软土地区竖直地埋管换热器参数与换热性能分析

地埋管换热器是土壤源热泵系统的关键部件,选择合理的设计参数是对土壤源热泵系统工程进行优化设计的基础。依托上海市典型地质条件建立单U垂直地埋管换热器三维非稳态传热模型,分别分析了上海市地质条件、换热器结构、循环流体、运行模式等因素对地埋管换热器换热性能的影响,并通过回归分析得到地埋管换热器换热效率的变化率与主要影响因素的关系式,使各影响因素的敏感度得到量化,对于影响地埋管换热器换热性能的参数,根据影响程度由大到小依次为循环水平均温度、原始地温、换热孔深度、岩土体导热系数、回填料导热系数。研究结果为土壤源热泵系统的实际应用提供参考。     

不同地质条件下地埋管热响应测试分析与比较

为获取不同地质条件下地埋管换热器换热特性,选取位于夏热冬冷地区的三个测试地点:浙江莫干山、上海浦江镇、江苏盐城。分别对所在地的地源热泵系统地埋管换热器开展热响应试验,利用线热源模型进行计算分析。得到莫干山、浦江、盐城的岩土导热系数分别为2.40 W/(m·℃)、1.92 W/(m·℃)、1.84 W/(m·℃)。岩石地质所具有的高热扩散性对地埋管换热器换热效果有促进作用;不能用"单位延米换热量"这个单一参数分析地埋管换热器换热效果。     

废弃冻结管作为地源热泵换热器换热量的计算研究

采用废弃的冻结管替代传统地埋管地源热泵换热器,实现浅部和深部的冷量和热量的利用是热泵技术一种新的取热途径。阐述了地埋管地源热泵换热器的换热量的计算过程,并以某工程实例为依据,采用换热量计算模型,对该工程地埋管地源热泵进行了冬夏季换热量测试。通过对测试数据比较,最终得到了该工程可靠的地埋管换热性能参数。     

不同形式地埋管换热器运行性能影响因素研究

为研究单U和双U地埋管换热器运行性能的影响因素,运用能耗模拟软件TRNSYS建立地埋管换热器模型对其运行特性进行仿真模拟后,与实际工程数据对比,验证模型有效性,继而以换热器能效系数为评价指标,分别对钻孔深度、U型管两管间距和回填材料导热系数等影响因素进行分析研究,得到:双U地埋管换热器的单位延米换热量为106.72 W/m,单U地埋管换热器的单位延米换热量为86.39 W/m;适当增加钻井深度、埋管间距和回填材料导热系数对整体换热效果有促进作用,且在双U埋管情况下促进作用更显著的结论.     

青岛地区水平埋管换热器换热特性分析

水平埋管换热器的换热性能对地源热泵系统的运行节能性有重要影响。以青岛棕壤地区为例,建立了考虑太阳辐射、地表温度波动、土壤竖直方向上的温度梯度等因素的水平埋管换热器传热模型,模拟了埋管深度、土壤导热系数对水平埋管换热器换热性能的影响。研究结果表明:热泵系统制冷工况下,埋深2 m和2.5 m时对应的换热量较埋深1.5 m时的对应值分别增加了10.4%和15.4%;土壤导热系数由1.278 W/(m·K)增加到3.278 W/(m·K)时,水平埋管的换热量增加了68.24%;HGHE的传热效率随着埋深和导热系数的增加而不断提高。以埋深2 m为界,上部区域的土壤温度受气温和太阳辐射的影响较大,下部区域主要受地埋管换热影响。     

西北地区地源热泵工程运用分析

结合具体工程案例,本文分析了地源热泵技术在西北地区的工程运用及设计步骤。文中详细介绍了地埋管换热器的换热特性测试系统及测试方法,并依照测试结果,设计地源热泵联合太阳能系统。通过对垂直U形地埋管换热器冬夏季取热和排热测试,分析不同换热工况下单双U管换热器换热特性,结合当地钻孔难度大以及地埋管换热器取热量小的特点,采用双U地埋管换热器。测试过程中,对不同入口温度、流量工况换热能力进行测试分析,提出在某些地区,地源热泵工程设计采用"大流量,小温差"的设计理念。     

陕西渭南地区岩土热响应测试与分析

地层热物性参数的确定对浅层地热能地源热泵系统的设计至关重要。依托陕西渭南某地源热泵项目,应用恒流法进行了岩土热响应测试,测试共进行了58 h,根据线热源理论对测试数据进行分析计算,求得准确的地层热物性参数,得到工区地层导热系数为2.16 W/(m∙K),比热容为2.39 MJ/(m³∙K),单U地埋管每延米换热量45 W。该研究结果可为项目后期设计与施工提供了相关参考和依据。     

同轴地埋管换热器岩土热响应试验研究

岩土热物理性质是影响地源热泵系统设计和运营的关键因素,对位于武汉市洪山区的2口不同深度的同轴地埋管换热孔分别进行48 h的热响应试验,并对同轴地埋管换热器内外管之间环形空间中的平均流体温度进行测试.根据同轴地埋管换热器的几何特性,以简便实用的方式测量同轴地埋管换热器环状空间传热流体的平均温度,结合同轴地埋管换热器钻孔热...     

