Temperature distributions in boreholes of a vertical ground-coupled heat pump system

The objective of this study is to show the temperature distribution development in the borehole of the ground-coupled heat pump systems (GCHPs) with time. The time interval for the study is 48 h. The vertical GCHP system using R-22 as refrigerant has a three single U-tube ground heat exchanger (GHE) made of polyethylene pipe with a 40 mm outside diameter. The GHE was placed in a vertical borehole (VB) with 30 (VB1), 60 (VB2) and 90 (VB3) m depths and 150 mm diameters. The experimental results were obtained in cooling and heating seasons of 2006–2007. A two-dimensional finite element model (FEM) was developed to simulate temperature distribution development in the soil surrounding the GHEs of GCHPs operating in the cooling and the heating modes. The finite element modelling of the GCHP system was performed using the ANSYS code. The FEM incorporated pipes, the grout and the surrounding formation. From the cases studied, this approach appears to be the most promising for estimation the temperature distribution response of GHEs to thermal loading.     

Heat transfer analysis of boreholes in vertical ground heat exchangers

A ground heat exchanger (GHE) is devised for extraction or injection of thermal energy from/into the ground. Bearing strong impact on GHE performance, the borehole thermal resistance is defined by the thermal properties of the construction materials and the arrangement of flow channels of the GHEs. Taking the fluid axial convective heat transfer and thermal “short-circuiting” among U-tube legs into account, a new quasi-three-dimensional model for vertical GHEs is established in this paper, which provides a better understanding of the heat transfer processes in the GHEs. Analytical solutions of the fluid temperature profiles along the borehole depth have been obtained. On this basis analytical expressions of the borehole resistance have been derived for different configurations of single and double U-tube boreholes. Then, different borehole configurations and flow circuit arrangements are assessed in regard to their borehole resistance. Calculations show that the double U-tubes boreholes are superior to those of the single U-tube with reduction in borehole resistance of 30-90%. And double U-tubes in parallel demonstrate better performance than those in series.     

Vertical-borehole ground-coupled heat pumps: A review of models and systems

A large number of ground-coupled heat pump (GCHP) systems have been used in residential and commercial buildings throughout the world due to the attractive advantages of high efficiency and environmental friendliness. This paper gives a detailed literature review of the research and developments of the vertical-borehole GCHP technology for applications in air-conditioning. A general introduction on the ground source heat pump system and its development is briefly presented first. Then, the most typical simulation models of the vertical ground heat exchangers currently available are summarized in detail including the heat transfer processes outside and inside the boreholes. The various design/simulation programs for vertical GCHP systems primarily based on the typical simulation models are also reviewed in this paper. Finally, the various hybrid GCHP systems for cooling or heating-dominated buildings are well described. It is found that the GCHP technology can be used both in cold and hot weather areas and the energy saving potential is significant.     

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。     

Transient heat transfer in a U-tube borehole heat exchanger

A transient heat transfer model has been development for a thermal response test (TRT) on a vertical borehole with a U-tube. Vertical borehole heat exchangers are frequently coupled to ground source heat pumps, which heat and cool buildings. The model provides an analytical solution for the vertical temperature profiles of the circulating fluid through the U-tube, and the temperature distribution in the ground. The model is verified with data sets from a laboratory sandbox and field TRTs, as well as a previously reported numerical solution. Unlike previous analytical models, the vertical profiles for the circulating fluid are generated by the model without any assumption of their functional form.     

Study on hybrid ground-coupled heat pump system for air-conditioning in hot-weather areas like Hong Kong

The ground-coupled heat pump (GCHP) system is becoming attractive for air-conditioning in some moderate-weather regions due to its high energy efficiency and reliable operation capability. However, when the technology is used in buildings where there is only cooling load in hot-weather areas like Hong Kong, the heat rejected into the ground by the GCHP systems will accumulate around the ground heat exchangers (GHE). This heat accumulation will result in degradation of system performance and increment of system operating costs. This problem can be resolved by using the hybrid ground-coupled heat pump (HGCHP) system, which uses supplemental heat rejecters to reject the accumulated heat. This paper presents a practical hourly simulation model of the HGCHP system by modeling the heat transfer process of the system’s main components. The computer program based on this hourly simulation model can be used to calculate the hour-by-hour operation data of the HGCHP system. As a case study, both a HGCHP system and a traditional GCHP system are designed for a hypothetic private residential building located in Hong Kong, and the economic comparisons are conducted between these two types of systems. The simulation results show that the HGCHP system can effectively solve the heat accumulation problem and reduce both the initial costs and operating costs of the air-conditioning system in the building.     

