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Ground source heat pump systems (GSHPs) for soil heat utilization have been widely concerned in the world due to their energy saving, high efficiency, and no pollutant emission. The thermal parameters of the local soil must be known first when designing GSHPs, and the in-situ thermal response test (TRT) is currently the conventional method for determining their values. Given this, this paper deals with the related contents of parameter identification based on TRT from four aspects. First, various modeling methods for ground heat exchanger (GHE) and inversion algorithms are introduced and compared. Next, many factors that influence the identification accuracy are summarized, including test conditions, external disturbance, groundwater seepage and subsurface natural convection. Then, the negative influence of correlation between parameters on identification results is analyzed, and the research progress of the uncertainty assessment of identification results is introduced, which can improve the reliability of the results. Furthermore, the characteristics of various advanced TRT setups are also summarized. Finally, several imperfects and objectives for improving the reliability of parameters identification are discussed. This work provides useful information for improving the identification accuracy of soil thermal parameters so that the GHSPs can be designed more reliably.

Use of ground source heat pumps has increased significantly in recent years for space heating and cooling of residential houses and commercial buildings, in both heating (i.e., cold region) and cooling (i.e., warm region) dominated climates, due to its low carbon footprint. Ground source heat pumps exploit the passive energy storage capacity of the ground for heating and cooling of buildings. The main focus of this paper is to critically review how different construction and operation parameters (e.g., pipe configuration, pipe diameter, grout, heat injection rate, and volumetric flow rate) have an impact on the thermal efficiency of the vertical ground heat exchanger (VGHE) in a ground source heat pump (GSHP) system. The published literatures indicate that thermal performance of VGHEs increases with an increase of borehole diameter and/or pipe diameter. These literatures show that the borehole thermal resistance of VGHEs decreases within a range of 9% to 52% due to pipe configurations and grout materials. Furthermore, this paper also identifies the scope to increase the thermal efficiency of VGHE. The authors conclude that in order to enhance the heat transfer rate in VGHE, any attempt to increase the surface area of the pipe configuration would likely be an effective solution.

Knowledge of borehole heat exchanger efficiency is necessary to optimize the design and performance of ground source heat pump systems. To evaluate the heat transfer performance of the wildly-used vertical U-pipe BHE, a novel one-dimensional numerical model was developed to assess the thermal transfer performance of the BHE from short-term (thermal response test) to long-term (a heating period) for engineering application. The proposed numerical model took into account the internal capacity of the borehole and the thermal resistance between the two legs of U-pipe which are often negligible in traditional one-dimensional numerical models. A thermal response test data from a case at Vorbasse, Denmark and the data from the TRNSYS model were used to validate the feasibility and reliability of the presented model. Then, both a short-term thermal response and a long-term temperature development of the fluid in BHE and surrounding ground were simulated and analyzed based on the model. Additionally, this study did a sensitivity analysis to see what effect the parameters have on the BHE efficiency. Hereby the impact of given assumptions can be able to estimate and the results can serve for the optimum design and control of GSHP systems.

Ground thermal properties (thermal conductivity and heat capacity) are the key parameters for design of borehole heat exchanger (BHE) in ground source heat pump (GSHP). This study presented a novel sequential estimation for ground thermal properties of coaxial BHE by using distributed thermal response test (DTRT) data. Firstly, a comprehensive heat transfer model for coaxial BHE was built to obtain the vertical temperature files. Then a new sensitivity analysis using Spearman correlation method was performed to obtain the correlation between the thermal properties and fluid vertical temperature, determining the estimation sequence. Lastly, according to the sequence, Monte Carlo stochastic method was used to estimate the thermal properties by applying the DTRT data. In addition, the effect of borehole depth and random samples on estimations was investigated. The simulations revealed that the heat input into BHE was mainly used to heat the circulating fluid during the earlier time. It was found that the estimation steps following the thermal conductivity first, then heat capacity got higher precision than other estimation steps with 0.6% and 4% respectively for the two parameters. In addition, the effect of borehole depth on estimation error could be eliminated by using the DTRT data.

L’articolo presenta le fasi di progettazione, installazione e collaudo di un geoscambiatore innovativo, pensato per gallerie realizzate con TBM. Il prototipo è stato sviluppato nell’ambito di una collaborazione tra BBT-SE, coinvolta nella costruzione della Galleria di Base del Brennero tra Italia e Austria, e l’Università di Bologna. Esso consiste in un sistema orizzontale a circuito chiuso, di tipo modulare, collocato nello spazio dedicato alla raccolta delle acque di drenaggio all’interno del rivestimento in arco rovescio del cunicolo esplorativo. Il prototipo è stato denominato “Smart Flowing”, in virtù del processo di funzionamento, basato sullo scambio termico con il flusso di acqua, della configurazione compatta dei moduli e della semplicità di installazione durante l’avanzamento. L’affidabilità e l’efficienza del sistema sono state dimostrate attraverso una serie di prove sperimentali, volte a simulare il funzionamento di un impianto a pompa di calore sia in modalità riscaldamento sia in raffrescamento. Infine, è stata realizzata una valutazione preliminare del potenziale economico della soluzione proposta e della sua sostenibilità ambientale. The article presents the design, installation and testing of an innovative geoexchanger, tailored for tunnels excavated by TBM. The prototype was developed through the joint efforts of BBT-SE, involved in the construction of the Brenner Base Tunnel connecting Italy and Austria, and the University of Bologna. It consists in a modular horizontal closed-loop system located in the lining invert space dedicated to the drainage water flow in the exploratory tunnel. Due to the manner in which it functions, based on the heat exchange process with the drainage water, and to its compact design and straightforward installation procedure, the prototype was called "Smart Flowing". Modules were built outside and aflerwards assembled in the tunnel system, concurrently to the advancement of the TBM. Specific tests were performed, simulating a heat pump conditioning system's operation in both heating and cooling modes, to prove the prototype's reliability and efficiency. Finally, a preliminary economic and environmental potential assessment was carried out.

