Long-term heat transfer performance of underground coaxial heat exchanger for medium-deep geothermal

Authors

LI Fengcui, School of Energy and Building Environment Engineering, Henan University of Urban Construction, Pingdingshan 467036, China
HAN Ershuai, School of Energy and Building Environment Engineering, Henan University of Urban Construction, Pingdingshan 467036, China
LIANG Lei, School of Energy and Building Environment Engineering, Henan University of Urban Construction, Pingdingshan 467036, China
WU Jing'an, School of Energy and Building Environment Engineering, Henan University of Urban Construction, Pingdingshan 467036, China
LU Bingxue, School of Chemistry and Material Engineering, Henan University of Urban Construction, Pingdingshan 467036, China

Abstract

Underground coaxial heat exchanger(UCHE) for medium-deep geothermal has the characteristic of the high heat transfer capacity and outlet water temperature, so it has attracted great attention in the heating industry. However, the current research mainly focus on the heat transfer performance of the UCHE during a single heating period, with insufficient analysis on the long-term heat transfer performance. In this paper, considering the operation characteristics of heat supply in heating season and stopping working in non-heating season for underground coaxial heat exchanger for medium-deep geothermal, a numerical heat transfer model is proposed based on the energy conservation equation. The numerical model is solved by the finite volume method and calculated in Matlab platform, which is used to analyze the heat transfer performance of UCHE during the long-term operation. The results show that the heat transfer performance of the heat exchanger declines with the operating years and the degree of decline decreases year by year, and finally reaches the quasi-steady state. Among them, the decrease proportion of the average heat transfer in the next year is the largest, and the greater the buried depth of the heat exchanger, the smaller the decrease proportion of the heat exchanger. The decline ratios of HTC in the second year are 4.00%, 3.78% and 3.56% for the heat exchangers with lengths of 2 000, 2 500, and 3 000 m, respectively, and the corresponding decline ratios of 30-year operations are 13.7%, 13.1% and 12.4%, respectively. Rock-soil temperature declines and its disturbed radius increase with operating years. During 30-year operations, the disturbed radius increases from 13 m to 105 m for heat exchanger with 2 000 m depth. Furthermore, the disturbed radius is larger for the deeper heat exchanger. The research results demonstrate the evolution of the heat transfer performance of UCHE during the long-term operation, which has the guiding significances to the UCHE design.

Keywords

geothermal wells, medium-deep, coaxial, heat exchanger, long-term heat transfer

DOI

10.3969/j.issn.1001-1986.2021.02.024

Recommended Citation

LI Fengcui, HAN Ershuai, LIANG Lei, et al. (2021) "Long-term heat transfer performance of underground coaxial heat exchanger for medium-deep geothermal," Coal Geology & Exploration: Vol. 49: Iss. 2, Article 25.
DOI: 10.3969/j.issn.1001-1986.2021.02.024
Available at: https://cge.researchcommons.org/journal/vol49/iss2/25

Reference

[1] FLORIDES G,KALOGIROU S. Ground heat exchangers:A review of systems,models and applications[J]. Renewable Energy,2007,32(15):2461-2478.

[2] FALCONE G,LIU X,OKECH R,et al. Assessment of deep geothermal energy exploitation methods:The need for novel single-well solutions[J]. Energy,2018,160:54-63.

[3] 韩二帅,张家护,鲁冰雪,等. 中深层地热能供热技术综述及工程实例[J]. 区域供热,2019(2):79-83. HAN Ershuai,ZHANG Jiahu,LU Bingxue,et al. Summary and engineering cases of medium-depth geothermal heating technology[J]. District Heating,2019(2):79-83.

[4] 邓杰文,魏庆芃,张辉,等. 中深层地热源热泵供暖系统能耗和能效实测分析[J]. 暖通空调,2017,47(8):150-154. DENG Jiewen,WEI Qingpeng,ZHANG Hui,et al. On-site measurement and analysis on energy consumption and energy efficiency ratio of medium-depth geothermal heat pump systems for space heating[J]. Journal of Heating Ventilation and Air condition,2017,47(8):150-154.

[5] 刘俊,蔡皖龙,王沣浩,等. 深层地源热泵系统实验研究及管井结构优化[J]. 工程热物理学报,2019,40(9):2143-2150. LIU Jun,CAI Wanlong,WANG Fenghao,et al. Experimental study and tube structure optimization of deep borehole ground source heat pump[J]. Journal of Engineering Thermophysics,2019,40(9):2143-2150.

