热响应测试在土壤热交换器设计中的应用

分析了土壤热交换器系统的影响因素以及设计与施工中存在的问题,介绍了自主研制的移动式地源热响应测试装置原理与构成。针对天津市某地源热泵项目,阐述了热响应测试的方法与步骤,得到了项目所在地的无干扰地温以及地埋管系统的供回水温度响应曲线。利用线源理论,得到了地埋管换热器钻孔的导热系数及热阻,分析了测试装置与环境的热损失和热增益、测试时间、供电稳定性、无干扰地温、不同深度土壤热导率的变化以及地下水流动对热响应测试造成的影响。测试结论对于指导土壤热交换器设计与施工具有一定的参考价值。     

An in situ thermal response test for borehole heat exchangers of the ground-coupled heat pump system

Thermal response tests (TRTs) are crucial for the estimation of the ground thermal properties and thermal performance of the borehole heat exchanger (BHE) of the ground-coupled heat pump (GCHP) system. In this article, a TRT apparatus was designed and built to measure the temperature response of inlet and outlet sections of BHE in the test borehole, the apparatus can effectively operate under both constant heating flux modes and heat injection and extraction modes with a constant inlet temperature. A TRT for a project of GCHP located in the Jiangsu province of China was carried out by the experimental apparatus. Based on the experimental data, the heat transfer performances of BHE under heating and cooling modes were evaluated, and the ground thermal properties, which include the ground thermal conductivity, ground volumetric specific heat, borehole thermal resistance and effective soil thermal resistance, were determined by the line source model. The results indicate that the experimental device and analysis model proposed in this article can be effectively applied to estimate the ground thermal properties and thermal performance of BHE. During the process of thermal response of ground, the fluid temperatures vary acutely at the start-stage of 8 h, and then tend to be a steady state after 40 h. The test data during the start-stage should be discarded for improving the estimation accuracy of ground thermal properties. At the same time, the effective soil thermal resistance increases continuously with time and a steady-state value would be reached after the start-time, and this steady-state thermal resistance can be used to evaluate the required length of BHE. In addition, the heat transfer rate of the BHE under different operating conditions can be used for the further evaluation on long-term operation performance of GCHPs.     

Predicting the fluid temperature at the exit of the vertical ground heat exchangers

The energy analysis of ground source heat pump systems is based on the instantaneous fluid temperature at the ground heat exchanger outlet. This temperature defines the ground source heat pump coefficient of performance (COP) and hence the electricity consumption required in order to fulfill the energy demands of the building. The aim of this work is to present a model able to predict the fluid temperature at the ground heat exchanger outlet, taking into account the heat transfer phenomena in the soil and the temporal variation of the thermal load of the ground heat exchanger. The model developed was verified using experimental data, expanding over a three years period, of a vertical ground heat exchanger. It is proved that the model is able to satisfactorily predict the recorded temperature values throughout the verification period. The differences between measured and estimated outlet water temperatures impose a deviation between the estimated and the actually recorded electricity consumption of less than 4%.     

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。     

竖直埋管地热换热器钻孔内的传热分析

准三维模型为竖直埋管地热换热器的结构优化提供了较为精确的理论基础。利用准三维模型对竖直埋管地热换热器进行分析与研究得出，不同的行程布置对双U型埋管地热换热器的传热性能有较大影响。就钻孔内热阻的对比，双U型埋管比单U型埋管钻孔内的热阻低，因而双U型埋管地热换热器较单U型埋管地热换热器更为合理。     

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.     

Analysis and thermal response test for vertical ground heat exchanger with two U‐loop configuration

An in situ thermal response test (TRT) is applied to evaluate the thermal performance of the vertical ground heat exchanger (GHX) with two U‐loop configuration. A line source method is used to derive the thermal conductivity and borehole thermal resistance from the measured data. Analyses are made to improve the interpretation of TRT data and to investigate the active area of interest in the borehole. Load tests of the GHX are performed to examine the daily variations of ground and mean fluid temperatures associated with daily intermittent operation of ground source heat pump system. Results show that while the ground thermal conductivity of two U‐loop GHX is moderately increased, the borehole thermal resistance is significantly reduced, compared with the single U‐loop GHX. Of the borehole thermal resistance components evaluated, the grout thermal resistance is the most governing one in the borehole heat transfer (77% of the total borehole thermal resistance), whereas the convective thermal resistance in the tube is almost negligible (less than 2%). Copyright © 2015 John Wiley & Sons, Ltd.     

