Influence of graphite volume fraction in phase change backfills on heat transfer performance of PHC energy piles

WANG Haoyu, ZhANG Dan, QIAN Zhengyu

doi:10.16511/j.cnki.qhdxxb.2024.21.010

Journal of Tsinghua University(Science and Technology) >

2024 , Vol. 64 >Issue 5: 831 - 840

DOI: https://doi.org/10.16511/j.cnki.qhdxxb.2024.21.010

SPECIAL SECTION: ENERGY GEOSTRUCTURE AND ENGINEERING

Influence of graphite volume fraction in phase change backfills on heat transfer performance of PHC energy piles

Expand

School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China

Received date: 2023-12-12

Online published: 2024-04-22

Fold

Abstract

[Objective] The global energy crisis is an urgent issue, highlighting the importance of shallow geothermal energy as a pivotal renewable energy source to support sustainable social development. Energy piles, a key method for harnessing shallow geothermal energy, hold enormous potential for growth and widespread application in global energy conservation and emission reduction projects. Research into the thermal performance of precast high-strength concrete (PHC) energy piles has advanced rapidly. Notably, phase change materials (PCMs) can absorb or release latent heat at a constant temperature during phase transitions, which makes PCMs an ideal backfill material to improve the heat transfer performance of PHC energy piles. However, a key drawback of PCMs is their low thermal conductivity, which could compromise the heat transfer performance of the PHC energy piles. To optimize the use of PCM in backfill materials, researchers have been exploring the addition of substances with high thermal conductivity. Graphite, in particular, has emerged as one of the most preferred fillers to enhance PCM thermal conductivity. Notwithstanding these advancements, there is limited research on how varying graphite volume fractions in PCM backfill materials affect the heat transfer performance of precast PHC energy piles. We established an indoor model of PHC energy piles and conducted tests using different backfill materials. To calculate the thermal conductivity of the backfill materials with different graphite volume fractions, we proposed and validated a theoretical calculation model based on the Maxwell model. A finite numerical model was developed using COMSOL Multiphysics, underpinned by empirical validation. Leveraging these models and experimental investigations, we analyzed the effects of different graphite volume fractions on several parameters, including the temperature difference between the inlet and outlet of the PHC energy pile, pile temperature, and the phase change state of the backfill material. The temperature difference increases with the graphite volume fraction. During the heat transfer process, the temperature difference gradually decreases until it stabilizes. A higher graphite volume fraction accelerates the heat exchange process within the PHC energy pile system, thus contributing to improved heat transfer performance. We observed a strong linear correlation between the thermal conductivity of the backfill materials and the temperature difference. As the graphite volume fraction increases, the pile temperature rises rapidly during the heat transfer process. When the operation time of the PHC energy pile remains constant, the pile temperature increases with increasing graphite volume fraction, whereas the axial temperature increment of the pile shows no significant change and exhibits a relatively uniform distribution. The use of the backfill materials designed in the numerical models ensures the stable operation of the PHC energy piles. The rate of phase change can be accelerated, and the recovery time can be shortened by increasing the graphite volume fraction of the backfill materials. Phase change backfill materials with a high graphite volume fraction can improve the heat transfer performance of PHC energy piles. This improvement is pivotal in meeting the high endurance energy requirements of buildings, thereby facilitating the efficient extraction of geothermal energy.

Key words： pre-stressed high-strength concrete (PHC); energy pile; phase change material; graphite; heat transfer performance

Cite this article

WANG Haoyu, ZhANG Dan, QIAN Zhengyu . Influence of graphite volume fraction in phase change backfills on heat transfer performance of PHC energy piles[J]. Journal of Tsinghua University(Science and Technology), 2024 , 64(5) : 831 -840 . DOI: 10.16511/j.cnki.qhdxxb.2024.21.010