套管式中深层地埋管换热器传热建模及取热分析

为了研究无干扰换热条件下,中深层地热能的实际取热性能,文章通过数值模拟方法模拟计算了套管式中深层地埋管换热器的名义取热量。模拟结果表明,套管式中深层地埋管换热器的名义取热量随着钻孔深度、大地热流、循环水流量、当地大气年平均温度的增加而增加。套管式中深层地埋管换热器周围土层的地质条件分布也影响着中深层地埋管换热器的名义取热量,具体表现为浅层土层的导热系数越小,中深层地埋管换热器的名义取热量越大;深层土层的导热系数越大,中深层地埋管换热器的名义取热量也越大。通过调整地埋管换热器的相关参数,并选择合适的地埋管埋设地点等优化措施,可使套管式中深层地埋管换热器达到可观的名义取热量。     

分层热物性测试在热泵工程的应用分析

以工程场地测试孔内岩土体垂直分布的热物性参数为研究对象。在传统热物性测试仪器中加入分布式光纤测温系统,进行岩土体初温测试和热物性测试,测定不同深度岩土体初温和加热试验中不同深度循环流体平均温度,并与传统测试结果进行对比分析。结果显示,分层热物性测试结果较为准确,可分析岩土体垂直方向温度分布,进而得出地埋管换热器在垂直方向上导热系数和传热系数。     

地埋管间断工作进出孔平均温度预测方法研究

按不改变每天总换热量的原则,将单位长度地埋管换热孔在制冷季或制热季中每天的实际释热或取热过程简化为一个矩形释热或取热脉冲,脉冲大小为单位长度地埋管换热孔的设计释热量或设计取热量,时间为每天的等效满负荷释热或取热小时数。采用线热源理论和热流叠加原理,推导若干个矩形脉冲负荷作用后地埋管换热器进出口温度平均值的计算公式,并通过长期现场岩土热响应试验对该公式进行了验证。在已知制冷季或制热季天数和地埋管换热器每天等效满负荷工作小时的基础上,通过设定地埋管在制冷季和制热季传热流体的最高或最低温度,可用该公式计算单位长度地埋管换热孔的设计释热量或取热量。     

The quantitative evaluation of design parameter's effects on a ground source heat pump system

The main solution for the reduction of energy consumption in the field of HVAC is the development of new and renewable energy technologies. Among the various renewable energy systems, ground source heat pump (GSHP) systems have been spotlighted as efficient building energy systems because of their great potentials for energy reduction in building air conditioning and reducing CO₂ emissions. However, higher initial cost works as a barrier to the promotion of their use. Therefore, it is critical to reduce the initial costs by optimizing the design of the system. In this paper, parameters that affect the performance of the GSHP system and the size of ground loop heat exchanger (GLHX) have been investigated. Ratio of GLHX length to unit capacity (L/Q) decreased according to increasing value of thermal conductivity, but L/Q increased according to increasing value of borehole heat transfer resistance. In cooling mode, L/Q decreased according to increasing EWT of underground circulating water and borehole distance but increased in heating mode. The value of L/Q tended to increase according to increasing underground initial temperature in cooling mode, but decreased in heating mode. L/Q decreased according to increasing U-tube separation distance and decreasing underground circulating water flow rate, because the thermal interference effect of underground circulating water and heat absorption and emission rate from the ground decreased. The reduction of the size of GLHX is very important in the aspect of saving total installation cost of a GSHP system. Therefore, the size of GLHX and the performance of GSHP system should be considered together for optimum design of the GSHP system.     

Experimental performance evaluation of a closed‐loop vertical ground source heat pump in the heating mode using energy analysis method

An experimental study is performed to determine the performance of a ground source heat pump (GSHP) system in the heating mode in the city of Erzurum, Turkey. The GSHP system using R‐134a as refrigerant has a single U‐tube ground heat exchanger (GHE) made of polyethylene pipe with a 16 mm inside diameter. The GHE was placed in a vertical borehole with 55 m depth and 203.2 mm diameter. The average coefficients of performance (COP) of the GSHP system and heat pump in heating mode are calculated as 2.09 and 2.57, respectively. The heat extraction rate per meter of the borehole is determined as 33.60 W m^?1. Considering the current gas and electric prices in Erzurum city, the equivalent COP of the GSHP system should be 2.92 for the same energy cost comparing with natural gas. The virgin ground in Erzurum basin has high permeability and low thermal conductivity. In order to improve the thermal efficiency of GHE and thus improve COP of a GSHP in the basin, the borehole should be backfilled with sand as low‐cost backfill material and a 1 to 2 m thick surface plug of clay should be inserted. Copyright © 2007 John Wiley & Sons, Ltd.     

