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Deep coaxial borehole heat exchangers (CBHEs) are being widely used in clean space heating of buildings in winter. Here we report a new field test on a 1800m deep CBHE in the southern Daxing uplift, Beijing, a representative geological region in North China. Under the economic flow rate of 20–25 m3/h, the steady heat extraction power is 237.24–256.54 kW. Further, numerical modelling and simulation calculations are performed to compare the applicability of the Eskilson-Claesson model and Al-Khoury model for different operation patterns and then analyze the operation performance of deep CBHE during the whole heating season. For typical flow rates, compared with the continuous pattern, the intermittent pattern can increase the heat extraction power per unit depth by 25.4–31.0% and reduce the thermal influence radius by 1.46–1.89 m due to a better ground temperature recovery around the borehole. In addition, a new contribution factor (CF) is introduced to evaluate the temperature contribution of different ground layers during the heat extraction, which can reveal the influences of flow conditions, geothermal gradient, thermal properties, and even groundwater flow. The present results support that deep CBHEs have more obvious advantages for space heating of buildings with intermittent heating requirements. [ABSTRACT FROM AUTHOR]

The ground thermal properties are the basic parameters for the design of borehole heat exchanger (BHE) in ground–coupled heat pump system (GCHPs), and their accuracy directly affects the economy and reliability of the heat pump system. In traditional design, ground is usually regarded as an isotropic homogeneous medium, and its thermal properties are obtained by solving the inverse heat transfer problem in BHE through in-situ thermal response test (TRT). In fact, different geological layers can be observed along the depth of BHE, and thermal physical properties of each layer are different based on the ground geological conditions. In order to investigate the effect of geological stratification on estimated accuracy of ground thermal parameters in TRT, a validated numerical layered BHE model (NLBM) is presented to simulated TRT, and the fluid temperature response is used to estimate effective ground thermal conductivity and borehole thermal resistance based on line source model (LSM). Secondly, the distributions of fluid temperature in U-pipe and heat flux of BHE along the depth are compared and analyzed based on the NLBM and the numerical homogeneous BHE model (NHBM). At last, borehole thermal resistance from the NLBM and estimated borehole thermal resistance from LSM are compared. The results show the maximum heat flux in the 3rd layer of the NLBM is 21.1% higher than that of NHBM, and the minimum heat flux in the 1st layer is reduced by 46.9% in the 100 h duration. The estimated λ LSM using the fluid temperature responses in the NLBM is 1.3% higher than the thickness-weighted thermal conductivity in the NLBM. The minimum relative error between R LSM and R b is still up to 10.45% in the duration of 100 h even though extending the duration is helpful to improve the estimated accuracy of R LSM. • A numerical layered BHE model is proposed and validated by the experiment. • The distributions of fluid temperatures are compared from the two models. • The differences of heat flux are illustrated in the two models. • Effect of geological stratification on estimated accuracy is investigated in TRT. [ABSTRACT FROM AUTHOR]

U-bend deep borehole heat exchanger (DBHE) is a new promising kind of ground heat exchanger, but nowadays there are lack of accurate efficient models to simulate U-bend DBHE. This paper has developed a semi-analytical model of U-bend DBHE, which considers the unsteady heat transfer of borehole, heterogenous soil thermal properties and geothermal gradient. A 2D numerical model and an analytical model based on infinite line-source theory are also developed, and the three models are compared with experimental data for different boundary conditions. Based on the soil thermal properties estimated by logging data and rock identification, the three models have large errors when compared with experimental data. Based on a soil thermal conductivity λ s estimated by matching experimental data, the three models match better with experimental data, and it is inferred that λ s should be measured by thermal response test instead of rock identification. The semi-analytical model and 2D numerical model match very well for all the time, but the analytical model has large differences with 2D numerical model, which is because the analytical model ignores borehole heat capacity and time-varying heat flows at the borehole wall. This paper could provide a cost-effective semi-analytical model and some advices for U-bend DBHE. [ABSTRACT FROM AUTHOR]

U-bend deep borehole heat exchanger (DBHE) is a new promising kind of ground heat exchanger, but nowadays there are lack of accurate efficient models to simulate U-bend DBHE. This paper has developed a semi-analytical model of U-bend DBHE, which considers the unsteady heat transfer of borehole, heterogenous soil thermal properties and geothermal gradient. A 2D numerical model and an analytical model based on infinite line-source theory are also developed, and the three models are compared with experimental data for different boundary conditions. Based on the soil thermal properties estimated by logging data and rock identification, the three models have large errors when compared with experimental data. Based on a soil thermal conductivity λ s estimated by matching experimental data, the three models match better with experimental data, and it is inferred that λ s should be measured by thermal response test instead of rock identification. The semi-analytical model and 2D numerical model match very well for all the time, but the analytical model has large differences with 2D numerical model, which is because the analytical model ignores borehole heat capacity and time-varying heat flows at the borehole wall. This paper could provide a cost-effective semi-analytical model and some advices for U-bend DBHE. [ABSTRACT FROM AUTHOR]