References

[1] 陈家威, 张国柱, 郭易木, 等. 层状地层能源管桩传热性能试验研究[J]. 岩石力学与工程学报, 2020, 39(增刊2):3615-3626. CHEN J W, ZHANG G Z, GUO Y M, et al. Investigation on heat transfer characteristics of PHC energy piles in multi-layer strata[J]. Chinese Journal of Rock Mechanics and Engineering, 2020, 39(S2):3615-3626. (in Chinese)
[2] RYBACH L. Geothermal energy:Sustainability and the environment[J]. Geothermics, 2003, 32(4-6):463-470.
[3] SHORTALL R, DAVIDSDOTTIR B, AXELSSON G. Geothermal energy for sustainable development:A review of sustainability impacts and assessment frameworks[J]. Renewable and Sustainable Energy Reviews, 2015, 44:391-406.
[4] LALOUI L, MORENI M, VULLIET L. Behavior of a dual-purpose pile as foundation and heat exchanger[J]. Canadian Geotechnical Journal, 2003, 40(2):388-402.
[5] 王雪松. PHC能源桩换热的离散元数值模拟研究[D]. 杭州:浙江大学, 2022. WANG X S. Discrete element numerical simulation of PHC energy pile heat transfer[D]. Hangzhou:Zhejiang University, 2022. (in Chinese)
[6] 郭易木, 钟鑫, 刘松玉, 等. 自由约束条件下分层地基中PHC能源桩热力响应原型试验研究[J]. 岩石力学与工程学报, 2019, 38(3):582-590. GUO Y M, ZHONG X, LIU S Y, et al. Prototype experimental investigation on the thermo-mechanical behaviors of free constrained full-scale PHC energy piles in multi-layer strata[J]. Chinese Journal of Rock Mechanics and Engineering, 2019, 38(3):582-590. (in Chinese)
[7] GO G H, LEE S R, YOON S, et al. Design of spiral coil PHC energy pile considering effective borehole thermal resistance and groundwater advection effects[J]. Applied Energy, 2014, 125:165-178.
[8] 王荣, 杨晨磊, 董世豪, 等. 回填材料热物性对地埋管换热器换热性能影响综述[J]. 建筑热能通风空调, 2021, 40(4):35-40. WANG R, YANG C L, DONG S H, et al. Influence of backfill thermo-physical properties on thermal performance of ground heat exchanger-a brief review[J]. Building Energy & Environment, 2021, 40(4):35-40. (in Chinese)
[9] 唐皓, 韦彬, 张国柱, 等. 制冷工况下PHC能源桩的换热性能分析[J]. 深圳大学学报(理工版), 2022, 39(1):51- 58. TANG H, WEI B, ZHANG G Z, et al. Analysis of the heat exchange performance of PHC energy pile under cooling condition[J]. Journal of Shenzhen University (Science and Engineering), 2022, 39(1):51-58. (in Chinese)
[10] PARK H, LEE S R, YOON S, et al. Evaluation of thermal response and performance of PHC energy pile:Field experiments and numerical simulation[J]. Applied Energy, 2013, 103:12-24.
[11] CUI Y P, XIE J C, LIU J P, et al. A review on phase change material application in building[J]. Advances in Mechanical Engineering, 2017, 9(6):1687814017700828.
[12] BENLI H. Energetic performance analysis of a ground-source heat pump system with latent heat storage for a greenhouse heating[J]. Energy Conversion and Management, 2011, 52(1):581-589.
[13] QI D, PU L, SUN F T, et al. Numerical investigation on thermal performance of ground heat exchangers using phase change materials as grout for ground source heat pump system[J]. Applied Thermal Engineering, 2016, 106:1023-1032.
[14] RABIN Y, KORIN E. Incorporation of phase-change materials into a ground thermal energy storage system:Theoretical study[J]. Journal of Energy Resources Technology, 1996, 118(3):237-241.
[15] BOTTARELLI M, BORTOLONI M, SU Y H, et al. Numerical analysis of a novel ground heat exchanger coupled with phase change materials[J]. Applied Thermal Engineering, 2015, 88:369-375.
[16] 王畅, 曹晓玲, 袁艳平, 等. 夏季间歇运行工况下相变温度对相变回填地埋管换热器传热性能的影响[J]. 太阳能学报, 2020, 41(3):234-241. WANG C, CAO X L, YUAN Y P, et al. Study on thermal performance of ground heat exchanger backfilled with phase change material under intermittent operation in summer[J]. Acta Energiae Solaris Sinica, 2020, 41(3):234-241. (in Chinese)
