A two-region simulation model of vertical U-tube ground heat exchanger and its experimental verification

Heat transfer around vertical ground heat exchanger (GHE) is a common problem for the design and simulation of ground coupled heat pump (GCHP). In this paper, an updated two-region vertical U-tube GHE analytical model, which is fit for system dynamic simulation of GCHP, is proposed and developed. It divides the heat transfer region of GHE into two parts at the boundary of borehole wall, and the two regions are coupled by the temperature of borehole wall. Both steady and transient heat transfer method are used to analyze the heat transfer process inside and outside borehole, respectively. The transient borehole wall temperature is calculated for the soil region outside borehole by use of a variable heat flux cylindrical source model. As for the region inside borehole, considering the variation of fluid temperature along the borehole length and the heat interference between two adjacent legs of U-tube, a quasi-three dimensional steady-state heat transfer analytical model for the borehole is developed based on the element energy conservation. The implement process of the model used in the dynamic simulation of GCHPs is illuminated in detail and the application calculation example for it is also presented. The experimental validation on the model is performed in a solar-geothermal multifunctional heat pump experiment system with two vertical boreholes and each with a 30 m vertical 1 1/4 in nominal diameter HDPE single U-tube GHE, the results indicate that the calculated fluid outlet temperatures of GHE by the model are agreed well with the corresponding test data and the guess relative error is less than 6%.     

别墅型建筑地源热泵空调系统设计

地源热泵是一种利用土壤所储藏的太阳能资源作为冷热源进行能量转换的供暖制冷空调系统,通过输入少量的高品位能源(如电力、机械功、燃气和液体燃料),实现热量从低温热源向高温热源的转移.以上海某小型别墅为对象,设计了一套家用地源热泵空调系统.首先计算了夏季冷负荷和冬季热负荷,然后根据冷、热负荷选择一套水源热泵机组(MWH080CR型机组)和相应的风机盘管,进行了室内水管环路系统、土壤热交换器和地板采暖的设计选型,最后对系统的能效比进行了计算.结果表明,该空调系统具有节能环保、稳定可靠、舒适耐用等优点.     

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.     

基于预测的复合地源热泵系统控制方法实现

在冬冷夏热且夏季冷负荷远大于冬季热负荷的地区常采用带有冷却塔的复合式地源热泵系统,其控制策略存在极大的优化空间。文章提出了直接比较冷却塔和与土壤换热器相连的板式换热器的出口温度的控制方法,并通过人工神经网络预测板式换热器机组侧的出口水温来实现此控制方法。通过FLUENT软件建立复合式地源热泵系统动态数值模型,获取建立神经网络的数据,采用3层BP网络,建立了多个预测板式换热器机组侧出口温度的模型。研究结果表明,采用神经网络可以准确实现此预测,绝对误差不超过0.4℃。     

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 investigation of the heat pump performance and ground temperature of a piled foundation heat exchanger system for a residential building

Novel methods are sought to provide greater efficiency of the installation of ground heat exchangers for GSHPs (ground source heat pumps) in domestic buildings. An economically viable option is to utilise concrete foundation piles as ground heat exchangers. The objective of this study is to investigate the operation of utilising a piled foundation structure as a ground heat exchanger. A test plot of 72 m² (ground floor area) was produced with 21 × 10 m deep concrete piles, with a single U tube pipe in each. Ground heat was extracted by a heat pump with the heat loading being varied in line with the date and the average air temperature. Over the 2007/2008 heating season this study had investigated the temperature changes in the foundation piles and the surrounding ground in addition to the heat pump operational performance. The temperature changes observed in the region of the test plot were compared with variations naturally experienced in the ground due to the seasonal climatic influence. The SPF (seasonal performance factor) of the heat pump was 3.62 and the ground temperature at a distance of 5 m from the test plot was seen to be undisturbed by the heat extraction and followed the predicted seasonal variation.     

Potential of autonomous ground-coupled heat pump system installations in Greece

The HVAC systems utilizing renewable energy sources are one of the main contributors towards the fossil fuel dependency reduction. Among these, the ground source heat pump systems, especially those based on vertical ground heat exchanger, are very attractive, due to their high efficiency.     

