摘要
能源桩将地源热泵系统与传统桩基结合,具有换热效率高、占用空间少和成本低等优点。梳理近年来关于能源桩的传热和承载性能两个方面的研究进展,分析能源桩传热性能的影响因素及机理、热—力耦合状态下的结构响应及传热、结构响应的数值模型。在此基础上,提出能源桩下一步的研究建议与展望。分析指出:螺旋形管体的换热面积最大,换热效率最佳;在循环剪切作用下,能源桩的桩—土界面侧摩阻力衰减,使桩基承载力不断弱化;此外,一个周期中的制冷/制热需求不均可能引起地表温度失衡,从而影响换热效率。能源桩的优化设计及长期服役的可靠性将是未来研究的重点。
随着社会经济的快速发展,各行各业对能源的需求量急剧上升。当前主要供应能源为煤炭、石油和天然气等化石能源。传统的化石能源虽能满足目前人类社会对能源的大部分需求,但也向大气中排放了大量温室气体,加剧了气候变
能源桩的传热是一个复杂的水—热耦合过程,伴随着热传导和对流换热等传热形式,在持续换热过程中,能源桩在桩与桩周围土体中产生温度场,使桩体承受了额外的温度应力。笔者针对能源桩传热过程中涉及的影响因素、传热模型及温度载荷作用下的结构响应状态进行系统地归纳总结,并指出目前研究中的不足和未来工作的方向,供研究人员参考。
流体与换热管间通过热对流及热传导进行热交换,通常用换热率和单位管长换热率()来衡量换热性能的大小,分别用
(1) |
(2) |
式中:为换热率,W;为单位管长换热率,W/m;C为管内流体的比热容,J/(kg·K);为流量,kg/s;为进出口流体温度之差,K;L为管体长度,m。
换热率表示单位时间内能源桩与外界发生热交换的规模,数值越大表明桩体与外界热交换越充分;单位管长换热率则描述了管体单位长度的换热量,是衡量换热效率的指标。一般来说,二者数值越大,换热性能越好。影响换热性能的主要因素有管型、螺旋形管桩螺距、循环介质流速和进水口温度等。
1)管型。能源桩管型是影响流体与管壁热对流的决定性因素。管型的设计原则是使管的换热面积最大化。常见的管型有单U型、双U型、三U型、W型、螺旋型等,详见

图1 换热管布设形式示意图
Fig. 1 Schematic diagram of heat exchange tube layout forms
2)螺旋形管桩螺距。Park
3)循环介质流速。在流速水平较低时,流体流动状态近似层流,换热率很低。此时增大流体流速会使流体与管壁的对流换热系数提高,换热量增加,换热率明显提高。但流速增加到一定值后,由于流体在管内的换热时间缩短,换热不充分,换热率反而降
4)进水口温度。若无外界因素干扰,进口温度主要由当地气温决定。由
混凝土热物性和桩体几何尺寸是制约桩体内导热特性的主要因素。混凝土是桩身与土进行热交换的介质,其传热性能通过导热系数来衡量。研究表明,当混凝土导热系数从1.2 W/(m·K)增加到2.5 W/(m·K)时,能源桩的传热性能可提高42
总的来看,适当增加含水率有利于增强土壤的储热和传热能力。一方面,随着含水率的增加,一部分孔隙被水填充,显著提高了土壤比热容,增强了地面对冷能(热能)的储存能
土壤固相对导热系数的影响可以通过土的矿物成分和干密度进行分析。不同土矿物成分导热系数有明显差别,如石英的导热系数约为7~9 W/(m∙K),而云母、高岭石及长石约2~3 W/(m∙K)。为了量化由不同矿物成分组合而成的土粒的导热系数,Johanse
土的干密度越大,其单位体积的孔隙就越少,即孔隙率越小。土壤中气体主要以自由态存在于土壤的孔隙中,极少部分以吸附态吸附在土壤颗粒表面或以溶解态溶解在水中。总体而言,土壤固体颗粒形状、结构和排列方式决定孔隙性状、尺寸和分布,进而影响导热系