[6] WANG Zhihua,WANG Fenghao,LIU Jun,et al. Field test and numerical investigation on the heat transfer characteristics and optimal design of the heat exchangers of a deep borehole ground source heat pump system[J]. Energy Conversion and Management,2017,153:603-615.

[7] ALIYU M D,CHEN Huapeng. Sensitivity analysis of deep geothermal reservoir:Effect of reservoir parameters on production temperature[J]. Energy,2017,129:101-113.

[8] SONG Xianzhi,WANG Gaosheng,SHI Yu,et al. Numerical analysis of heat extraction performance of a deep coaxial borehole heat exchanger geothermal system[J]. Energy,2018,164:1298-1310.

[9] LIU Jun,WANG Fenghao,CAI Wanlong,et al. Numerical study on the effects of design parameters on the heat transfer performance of coaxial deep borehole heat exchanger[J]. International Journal of Energy Research,2019,43:6337-6352.

[10] SHENG Pan,KONG Yanlong,CHEN Chaofan,et al. Optimization of the utilization of deep borehole heat exchangers[J]. Geothermal Energy,2020,8(1):6.

[11] 鲍玲玲,徐豹,王子勇,等. 中深层同轴套管式地埋管换热器传热性能分析[J]. 地球物理学进展,2020,35(4):1217-1222. BAO Lingling,XU Bao,WANG Ziyong,et al. Heat transfer performance analysis of the middle-deep coaxial casing ground heat exchanger[J]. Progress in Geophysics,2020,35(4):1217-1222.

[12] 李永强,徐拴海,张卫东,等. 套管式地埋管换热器热短路及换热性能[J]. 煤田地质与勘探,2020,48(1):183-188. LI Yongqiang,XU Shuanhai,ZHANG Weidong,et al. Thermal short-circuiting and heat transfer performance of coaxial borehole heat exchanger[J]. Coal Geology & Exploration,2020,48(1):183-188.

[13] 王景刚,马一太,张子平,等. 地源热泵的运行特性模拟研究[J]. 工程热物理学报,2003,24(3):361-366. WANG Jinggang,MA Yitai,ZHANG Ziping,et al. Operating performance simulation of ground source heat pump system[J]. Journal of Engineering Thermophysics,2003,24(3):361-366.

[14] FANG Liang,DIAO Nairen,SHAO Zhukun,et al. A computationally efficient numerical model for heat transfer simulation of deep borehole heat exchangers[J]. Energy and Buildings,2018,167:79-88.

[15] GNIELINSKI V. New equations for heat and mass transfer in turbulent pipe and channel flow[J]. International Chemical Engineering,1976,16(2):359-368.

[16] HOLMBERG H,ACUNA J,NAESS E,et al. Thermal evaluation of coaxial deep borehole heat exchangers[J]. Renewable Energy,2016,97:65-76.

[17] 陶文铨. 数值传热学[M]. 西安:西安交通大学出版社,2001. TAO Wenquan. Numerical heat transfer[M]. Xi'an: Xi'an Jiaotong University Press,2001.

[18] MORITA K,BOLLMEIER W,MIZOGAMI H. An experiment to prove the concept of the downhole coaxial heat exchanger(DCHE) in Hawaii[J]. Geothermal Resources Council Transactions,1992,16:9-16.

[19] 於仲义,陈焰华,胡平放,等. 土壤分层对U形地埋管换热器换热能效特性的影响[J]. 暖通空调,2013,43(7):78-81. YU Zhongyi,CHEN Yanhua,HU Pingfang,et al. Heat transfer energy efficiency characteristics of U-tube ground heat exchangers in stratified soil[J]. Journal of Heating Ventilation and Air condition,2013,43(7):78-81.

[20] 陕西省住房和城乡建设厅. 中深层地热地埋管供热系统应用技术规程:DBJ 61/T 166-2020[S]. 北京:中国建材工业出版社,2020. Department of Housing and Urban-Rural Development of Shaanxi. Technical regulation for medium deep geothermal buried pipe heating system:DBJ 61/T 166-2020[S]. Beijing:China Building Materials Press,2020.