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Ground (or geothermal) heat exchangers are attracting a great deal of attention as a way of using shallow geothermal energy. This paper provides not only a critical review but also a thorough introduction to the analysis of heat transfer by borehole and foundation pile ground heat exchangers, with an emphasis on different analytical models. The literature is reviewed in a time-scale framework because of the diversity of the time and space scales involved in the thermal processes of ground heat exchangers. We summarize, discuss, and evaluate major advances in this field, including heat-source models, short-time models, models for energy piles, in situ thermal-response tests, indoor sandbox experiments, and parameter estimation as an inverse problem. Of particular note is that the unit-step temperature response (i.e., G -function) of a ground heat exchanger with one U-shaped pipe is calculated; and six analytical models are compared: an infinite cylinder-source model, two infinite line-source models, two finite line-source models, and a composite-medium line-source model. This paper closes by identifying several unsolved problems that require solutions. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

The ground and grout thermal parameters are very important for the Ground-Coupled Heat Pump system (GCHPs) design and simulation. An accurate ground and grout thermal parameter will result in a reasonable GCHPs design and operation control strategy. In this paper, a new transient ground and grout parameter estimation method was proposed. In addition to the ground thermal parameters, the transient borehole thermal resistance and the grout thermal parameters were firstly estimated by analytical models with the proposed method. Compared to the measured values by the thermal probes, the parameters estimated by the proposed method was reasonable and much more accurate than the conventional method. Accompany with the transient borehole thermal resistance estimated by the proposed method, the fluid temperature predicted by the ILS model was acceptable for both the early regime and late regime, and the maximum relative error for the fluid prediction result was up to 2.82%. [ABSTRACT FROM AUTHOR]

In recent years, ground-coupled heat pump system has been widely used in engineering practice. For saving the area, arrangement of buried pipes under building foundation began to attract public attention. However, under the influence about the alternation of soil temperature field and the base compression, the overall strength and stability of the foundation is weakened, the foundation is settled again with the use of the building, which may affect the use of the building structure. In this paper, the settlement has been analyzed with the effect by porosity and temperature field. The best placement depth of U-tube under the foundation is also explored. The results are as follow. The settlements of cold and heat accumulation are consistent with the characteristics of soil thermal expansion and contraction. Until the settlements of buildings whose foundation have cold accumulation don’t satisfy the standard requirements, the depth of the U-tube heat exchanger need to be buried under the depth of settlement calculation regardless without the effects of temperature. [ABSTRACT FROM AUTHOR]

This paper presents a novel solar assisted ground-coupled heat pump indirect heating system (SA-GCHPIHS) designed for a residential building in Beijing for space heating and supplying domestic hot water. The recharging strategy for ground heat exchanger (GHE) is analyzed and determined based on system energy performance. The impacts of operation strategies and heating load ratio are simulated and compared, and the system with heat pump and water tank installed in series (SA-GCHPIHS(s)) showed better performance than SA-GCHPIHS(r) when domestic hot water (DHW) load dominates the heating load. And more energy would be saved by SA-GCHPIHS(s) as DHW load grows larger. [ABSTRACT FROM AUTHOR]

Horizontal geothermal heat exchangers (HGHEs) with vertical spiral coils have been increasingly used in ground source heat pump (GSHP) systems. Research studies have been conducted using field tests and simulations trying to reveal the heat transfer mechanism of HGHEs with vertical spiral coils and to provide a good heat transfer performance. However, there is no analytical solution has been established for HGHEs with vertical spiral coils. Of particular interest is the temperature variation at the soil surface. The study presented in this paper proposes the development of a mathematical vertical ring-coil model used to describe the heat transfer process of HGHEs with vertical spiral coils. In this new model, the spiral heat exchanger is simplified as a series of ring coils that inject/extract heat in/from a semi-infinite medium. A single ring model in an infinite medium is first introduced. Using images and superposition, a multiple ring-coils analytical solution is then given. The significant influence on the heat transfer performance of HGHEs temperature variation of the ground surface is then considered. A periodically changed boundary condition is being taken into consideration in the modeling process. The validity of the model was examined by a field test and a 3-D simulation model. The analytical solutions may provide a desirable and better tool for the simulation/design HGHEs with vertical spiral coils. [ABSTRACT FROM AUTHOR]

One potential problem for the ground coupled heat pump is that the ground thermal properties is hardly to be known due to the complicated ground construction. The p ( t )-linear average method has been proved that it can improve the accuracy of borehole thermal resistance. However, the p ( t )-linear fluid temperature distribution approximation is not agree well with the temperature profile measured by the fiber cable. Thus, in this paper, an improved p ( t )-linear average method in which the fluid temperature distribution approximation based on the vertical temperature profile is proposed. With the new vertical temperature profile simulation model, the accuracy for the borehole thermal resistance estimation will be improved comparing to the true value. Besides that, the estimation results are sensitive with the distance between two pipes, and together with the borehole thermal resistance, the distance will be optimized by the outlet fluid temperature. The life cycle cost analysis results of a case study for an office building in Hunan University show that, although the operation cost will be increased, the total cost during the whole lifetime will be reduced with a lower initial investment. [ABSTRACT FROM AUTHOR]