图2 不同含水率的砂土/黏壤土导热系数与干密度的关
Fig. 2 Relationship between thermal conductivity and dry density of sandy/clay loam with different moisture conten
在地源热泵系统中,热辐射传导的热量忽略不计,主要由热传导和热对流产生传热作用。一般来说,桩—土间的传热方式主要为热传导,但如果土层中存在丰富的地下水流动参与换热,换热方式将以热对流为主。地下换热器的传热模型是发展地源热泵系统的前提,一个准确而恰当的模型能为系统的整个传热过程提供有效的预测和指导。
线热源模型将桩—土间的传热过程简化为以一个无限长的恒定热流密度的热源为中心,以辐射状向外传热。线热源模型可分为无限长线热源模型与有限长线热源模型。Mogensen于1983年提出了无限长线热源模型的解析解。在此基础上,Eskilson于1987年提出了有限长热源模型,该模型能更好地描述地源热泵系统埋地部分长时间运行状况下的传热过程。Zeng
圆柱热源模型分为空心和实心两类。王子阳
一般认为,地下水的存在对能源桩传热性能有积极影响。地下水位(土中孔隙的位面和饱和水分的位面之间的分界线)以下的水分在土层颗粒之间移动,形成水平流,而水平流可以降低桩周土温度,缓解地层冷热积累,改善传热性能,最终使土温趋于稳定。笔者以时间为序整理了若干瞬态传热模型,如
与常规桩基础不同,承载时能源桩,与外界发生持续的换热作用,产生额外温度载荷。热量的转移将改变桩体温度,使桩体膨胀(收缩)。受限于桩周围岩土体的约束,桩基无法发生自由变形,在桩内部会产生额外的温度应力。实际工程中,能源桩的桩型大多是摩擦桩,桩周土体施加的摩阻力和桩端约束力与外力相平衡。Amatya

(a) 升温工况下的应力

(b) 升温+上覆载荷工况下的应力

(c) 降温工况下的应力

(d) 降温+上覆载荷工况下的应力
图3 有温度载荷作用时桩基轴力分
Fig. 3 Axial force distribution of the energy pile under temperature loadin
一般来说,上覆载荷水平较低时,随着摩阻力的积累,轴向应力逐渐减小,到达一定深度后变为零,该零点处往下部分的应力水平主要受温度载荷影响。现场试验结果也证实了这一点。Laloui
影响能源桩应力和位移的主要因素包括上部荷载刚度、桩端土体刚度以及桩内埋管形式等。王成龙
能源桩与土发生热传导,引起土体温度沿桩体径向变化,从而引起土体物理力学性质发生改变。从土的角度看,影响桩的承载力和性能的主要因素包括水—热耦合过程中土壤的热硬化、热诱导水流、超静孔隙水压力的发展和热固结后的体积变化。对于正常固结土,升温条件下塑性应变的硬化效应与等应力排水条件下边界面产生的软化效应相抵消,土体产生塑性压缩,塑性压缩变形远大于土体骨架热弹性膨胀变形,故总体表现为体积缩小。加热导致土体在孔隙水排出过程中发生固结,土粒之间结合更加紧密,抗剪强度增
Yazdani

图4 热循环状态下土的温度与孔隙压力呈周期性变
Fig. 4 The temperature and pore pressure of soil under thermal cycling state change periodicall
对于不会发生固结或固结程度较低的砂土,温度变化引起的体积膨胀(收缩)很
在岩土工程设计中往往需要考虑桩基沉降。过度的桩基均匀沉降使得建筑物高程降低,影响建筑正常使用,而不均匀沉降更会使建筑产生附加应力而引起裂缝,甚至局部构件断裂。桩体沉降由桩本身的弹性压缩和桩端土体压缩产生的桩端沉降组成。McCartney