提高地能利用的强化传热试验研究

以太阳辐射积聚大量能量和冷热良性循环蓄集巨大能量的地下是一个庞大的可再生能源库，也是一项可充分利用的自然能量资源。该文介绍了在地源热泵系统中开展的地下100m和200m竖直井闭式循环传热的研究工作，提出利用地下螺旋芯管束新方法，加强旋流流动，提高地下换热能力。试验表明，在放热和吸热过程中，传热均得到显著提高。所提出的可控制间歇过程，将充分发挥换热井的换热能力，实现最少的井数布置，保证良好的地源热泵机组运行工况。     

Exergetic performance evaluation of heat pump systems having various heat sources

In this study heat pump systems having different heat sources were investigated experimentally. Solar‐assisted heat pump (SAHP), ground source heat pump (GSHP) and air source heat pump (ASHP) systems for domestic heating were tested. Additionally, their combination systems, such as solar‐assisted‐ground source heat pump (SAGSHP), solar‐assisted‐air source heat pump (SAASHP) and ground–air source heat pump (GSASHP) were tested. All the heat pump systems were designed and constructed in a test room with 60 m² floor area in Firat University, Elazig (38.41°N, 39.14°E), Turkey. In evaluating the efficiency of heat pump systems, the most commonly used measure is the energy or the first law efficiency, which is modified to a coefficient of performance for heat pump systems. However, for indicating the possibilities for thermodynamic improvement, inadequate energy analysis and exergy analysis are needed. This study presents an exergetic evaluation of SAHP, GSHP and ASHP and their combination systems. The exergy losses in each of the components of the heat pump systems are determined for average values of experimentally measured parameters. Exergy efficiency in each of the components of the heat pump systems is also determined to assess their performances. The coefficient of performance (COP) of the SAHP, GSHP and ASHP were obtained as 2.95, 2.44 and 2.33, whereas the exergy losses of the refrigerant subsystems were found to be 1.342, 1.705 and 1.942 kW, respectively. The COP of SAGSHP, SAASHP and GSASHP as multiple source heat pump systems were also determined to be 3.36, 2.90 and 2.14, whereas the exergy losses of the refrigerant subsystems were approximately 2.13, 2.996 and 3.113 kW, respectively. In addition, multiple source heat pump systems were compared with single source heat pump systems on the basis of the COP. Exergetic performance coefficient (EPC) is introduced and is applied to the heat pump systems having various heat sources. The results imply that the functional forms of the EPC and first law efficiency are different. Results show that Ex_loss,total becomes a minimum value when EPC has a maximum value. Copyright © 2008 John Wiley & Sons, Ltd.     

Back‐analyses of in‐situ thermal response test (TRT) for evaluating ground thermal conductivity

In this study, a series of computational fluid dynamics (CFD) numerical analyses was performed in order to evaluate the performance of six full‐scale closed‐loop vertical ground heat exchangers constructed in a test bed located in Wonju, South Korea. The high‐density polyethylene pipe, borehole grouting and surrounding ground formation were modeled using FLUENT, a finite‐volume method program, for analyzing the heat transfer process of the system. Two user‐defined functions accounting for the difference in the temperatures of the circulating inflow and outflow fluid and the variation of the surrounding ground temperature with depth were adopted in the FLUENT model. The relevant thermal properties of materials measured in laboratory were used in the numerical analyses to compare the thermal efficiency of various types of the heat exchangers installed in the test bed. The numerical simulations provide verification for the in‐situ thermal response test (TRT) results. The numerical analysis with the ground thermal conductivity of 4.0 W/m?K yielded by the back‐analysis was in better agreement with the in‐situ TRT result than with the ground thermal conductivity of 3.0 W/m?K. From the results of CFD back‐analyses, the effective thermal conductivities estimated from both the in‐situ TRT and numerical analysis are smaller than the ground thermal conductivity (=4.0 W/m?K) that is input in the numerical model because of the intrinsic limitation of the line source model that simplifies a borehole assemblage as an infinitely long line source in the homogeneous material. However, the discrepancy between the ground thermal conductivity and the effective thermal conductivity from the in‐situ TRT decreases when borehole resistance decreases with a new three pipe‐type heat exchanger leads to less thermal interference between the inlet and outlet pipes than the conventional U‐loop type heat exchanger. Copyright © 2012 John Wiley & Sons, Ltd.     