地埋管换热器分区运行对地源热泵系统运行经济性影响的模拟研究

针对管群地埋管换热器,在不同夏冬季建筑负荷比运行条件下,对一种简单分区运行与不分区运行方式的循环水温度变化及系统能耗情况进行对比。结果表明:与不分区运行相比,采用简单分区运行方式时,地埋管循环水温度随运行时间的变化幅度相对较小,能耗降低,节能率随运行时间的增加而增大;随着夏冬季负荷比的增大,分区运行节能效果呈先增大后减小的趋势。研究结果表明,对于冬夏季负荷不平衡时,采用简单的地埋管换热器分区运行方式,可提升系统运行效益及可靠性。     

地埋管换热器分区运行对地源热泵系统运行经济性影响的模拟研究

针对管群地埋管换热器,在不同夏冬季建筑负荷比运行条件下,对一种简单分区运行与不分区运行方式的循环水温度变化及系统能耗情况进行对比。结果表明:与不分区运行相比,采用简单分区运行方式时,地埋管循环水温度随运行时间的变化幅度相对较小,能耗降低,节能率随运行时间的增加而增大;随着夏冬季负荷比的增大,分区运行节能效果呈先增大后减小的趋势。研究结果表明,对于冬夏季负荷不平衡时,采用简单的地埋管换热器分区运行方式,可提升系统运行效益及可靠性。     

Efficiency of Vertical Geothermal Heat Exchangers in the Ground Source Heat Pump System

Taking the fluid temperature distribution along the borehole depth into account, a new quasi-three-dimensional model for vertical ground heat exchangers has been established, which provides a better understanding of the heat transfer processes in the geothermal heat exchangers. On this basis the efficiency of the borehole has been defined and its analytical expression derived. Comparison with the previous two-dimensional model shows that the quasi-three-dimensional model is more rational and more accurate to depict the practical feature of the conduction of geothermal heat exchanger, and the efficiency notion can be easily used to determine the inlet and outlet temperature of the circulating fluid inside the heat exchanger.     

土壤源热泵的研究与开发

土壤源热泵是利用地下土壤能源资源来进行供暖空调的一种高效、节能、环保型空调技术，近年来得到了快速的发展；文章介绍了它的国内外研究状况，分析了土壤的传热特性、土壤的温度分布状况及埋地盘管的传热特性，建立了埋地盘管的传热数学模型，指出了土壤源热泵研究与发展中的关键性问题，最后展望了其应用前景。     

Validity ranges of three analytical solutions to heat transfer in the vicinity of single boreholes

In ground-coupled heat pump systems, accurate prediction of transient ground heat transfer is important to establish the required borehole length and to determine precisely the resulting fluid temperature. Three analytical solutions to transient heat transfer in the vicinity of geothermal boreholes are presented. These solutions are referred to as the infinite line source (ILS), the infinite cylindrical source (ICS) and the finite line source (FLS) models, which vary in complexity and are based on simplifications of the borehole geometry. The results of these models are compared and their validity domains are determined.     

A study on utilization of ground source energy for space heating using a nanofluid as a heat carrier

Ground source heat pump (GSHP) systems are well established as an energy-efficient space conditioning device. However, for better utilization of the ground source, improvement in GSHP performance is desirable, which limits the small temperature difference between the ground and the circulating fluid. In this study, efforts have been made to investigate the performance of a ground heat exchanger (GHX) with a nanofluid as a heat carrier. Mathematical modeling is performed for the closed-loop vertical U-tube GHX with six different (Al₂O₃, CuO, graphite, multiwalled carbon nanotube, graphene, and Cu) water-based nanofluids. The effect of different operating parameters on GHX length, fluid temperature, and pressure drop with nanofluids is determined. On the basis of the analytical results, it is found that the graphite particle-based nanofluid plays a prominent role to enhance the performance of the GHX as compared with other nanoparticles. The maximum enhancement in the increase in outlet fluid temperature and reduction in pipe length with graphite particle-based nanofluid are 68.3% and 63.3%, respectively, for an increase in temperature difference from 7°C to 15°C between the atmosphere and the ground. Also, with the graphite particle-based nanofluid and the increase in pipe diameter from 20 to 50 mm, the fluid outlet temperature increases up to 11.2%, and the requirement in GHX length reduces up to 55%.     

Computer simulation of borehole ground heat exchangers for geothermal heat pump systems

Computer simulation of borehole ground heat exchangers used in geothermal heat pump systems was conducted using three-dimensional implicit finite difference method with rectangular coordinate system. Each borehole was approximated by a square column circumscribed by the borehole radius. Borehole loading profile calculated numerically based on the prescribed borehole temperature profile under quasi-steady state conditions was used to determine the ground temperature and the borehole temperature profile. The two coupled solutions were solved iteratively at each time step. The simulated ground temperature was calibrated using a cylindrical source model by adjusting the grid spacing and adopting a load factor of 1.047 in the difference equation. With constant load applied to a single borehole, neither the borehole temperature nor the borehole loading was constant along the borehole. The ground temperature profiles were not similar at different distances from the borehole. This meant that a single finite difference scheme was not sufficient to estimate the performance of a borefield by superposition. The entire borefield should be discretized simultaneously. Comparison was made between the present method and the finite line source model with superposition. The discrepancies between the results from the two methods increased with the scale of borefield. The introduction of time schedule revealed a discrepancy between the load applied to the ground heat exchanger and that transferred from the borehole to the ground, which was usually assumed to be the same when using analytical models. Hence, in designing a large borefield, the present method should give more precise results in dynamic simulation.     