图5 不同温度下桩端沉降量与轴向载荷的关
Fig. 5 Relationship between pile tip settlement and axial load at different temperature
陆浩杰
对于群桩,温度荷载会引起桩群的内力重分布。随着附加沉降的累积,能源桩侧阻力及桩端阻力退化,导致常规桩的承载压力增大,可能导致基础失效。Peng
热塑性效应在土体热机械响应中发挥着重要作用,非等温条件下的土体力学行为建模需要先进的弹塑性应力—应变理论框架。Hueckel
基于有限元方法的数值模型是除试验之外的有利补充。Rotta Loria
传热性能是能源桩的核心性能,不少学者针对能源桩传热状态的影响因素做了大量研究。但总的来看,仍存在一些不足:
1)考虑到换热范围有限,能源桩的作用对象大多为低层建筑,因此,设计桩长一般不大。在一定土层深度范围内,土体温度梯度变化的范围有限。在寒冷地带,环境温度与土层温度相差较大,地源热泵热效率较高,因此,较为成功的地源热泵相关案例大都在偏冷地区;而在温度较高的地区,如热带和亚热带地区,地源热泵换热效率较低。作为地源热泵的特殊形式,能源桩在温暖地带的经济性值得探讨。
2)长期换热的稳定性是能源桩急需解决的问题。季节性负荷(冷暖季供暖与制冷需求不平衡导致的地层温度失衡)是影响能源桩长期换热性能的主要因素。在地下水丰富的区域,地下水的流动可在相当程度上缓解地层温度的失衡,而在无地下水区域,则需对土层进行一定程度的热量补偿,但有效的热量补偿形式有待设计和完善。
3)换热管长期承受循环介质的冲刷与腐蚀,其耐久性需引起重视。管体破损除了影响换热效率外,由于流体直接与桩体材料接触,还可能降低混凝土桩体的承载力和耐久性。此外,换热管的更替技术也有待研究。
4)换热管、桩间距布置不当将会产生热干扰现象,但关于二者对换热效率影响程度的量化研究仍有待深入。在此基础上,应结合工程实际,兼顾成本、结构安全等要素,优化基础设计,确保桩体换热与承载性能长期稳定。
无限长线热源模型运用于地源热泵系统将会受到一定的限制,计算结果与实际有一定偏离。原因在于该模型只是孤立地考虑单管传热,而对于管间的热干扰及运行时间对周围土体的影响都没有考虑。但由于该解析式简单易用,至今在地下换热器中的应用仍十分广泛。用有限长线热源模型可得稳态温度场,系统长时间运行后热源周边温度场趋于稳定,较之无限长线热源模型结果更为精确,但计算更为复杂。
空心圆柱热源模型考虑了钻孔实际形状的导热影响,比线热源模型更为完善。但当传热时间较短或圆柱体尺寸过大、回填材料及埋管的导热性不可忽视的时候,会与实际产生较大误差。实心圆柱模型主要针对螺旋埋管桩和时间步长较短的加热过程。该模型的解析解表达式简单,便于数值计算。有地下水存在的情形下,换热条件较为复杂,考虑水流流速、土体渗流特性及桩基轴向传热,提出瞬态热源模型,大大提高了能源桩传热模型的适用性。
除能源桩本身的几何构型外,影响建模准确度的因素还很多,如地面温度分布、土壤含水量及其热物性、地下水运动等。为简化分析过程,当前大多数模型都忽略了上述变量的影响。
从时间上看,地面温度是一个变量,时间跨度较长时,地温的变化不可忽略;从空间上看,地面温度场也并非均匀分布,水平与竖直方向的分布有所差异。常见的模型都将地表温度设为恒定(一般为0 ℃),但若地表温差形成的温度梯度过大时,将对模型的精确度造成相当程度的影响。
土壤含水率及其热物性与土壤的导热系数息息相关,当前主要通过假设土层为均匀介质,测试几种不同类型的天然土的导热系数,引入参数,按建议参数取值并保持不变,但实际上土的导热系数在不同类型的土和不同含水率下的连续变化很难通过参数取值进行预测,同时,热量的传递将引起土的渗透性和力学特性的改变,这种变化又反过来影响土的热物性。如何描述土的热物性是建立模型的挑战之一。