R&D of the ground-coupled heat pump technology in China

The ground-coupled heat pump (GCHP) systems have been identified as one of the best sustainable energy technologies for space heating and cooling in residential and commercial buildings. In this paper, research on and development of the GCHP technology in China are summarized. New models are presented for efficient thermal analysis of ground heat exchangers, of which one- and two-dimensional solid cylindrical source models and their analytical solutions are devised to deal with pile ground heat exchangers. Analytical solutions are also derived for vertical and inclined finite line source models as well as for a groundwater advection model. Explicit solutions of a quasi-three-dimensional model can be used to better evaluate the thermal resistance inside boreholes. Studies on hybrid GCHP systems and the thermal response test in China are also commented.     

Feasibility of using ground-coupled condensers in A/C systems

The feasibility of using ground-coupled condensers for air-conditioning (A/C) systems in the Kingdom of Saudi Arabia has been investigated. Temperatures and soil properties required for the performance analysis of one of these condensers were determined experimentally. Thermal response tests were conducted to evaluate the effective thermal conductivity of the ground. The measurements required the installation of a closed loop vertical ground heat exchanger. The results indicated a significant difference between the ambient air and ground temperatures, which would result in an increase in the coefficient of performance and in a reduction in the energy consumption of an A/C unit using a vertical ground heat exchanger rather than an air-cooled condenser. The cost analysis indicated that the use of ground-source heat pumps in Saudi Arabia is not economical at the present time because of low electricity prices and high drilling costs.     

土壤源热泵U型埋管换热器传热的非稳态数值模拟

本文基于能量平衡方程,建立了土壤源热泵U型埋管换热器周围土壤的非稳态传热模型,用所建立的传热模型对土壤源热泵冬季取热过程进行了动态模拟.研究了不同物性土壤温度及U型埋管出口流体温度的变化规律,分析了土壤物性对换热器换热性能的影响,采用间歇运行方式提高了换热器换热能力.通过建立数学模型得出的结果,可供设计参考.     

Heat and mass transfer model of a ground heat exchanger: validation and sensitivity analysis

Validation of ground heat exchanger (GHE) model is presented using the experimental data obtained for both single and double pipe horizontal GHE. Sensitivity analysis of the GHE model shows the influence of the variation in the soil thermal conductivity, specific heat and density on the thermal performance of a GHE. Finally, the thermal performance of a GHE is analysed using both heat and mass transfer models, and conduction only model, as well as the influence of the initial soil moisture content on the thermal performance of a GHE. © 1998 John Wiley & Sons, Ltd.     

地源热泵地下岩土热物性的测定方法

提出了可用于现场测量计算地下岩土综合热物性参数的方法。垂直埋设在地下的换热器与周围岩土换热过程可以近似地看作半无限大介质中常热流边界条件下的非稳态导热过程来处理。这种方法克服了其他常用研究模型对钻孔中埋管的具体位置、上升管及下降管之间的距离、换热器材料和回填材料的物性参数的要求,相应地消除了上述各个参数所带来的误差。通过测量地埋换热器的循环水流量、进出水口温度,加热器的加热功率等数据确定地下岩土综合的热物性参数。     