A ground-coupled heat pump (GCHP) is a clean and energy-saving approach to providing air conditioning and domestic hot water. For buildings with a dominant heating load, more heat is extracted from the soil and thermal imbalance becomes a crucial problem, in turn reducing efficiency and impeding the wide application of GCHP. Due to the disadvantages of the existing solutions, a multi-mode air-source heat compensator (AHC) integrated with a GCHP system to make a hybrid AHC-GCHP system is proposed in this paper. The principles and operating strategy of AHC-GCHP are illustrated in detail and its performance is simulated in the transient system simulation tool TRNSYS. AHC can not only compensate heat into the ground but also supply heat for space heating and domestic hot water in different modes through full use of the energy in the air with the average coefficient of performance (COP) ranging from 4.49 to 15.09. The AHC-GCHP system effectively keeps the soil thermal balance, and saves 23.86% energy compared with a traditional “boiler + split air conditioner” system, and costs less than half as much as a “hybrid GCHP assisted with solar collector” system. Consequently, the AHC-GCHP system is a potentially efficient and economic approach for buildings in cold regions. [ABSTRACT FROM AUTHOR]

Ground-Coupled Heat Pump (GCHP) systems used in commercial buildings are conventionally controlled by some time schedule strategies. So, a reliable method of modeling underground temperature profiles is of great importance for the operation of GCHP systems. Underground temperature profiles are usually predicted by commercial numerical software. This approach is time consuming and lack the flexibility of modeling various cases of GCHP systems. In this paper, a new analytical model based on a composite-medium line-source model is proposed. The new analytical model agrees well with in situ experimental data. An exhaustive simulation is conducted to explore influencing factors on the ground temperature profile, such as operating time, on-off ratio, heat fluxes and thermal conductivity of back-filled grout. These results indicate that a proper operating schedule and heat fluxes are helpful for maintaining the balance of ground temperature, which may offer an opportunity to develop a control strategy based on the heat flux prediction with the help of GCHP system intermittent operation. [ABSTRACT FROM AUTHOR]

The paper makes heat transfer analysis on borehole ground heat exchangers (GHEs) of ground-coupled heat pump system, for one thing, the thermal transmission on two cases including sing U-tube and double U-tubes inside borehole are illustrated, for another, the heat transfer characteristics outside borehole are presented based on the mathematical model, it is meaningful to obtain the analytical solution of temperature response in the underground medium. The relevant analysis lays a firm foundation for the optimal design of borehole GHEs because it can provide theoretical guidance for the calculation and design. In addition, some factors which can exert influences on the heat exchange performance of GHEs are pointed out, such as the arrangement of boreholes GHEs, the type of circulating liquid and so on. Afterwards the impacts degrees of these factors are studied according to simulation. It is necessary to optimize the design of borehole GHEs because in such a way can the size of GHEs be reduced so that the initial cost of the system is lowered, this is favourable for the application of ground-coupled heat pump technology. [ABSTRACT FROM AUTHOR]

With widespread application of ground-coupled heat pump (GCHP) system, heat transfer process of pile ground heat exchanger (PGHE) has attracted much attention. Previous researches, studying the impact of groundwater flow, mainly focus on the PGHE with cast-in U-tubes. However, compared with the PGHE with U-tubes, PGHE with cast-in spiral coils has better heat transfer performance and is more suitable for a modern city with high building density. Therefore, in this paper, a 3-D simulation model and a simplified analytical model are established to investigate the influence of groundwater flow on the thermal performance of PGHE with cast-in spiral coils. A comparison between these two models is carried out. Both numerical and analytical results show that the groundwater flow has an enhancing effect on the heat transfer performance of the PGHE with spiral coils and can accelerate the heat transfer process into stability. When the seepage velocity is 1e-5 m/s, the average enhancing rate is 22.98%. [ABSTRACT FROM AUTHOR]

Abstract: A composite-medium line-source model has been recently developed for modeling short-time thermal processes of borehole ground heat exchangers. The composite-medium line-source model is attractive for applications where the short-term or high-frequency responses of borehole ground heat exchangers are important. In this paper, the short-time performance of this analytical model is examined in depth by comparing it with a two-dimensional finite volume model. The numerical model can address the heat capacity of circulating water by using a novel time-varying boundary condition on the inner walls of the U-shaped tube, in addition to the heat capacities of U-shaped tube and grouting material. Both the numerical and the analytical composite-medium models are validated using an independent sandbox experiment. Comparisons between the two models are also made for different parameters, including heating rate, the spacing of the U-tube legs, thermal properties, and borehole radius. Overall, the composite-medium line-source model gives the virtually same results as the numerical model except for the first several minutes: the analytical solution overestimates temperature responses during the first 3 min, which is caused by the line-source assumption ignoring the heat capacities of the fluid and the U-shaped pipe. [Copyright &y& Elsevier]

