垂直U型土壤换热器长期供冷问题的研究
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摘要
土壤源因其清洁、环保、可再生、能量大等优点,被认为是21世纪一项最具有发展前途的、具有节能和环保意义的自然冷(热)源。该能源作为土壤源热泵系统的主要能量成为越来越多的学者关注的焦点。目前,土壤源热泵的系统已不完全局限于传统的热泵系统,应用的领域也扩大到工业领域。制约土壤热源利用的因素很多。其中,地下埋管换热器换热与土壤热平衡问题一直是研究的关键。
本文在总结了国内外土壤源热泵技术的基础上,针对某工艺的特殊要求对垂直U型土壤换热器长期取冷时,地下温度的变化和分布情况进行了模拟分析。本文以该系统的地下埋管换热器作为研究对象,主要从以下几个方面进行了研究:
首先,利用有限容积法建立了单管土壤换热器三维数理模型,模拟了哈尔滨地区系统连续运行10年土壤温度场的变化情况。比较分析在不同换热量,不同流速、不同回填料和不同导热系数时流体温度变化和土壤温度分布的影响。其中,回填料对系统长期运行的换热效果影响并不明显,在进行钻井回填时,可考虑对U型管的固定作用,不必刻意斟酌加强换热,因此可采用原土壤回填。
其次,在所建立的数理模型基础上,本文对天津、武汉、广州等几个典型的地区进行了模拟比较分析。对各个地区在本文设定工况下的极限换热量进行了计算,并对其进行比较。其中,天津地区U型管换热器长期供冷的极限换热量为1006.51W,单位管长换热量为22.13W/m;武汉地区U型管换热器长期供冷的极限换热量为935.17W,单位管长换热量为18.70W/m;广州地区U型管换热器长期供冷的极限换热量为100.20W,单位管长换热量为2W/m。由此可以看出,对于广州地区,采用U型管换热器长期进行直接供冷,能够获得的冷量非常有限,说明该系统并不适合在广州(夏热冬暖地区)地区使用。
再次,在单管的基础上建立双管和多管系统的传热模型,对不同管间距埋管的换热量进行模拟比较。双管和多管布置时,由于管间的热干扰作用,系统的换热量将不同程度降低。其中双管布置时,较相同工况下单管换热时换热量减少20%以上;多管布置时,换热量减少50%以上。
利用土壤热源长期应用于工艺供冷的系统是一种新型的系统,目前该系统在实际中还得不到广泛的应用,但是随着常规能源的日益枯竭、能源价格的逐渐上涨、环保压力的不断增加和应用技术的进步,该系统将逐渐成为一种有竞争力的替代传统供冷的新方式。本文的工作可以为今后土壤热源长期直接供冷系统的应用提供理论基础和技术参考。
Due to its advantages such as clean, environment-friendly, renewable, large amount, ground thermal source is considered the most potential natural heat/ cold source for the 21st century. Acting as the main heat source of soil-source heat pump systems, soil thermal energy has been a cynosure of researchers. Currently, ground-source heat pump systems are no longer constricted in conventional heat pump systems; its application has been extended to industrial realm. There are many factors which restrict the application of soil-source heat pump systems, among these factors, heat exchange of ground heat exchanger and ground thermal balance are the crucial ones.
In this paper, ground-source heat pump techniques in our country and abroad are generalized, and then based on this work; underground temperature variation and distribution are analyzed by simulation, when U-tube underground heat exchanger is used for long-term cooling for a special process. This paper takes underground heat exchanger as research subject; the research focuses on the following aspects:
Firstly, a 3-D numerical model of single U-tube using Finite Volume Method is developed, to simulate the underground temperature variation of a ground-source heat pump system which has been continuously running for 10 years in Harbin. The temperature variation and underground temperature distribution are analyzed and compared with different△t, flow rates, grouting materials, and heat transfer coefficients. The influence of grouting materials on heat transfer is not obvious, so in the process of grouting, it is not necessary to consider about intensifying heat transfer but only fix function, so original soil can be used for grouting.
Secondly, on the basis of the established numerical model, in this paper, heat exchange of ground heat exchanger and ground thermal balance of ground-source heat pump in typical regions such as Tianjin, Wuhan, Guangzhou, etc. are simulated and analyzed. The limit heat transfers in these regions are calculated in this paper under setting conditions, and comparisons are made. The limit heat transfer of U-tube heat exchanger for long-term cooling in Tianjin is 1006.51W (22.13W/m); in Wuhan, it is 935.17W (18.7W/m); but in Guangzhou, it is only 100.2W (2W/m). From these we can get that in Guangzhou, the cold source available is very limited if using U-tube heat exchanger for long-term direct cooling, so this type of system is not suitable to be using in Guangzhou which is hot in summer and warm in winter.