This paper concerns the modeling of vertical Borehole Heat Exchangers (BHEs) for Ground Source Heat Pump (GSHP) applications. Focus is devoted to the analyses of Thermal Response Test (TRT) simulations aimed at understanding the main factors that influence the ground thermal conductivity and the effective borehole thermal resistance estimations. The conventional infinite line-source (ILS) model does not include any influence of the external heat transfer rate on the BHE/ground property evaluation. Analyses of numerically simulated TRTs show this omission can sometimes produce an error in the estimate of the ground thermal conductivity. The error may be between ±10% and ±22% for coaxial boreholes (800 m depth), if the ground has a significant geothermal gradient. On the other hand, for single and double U-pipe BHEs the error is less than ±5% under similar conditions. The parameter qratio is identified as an indicator of when the error is significant. This parameter is equal to the external heat rate (injection or extraction) divided by a natural heat rate that is related to the geothermal gradient. Errors greater than ±10% tend to occur for coaxial boreholes with a center-pipe fluid inlet when |qratio

The knowledge of the ground thermal properties, and in particular the ground thermal conductivity is fundamental for the correct sizing of the Ground Coupled Heat Pump (GCHP) plant. The Thermal Response Test (TRT) is the most used experimental technique for estimating the ground thermal conductivity. This paper presents an experimental setup aimed to realise a suitable scale prototype of the real borehole heat exchanger (BHE) and the surrounding ground for reduced scale TRT experiments. The scaled ground volume is realised with a slate block. Numerical analyses were carried out to correctly determine suitable geometric and operational parameters for the present setup. The scaled heat exchanger, inserted into the block, is created with additive technology (3D printer) and equipped with a central electrical heater along its entire depth and with temperature sensors at different radial distances and depths. Present measurements highlight the possibility to reliably perform a TRT experiment and to estimate the slate/ground thermal conductivity with an agreement of about +12% with respect to measurements provided by a standard commercial conductivity meter on proper cylindrical samples of the same material and onto 10 different portions of the slate block.

A ground source heat pump system (GSHP) with a ground heat exchanger (GHE) is a renewable and green technology used for heating and cooling residential and commercial buildings. An innovative U-Tube pipe configuration is suggested to enhance the heat transfer rate in the vertical ground heat exchanger (VGHE). Laboratory experiments are conducted to compare the thermal efficiency of VGHEs with two different pipe configurations: (1) an innovative U-Tube pipe configuration (single U-Tube with two outer fins) and (2) a single U-Tube. The results show that the difference between the inlet and outlet temperatures for the innovative U-Tube pipe configuration was 0.7 °C after 60 h, while it was 0.4 °C for the single U-Tube after the same amount of time. The borehole thermal resistance for the innovative U-Tube pipe configuration was 0.680 m·K/W, which is 29.22% lower than that of the single U-Tube. The heat exchange rate in the innovative U-Tube pipe configuration is increased by 57.95% compared to the conventional single U-Tube. Measured ground temperatures indicate that compared to single U-Tube pipe configuration, the innovative U-Tube pipe configuration has superior heat transfer performance. Based on the experimental results presented in this paper, it was concluded that increasing the surface area significantly by introducing external fins to the U-Tube enhances the heat transfer rate, resulting in increased thermal efficiency of the VGHE.

The in-site thermal response test (TRT) is commonly applied in engineering, but it treats the complex soil with groundwater as the homogeneous soil without groundwater. Although the short-term heat transfer of the two soils may be equivalent, its impact on the long-term performance of the ground source heat pump system (GSHPS) remains to be proved. In this paper, a three-dimensional numerical simulation platform was established. Based on the platform, the equivalence of the TRT and its impact on the long-term performance of the system were analyzed. The results showed that the short-term heat transfer between the actual and equivalent conditions is similar, but the long-term performance of the GSHPS differs greatly, better in the actual condition. The difference of the heat transfer of compact clay, lightweight sand and dry shale between the actual and equivalent conditions were 2.65%, 2.75% and 3.07%, respectively. Under the actual condition of dense clay, the average cooling coefficient of performance (COP) in the tenth year is 29.01% smaller than the equivalent condition, but the average heating COP was 12.38% higher. Compared with first year, the heating COP in the tenth year under actual condition decreased by 1.67%, but 8.70% under the equivalent condition.

The U-type ground heat exchanger is a significant component of the ground source heat pump (GSHP) system, transferring heat from the soil for the air conditioning requirements. Heat accumulation becomes a common problem for the practical application of such systems in southern China. Moreover, the buried pipes containing heat exchangers are constructed in a group form, making the heat accumulation situation more severe and complex. A 3-dimensional simulation model of a 9-well buried pipe group was established and validated by a thermal response test conducted in Nanjing, China. On the basis of this model, the influence of underground seepage water temperature and flow rate on thermal distribution was investigated. Subsequently, the heat transfer capacity of single-well depth in a heat exchanger under varying conditions was calculated. It is concluded that a serious heat accumulation occurs without groundwater seepage. With seepage flow, the heat accumulation can be dissipated. Lower seepage temperature has a modestly beneficial effect on the heat dissipation. The heat accumulation can be better dissipated and the heat transfer capacity per well depth can be improved with greater seepage flow rate.

