Numerical Study on Energy Pile Groups with Deep Penetration 1-U-Shape Heat Exchangers under Different Operation Modes
Publication: International Journal of Geomechanics
Volume 23, Issue 12
Abstract
Energy piles, as an environmentally friendly means to exploit renewable energy, have attracted the attention of global researchers over the past years. Traditional heat exchanger configurations (1-U-shape, 1-W-shape, and multi-U-shape in series or parallel) have been utilized. Traditional 1-U-shape is limited by pile length, short heat transfer path, and low total heat transfer rate. Traditional 1-W-shape heat exchange tubes will cause thermal interference. Moreover, the heat exchange rate per unit tube length is not high for multi-U-shape in series or parallel. Therefore, this paper presents an efficient pile foundation heat exchanger configuration, i.e., deep penetration 1-U-shape configuration. Through a numerical study, energy pile groups with deep penetration 1-U-shape heat exchangers are investigated under continuous operation mode and intermittent operation mode. The temperature change of the piles and soil under intermittent operation mode is analyzed. Thermal performances (e.g., outlet water temperature and heat transfer efficiency) are compared under the two operation modes. Four typical monitoring points under the two operation modes are compared, and the difference in temperature variation is obtained to study the influence on the temperature field of the piles and soil. Results show that in summer mode, the temperature of the pile under intermittent operation mode presents restorability while the temperature of the soil increases gradually. Intermittent operation mode yields a lower temperature of outlet water temperature than continuous operation mode does, and therefore intermittent operation mode is recommended.
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Acknowledgments
This research was supported by the Hubei Technological Innovation Special Fund (Grant Nos. 2017AAA128 and 2018AAA028). This support is gratefully acknowledged.
Author contributions: Conceptualization, Weidong Lyu; methodology, Min Xia and Mingjian Liu; software, Zhangshuo Luo and Dingbao Song; formal analysis, Weidong Lyu; writing—original draft preparation, Weidong Lyu and Min Xia; writing—review and editing, Weidong Lyu and Min Xia; and supervision, Weidong Lyu. All authors have read and agreed to the published version of the manuscript.
Notation
The following symbols are used in this paper:
- A
- cross-sectional area of the tube (m2);
- A0
- maximum annual amplitude of temperature (K);
- cs
- specific heat capacity (J/kg · K);
- cw
- specific heat capacity of water (J/kg · K);
- D
- hydraulic diameter of the tube (m);
- d
- damping depth of annual fluctuation of temperature (m);
- div
- divergence operator;
- fd
- friction factor;
- grad
- gradient operator;
- hint
- convective heat transfer coefficient inside the tube (W/m2 · K);
- (hZ)eff
- thermal transmittance of the tube (W/K);
- kDf
- shape factor related to tube’s curvature;
- ks
- thermal conductivity (W/m · K);
- kt
- thermal conductivity of the tube (W/m · K);
- kw
- thermal conductivity of water (W/m · K);
- p
- water pressure (Pa);
- Qwall
- exchanged heat transfer rate per unit of length through the tube wall (W/m);
- r0
- internal radii of the tube (m);
- r1
- external radii of the tube (m);
- T
- temperature (K);
- Tave
- annual average temperature (K);
- Text
- pile temperature (K);
- Tin
- inlet water temperature (K);
- Tout
- outlet water temperature (K);
- t
- time (day);
- t
- time (s);
- u
- flow velocity of water (m/s);
- |u|
- norm of water flow velocity (m/s);
- v
- volumetric flow rate of water (m3/s);
- z
- depth (m);
- Δm
- water mass per unit time (kg/s);
- ω
- annual radial frequency (1/day);
- ρs
- density of soil or pile (kg/m3); and
- ρw
- density of water (kg/m3).
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Published In
International Journal of Geomechanics
Volume 23 • Issue 12 • December 2023
Copyright
© 2023 American Society of Civil Engineers.
History
Received: Oct 13, 2022
Accepted: Jun 25, 2023
Published online: Oct 4, 2023
Published in print: Dec 1, 2023
Discussion open until: Mar 4, 2024
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