Finally, numerical heat transfer models of double and multi-U-tube ground heat exchangers are developed on the basis of that of single-U-tube ground heat exchanger, and the heat transfers of pipes with different spans are simulated and compared. When double and multi-U-tube ground heat exchangers are used, heat transfers of the system decreases due to the thermal disturbance among the tubes. Compared with single-U-tube ground heat exchanger, under the same running status, heat transfer decreased above 20% for double and 50% for multi-U-tube ground heat exchangers.
Using ground-thermal energy for long-term cooling is a new type of heat pump system; currently this type has not been widely applied in practical, but with the exhausting of conventional sources of energy, the roaring of energy prices, the increasing of pressure of environmental protection, and the advancement of technology, this system will gradually become a competitive alternative term of conventional cooling systems. The research work in this paper can supply theoretical support and technological references to future application of ground-source heat pump systems for long-term direct cooling.
本文在总结了国内外土壤源热泵技术的基础上,针对某工艺的特殊要求对垂直U型土壤换热器长期取冷时,地下温度的变化和分布情况进行了模拟分析。本文以该系统的地下埋管换热器作为研究对象,主要从以下几个方面进行了研究:
首先,利用有限容积法建立了单管土壤换热器三维数理模型,模拟了哈尔滨地区系统连续运行10年土壤温度场的变化情况。比较分析在不同换热量,不同流速、不同回填料和不同导热系数时流体温度变化和土壤温度分布的影响。其中,回填料对系统长期运行的换热效果影响并不明显,在进行钻井回填时,可考虑对U型管的固定作用,不必刻意斟酌加强换热,因此可采用原土壤回填。
其次,在所建立的数理模型基础上,本文对天津、武汉、广州等几个典型的地区进行了模拟比较分析。对各个地区在本文设定工况下的极限换热量进行了计算,并对其进行比较。其中,天津地区U型管换热器长期供冷的极限换热量为1006.51W,单位管长换热量为22.13W/m;武汉地区U型管换热器长期供冷的极限换热量为935.17W,单位管长换热量为18.70W/m;广州地区U型管换热器长期供冷的极限换热量为100.20W,单位管长换热量为2W/m。由此可以看出,对于广州地区,采用U型管换热器长期进行直接供冷,能够获得的冷量非常有限,说明该系统并不适合在广州(夏热冬暖地区)地区使用。
再次,在单管的基础上建立双管和多管系统的传热模型,对不同管间距埋管的换热量进行模拟比较。双管和多管布置时,由于管间的热干扰作用,系统的换热量将不同程度降低。其中双管布置时,较相同工况下单管换热时换热量减少20%以上;多管布置时,换热量减少50%以上。
利用土壤热源长期应用于工艺供冷的系统是一种新型的系统,目前该系统在实际中还得不到广泛的应用,但是随着常规能源的日益枯竭、能源价格的逐渐上涨、环保压力的不断增加和应用技术的进步,该系统将逐渐成为一种有竞争力的替代传统供冷的新方式。本文的工作可以为今后土壤热源长期直接供冷系统的应用提供理论基础和技术参考。
Due to its advantages such as clean, environment-friendly, renewable, large amount, ground thermal source is considered the most potential natural heat/ cold source for the 21st century. Acting as the main heat source of soil-source heat pump systems, soil thermal energy has been a cynosure of researchers. Currently, ground-source heat pump systems are no longer constricted in conventional heat pump systems; its application has been extended to industrial realm. There are many factors which restrict the application of soil-source heat pump systems, among these factors, heat exchange of ground heat exchanger and ground thermal balance are the crucial ones.
In this paper, ground-source heat pump techniques in our country and abroad are generalized, and then based on this work; underground temperature variation and distribution are analyzed by simulation, when U-tube underground heat exchanger is used for long-term cooling for a special process. This paper takes underground heat exchanger as research subject; the research focuses on the following aspects:
Firstly, a 3-D numerical model of single U-tube using Finite Volume Method is developed, to simulate the underground temperature variation of a ground-source heat pump system which has been continuously running for 10 years in Harbin. The temperature variation and underground temperature distribution are analyzed and compared with different△t, flow rates, grouting materials, and heat transfer coefficients. The influence of grouting materials on heat transfer is not obvious, so in the process of grouting, it is not necessary to consider about intensifying heat transfer but only fix function, so original soil can be used for grouting.
Secondly, on the basis of the established numerical model, in this paper, heat exchange of ground heat exchanger and ground thermal balance of ground-source heat pump in typical regions such as Tianjin, Wuhan, Guangzhou, etc. are simulated and analyzed. The limit heat transfers in these regions are calculated in this paper under setting conditions, and comparisons are made. The limit heat transfer of U-tube heat exchanger for long-term cooling in Tianjin is 1006.51W (22.13W/m); in Wuhan, it is 935.17W (18.7W/m); but in Guangzhou, it is only 100.2W (2W/m). From these we can get that in Guangzhou, the cold source available is very limited if using U-tube heat exchanger for long-term direct cooling, so this type of system is not suitable to be using in Guangzhou which is hot in summer and warm in winter.