An accurate estimate of site-specific underground thermal properties from thermal response tests (TRTs) is vital to the efficient and sustainable use of ground source heat pump (GSHP) systems. During a conventional TRT, the ground is perturbed by a heat injection and the response is measured in time. Despite the simplicity of the concept, conventional TRT fails to take into account the ground thermal response to heat extraction, which may affect the reliability of the thermal properties derived. To address the problem, an improved TRT based on both heat injection and extraction was proposed in this study. The improvements were demonstrated with an in-situ test carried out in Taiyuan, China. The measurement results showed that the ground thermal conductivity in Taiyuan was 1.56 W/(m K) and the borehole thermal resistance was 0.22 (m K)/W. The results also showed that, in the case of Taiyuan, ground thermal conductivities derived solely from heat injection test and heat extraction test were, respectively, 7.6% higher and 8.3% lower than that from the improved TRT. Thus, compared with the conventional TRT, the improved TRT consisting of both heat injection and extraction tests can provide test data more accurately reflect the annual performance of GSHP systems.

The design of ground-coupled heat pump systems requires the knowledge of the mean temperature Tfm of the working fluid in borehole heat exchangers. This quantity is usually approximated by the arithmetic mean of inlet and outlet temperature. The approximation yields an overestimation of the thermal resistance of the heat exchanger in thermal response tests, as well as errors in the estimation of the outlet fluid temperature in the dynamic simulation of ground-coupled heat pumps. Recently (Applied Energy 206, 2017, 1406–1415), Zanchini and Jahanbin determined, by finite element simulations, correlations that allow an immediate evaluation of Tfm in any working condition, with reference to double U-tube boreholes with length 100 m and shank spacing 85 mm. In this paper, the analysis presented there is extended to double U-tube boreholes with any length, between 50 and 200 m, and any shank spacing, between 65 and 105 mm. The results hold for every thermal conductivity of the sealing grout between 0.9 and 1.6 W/(m K), every volume flow rate between 12 and 24 L per minute, every diameter of the BHE and every working condition (heating, cooling, thermal response test), both in quasi-stationary and in transient regime.

The paper presents the results of an ongoing research project aimed at developing an innovative technology for energy geostructures for the exploitation of low-enthalpy geothermal energy: the energy micro-piles (EMP). The EMP has the double role of structural support and heat exchanger since it is equipped with a primary circuit of a traditional ground source heat pump system. Given the particular nature of the EMP considered, in terms of pile geometries and characteristics of the primary circuit, its performance has been investigated by means of both field testing, carried out on a full-scale prototype, and 3D finite element simulations. The numerical model has been calibrated on the results of a thermal response test carried out on the full-scale prototype. Then, a parametric study has been conducted to gain useful insight on the performance of the EMP under long-term operating conditions, by varying: (i) the mass flow rate of the heat-carrier fluid; (ii) the soil thermal conductivity; (iii) the thermal properties of the circulation pipe; (iv) the material filling the bottom portion of the prototype. The results of the experimental and numerical activities showed that the value of the specific heat flux generated by the EMP falls in the same range of that of conventional energy piles and that the performance of the EMP is mainly affected by the thermal properties of the foundation soil.

In this paper, a novel U-tube Borehole Heat Exchanger (BHX) with an oval cross-section is proposed for improving the performance of Ground Source Heat Pump systems (GSHP). To study the thermo-hydraulic performance of Borehole Heat Exchanger (BHX) with an oval U-tube: A three-dimensional, symmetric and unsteady state multi-physics Computational Fluid Dynamic (CFD) simulations are carried out by ANSYS FLUENT environment. The conjugate heat transfer and fluid flow are simulated. Borehole thermal resistance, fluid inlet and outlet temperatures, fluid temperature profile along pipe length and pressure drop inside the tube are calculated and analyzed. The oval shape dimensions are adapted, then a comparison between the circle and the oval cross-sections is carried out; four different cases are investigated. As a result, the CFD results show a good agreement with their experimental pairs via Thermal Response Test (TRT) test which was done during September 2017 by our laboratory members. Besides, the results show that the oval shape enhances the heat transfer process and achieves the lowest borehole thermal resistance of 0.125 m K/W, also, it decreases the thermal resistance by 18.47% compared with the circular one. Besides, the oval shape shows a pressure drop of 32% less than the circular tube. Consequently, the oval shape could enhance the coefficient of performance (COP) of GSHP, decrease the borehole length, decrease the operating pressure and cut down initial and operating costs. Also, it fit easily inside tight borehole, so it is preferable in high-densely urban areas.

Laboratory and in-situ tests were carried out with the aim of investigating underground thermal properties and mechanisms of heat transfer of a pilot ground-source heat pump system consisting of two borehole heat exchangers. Samples of grouts used for filling the boreholes and of the main lithotypes penetrated by drilling were analysed in the lab for assessing their thermal properties. Grouts with different mechanical characteristics and similar mineral composition were used in the two holes. The grout thermal conductivity ranged from 1.65 to 2.13 W m−1 K−1 and thermal diffusivity from 0.61 to 0.80 μm2 s−1. Thermophysical measurements on the lithotypes showed the largest thermal conductivity (2.66 W m−1 K−1) in sandstones, whereas pelite denoted the lowest value (1.82 W m−1 K−1). The average thermal diffusivity was 0.73 μm2 s−1. The in-situ experiments included thermal logs, to evaluate the undisturbed underground temperature, and thermal response tests, which were interpreted with the moving line-source model to infer the underground thermal conductivity, the borehole thermal resistance and the groundwater velocity. The inferred thermal conductivity varied from 2.09 to 2.48 W m−1 K−1 and thermal resistance from 0.188 to 0.193 m K−1 W−1. Values of Darcy velocity ranged from 5 to 9 × 10−7 m s−1. After the heat injection tests, the temperature variation with time was recorded at 20-m-depth intervals in both holes. The temperature-time series were used to assess the thermal conductivity variation with depth. In both boreholes, thermal conductivity was little variable from 20 to 80 m depth (2.14–2.34 W m−1 K−1) and it increased (2.49–2.68 W m−1 K−1) at the hole bottom. The average thermal conductivity for the two boreholes was quite similar (2.29–2.32 W m−1 K−1) and consistent with the values obtained with the moving line-source model. The results obtained with this approach were also consistent with the laboratory measurements and gave estimates of thermal conductivity at different depths.