Development of a numerical model for the simulation of vertical U-tube ground heat exchangers

Vertical U-tube ground heat exchangers are a key component in geothermal energy utilization systems like ground source heat pumps (GSHPs). This paper presents a three-dimensional unstructured finite volume model for them. The model uses Delaunay triangulation method to mesh the cross-section domain of the borefield (borehole field), and consequently may intactly retain the geometric structure in the borehole. To further improve the computational accuracy, the soil is divided into many layers in the vertical direction in order to account for the effect of changing fluid temperature with depth on the thermal process in the borefield. The inlet temperature of the ground heat exchanger (GHE) is used as a boundary condition, and the inside and outside surfaces of the U-tube pipes are treated as the conjugated interfaces in the domain. Thus, the conjugate thermal processes between the fluid in the pipes and the soil around it and between the two pipe legs may be accounted fully. A comparison of the model predictions and experimental data shows that the model has good prediction accuracy.     

Extremum seeking control for efficient operation of hybrid ground source heat pump system

The Hybrid Ground Source Heat Pump (GSHP) systems combine the renewable geothermal energy and cooling tower for rejecting the cooling load, which is often adopted for high cooling demand. Model based control can be limited due to variations in ambient conditions, ground-loop heat exchanger (GHE) and equipment characteristics, cost and reliability of sensors. A self-optimizing control scheme is proposed for efficient operation of the hybrid GSHP based on Extremum Seeking Control (ESC), with feedback of the total power consumption and the control inputs of the relative flow rate of cooling tower and the water pump speed. The cooling capacity of the heat pump regulates the evaporator leaving water at 7 °C. A Modelica based dynamic simulation model is developed for a Hybrid GSHP system, with the vertical GHE model adopted from Modelica Buildings Library. The transient heat transfer is implemented with a finite volume method inside and outside the borehole. The proposed ESC scheme is evaluated under the scenarios of fixed cooling load, ramp change in the evaporator inlet water temperature, diurnal sinusoidal cycle of air wet-bulb temperature, and realistic ambient and cooling load condition. Simulation results show the proposed ESC strategy effectively achieves nearly optimal efficiency without the need for plant model.     

Development of suitability maps for ground-coupled heat pump systems using groundwater and heat transport models

The thermophysical properties of subsurface materials (soils, sediments and rocks) and groundwater flow strongly affect the heat exchange rates of ground heat exchangers (GHEs). These rates can be maximized and the installation costs of the ground-coupled heat pump (GCHP) systems reduced by developing suitability maps based on local geological and hydrological information. Such maps were generated for the Chikushi Plain (western Japan) using field-survey data and a numerical modeling study. First, a field-wide groundwater model was developed for the area and the results matched against measured groundwater levels and vertical temperature profiles. Single GHE models were then constructed to simulate the heat exchange performance at different locations in the plain. Finally, suitability maps for GCHP systems were prepared using the results from the single GHE models. Variations in the heat exchange rates of over 40% revealed by the map were ascribed to differences in the GHE locations, confirming how important it is to use appropriate thermophysical data when designing GCHP systems.     

土壤源热泵地下垂直换热埋管周围非稳态温度场的数值模拟

针对土壤源热泵地下垂直U型换热埋管,建立了周围土壤的非稳态温度场的数学模型,并利用隐式有限差分法进行了数值模拟。通过对制冷和制热工况的模拟,得到土壤温度沿径向的变化规律、埋管出水温度的变化规律及埋管的热作用半径的变化规律。     

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.     

Feasibility study on novel hybrid ground coupled heat pump system with nocturnal cooling radiator for cooling load dominated buildings

When the ground coupled heat pump (GCHP) system is utilized for air conditioning in cooling load dominated buildings, the heat rejected into ground will accumulate around the ground heat exchangers (GHE) and results in system performance degradation. A novel hybrid ground coupled heat pump (HGCHP) system with nocturnal cooling radiator (NCR) works as supplemental heat rejecter is proposed in this paper to resolve this problem. The practical analytical model of NCR and novel HGCHP system are established. The computer program based on established model is developed to simulate the system operation performance. The novel HGCHP system is designed and simulated for a sample building located in Hong Kong, and a simple life cycle cost comparisons are carried out between this system and conventional GCHP system. The results indicate that it is feasible to use NCR serves as supplemental heat rejecter of the novel HGCHP system for cooling load dominated buildings even those located in humid subtropical climate areas. This novel HGCHP system provides a new valuable choice for air conditioning in cooling load dominated buildings, and it is especially suitable for buildings with limited surface land areas.     