[17] CHEN F, MAO J F, CHEN S Y, et al. Efficiency analysis of utilizing phase change materials as grout for a vertical U-tube heat exchanger coupled ground source heat pump system[J]. Applied Thermal Engineering, 2018, 130:698-709.
[18] MARÍN J M, ZALBA B, CABEZA L F, et al. Improvement of a thermal energy storage using plates with paraffin-graphite composite[J]. International Journal of Heat and Mass Transfer, 2005, 48(12):2561-2570.
[19] 曾军, 李翔晟, 欧阳立芳, 等. 石蜡/膨胀石墨/碳化硅复合相变材料的制备及热性能研究[J]. 化工新型材料, 2023, 51(4):115-118, 125. ZENG J, LI X S, OUYANG L F, et al. Preparation and thermal properties studies of paraffin/expanded graphite/silicon carbide composite phase change materials[J]. New Chemical Materials, 2023, 51(4):115-118, 125. (in Chinese)
[20] LING Z Y, CHEN J J, XU T, et al. Thermal conductivity of an organic phase change material/expanded graphite composite across the phase change temperature range and a novel thermal conductivity model[J]. Energy Conversion and Management, 2015, 102:202-208.
[21] 胡定华, 许肖永, 林肯, 等. 石蜡/膨胀石墨/石墨片复合相变材料导热性能研究[J]. 工程热物理学报, 2021, 42(9):2414-2418. HU D H, XU X Y, LIN K, et al. Study on heat conductivity of paraffin/expanded graphite/graphite sheet composite material[J]. Journal of Engineering Thermophysics, 2021, 42(9):2414-2418. (in Chinese)
[22] CAO Z M, ZHANG G Z, LIU Y P, et al. Thermal performance analysis and assessment of PCM backfilled precast high-strength concrete energy pile under heating and cooling modes of building[J]. Applied Thermal Engineering, 2022, 216:119144.
[23] DE FREITAS MURARI M C, DE HOLLANDA CAVALCANTI TSUHA C, LOVERIDGE F. Investigation on the thermal response of steel pipe energy piles with different backfill materials[J]. Renewable Energy, 2022, 199:44-61.
[24] 何伟, 赵万方, 郑晓宇. 新型相变材料回填的地埋管传热特性研究[J]. 建筑热能通风空调, 2021, 40(8):18-24. HE W, ZHAO W F, ZHENG X Y. Research on heat transfer of a borehole ground heat exchanger with novel phase change backfill materials[J]. Building Energy & Environment, 2021, 40(8):18-24. (in Chinese)
[25] 杨卫波, 杨彬彬, 李晓金. 取放热不平衡条件下相变材料回填地埋管换热器传热特性研究[J]. 流体机械, 2021, 49(6):72-78. YANG W B, YANG B B, LI X J. Study on heat transfer characteristics of ground heat exchanger with PCM backfill under imbalance condition of heat absorption and release[J]. Fluid Machinery, 2021, 49(6):72-78. (in Chinese)
[26] 白丽丽, 裴华富, 宋怀博, 等. 相变能量桩段模型传热模拟[J]. 防灾减灾工程学报, 2019, 39(4):684-690. BAI L L, PEI H F, SONG H B, et al. Heat transfer simulation of phase change energy pile[J]. Journal of Disaster Prevention and Mitigation Engineering, 2019, 39(4):684-690. (in Chinese)
[27] 谢敬礼, 佟强, 鲁文玥. 膨润土-石墨混合材料导热性能试验研究[J]. 硅酸盐通报, 2020, 39(5):1689-1693. XIE J L, TONG Q, LU W Y. Experimental study on thermal property of bentonite-graphite mixtures[J]. Bulletin of the Chinese Ceramic Society, 2020, 39(5):1689-1693. (in Chinese)
[28] 李润丰, 刘艳军, 涂玉波, 等. 石墨烯增强复合相变储能材料的热学性能研究[J]. 硅酸盐通报, 2022, 41(7):2542-2548. LI R F, LIU Y J, TU Y B, et al. Thermal properties of graphene enhanced composite phase change materials for energy storage[J]. Bulletin of the Chinese Ceramic Society, 2022, 41(7):2542-2548. (in Chinese)
[29] 屈春来, 辛悦, 党承华. 石墨掺量及骨料种类对混凝土导热系数的影响与内在机理研究[J]. 混凝土, 2020(7):70-73, 77. QU C L, XIN Y, DANG C H. Influence of graphite content and aggregate type on thermal conductivity of concrete and study of its internal mechanism[J]. Concrete, 2020(7):70-73, 77. (in Chinese)
[30] FAIZAL M, BOUAZZA A, HABERFIELD C, et al. Axial and radial thermal responses of a field-scale energy pile under monotonic and cyclic temperature changes[J]. Journal of Geotechnical and Geoenvironmental Engineering, 2018, 144(10):04018072.

Options

Outlines

模态框（Modal）标题

Abstract

Cite this article

References

Visited