Performance of shallow geothermal systems such as borehole thermal energy storage (BTES) and ground source heat pump (GSHP) mainly depends on the thermal properties of the subsurface and proper design of borehole heat exchangers (BHE). This paper introduces a novel hybrid approach for measuring the effectiveness of BHEs and surrounding subsurface thermal properties, which combines traditional thermal response test (TRT) with the borehole temperature relaxation method (BTR), based on two dimensional radial conductive heat transfer. The new method allows for: (1) evaluation of how convective heat loss at groundwater layers influence estimation of subsurface thermal properties; (2) examination of non-uniform heat transfer through a BHE to stratified subsurface layers; and, (3) calculation of depth-dependency of thermal properties of unsaturated subsurface layers. The hybrid approach was tested using a 50 m U-type BHE, the results of which indicated that convective heat transfer at the groundwater level altered the real value of effective thermal conductivity from 0.45 to 1.56 W/m K. The non-uniformity of heat transfer along the BHE was confirmed by calculations that showed subsurface thermal conductivities were depth dependent, varying between 0.34 and 0.61 W/m K.

Since ground-coupled heat pump system (GCHPs) has been more and more widely applied, it is vital to build an accurate method for sizing borehole heat exchangers (BHEs), and the methods for determining the heat transfer between the boreholes and surrounding ground under time-varying loads in different time scales are the kernel of optimizing design of BHEs, which is helpful to obtain accurate results and significantly decrease computation time consuming. This paper summarizes computational methods for the ground thermal response of BHE under time-varying loads and thermal interaction of multiple BHEs, compares the representative design tools in terms of their performance characteristics, and analyzes the direct applications of computational methods in thermal response test (TRT) and operating performance simulation of GCHPs. Finally, some recommendations for future development of computational methods are proposed.

[EN] The determination of ground thermal properties is essential to design competitive commercial ground source heat pump systems. A thermal response test (TRT) carried out on site allows the determination of both the conductivity of the ground and the borehole thermal resistance by means of analytical approaches, which require several simplifying assumptions as well as TRT durations of at least fifty hours and constant heat injection. In this context, a detailed dynamic numerical model of the borehole can help reducing both the uncertainties, associated with these simplifying assumptions, and the required test duration. This paper presents a TRNSYS tool to obtain the grout and ground thermal properties by means of a parameter estimation technique in conjunction with a two-dimensional dynamic numerical model, the B2G model, which is able to provide accurate results with a much shorter testing time and without the necessity of a constant heat injection. The methodology to estimate the borehole and ground characteristics has been validated thanks to the analysis of a set of experimental TRTs for U-pipe vertical boreholes carried out in different type of soils and borehole geometries. Results show that, for the analysed tests, it is possible to obtain an accurate and fast estimation of the ground thermal conductivity with reductions of the necessary TRT duration of up to a 70%, with a total uncertainty between +/- 10% and +/- 18%, considering not only the uncertainty introduced by the reduction of the test duration, but also the main sources of uncertainty.

The correct sizing of the borehole field in closed-loop Ground Source Heat Pump (GSHP) systems requires accurate value of the ground thermal properties, especially the thermal conductivity. One of two methods are generally applied to evaluate this parameter: the Thermal Response Test (TRT) and databases reported in guidelines or Standards for lithologies identified in the local stratigraphy. This paper presents a database of more than 100 Thermal Response Tests performed in Italy. The equivalent thermal conductivities derived from the TRT outputs are here directly compared with the value estimated through knowledge of the local geological stratigraphy. The obtained results are analyzed in terms of geological setting. A sensitivity analysis on the borefield design has been conducted to evaluate the effects of the equivalent thermal conductivity estimation error on the calculation of the required total borehole length. The borehole length, in turn, strongly affects the initial investment costs as well as the operating conditions of the heat pump over the long term. The obtained results, in some cases significant, highlight the importance of performing the TRT not only when the thermal capacity of the GSHP is high but also in the case of strong geological uncertainty, or in particular geological settings such as high plain areas, alpine valley floors and rocky environments. In the other cases as low plain areas, this database can provide an initial estimate of the range of the expected equivalent thermal conductivity value; therefore, it can be useful for designers of both GSHP systems and other applications where the knowledge of the underground thermal behavior is necessary.

