground Heat Exchangers for Civil Infrastructures

  • 이철호 (대학원 건축·사회환경공학과 지반공학전공)

초록/요약

The need for developing alternative energy sources in Korea is dramatically increasing in order to prepare the era of hefty oil prices because the shortage of energy and resources enforces us to import about 97% of fossil fuels used in Korea. Among the various types of new and renewable energy resources, geothermal energy which is defined as thermal energy generated and stored in the Earth is considered to be a relatively clean and inexhaustible resource. Recently, ground-source heat pump (GSHP) systems have been increasingly used around the world, because they are among the most energy- and cost-efficient heating and cooling systems for residential and commercial buildings. A typical GSHP system mainly consists of a conventional water-source heat pump unit coupled with a group of ground heat exchangers (GHEXs) where heat exchange occurs between a working fluid circulating through the GHEX and the ground formation. A GHEX, where heat extraction or injection of thermal energy occurs from/into the ground, is an important device that determines the performance of a GSHP system and its initial installation cost. In this study, conventional system (borehole heat exchanger or closed-loop vertical ground heat exchanger) is examined by performing field test, laboratory test and numerical analysis to improve thermal capability of GHE with consideration of construction cost. And two possible thermo-active energy structures, i.e., tunnel and cast-in-place pile are also employed to compare with conventional system for heat exchange rate. Field tests and numerical analyses are used to evaluate performance of two thermo-active structures. To evaluate the effective ground thermal conductivity and investigate the thermal efficiency of the GHEX, six test boreholes were constructed with different installation conditions: such as different grouting materials (cement vs. bentonite), different shape of pipe-sections (conventional U-loop type vs. new 3 pipe-type), and different additives (silica sand vs. graphite), at a test bed in Wonju, South Korea. Thereafter, the thermal efficiency of each borehole is evaluated by conducting a series of in-situ thermal response tests. Lastly, the design length of GHEXs for an artificial heating/cooling load for an imaginary building is estimated based on the in-situ TRT results of each borehole. A series of cost analyses has been carried out to show the applicability of the cement grouting, the graphite additive, and the new 3-pipe type of heat exchange pipe. A new geothermal energy source obtained from a tunnel structure has been studied in this study. The geothermal energy is extracted through a textile-type ground heat exchanger named "Energy Textile" that is fabricated between a shotcrete layer and a guided drainage geotextile. To evaluate the thermal performance of the energy textile, a test bed was constructed in an abandoned railway tunnel located in Seocheon, South Korea. A series of laboratory experiments was performed to measure the thermal conductivity of tunnel wall materials (i.e., shotcrete, lining material). A test bed was then constructed to facilitate an evaluation of the thermal performance of the energy textile under the field condition. To observe a long-term thermal behavior, the difference in the temperatures of the inlet and outlet fluid of the energy textile is monitored using a constant-temperature water bath (CWB) and a real simulation system for both cooling and heating operating. Based on the field experiment results, numerical analyses considering field conditions such as arrangement of pipes, property of lining material and temperature of the air in tunnel are also conducted to evaluate the performance of the geothermal energy extraction system that utilizes the energy textile under the field condition. In addition, thermal stress due to the change of temperature is estimated because original function of the tunnel lining is a structural stability and finishing structure of the tunnel therefore additional condition, i.e. energy textile, should avoid affecting negative influence to the tunnel lining. Energy piles set up the heat exchange pipe inside the pile foundation and allow circulating fluid through the pipe leading to heat exchange with the subsurface ground formation. In this study, a cast-in-place concrete energy pile is considered, which functions not only as a structural foundation but also as an underground heat exchanger. The energy pile is a large-diameter drilled shaft equipped with two types of heat exchange pipe installation, i.e., a W-shape and an S-shape. The drilled shaft reaches to the depth of 60 m whilst the heat exchange pipes are inserted to about 30 m deep from the ground surface. The W-shape and S-shape pipes were installed in the opposite sections in the drilled shaft. In-situ thermal performance on the cast-in-place concrete energy pile was evaluated by carrying out a series of in-situ thermal response tests (TRTs) and CFD numerical analyses. TRTs were separately performed for both the W-shape and an S-shape heat exchange pipes, and the heat exchange rate was relatively compared each other. In addition, two closed-loop vertical ground heat exchangers of 40 and 60 m in depth were symmetrically constructed adjacent to the cast-in-place concrete energy pile at the distance of 1.5 m, respectively. Based on the results of the numerical simulations, it is shown that CFD analysis can be used to simulate heat exchange behavior of energy pile and to optimize the design factors by performing parametric studies.

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목차

Abstract I
Acknowledgements III
Dedication IV
Table of Contents V
List of Figures VIII
List of Tables XV
Chapter 1. Introduction 1
1.1 Overview 1
1.2 Organization 3
Chapter 2. Literature review 4
2.1 Heat transfer in the ground 4
2.2 Line source model 9
2.3 Line source model - Ribbon type 12
2.4 Cylinder source model 15
2.5 Ring coil source model 17
2.6 Computational Fluid Dynamics (CFD) analysis 19
2.7 In-situ thermal response test 22
2.8 Closed-loop vertical ground heat exchanger 27
2.9 Energy foundations and other thermo-active underground structures 31
Chapter 3. Closed-loop Vertical GHEX 35
3.1 Properties of grout materials 38
3.1.1 Materials 38
3.1.2 Thermal Conductivity 40
3.1.3 Viscosity 43
3.1.4 Salinity effect on volume reduction 45
3.2 Field test 48
3.2.1 In-situ thermal response test 48
3.2.2 Equipment of thermal response test 50
3.2.3 Field conditions 51
3.2.4 Field testing procedure 55
3.3 Results of Field test 58
3.3.1 In-situ thermal response test 58
3.3.2 Subsurface temperature distribution during TRT 65
3.4 Back-analyzing in-situ thermal response test 66
3.4.1 Numerical Modeling 66
3.4.2 Results and comparison of the numerical model 69
3.5 GHEX Design with results from field test 74
3.5.1 Method for designing total pipe length of GHEX 74
3.5.2 Designing total pipe length of GHEX 75
3.5.3 Designed pipe length of GHEX and cost analysis 76
Chapter 4. Ground heat exchanger in tunnel 81
4.1 Laboratory test 83
4.2 Preliminary study using numerical model 86
4.2.1 Numerical model 86
4.2.2 Results of numerical analyses 87
4.3 Test bed construction and field experiment 89
4.3.1 Test bed construction 89
4.3.2 In-situ thermal response test 95
4.3.3 Long-term monitoring for heat exchange rate 97
4.4 Results of field experiment 100
4.4.1 Short-term monitoring using TRT 100
4.4.2 Long-term monitoring for heat exchange rate 102
4.5 Numerical simulation 123
4.5.1 Numerical model 123
4.5.2 Results from numerical analysis 128
4.5.3 Thermal stress analysis with CFD results 133
4.6 Design of energy textile 136
4.6.1 Method for designing energy textile 136
4.6.2 Design results 139
Chapter 5. Ground heat exchanger in pile 145
5.1 Laboratory test to measure thermal conductivity of concrete used in field 146
5.2 Construction of energy pile 147
5.3 Field experiments 151
5.3.1 Hydration heat in cast-in-place concrete energy pile 151
5.3.2 In-situ thermal response tests 152
5.4 Numerical simulation of energy pile 157
Chapter 6. Summary and Conclusions 170
References 175

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