竖直U型埋管地热换热器热短路现象的影响参数分析

通过引入换热器出口最高流体温度的概念,对地源热泵竖直U型埋管地热换热器的热短路现象进行了量化,基于竖直U型埋管周围的瞬时有限元模型,对影响热短路现象的主要参数(支管间距和回填料导热系数)进行了模拟分析,得出了量化结果。结果表明,增大支管间距可降低换热器出口最高流体温度,减小由热短路现象引起的热损失;回填料的导热系数对热短路现象的影响较大,当回填料导热系数小于周围土壤的导热系数时,增大回填料导热系数对减小热短路损失有较大作用,而当回填料导热系数大于土壤导热系数时则作用不大,推荐使用导热系数与周围土壤导热系数接近的回填材料。     

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.     

基于线热源理论的垂直U型埋管换热器传热模型的研究

基于经典常热流线热源理论,通过引入叠加原理、阶跃负荷及孔洞热阻思想将其发展为能够适用于变热流埋管换热与地源热泵系统模拟的变热流线热源模型,并与改进的经实验与理论验证的圆柱源理论模型进行了比较与分析。结果表明:所发展的变热流线热源模型能够有效地模拟地下埋管的换热过程,可作为地下垂直U埋管换热过程的计算模型,为地源热泵地下埋管换热器的设计计算及地源热泵系统的模拟提供了一种新的简单而又实用的计算方法。     

Unit-response function for ground heat exchanger with parallel,series or mixed borehole arrangement

A novel approach is presented that allows to predict fluid temperatures entering a Ground Heat Exchanger (GHE) for parallel, series and mixed arrangements of boreholes. The method determines at each time step the heat transfer rates occurring at each borehole so as to reproduce the fluid temperature at the GHE inlet for a specific borehole arrangement. The analytical finite line source model is used to compute the borehole wall temperatures, whereas the fluid temperatures are assumed to vary linearly along the pipes. The method requires to solve a linear system of equations at a small number of time steps. The different systems of equations for each arrangement are determined. A comprehensive 3D finite element numerical model shows good agreement with the computed fluid temperatures. The proposed approach is computationally very efficient. The fluid temperature unit response function can be convolved with any desired heat load to estimate fluid temperatures at the GHE inlet for a wide variety of scenarios.     

土壤源热泵U型垂直埋管传热过程的数值研究

针对土壤源热泵地下U型垂直埋管,建立了周围土壤的非稳态温度场的数学模型,并利用Matlab软件进行求解。通过对夏季制冷工况的模拟,研究了管内流量和回填材料对出水温度的影响,还研究了埋管间距和热作用半径对单位管长换热量的影响。     

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.     

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.     

Simulation and experimental analysis of optimal buried depth of the vertical U-tube ground heat exchanger for a ground-coupled heat pump system

This paper presents a numerical heat transfer model for vertical U-tube Ground Heat Exchangers (GHE). This model is uniquely able to take into to account different initial soil temperatures and physical properties at different depths. The model has been validated based on an experimental case study and has been used to simulate the thermal performance of GHEs. The simulation results show that for a 100-m vertical GHE, the first 70 m of the vertically buried GHE has a higher heat transfer capability than its last 30-m section. In addition, the validated model is used to investigate the optimal depth of vertical GHEs in five case studies ranging from 60 to 100 m length. Among them, the simulation results demonstrate that the GHE with buried depth of 70 m is able to provide the highest heat exchange rate per unit depth (54.1 and 47.0 W/m under heat rejection/extraction mode). It consequently results in shortest total GHE length of 11,388 m and the minimum cost of 1.82 million Yuan. However, a larger area is needed for boreholes and GHEs installation. Therefore, the optimal buried depth of the vertical U-tube GHEs for the studied case is 70 m on the condition of allocation of an abundant area to set up the boreholes.