This chapter presents the operation principle of a heat pump (HP) and the necessity for using HPs in the heating/cooling systems of buildings, discusses the vapour compression-based HP systems and describes the thermodynamic cycle and they calculation, as well as operation regimes of a vapour compression HP with electro-compressor. The calculation of greenhouse gas emissions of HPs and energy and economic performance criteria that allow for implementing an HP in a heating/cooling system is considered. A detailed theoretical study and experimental investigations on ground source HP technology concentrating on ground-coupled heat pump (GCHP) systems are also included. Additionally, an analytical model for evaluation of the ground thermal conductivity and the borehole thermal resistance using a thermal response test is developed and the Earth Energy Designer (EED) simulation program is used to calculate the fluid temperature for a case study of the ground heat exchanger. An experimental study is performed to test the energy efficiency of the radiator or radiant floor heating system for an office room connected to a GCHP. Experimental measurements are also used to test the performance of a reversible vertical GCHP system at different operating modes. Fundamental efficiency parameters (coefficient of performance (COP) and CO2 emission) are obtained for one month of running using two control strategies of the GCHP: standard and optimised regulation of the water pump speed, and a benchmarking of these parameters is achieved. Exploratory research has indicated higher efficiency of the system for the flow regulation solution using a buffer tank and programmed control device for the circulation pump speed compared with the standard regulation solution (COPsys with 7–8% increase, and CO2 emissions with 7.5–8% decrease). Finally, two simulation models of thermal energy consumption in heating/cooling and domestic hot water operation were developed using TRNSYS software.

The purpose of this study was to determine the life cycle cost (LCC) of a ground source heat pump (GSHP) system based on the results of a multilayer thermal response test (TRT). Previous studies by our research team suggested the possibility of significantly reducing the total borehole length, based on the results of the multilayer TRT to identify the partial groundwater flow. In the present study, it was validated that the borehole heat exchangers reduced by the results of the partial groundwater flow could operate with a sustainable performance of the GSHP system over an extended period. LCCs of the GSHP system for 30 years were analyzed in three areas with different climatic and geological conditions. Based on these results, the proposed method demonstrates a reasonable cost reduction advantage compared with conventional TRT analysis. The multilayer TRT analysis was validated using root mean square error and temperature error, obtained from temperature comparison between the numerical simulation results and the TRT data. The required total borehole length was achieved when the performance of the GSHP system attained the target performance for heating, after 30 years of operation. The GSHP system, designed by considering the partial groundwater flow, can reduce 30–40% of the total borehole length and 15–20% of the LCC, compared with the system designed based on conventional TRT analysis.

A ground source heat pump system is a highly efficient renewable heating, cooling, and ventilation system that utilizes the ground as a heat source or sink via ground heat exchangers. Energy pile is an energy geotechnical structure that couples a ground heat exchanger with a geotechnical structure, leading to low capital cost. The design of energy piles can be challengeable due to their complicated geometries and the requirement of mechanical load. This study focuses on the heat transfer across the concrete&ndash, soil interface of energy piles in urban areas. Case studies from two projects, the Lambeth College and Shell Centre projects, are presented and discussed. The back analysis of two energy pile cases illustrated that the heat transfer coefficient at the pile&ndash, soil interface can differ between the cooling mode and the heating mode. It can be concluded that the difference in the heat transfer coefficient is influenced by a number of factors such as soil properties, concrete (grout) properties, and the installation method.

The authors developed a ground heat exchanger (GHE) calculation model influenced by the ground surface by applying the superposition theorem. Furthermore, a simulation tool for ground source heat pump (GSHP) systems affected by ground surface was developed by combining the GHE calculation model with the simulation tool for GSHP systems that the authors previously developed. In this paper, the outlines of GHE calculation model is explained. Next, in order to validate the calculation precision of the tool, a thermal response test (TRT) was carried out using a borehole GHE with a length of 30 m and the outlet temperature of the GHE calculated using the tool was compared to the measured one. The relative error between the temperatures of the heat carrier fluid in the GHE obtained by measurement and calculation was 3.3% and this result indicated that the tool can reproduce the measurement with acceptable precision. In addition, the authors assumed that the GSHP system was installed in residential houses and predicted the performances of GSHP systems using the GHEs with different lengths and numbers, but the same total length. The result showed that the average surface temperature of GHE with a length of 10 m becomes approximately 2 °C higher than the average surface temperature of a GHE with a length of 100 m in August.

This paper concerns the modeling of vertical coaxial heat exchangers for Ground Source Heat Pump (GSHP) applications. Vertical coaxial borehole heat exchangers (CBHEs) can be buried at depths which are even higher than the conventional ones. In this case they are referred as Deep Borehole Heat Exchangers (DBHEs). As it is known, there is indeed a strong recent tendency, especially in the Scandinavian regions, to use high depth (500–1000 m) underground heat exchangers for the GSHP applications. This study is aimed at the analysis of the BHE behaviour in the early period, say for Fourier numbers typical of the Thermal Response Test (TRT) measurements. The novelty of the present numerical results is related to the applicability of standard TRT methods when referred to DBHEs and different geothermal gradients can be found. To this aim a Fortran code has been developed for describing a 2D transient conduction and convection problem able to provide the fluid and ground temperature evolution as a function of a series of boundary conditions, including the initial and far field ground temperature distribution along the depth. The application of the present model is related to coaxial BHEs for the assessment of the effects of the undisturbed ground temperature profile and the direction of the carrier fluid on the ground thermal conductivity estimation in TRT experiments. It is here demonstrated that different BHE depths (ranging from 150 to 800 m) and different undisturbed temperature profiles (including zero and positive geothermal gradients) can severely affect the TRT ground conductivity estimation (errors up to 25%) if the flow direction is based on the annular pipe or the central pipe inlets.

Thermal Response Test (TRT) has become a very popular method of evaluating geothermal properties for ground-coupled heat pump systems. However, a test must be carried out under certain operating conditions of the project it serves, which is a strong restriction to its standardization. Few studies have previously focused on how to deal with TRT results in alternative conditions. Based on line source model, the influences of different factors on TRT results were analyzed, and a new algorithm for correcting TRT results was developed in this paper. The algorithm was applied to a case study, and the results calculated using the new algorithm show that thermal conductivity difference before and after comparison changed from −0.97 W/(m K) to 0.10 W/(m K) at different test conditions, and the relative error between the corrected values of ground thermal conductivity was reduced to 4.63%. Therefore, the new proposed correcting algorithm provides reference to the standardization of TRT and the generalization of test conditions, which can help save test time and cost caused by repeating tests.