Abstract: The technology of embedding heat exchange tubes in diaphragm walls is a new direction for Ground-coupled Heat Pumps (GCHP). This paper examines the heat transfer model and design method for geothermal heat exchangers in diaphragm walls, which are seldom investigated in the world. Two-dimensional (2D) heat transfer models for diaphragm wall heat exchangers (DWHE) are established. A design method for DWHE is further developed to calculate the hourly heat exchange capacity of DWHE and to get the reasonable design parameter of DWHE. The DWHE model over the excavation line has the same changing trend with numerical solutions, and the relative errors between them are less than 2%. The relative errors of calculated heat exchange rate per meter using DWHE model and measured data are less than 6% after running 10h. The relative errors of temperature between DWHE model under the excavation line and the measured data at different positions are no more than 9%, whereas the relative error between the tradition models in GCHP (line source model and finite-line source model) and the measured data may reach as much as 40%. [Copyright &y& Elsevier]

Abstract: Short-time responses of borehole ground heat exchangers (GHEs) are difficult to predict because they involve heat capacity effect and various arrangements of U-shaped tubes within boreholes. To meet this engineering challenge, this paper builds a composite-medium line-source model. The central idea of the new model is that the downward and upward channels of U-shaped tubes (not the borehole) are approximated as line sources or sinks of heat placed in a composite medium. This approach can account for not only the influence of the heat capacity of grouting material but also the difference between properties of soil and grout, whilst it is flexible enough to model various U-pipe configurations. The new composite-medium model is validated by using a reported laboratory experiment; it can yield predictions matching the experimental response profiles with acceptable accuracy for times as short as 5min. Several uncertainties which may contribute to discrepancies between the model prediction and the laboratory data are analyzed, including heat input rate, thermal conductivities and heat capacities of the composite medium. The potential errors caused by these uncertainties are comparable to the observed discrepancies. [Copyright &y& Elsevier]

Abstract: The application of ground heat exchangers (GHEs) has attracted much attention for its advantages in terms of energy-saving and emission-reduction. Therefore, it is significant to introduce heat transfer into investigation, the paper illustrated the existing heat transfer models in order to analyze the GHEs with buried coils. Line heat source, cylindrical heat source and ring-coil heat source are introduced step by step. For the present, the borehole GHEs are the mainstream, and in recent years pile foundation GHEs have been followed with interest on account of heat transfer superiority as well as lowering initial cost and saving land area. Concerning that groundwater flow exist in geological stratum sometimes, so that the thermal analysis about heat transfer should be composed of two cases, i.e. models under the condition of both pure conduction and combination that include conduction and advection. It is obvious that ring-coil model is on the verge of actual situation for pile foundation GHEs with buried coils from the academic research perspectives, nevertheless for some engineering computation problems which the result error can be permitted in the certain range, other heat transfer models can be chosen in order to simplify the calculation process. [Copyright &y& Elsevier]

Abstract: This paper investigates the transient heat conduction around the buried spiral coils which could be applied in the ground-coupled heat pump systems with the pile foundation as a geothermal heat exchanger. A transient ring-coil heat source model is developed, and the explicit analytical solutions for the temperature response are derived by means of the Green’s function theory and the image method. The influences of the coil pitch and locations are evaluated and discussed according to the solutions. In addition, comparisons between the ring-coil and cylindrical source models give that the improved finite ring-coil source model can accurately describe the heat transfer process of the pile geothermal heat exchanger (PGHE). The analytical solutions may provide a desirable and better tool for the PGHE simulation/design. [Copyright &y& Elsevier]

Abstract: This paper presents the experimental study of a solar-assisted ground-coupled heat pump system (SAGCHPS) with solar seasonal thermal storage installed in a detached house in Harbin. The solar seasonal thermal storage was conducted throughout the non-heating seasons. In summer, the soil was used as the heat sink to cool the building directly. In winter, the solar energy was used as a priority, and the building was heated by a ground-coupled heat pump (GCHP) and solar collectors alternately. The results show that the system can meet the heating–cooling energy needs of the building. In the heating mode, the heat directly supplied by solar collectors accounted for 49.7% of the total heating output, and the average coefficient of performance (COP) of the heat pump and the system were 4.29 and 6.55, respectively. In the cooling mode, the COP of the system reached 21.35, as the heat pump was not necessary to be started. After a year of operation, the heat extracted from the soil by the heat pump accounted for 75.5% of the heat stored by solar seasonal thermal storage. The excess heat raised the soil temperature to a higher level, which was favorable for increasing the COP of the heat pump. [ABSTRACT FROM AUTHOR]