Finally, numerical heat transfer models of double and multi-U-tube ground heat exchangers are developed on the basis of that of single-U-tube ground heat exchanger, and the heat transfers of pipes with different spans are simulated and compared. When double and multi-U-tube ground heat exchangers are used, heat transfers of the system decreases due to the thermal disturbance among the tubes. Compared with single-U-tube ground heat exchanger, under the same running status, heat transfer decreased above 20% for double and 50% for multi-U-tube ground heat exchangers.
Using ground-thermal energy for long-term cooling is a new type of heat pump system; currently this type has not been widely applied in practical, but with the exhausting of conventional sources of energy, the roaring of energy prices, the increasing of pressure of environmental protection, and the advancement of technology, this system will gradually become a competitive alternative term of conventional cooling systems. The research work in this paper can supply theoretical support and technological references to future application of ground-source heat pump systems for long-term direct cooling.
引文
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7 Gu Y, O′Neal D L. An Analytic Solution to Transient Heat Conduction in a Composite Region with a Cylindrical Heat Source. Journal of Solar Energy Engineering; Transactions of the ASM, 1995, 117(3): 242-248
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9 Gu Y, O′Neal D L. Modeling the Effect of Backfills on U-tube Ground Coil Performance. ASHRAE Transactions, 1998, 104(2): 356-365
10 Allan M L. Materials Characterization of Super Plasticized Cement-sand Ground. Cement and Concrete Research, 2000, 30(6):937-942
11 Sun SC. Heat Transfer and Soil Thermal Stability as Related to a Plate-type Heat Exchanger in Ground coupled Heat Pump System. Lexington(USA): University of Kentucky, 1998:74-76
12 Elliott H S. Ground-coupled Heat Pump Loop Design Using Thermal Conductivity Testing and the Effect of Different Back Fill Materials on Vertical Bore Length. ASHRAE Transactions, 1998, 104(2): 775-779
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2 Ingersoll L R, Plass H J. Theory of the ground pipe heat source for the heat pump. HPAC. 1948, 20(7):119-122
3 Ingersoll L R, Zobel O J, Ingersill A C. Heat Conduction with Engineering, Geological and Other Applications. New York: McGraw-Hill Co, 1954:45-53
4 Met Z P. Development of a Validated Model of Ground Coupling. Proceedings of the Annual Meeting-American Section of the International Solar Energy Society,1980, 3(1): 344-348
5 Carslaw H S, Jaeger J C. Conduction of Heat in Solids. Oxford: Claremore Press, 1947:260-265
6 Mei V C, Fischer S K. Vertical concentric tube ground coupled heat exchangers.ASHRAE Trans,1983,89(2):391-406
7 Gu Y, O′Neal D L. An Analytic Solution to Transient Heat Conduction in a Composite Region with a Cylindrical Heat Source. Journal of Solar Energy Engineering; Transactions of the ASM, 1995, 117(3): 242-248
8 Gu Y, O′Neal D L. Development of an Equivalent Diameter Expression for Vertical U- tubes used in Ground-coupled Heat Pumps. ASHRAE Transactions. 1998,104(2): 347-355
9 Gu Y, O′Neal D L. Modeling the Effect of Backfills on U-tube Ground Coil Performance. ASHRAE Transactions, 1998, 104(2): 356-365
10 Allan M L. Materials Characterization of Super Plasticized Cement-sand Ground. Cement and Concrete Research, 2000, 30(6):937-942
11 Sun SC. Heat Transfer and Soil Thermal Stability as Related to a Plate-type Heat Exchanger in Ground coupled Heat Pump System. Lexington(USA): University of Kentucky, 1998:74-76
12 Elliott H S. Ground-coupled Heat Pump Loop Design Using Thermal Conductivity Testing and the Effect of Different Back Fill Materials on Vertical Bore Length. ASHRAE Transactions, 1998, 104(2): 775-779
13范萍萍,端木琳.土壤源热泵的发展与研究现状.煤气与热力. 2005:66-69
14李道强.地源热泵空调系统的技术经济评价及地热换热器优化研究.西安建筑科技大学硕士学位论文. 2004:4-5
15朱仕武.地源热泵埋地换热器动态特性及地上机组节能研究.上海海运学院硕士学位论文. 2002:8
16徐秋敏. U型垂直埋管式地源热泵地下传热特性的实验研究.大连理工大学硕士学位论文. 2005
17张昆峰,马芳梅,金六一等.土壤热源与热泵联结运行冬季工况的试验研究.华中理工大学学报. 1996, 24(1): 23-26
18张旭.土壤耦合热泵的实验及其相关基础理论研究.现代空调. 2001(3):1-8
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