A new-shaped U-tube with an oval cross-section has been developed to boost the heat transfer inside the borehole heat exchanger and to reduce the heat exchanger depth and the drilling cost. In addition, it could fit efficiently inside tight boreholes, so it is fitting for installing GSHPs at urban areas. This research evaluated the effectiveness in situ by thermal response tests at total 20 borehole heat exchangers in three sites, Japan. The in situ borehole thermal resistances were about 10–30% smaller with the oval U-tube than the circle U-tube. However, the spacers had an imperceptible impact probably because of the soil pressure to the U-tubes. Also, transient numerical CFD simulations were carried out to calculate the long-term performance of a household GSHP system with oval U-tubes. It indicated that the required lengths of a borehole heat exchanger could be reduced by about 14% in the fine soils and 15–19% in the coarse soils.

The Centre of Sustainable Development and Energy Savings of the Faculty of Geology, Geophysics and Environmental Protection of AGH University of Science and Technology in Miekinia conducts research and educational activities in the field of renewable sources of energy, geothermal heat pumps especially. Growing interest in using the renewable energy sources (RES) is reflected upon the interest in such a discipline of studies. Many people declare intention of studying RES. This is a modern discipline, opened at AGH University of Science and Technology in 2003 as the first in Poland. Since 2012, when the Centre was open ca. 8000 people (including students, pupils, local governments and communities) were educated here. Not only geothermal energy is a main focus attention field. In the Centre other renewable sources of energy are also researched. This include solar energy (both photovoltaic and thermal), solid fuel boilers and wind energy. The Centre is heated by heat pumps with ground sources which are simultaneously use by students for practice and measurements. There are didactic heat pumps with temperature and pressure sensors and electric meters to be used by students during measurements. The Centre is in disposal of self-constructed Thermal Response Test device for thermal parameters measurements. In a stand for testing ground-source brine heat pumps for central heating and hot domestic water preparation research are conducted on the COP heat pump efficiency in accordance with the PN-EN 14511 standard. The Centre works on the prototyping of new, innovative heat pumps.

This paper presents a new parameter identification (PI) algorithm for estimating effective and detailed thermal parameters of ground source heat pump systems using data obtained from the well-known thermal response test. The PI comprises an iterative scheme coupling a semi-analytical forward model to an inverse model. The forward model is formulated based on the spectral element method to simulate transient 3D heat flow in ground source heat pump (GSHP) systems, and the inverse model is formulated based on the interior-point optimization method to minimize the system objective function. Compared to existing interpretation tools for the thermal response test, the proposed PI algorithm has several advanced features, including: it can handle fluctuating heat pump power and inlet temperatures; interpret data obtained from multiple heat injection or extraction signals; produce accurate backcalculation for short and long duration experiments; and handle multilayer systems. The PI algorithm is tested against synthesized data, using a wide range of random noise, and versus an available laboratory experiment. The computational results show that the PI algorithm is accurate, stable and exhibiting relatively high convergence rate.

Ground thermal properties, including the thermal conductivity and diffusivity of both soil and grout, are significant considerations for the design of a ground-coupled heat pump (GCHP) system. However, as a result of the limitations inherent in available response models, few in-situ thermal response tests (TRTs) can identify grout thermal conductivity and diffusivity. This paper proposes a new method to estimate the thermal conductivity and diffusivity of both soil and grout simultaneously using the recently developed infinite composite-medium line source (ICMLS) model. Firstly, a linear dependence analysis is performed on the aforementioned four parameters to ensure the feasibility of the proposed method, leading to an estimation of the minimum TRT duration. Secondly, uncertainty analysis is carried out to analyze the influence of U-pipe shank spacing, as it is considered a sensitive parameter in modeling the heat transfer of ground heat exchangers (GHEs). Thirdly, a genetic algorithm is used to identify these four parameters using the data collected from a TRT. The proposed method is verified using a well-designed sandbox experiment. Finally, its application is demonstrated and evaluated by applying it to the design of a GCHP system for an office building at Hunan University.

An experimental geothermal monitoring, performed on the ground source heat pump (GSHP) system running in a nearly zero energy building (NZEB), has been realized in the protected hydrothermal area of Ponte Arche (Italian Alps), at the request of the competent geological authority. The scope is to assess if a thermal interference could appear between the exploitation of the shallow underground by borehole heat exchangers (BHEs) and nearby thermal baths. Currently, in this area an absolute prohibition for shallow geothermal applications is in force, the area is categorized “potentially subject to geothermal manifestations”. However, in light of new geological and hydrogeological knowledge, supported by long-term monitoring, the protection criterion and constraint geometry could be reviewed. Preliminary to the experimental geothermal monitoring, a detailed ground investigation campaign was performed, including: a geological setting overview based on existing cartography, one continuous core drilling, three downhole geophysical surveys, one thermal response test (TRT). The monitored parameters comprehend the underground temperature at different depths, the outdoor air temperature, and the ground-side heat flow of the GSHP. The collected data are presented and discussed, including considerations about the underground sustainability of the BHE field, and about the real heating and cooling operation of the GSHP ground-side.