Optimisation for interconnected energy hub system with combined ground source heat pump and borehole thermal storage

Ground source heat pumps (GSHP) give zero-carbon emission heating at a residential level. However, as the heat is discharged, the temperature of the ground drops, leading to a poorer efficiency. Borehole inter-seasonal thermal storage coupled with GSHP maintains the efficiency at a high level. To adequately utilize the high performance of combined GSHP and the borehole system to further increase system efficiency and reduce cost, such a combined heating system is incorporated into the interconnected multi-carrier system to support the heat load of a community. The borehole finite element (FE) model and an equivalent borehole transfer function are proposed and respectively applied to the optimisation to analyze the variation of GSHP performance over the entire optimisation time horizon of 24 h. The results validate the borehole transfer function, and the optimisation computation time is reduced by 17 times compared with the optimisation using the FE model.     

Evaluation of heat exchange rate of GHE in geothermal heat pump systems

Total thermal resistance of ground heat exchanger (GHE) is comprised of that of the soil and inside the borehole. The thermal resistance of soil can be calculated using the linear source theory and cylindrical source theory, while that inside the borehole is more complicated due to the integrated resistance of fluid convection, and the conduction through pipe and grout. Present study evaluates heat exchange rate per depth of GHE by calculating the total thermal resistance, and compares different methods to analyze their similarities and differences for engineering applications. The effects of seven separate factors, running time, shank spacing, depth of borehole, velocity in the pipe, thermal conductivity of grout, inlet temperature and soil type, on the thermal resistance and heat exchange rate are analyzed. Experimental data from several real geothermal heat pump (GHP) applications in Shanghai are used to validate the present calculations. The observations from this study are to provide some guidelines for the design of GHE in GHP systems.     

A review of methods to evaluate borehole thermal resistances in geothermal heat-pump systems

In the design of a ground-source heat pump (GSHP) system, the heat transfer from the fluid to the ground is influenced by the thermal borehole resistance between the fluid and the borehole surface and also by the interference resistance between the two (or four) pipes inside the borehole. Several authors have proposed empirical and theoretical relations to evaluate these resistances as well as methods to evaluate them experimentally. The paper compares the different approaches and proposes good practice to evaluate the resistances. The impact of the different approaches on the design of heat exchanger is also examined. Two-dimensional and fully three-dimensional numerical simulations are used to evaluate the different methods. A new method is also proposed to evaluate the borehole resistances from in situ tests.     

Reference data sets for vertical borehole ground heat exchanger models and thermal response test analysis

Ground source heat pump systems often use vertical boreholes to exchange heat with the ground. Two areas of active research are the development of models to predict the thermal performance of vertical boreholes and improved procedures for analysis of in situ thermal conductivity tests, commonly known as thermal response tests (TRT). Both the models and analysis procedures ultimately need to be validated by comparing them to actual borehole data sets. This paper describes reference data sets for researchers to test their borehole models. The data sets are from a large laboratory “sandbox” containing a borehole with a U-tube. The tests are made under more controlled conditions than can be obtained in field tests. Thermal response tests on the borehole include temperature measurements on the borehole wall and within the surrounding soil, which are not usually available in field tests. The test data provide independent values of soil thermal conductivity and borehole thermal resistance for verifying borehole models and TRT analysis procedures. As an illustration, several borehole models are compared with one of the thermal response tests.     

Sequential estimation of borehole resistance and ground thermal properties through thermal response test

Borehole thermal resistance and ground thermal properties (thermal conductivity and heat capacity) are the key parameters to implement the ground source heat pump (GSHP), usually obtained by thermal response test. In this study, a novel sequential parameters estimation method for the above three parameters is proposed, and the sensitivity analysis by using a special correlation method is performed to decide the best estimation sequences. At first, the Spearman partial rank correlation coefficient was used to represent the correlation between the estimated thermal properties and fluid temperature for the line source model (ILS), then the estimation sequence for the three parameters could be determined by the correlation results. Lastly, with the estimation step, Monte Carlo method was adopted to determine the parameters replacing conventional iterative algorithms. In addition, the effect of value bounds and initial inputs as well as random samples was investigated. The results showed that compared to the other estimation steps, the estimation sequence following borehole resistance firstly, then thermal conductivity, heat capacity lastly could get the best precision with 4.5%, 0.4%, 1% respectively. Specially, the estimation precision for ground heat capacity could be promoted by the sequential estimation. Also, the effect of value bounds on estimation precision was nearly eliminated by the proposed method.