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. • DTRT data was interpreted for ground thermal properties estimation. • Sequential parameters estimation was presented for DTRT analysis. • The estimation precision for ground heat capacity was improved to 4%. • Effect of borehole depth on estimate error was eliminated by using DTRT data. [ABSTRACT FROM AUTHOR]

This paper proposes an innovative algorithm using the idea of step-wise to estimate of multiple parameters of the shallow ground and U-shaped geothermal heat exchangers. This procedure uses an infinite composite-medium line-source solution. The short-term solution not only enables testing data collected during short times (< 10 h) to be used in parameter estimation but also provides a feasible approach to estimating geometric parameters and thermal properties inside a borehole. The step-wise estimation consists of four stages: 1) determine soil thermal conductivity k s , 2) estimate the thermal conductivity of grout k b , 3) estimate the distance between centers of U-tube pipe D , the thermal diffusivities of soil and grout, a s and a b , and 4) an iteration process if necessary. The outstanding feature of this step-wise algorithm is that the TRT data used in each step can be controlled separately, which can increase the accuracy of estimation because it alleviates the linear dependence among the sensitivities of parameters. It is shown that the accuracy of the estimates is ranked in descending order as follows: k b (± 1.0%), k s (± 2.0%), D (± 16.0%), a s (± 20.0%) and a b (± 70.0%). • New step-wise parameter estimation method is created and validated by TRTs data. • The new algorithm uses a compositemedium line-source solution. • Multi-parameter of the ground and GHE are determined in a set of procedures. • Early-time (<10 h) data is effectively and sufficiently used for estimations. [ABSTRACT FROM AUTHOR]

The soil moisture contents affect significantly the soil thermal properties and consequently the thermal efficiency of shallow geothermal systems. This effect becomes more complex to be evaluated for an Earth-Air Heat Exchanger (EAHE) because of its non-stable energy performance due to a large fluctuation of the temperature of air. In this study, the impact of soil moisture content and soil thermal properties has been investigated on the long-term energy performance of an instrumented EAHE site. First, a full-scale experimental EAHE site in University of Strasbourg as well as its measured data are presented. The thermal properties of different soil layers present in the site were experimentally and theoretically characterized with different soil moisture contents. Based on these results, an analytical solution was proposed to simulate the soil temperature of the field and the output air temperature of the EAHE. A computer program based on this analytical solution was developed to assess the performance of the system for a period of three years. The numerical calculation was validated for an average saturation condition by comparing simulation results with measured data. Different soil saturation conditions were also used in the numerical simulation to consider the effect of soil moisture on the system performance. The results show that if the turbulent flow of the circulating air is fully developed, the difference of the exchanged energy could reach more than 40%. • The long-term performance of a full-scale earth-air heat exchanger was assessed. • The soil thermal properties were characterized at different moisture contents. • An analytical solution was used to simulate the system for a period of 3 years. • Different soil moisture conditions were used in numerical simulations. • The difference of exchanged energies reached more than 40%. [ABSTRACT FROM AUTHOR]

To ensure techno-economically suitable installation of ground source heat pump (GSHP) systems, thermal and hydrogeological properties of the subsoil need to be investigated. In this paper, the geothermal potential for three types of GSHP installations in the urban area of Wuhan city is assessed based on preliminary geological investigations. The potential for shallow geothermal energy is evaluated for surface water heat pump systems (SWHP), groundwater heat pump systems (GWHP) and ground coupled heat exchanger heat pump systems (GCHP). The mapped shallow geothermal potentials provide essential information for the installation of GSHPs and for the management of geothermal resources of Wuhan city. Furthermore, the heat transfer rates for some typical configured borehole heat exchanger (BHE) are tested by field Thermal Response Tests (TRT). In order to understand the techno-economic feasibility of the GSHPs, different types of the installed systems are measured and analyzed. [ABSTRACT FROM AUTHOR]

Heat pumps combined with vertical borehole heat exchangers utilize the ground as a heat source or sink for the purpose of heating or cooling. The proper dimensioning of the borehole heat exchanger requires understanding of the thermophysical properties of the ground surrounding boreholes. Presented research was carried out in two geologically different regions with varying thermal parameters relevant for the utilization of ground-coupled heat pumps: Pannonian Basin System and the Dinarides. Eight different sites were selected based on characteristic geological settings and climate in order to include inland, seaside and mountainous regions. In boreholes, 100 and 130 m deep, double U-pipe borehole heat exchangers were installed. The improved method for evaluation of the ground thermal conductivity and thermal resistance of a borehole heat exchanger known as distributed thermal response test was used. This method employs optic fiber cables for the measurement of temperature profiles along the borehole heat exchangers. Measurements conducted at different locations show great variability of thermal properties and undisturbed ground temperatures. Obtained results indicate that the average thermal conductivity of sediments in the Pannonian part varies between 1.62 and 2.10 W/(m K), compared to Dinaridic carbonate and evaporite rocks' values between 2.01 and 3.19 W/(m K). [ABSTRACT FROM AUTHOR]