中深层地热Ｕ型井地埋管换热器的试验研究

地热能作为一种非碳基能源,具有储量丰富、清洁可再生等特点,开发利用地热能有助于碳达峰的实现。在中深层地源热泵领域,我国主要以单井同轴管为主,而相对高效的中深层地热U型井地埋管案例屈指可数。为了了解中深层地热U型井地埋管换热性能及井下换热参数变化,完成了新型的Ｕ型井地埋管换热器工程,并在此基础上进行了实验研究。首先,开展了地温测量,确定了研究区的地层温度,根据热储的物性条件选取了水平井段及对接位置;其次,分析空载循环试验工况下循环水的流量及井下温度的变化情况,研究了负载工况下供回水温度、流量、换热量、不同井段对换热的贡献率、井下温度的动态变化、U型井的恢复能力等因素。实验结果表明,中深层Ｕ型井地埋管换热器井底温度会随运行时间增长而降低,流量大且回水温度较低的情况下,换热器的换热量比较高,最高为1336.8kW;回水井对换热量的增加有限,每百米增加0.12℃,实际工程中可以考虑减小口径,降低建设费用。U型井地埋管换热器的地温恢复能力较强,停止运行24h左右井底温度与初始温度差为-13℃。研究结果有助于研究人员对中深层Ｕ型井地埋管换热器有更进一步的认识,从而推动中深层地热能的健康可持续发展。     

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...     

Performance of a spiral ground heat exchanger for heat pump application

A high-efficiency ground heat exchanger has been developed for use with ground-source heat pumps. The exchanger is made of copper tubing, shaped in the form of a spiral, which can be installed in a vertical borehole backfilled with sand. Thermal performance of a full-scale prototype indicated that this heat exchanger can achieve very high heat extraction rates if subfreezing operating temperatures are used. For most soil types cyclic freezing and thawing is not a problem; however, for the sensitive Leda clay in which the prototype tests were conducted, substantial settlement occurred after the first freeze-thaw cycle owing to initial collapse of the soil structure.     

Evaluation of the performance of a ground-source heat-pump system with series GHE (ground heat exchanger) in the cold climate region

Ground-source heat-pump systems provide a new and clean way of heating buildings in the world. They make use of renewable energy stored in the ground, providing one of the most energy-efficient ways of heating buildings. Consumption costs are lowered through the use of free energy from the environment, and the dependence on fossil fuels simultaneously reduces. The aim of this study is to evaluate the performance of vertical ground-source heat-pump system for climatic condition of Erzurum having cold climate in Turkey. For this purpose, an experimental set-up was constructed. The experimental apparatus consisted of a series GHE (ground heat exchanger), a liquid-to-liquid vapor compression heat pump, water circulating pumps and other measurement equipments. In this study, the performance of the system was experimentally investigated. The experimental results were obtained from October to May for the months of heating season of 2008–2009. The experimental results indicate that the average heat-pump COP and overall system’s COPS values are approximately 3.0 and 2.6 in the coldest months of heating season. This study also shows that this system could be used for residential heating in the province of Erzurum being a cold climate region of Turkey.     

An experimental study of a direct expansion ground‐coupled heat pump system in heating mode

In this paper, an experimental performance evaluation of a direct expansion ground‐coupled heat pump (DX‐GCHP) system in heating mode is presented. The DX‐GCHP uses R134a as the refrigerant, and consists of three single U‐tube copper ground heat exchangers (GHEs) placed in three 30 m vertical boreholes. During the on–off operations from December 25, 2007, to February 6, 2008, the heat pump supplied hot water to fan‐coil at around 50.4°C, and its heating capacity was about 6.43 kW. The energy‐based heating coefficient of performance (COP) values of the heat pump and the whole system were found to be on average 3.55 and 3.28 at an evaporating temperature of 3.14°C and a condensing temperature of 53.4°C, respectively. The second law efficiency on the DX‐GCHP unit basis was around 0.36. The exergetic COP values of the heat pump and the whole system were obtained to be 0.599 and 0.553 (the reference state temperature was set equal to the average outdoor temperature of ?1.66°C during the tests), respectively. The authors also discussed some practical points such as the heat extraction rate from the ground, refrigerant charge and two possible new configurations to simultaneously deal with maldistribution and instability of parallel GHE evaporators. This paper may reveal insights that will aid more efficient design and improvement for potential investigators, designers and operators of such DX‐GCHP systems. Copyright © 2009 John Wiley & Sons, Ltd.