Abstract: The ground-coupled heat pump (GCHP) systems have been identified as one of the best sustainable energy technologies for space heating and cooling in buildings. While the foundation piles of buildings are used to partly take the place of boreholes in the ground heat exchanger (GHE) in recent years, the classical approaches of the line heat source model and the “hollow” cylindrical heat source model for the borehole GHEs fail for thermal analysis and design of the pile GHEs. Evolved from the classical models, a new “solid” cylindrical source model is presented in this paper to consider both the radial dimension and the heat capacity of the borehole or pile. Expressions of the analytical solution are derived for 1-D and 2-D new models by means of the Green’s function method. Results obtained from the new 1-D model are compared with the classical line source and “hollow” cylindrical source models of the borehole GHE, and also validated by a numerical solution of the same model. While the 1-D and 2-D solid cylindrical source models can provide adequate tools for design and simulation of the pile GHEs, improvement can also be achieved in simulating the temperature response of the borehole GHEs, especially for short time steps. [Copyright &y& Elsevier]

This paper describes the applicability of artificial neural networks (ANNs) to predict performance of a horizontal ground-coupled heat pump (GCHP) system. Performance forecasting is the precondition for the optimal control and energy saving operation of heat pump systems. ANNs have been used in varied applications and they have been shown to be particularly useful in system modelling and system identification. In order to train the ANN, limited experimental measurements were used as training data and test data. In this study, in input layer, there are air temperature entering condenser unit and air temperature leaving condenser unit, and ground temperatures (1 and 2m); coefficient of performance of system (COPS) is in output layer. The back propagation learning algorithm with three different variants, namely Levenberg–Marguardt (LM), Pola–Ribiere conjugate gradient (CGP), and scaled conjugate gradient (SCG), and tangent sigmoid transfer function were used in the network so that the best approach can find. The most suitable algorithm and neuron number in the hidden layer are found as LM with seven neurons. For this number level, after the training, it is found that Root-mean squared (RMS) value is 1%, and absolute fraction of variance (R 2) value is 99.999% and coefficient of variation in percent (COV) value is 28.62%. It is concluded that, ANNs can be used for prediction of COPS as an accurate method in the systems. [Copyright &y& Elsevier]

Abstract: The in situ thermal response test provides an effective method to determine the ground thermal properties required for the design of a geothermal energy installation. This paper presents a unique compact testing device for in situ thermal response testing based on a previous, trolley-based unit. Modernisations in the presented new apparatus enable the time and cost of the test to be reduced and offer enhanced flexibility and application potential for in situ thermal response testing. The methodology adopted for the test, including the theoretical approach for the evaluation of test data, together with an outline of the design and workings of the apparatus are presented. A case study is also shown for the verification of the quality of the results. [Copyright &y& Elsevier]

Abstract: This article present a comparison of artificial neural network (ANN) and adaptive neuro-fuzzy inference systems (ANFIS) applied for modelling a ground-coupled heat pump system (GCHP). The aim of this study is predicting system performance related to ground and air (condenser inlet and outlet) temperatures by using desired models. Performance forecasting is the precondition for the optimal design and energy-saving operation of air-conditioning systems. So obtained models will help the system designer to realize this precondition. The most suitable algorithm and neuron number in the hidden layer are found as Levenberg–Marquardt (LM) with seven neurons for ANN model whereas the most suitable membership function and number of membership functions are found as Gauss and two, respectively, for ANFIS model. The root-mean squared (RMS) value and the coefficient of variation in percent (cov) value are 0.0047 and 0.1363, respectively. The absolute fraction of variance (R 2) is 0.9999 which can be considered as very promising. This paper shows the appropriateness of ANFIS for the quantitative modeling of GCHP systems. [Copyright &y& Elsevier]

Abstract: This paper presents the experimental study of a ground-coupled heat pump used in a 180m2 private residence and combined with thermal solar collectors. This process, called GEOSOL, meets domestic hot water and heating–cooling building energy needs. Solar heat is used as a priority for domestic hot water heating and when the preset water temperature is reached, excess solar energy is injected into the ground via boreholes. This system has the advantage to contribute to the balance of the ground loads, increasing the operating time of the solar collectors and preventing overheating problems. After 11 months in operation, the power extracted and injected into the ground had average values of 40.3 and 39.5W/m, respectively. Energy injected into the ground represents 34% of the heat extracted, and the heat pump''s coefficient of performance (COP) in heating mode had an average value of 3.75. In addition, the domestic hot water solar fraction had an average value higher than 60% for the first 11 months in operation. [Copyright &y& Elsevier]