The development and utilization of shallow geothermal energy have rapidly improved worldwide. The thermal physical parameters of rock and soil layers must be accurately measured to design a ground source heat pump system. The obtained thermal conductivity of rock and soil play a critical role in optimizing heat pump system and cost-saving design. In this study, detailed tests were performed on a borehole in Jixi City, Heilongjiang Province in northeast China. In situ geotechnical comprehensive thermal conductivity and layered rock thermal conductivities were obtained by traditional thermal response test (TRT) and distributed thermal response test (DTRT), respectively. A series of laboratory tests were also conducted on rock samples obtained from drilling and the thermal conductivities were compared with that obtained through DTRT. Changes in groundwater environment, moisture content contribute to the deviation of the laboratory experimental results from the value derived through in situ test. The influence of different factors and the analytic hierarchy process (AHP) method was analyzed to determine the weight value for correcting the thermal conductivity tested in the laboratory. The accuracy of the layered conductivity obtained by DTRT, and laboratory test before and after modification was examined by the numerical model based on TOUGH2-MP code. The heat exchange power was predicted on the basis of thermal conductivity obtained by TRT (2.29 kW), DTRT (2.197 kW) and laboratory test before and after being corrected (1.917 kW and 2.180 kW, respectively). The proposed method was proven to be useful in determining layered rock thermal conductivity.

The ground-source heat pump (GSHP) system using a horizontal ground heat exchanger (GHE) can be employed to reduce the installation cost and obtain a balance between efficiency and costs. Among the different types of horizontal GHEs, a spiral-coil-type GHE is one of the advantageous configurations in terms of thermal performance. However, there is no satisfactory guideline to design the horizontal spiral-coil GHEs, though design methods for other types of horizontal GHEs exist. Hence, in this study, a design method is proposed for the horizontal spiral-coil GHEs by modifying the boundary conditions of an existing equation. To verify the applicability of the proposed design equation, a laboratory thermal response test was conducted to validate the finite element model. Then, the validated numerical model was utilized for a computational fluid dynamics (CFD) simulation on an arbitrary building wherein a GSHP system with a horizontal spiral-coil GHE is operated. The entering water temperature (EWT) of 32.09 °C from the simulation result was lower than the design EWT criteria of 32.2 °C, implying that the thermal performance of the GHE for a month of operation is sufficient to cover the building load. The result provides the applicability of the proposed design method.

A 3-D numerical heat transfer model of a ground-source heat pump thermal response test is established in this paper and verified by testing. The numerical model is used to simulate the thermal response test. The soil thermo-physical parameters are estimated based on the cylindrical heat source and the linear heat source theory and contrasted with the values setting in the numerical heat transfer model. The effect of the identification model of the in-situ thermal response on the accuracy of the identification results is studied under different soil thermal properties. The results show that there is an optimal time to identify the soil thermo-physical parameters based on the two models. Identifying the physical parameters at the non-optimal time points will produce a large error. Under the same conditions, the optimal identification time of the cylindrical heat source model is earlier than that of the linear heat source model and is also related to the soil physical properties. Therefore, the test time should be selected to ensure the accuracy of the results. Furthermore, there is no evident influence of the initial soil temperature on the identification accuracy of the two models. This paper offers a reference for improving the thermal response test of ground source heat pumps.

Classic and extended step thermal response test were conducted on three different location in Zagreb. Measurements with the classical thermal response test were used to determine thermogeological properties of the ground and thermal resistance of the borehole for each location. Different values of thermal conductivity are result of differences in geological profile and depth of the sites. In addition, experimental research of steady-state thermal response step test (SSTRST) was carried out to determine heat rejection rates for passive and active cooling in steady state regime. Results showed that heat rejection rate is only between 8-11 W/m, which indicates that coaxial system is not suitable for passive cooling demands. Furthermore, the heat pump in passive cooling mode uses additional plate heat exchanger which causes additional temperature drop of the working fluid by approximately 1, 5 °C. Therefore, steady-state rejection rate for passive cooling is even lower for a real case project. Coaxial heat exchanger should always be designed for an active cooling regime with an operation of a heat pump compressor in a classical vapour compression refrigeration cycle.

A thermal response test (TRT) is commonly conducted at a site proposed for the installation of a ground-coupled heat pump system. The purpose of the TRT is to gather local data to infer thermal properties of the soil. Accurately estimating soil thermal properties enables designers to size the geofield appropriately and supports improved simulations of ground heat exchanger (GHX) systems to better estimate the system performance over both shorter-term and longer-term intervals. This paper reports on the use of a fiber optic distributed temperature sensing system to spatially resolve the temperatures along the entire length of a U-tube within a vertical bore geothermal well. The fiber optic probe inside the U-tube provides spatial temperature data at a 2-m resolution throughout the duration of a TRT. The spatial temperature data within the U-tubes provided by a fiber optic probe offers the potential to determine other characteristics of a test bore, such as ground properties as a function of depth, which will enable much more accurate ground heat exchanger models in the future. An added outcome of this technique is a direct means to observe stratification of fluid temperatures which is another indicator of differences in ground properties that may occur along the depth of a vertical bore.

Thermal response tests are used to estimate the thermal properties of the ground and the borehole heat exchanger being tested. They are thus important for the design of borehole thermal energy storages and ground source heat pump systems. In this study, a theoretical framework is proposed in order to investigate if noise on the heat rate leads to a bias in the parameter estimation. Under the sole assumption of a linear time-invariant system and the use of the sum of squared errors as cost function, it is shown analytically that estimates are in fact biased when the heat rate is corrupted by noise. To understand how large this bias can be, a Monte-Carlo study is performed. It includes more than 126,000 simulations with different noises, thermal parameters and heat rate profiles. Negative biases as high as − 0.44 W/(m K) (11%) and − 11 ⋅ 10 − 3 m K/W (4.1%) are observed for the thermal conductivity and borehole thermal resistance estimates, respectively. In addition, the parameter estimation is stochastic due to randomness of measurement noises. This cannot be ignored since only one thermal response test is performed, in general. Population of estimates with 95% confidence intervals as large as 1.0 W/(m K) (25%) and 24 ⋅ 10 − 3 m K/W (9.4%) appear in this study. Although the bias and confidence intervals are not significant in all simulated cases, they cannot be generally disregarded and one should therefore be mindful of this potential issue when analyzing thermal response tests. An observed trend is that the confidence intervals and bias are higher for higher parameter values, with a particular dependency on thermal conductivity. To reduce the bias and spread of the estimates, having larger heat rate per meter appears to be a good strategy. Having a higher sampling frequency and/or longer tests might also help, but only in reducing the spread of the estimates, not the bias.