能源桩在含水地层的热交换可以看作是土颗粒与孔隙水的热传导、地下水运动时与桩及桩周土的热对流。整个过程是各因素相互耦合的瞬态过程,但当前主要用稳态模型进行模拟,有相当的局限性。另外,出于成本和操作难度的考量,实际工程中很难获取准确的水文地质信息。综上所述,各传热影响因素间的相互耦合十分复杂,仍存在很多亟待解决的问题。
桩周土体的热响应对桩基承载力的影响是多方面的。温度变化引起桩体周期性的胀缩变形,改变了桩—土接触状态。可近似认为桩周土承受了循环剪切作用,桩—土接触应力可能随桩体温度循环而逐渐衰减,使桩侧摩阻力减小,不断弱化桩基的承载性能。另外,对于黏性土而言,热循环作用影响了土体的胀缩特性,使桩体产生刚性位移。热循环引起土中水分的定向迁移,使土体干缩/膨胀。温度变化将使土体产生弹性变形和塑性变形,温度升高时,同时存在弹性和塑性变形, 塑性变形随着土超固结度的增大而减小;温度降低时,则只有弹性变形。循环干缩/膨胀会使黏性土颗粒的集聚和排列方式发生变化,从而引起土体微观结构的改变。在早期的干湿循环中,由于土体结构连结效果减弱,土体胀缩特性较为敏感。随着循环次数的增加,达到某种平衡状态,土体的循环胀缩特性趋于稳定,但由于存在塑性变形,胀缩过程并不完全可逆。
区别于常规桩基,能源桩在热—力耦合下的承载特性有明显区别。桩基与土体存在热物性上的显著差异,在温度作用下会产生不同程度的热胀冷缩变形,使桩周土体对桩体的约束产生变化,桩土界面法向接触应力随之改变。目前提出的模型主要基于荷载传递法,即假设应力与位移的传递函数(主要为折线、双曲线模型及指数模型等),预估温度位移零点位置,将桩体单元由温度变化引起的变形作为变量,以桩体整体平衡条件作为控制方程,通过迭代计算得到桩的侧摩阻力及桩端阻力。但传统荷载传递模型不能反映桩—土界面加/卸载循环剪切性状和桩—土界面反向加载时的残余位移,也不能反映桩侧土体固结过程中桩—土界面法向应力递增时桩—土界面的剪切力学特性。上述传递函数均未考虑温度变化对土体力学行为和桩—土函数传递关系的影响,尤其对力学性质随温度有明显变化的黏性土来说,模型的有效性需要验证。模型还需要关注到土壤的热—力耦合效应,而当前考虑热效应的土壤本构模型比较复杂,再叠加能源桩的循环温度荷载,实际的桩—土工作状态的描述仍有待探讨。同时,考虑循环温度荷载、群桩效应、桩端约束形式和间歇运行等因素的桩基计算也是未来研究的重、难点。
针对开发利用地热能的迫切需求,深入分析了能源桩传热及承载特性,描述了传热效率的影响因素,介绍了桩基结构响应,阐述了传热模型和荷载传递模型的研究现状并进行了综合评价。
1)传热影响因素方面,当前研究主要集中在换热管管型和布置形式、流体温度和流速以及桩身和土体导热系数等宏观层面,笔者认为后续的研究应更多地考虑微观层面,如液—桩—土间的接触状态、桩身材料、桩周土颗粒形态和颗粒间的接触形式等,以传热学为基础,对能源桩传热全过程进行理论及试验分析,从根本上了解各因素对能源桩换热性能的影响。
2)传热模型方面,当前主流的模型皆以直线形、圆柱形热源模型为基础,二者有着各自的适用性和局限性,需视情况进行选取。考虑地温边界、土壤热物性变化和地下水等条件的模型尚待提出和完善,同时应更加注重原位试验,以验证模型的准确性。
3)结构响应方面,研究表明,温度应力作用下,桩基承载力会有一定衰减,但具体机理并不十分清楚。应从桩—土热交换本质入手,考虑桩—土接触机理和土的自然历史条件等因素,全面评估热交换对承载能力的影响。
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