Simple and accurate model for the ground heat exchanger of a passive house

This paper develops previous research on passive house (PH) space heating. A simple and accurate ground heat exchanger model is developed. It is based on a numerical transient bi-dimensional approach that allows to computing the ground temperature at the surface and at various depths. The new model was integrated into the existing theoretical approach and implemented within the computer code used to simulate the heating system operation in Pirmasens PH (Rhineland Palatinate, Germany). The heating and cooling potential of the system under real climatic conditions was investigated. The energy delivered by the ground heat exchanger depends significantly on different design parameters like pipe's depth, diameter and material.     

基于圆柱源理论模型的U型埋管换热器的模拟研究

分析了圆柱热源(热汇)理论模型,运用圆柱源理论对太阳能-土壤源联合运行热泵系统进行连续20d运行的状况进行模拟,得出联合运行模式较单独土壤源运行模式节能8%~10%,可为圆柱源理论模型应用和太阳能-土壤源热泵运行模式的选择提供参考。引入“负荷集合”思想对圆柱热源(热汇)理论模型进行改进,使其适合于对土壤源热泵的全年运行进行模拟,模拟过程更为快捷。     

Field-scale evaluation of the design of borehole heat exchangers for the use of shallow geothermal energy

A numerical model for the simulation of temperature changes in a borehole heat exchanger (BHE) with fluid circulating through U-tubes is developed. The model can calculate the thermal power transferred from heat pumps to BHEs while considering the nonlinear relationship between temperature of the circulating fluid and the thermal power. The use of the developed model enables the design of a geothermal heat pump (GHP) system with the view of pursuing efficiency and financial benefit. The developed model is validated by comparing two measurement datasets with their respective simulation results. The numerical evaluation of a real GHP system with 28 BHEs and 79 heat pumps involved consideration of a base case and modified cases. In all cases, the temperatures of the circulating fluid at the BHE inlet and outlet, heat pump efficiency, and the heating power and electric power of heat pumps were obtained. The estimated cost of electricity in the year 2030 is 0.146 US$/kW. The most cost-effective system in this case is for there to be 4, 6, and 6 BHEs on the first, second, and third floors, respectively.     

Modeling of horizontal geoexchange systems for building heating and permafrost stabilization

We present a new analytical model based on the finite line source that extends the steady state results for parallel horizontal pipes to the transient case and for any desired horizontal pipe layout. The analytical model is validated, when there is no freezing/thawing, by a 3D finite element numerical model. When the phase change is accounted for in the numerical model, the analytical model still provides good approximation to the ground temperature during the heating season and the heat extracted by the ground heat exchanger. However, summer ground temperature and thaw depth are overestimated by the analytical model. A case study for a typical building in Kuujjuaq (northern Canada) area is analyzed. The ground heat exchanger layout follows a spiral pattern characterized by three parameters: length L, depth D, and spacing S. The influence of each parameter on the amount of heat extracted from the ground and on the ground temperature at a control point is assessed. The results show that increasing depth D favors keeping the ground frozen at this depth and increases the amount of heat that may be extracted. Conversely, increasing S and/or L is beneficial for the amount of heat extracted, but it enhances the risk of thawing around the pipes. The model and case study provides useful ground heat exchanger design guidelines in cold regions for the double purpose of ground freezing and heat extraction.     

Temperature distribution and performance of ground-coupled multi-heat pump systems for a greenhouse

The primary objective of a greenhouse is to produce good plant-growth conditions such as temperature and humidity. One of the hot issues for the greenhouse is to provide an appropriate heating system which can achieve favorable temperature condition and save energy. In this study, the performance of a ground-coupled multi-heat pump system for the greenhouse heating was investigated. The ground-coupled multi-heat pump system was composed of GLHX (ground loop heat exchanger) and multi-heat pump unit which had one outdoor unit and two or more indoor units. The temperature distribution within the greenhouse using the ground-coupled multi-heat pump system was represented relatively uniform comparing to when the conventional heating system and GCHP system were adopted, because the capacity of each indoor unit could be changed linearly according to the variation of load. The temperature difference between the maximum and minimum temperatures and the standard deviation of inside temperature for the greenhouse were 2.1 °C and 1.2 °C, respectively. It is necessary to develop the multi-heat pump unit which can be operated with high performance at relatively low temperature setting conditions. The system COP of the ground-coupled multi-heat pump unit decreased greatly at part load condition due to relatively high power consumption of the ground circulation pump. Therefore, it is suggested that a control algorithm of the ground circulation flow rate has to be developed to maximize energy saving by applying the ground-coupled multi-heat pump system to the greenhouse.     

U型管桩埋换热器稳态传热模拟研究

采用能量平衡的方法建立了土壤层内U型管桩埋换热器稳态传热模型，并以天津市一地源热泵实际工程为背景，模拟计算了管脚热影响因子、土壤导热系数等对U型管桩埋换热器的传热特性的影响。