Ground-Source Heat Pump (GSHP) systems represent one of the most efficient renewable energy technologies. Their efficiency is highly influenced by the thermal properties of the ground, which are often measured in-situ using the Thermal Response Tests (TRTs). While three-dimensional mechanistic models offer significant advantages over analytical solutions for the numerical interpretation of TRTs, their computational cost represents a limiting factor. Moreover, most of the existing models do not include a comprehensive description of hydrological processes, which have proven to strongly influence the behavior of GSHP. Thus, in this study, we propose a computationally efficient pseudo-3D model for the numerical analysis and interpretation of TRTs. The numerical approach combines a one-dimensional description of the heat transport in the buried tubes of the exchanger with a two-dimensional description of the heat transfer and water flow in the surrounding subsurface soil, thus reducing the dimensionality of the problem and the computational cost. The modeling framework includes the widely used hydrological model, HYDRUS, which can simulate the movement of water, heat, and multiple solutes in variably-saturated porous media. First, the proposed model is validated against experimental data collected at two different experimental sites in Japan, with satisfactory results. Then, it is combined with the Morris method to carry out a sensitivity analysis of thermal properties. Finally, the model is exploited to investigate the influence of groundwater and lithologic heterogeneities on the thermal behavior of the GSHP.

Thermal properties of rock-soil are of great importance for the design of borehole heat exchangers (BHEs). In the process of testing thermal properties of rock-soil through an in-situ thermal response test (TRT), groundwater seepage directly affects the evaluation of thermal properties in TRTs and leads to a varied arrangement and engineering costs of BHEs. Therefore, comprehensive hydrogeological surveys of groundwater and TRTs to determine geothermal properties are necessary. In this study, we made a preliminary attempt to use hydrogeological surveys and correlated TRTs to investigate the heat exchange efficiency of a single U-tube BHE. The geological conditions were used to predict and analyze the groundwater form, groundwater depth, and flow trajectory in the test area based on the natural electric field frequency selection method. Using a comprehensive dataset, the groundwater velocity was determined with ANSYS. The results showed that the initial temperature of rock-soil was 21.8 °C and that the comprehensive thermal conductivity of rock-soil was 1.942 W/(m·°C). Confined and phreatic aquifers existed in the test area, and the depth of the confined aquifer was approximately 70 m. The groundwater velocity in the test area was considered to be approximately 2–2.5 m/d. The results of this study confirm that TRTs combined with groundwater surveys can improve the test accuracy and consequently provide guidance for engineering practices in ground-coupled heat pump systems.

This study aimed to investigate the effect of changing ambient conditions on the accuracy and stability of ground thermal conductivity, λs, using the thermal response test (TRT). First, a detailed numerical model considering the heat fluxes on the field surface of TRT was developed. Then, the numerical model was compared with that with the adiabatic surface boundary in two representative test seasons. After that, the impacts of three parameters associated with the surface heat fluxes, including the surface absorptivity, evaporation coefficient, and surface infrared emissivity on the λs were analyzed. Lastly, statistical analyses with various ambient conditions were employed to evaluate the degree of estimated λs deviating from its actual value in different specifications of TRT and actual ground thermal conductivities. Results show that the surface absorptivity can be generally seen as the most significant contributory factor to the uncertainty of λs. The λs is overestimated and the uncertainty of TRT is aggravated by considering the changing ambient conditions, especially when the depth of drilling test well is short and the actual λs is high. For such conditions, the heat insulation on the test site surface is highly recommended considering the acceptable accuracy of λs with the adiabatic boundary.

Ground thermal conductivity, λs, is one of the important design factors for the ground source heat pump system. This study aimed to investigate the effect of environmental thermal disturbance on the λs estimation during the thermal response test (TRT), which is interpreted by the infinite line source model. For this purpose, a detailed numerical model considering the heat exchange between the circulating fluid inside the above-ground pipe of TRT rig and the outdoor environment in two representative seasons was developed. The numerical model was compared with the model with the adiabatic boundary assigned to the outer surface of above-ground pipe to evaluate the fluctuations of λs on account of the atmospheric disturbance. The parametric and sensitivity analyses were carried out to quantify the impacts of TRT seasons and above-ground pipe parameters including the surface absorptivity, thickness, and thermal conductivity of insulation material, and the length of the above-ground pipe on the λs and corresponding design borehole length. An approach of prolonging the TRT duration was proposed to impair the disturbance when the pipe is not insulated. Results show that λs fluctuates strongly and is largely misestimated considering the atmospheric disturbance, especially when TRT duration is limited. The λs varies widely as the length of above-ground pipe, insulation surface absorptivity, and insulation thermal conductivity increase, insulation thickness decreases, and TRT starts in summer. The pipe length is the most significant factor to the uncertainty of λs. The test time is recommended to last at least 3 days in summer and 3.6 days in winter when the pipe is not wrapped by a layer of insulation material. This study quantitatively assesses the impacts of environmental thermal disturbance during the TRT and gives guidance to carry out TRT properly.