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A diagram of a borehole with coaxial
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Taking the fluid temperature distribution along the borehole depth and thermal short-circuiting into account, a new model for coaxial tube borehole heat exchangers has been established, which provides a better understanding of the heat transfer processes in the ground heat exchangers. On this basis the borehole resistance and efficiency of the bore...
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Inasmuch as the European Union promotes only energetically viable heat pumps in a given location, the aim of the work is an assessment of whether a ground-to-water heat pump (ground source heat pump: GSHP) can be considered as an ecological heat generator in Polish climatic conditions and those of the energy market. Here, as an estimator, the net s...
The worldwide need for improved energy efficiency in end use energy demand technologies catalyses the development of advanced household devices. Efficiency in the end use of energy also embraces the development of advanced household technologies with lower energy demand. Wet appliances are among the most energy intensive domestic devices in the dom...
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... The results imply that q decreases as L increases, which can be attributed to an amplified thermal shunt effect and the integration of the geothermal gradient effect. This phenomenon becomes particularly prominent at greater values of L, increasing R (see Fig. 6(b)), corroborating findings from former studies [10,43]. Furthermore, under all the given L values and along the operating timeframes, q oval surpasses q circle . ...
Ground-coupled heat pumps (GCHP) are highly impactful and promising systems for extracting geothermal energy. Nevertheless, the effectiveness of GCHP can be further improved by reshaping and optimizing the configuration of the coaxial ground heat exchanger (CGHE), which represents the core component of the GCHP and has a significant influence on the heat extraction performance of the system. This paper introduces a detailed comparative numerical study on a novel CGHE with an oval cross-section for enhancing the performance of the GCHP. The proposed design's simplicity stems from altering the outer tube to an oval shape, compared to the commercially used circular CGHE. The conjugate fluid flow and heat transfer are simulated utilizing a three-dimensional numerical model. Heat flux, GHE thermal resistance, and pressure drop within the tube are computed and analyzed. A comparison between circle and oval cross-sections is carried out. The adopted 3-D model shows a fit verification with their experimental pairs via a field experiment. Parametric study and sensitivity analysis are performed, and the effect of groundwater flow on heat transfer is also examined. An optimization approach is developed to explore the optimal inner tube position and incorporate multi-inner tubes that attain the maximum heat transfer efficiency of the oval-CGHE. The results reveal that oval-CGHE significantly surpasses the conventional circle-CGHE, improving the maximum and average heat transfer by 21.03% and 10.24%, corresponding to a reduction in thermal resistance by 28.83% and 9.06%, respectively. Additionally, optimizing the inner tube position and incorporating multi-inner tubes further enhances the performance of oval-CGHE by maximum and average heat transfer rates of 33.01% and 22.91% for inner tube position optimization and 39.08% and 28.84% for multi-inner tube optimization, respectively.
... There are fewer studies on the heat transfer changes in the rocky soil around the casing during operation. In similar studies, related scholars used numerical programming or finite element simulation software to complete relevant calculations, such as Fang et al. [6,7] established the analytical and numerical solutions for the heat exchanger of the medium-deep buried pipe based on the original theory of shallow buried management; Jia et al. [8] assumed the surrounding geotechnical layer as a homogeneous medium and studied the single-hole and multi-hole medium-deep (2,000 m) by numerical methods under multiple operating conditions The thermal response of the geotechnical soil around the geotechnical buried pipe; Renaud et al. [9] used numerical programming to calculate and analyze the thermal disturbance of the geotechnical rock during years of operation of a vertical buried pipe of high temperature geotechnical rock near the Icelandic subsurface magma. Lu et al. [10] used CFD software to construct a two-dimensional cluster well model and computationally analyzed the variation pattern of the geotechnical temperature field around the cluster well. ...
In order to study the temperature change trend of the surrounding geotechnical soil during the operation and thermal recovery of the medium-deep geothermal buried pipe and the influence of the geotechnical soil on the operational stability of the vertical buried pipe after thermal recovery. Based on the data of geological stratum in Guanzhong area and the actual engineering application of medium-deep geothermal buried pipe heating system in Xi’an New Area, the influence law of medium-deep geothermal buried pipe heat exchanger on surrounding geotechnical soil is simulated and analyzed by FLUENT software. The results show that: after four months of heating operation, in the upper layer of the geotechnical soil, the reverse heat exchange zone appears due to the higher fluid temperature; in the lower layer of the geotechnical soil, the temperature decreases more with the increase of depth and shows a linear increase in the depth direction; without considering the groundwater seepage, after eight months of thermal recovery of the geotechnical soil after heating, the maximum temperature difference after recovery is 3.02 ℃, and the average temperature difference after recovery is 1.30 ℃ The maximum temperature difference after recovery was 3.02 ℃ and the average temperature difference after recovery was 1.30 ℃. The geotechnical thermal recovery temperature difference has no significant effect on the long-term operation of the buried pipe, and it can be operated continuously and stably for a long time. Practice shows that due to the influence of various factors such as stratigraphic structure, stratigraphic pressure, radioactive decay and stratigraphic thermal conductivity, the actual stratigraphic temperature below 2000m recovers rapidly without significant temperature decay, fully reflecting the characteristics of the Earth’s constant temperature body.
... To analyze the heat transfer characteristics inside the borehole, thermal resistance was employed, as this has proven to be a useful method [30][31][32]. Before the analysis, some assumptions were made. ...
... Comparing the outlet temperature of the measured results with that of the simulated results, it can be observed that less of a difference appeared between the two. To better compare this difference, a relative error was employed, as calculated by Equation (32). It can be observed that the relative error is greater before 45 h and decreases after 45 h. ...
The coaxial borehole heat exchangers (CBHEs) are efficient geothermal energy utilization method, which has recently attracted increasing attention. However, this method still has no suitable or efficient analysis method, which limits the wide application of CBHEs. Moreover, the non-uniformity of the ground has less been considered in related analyses, which will lead to estimation errors in CBHEs heat exchange efficiency. Hence, an analytical solution for coaxial borehole exchangers considering non-uniformity of the ground with high calculation efficiency is introduced in this paper. By comparing the CBHEs analytical model with typical experimental results, the reliability of this model is verified. Next, using reliability theory, we further investigate the heat exchange efficiency of CBHEs considering the non-uniformity of the ground, the temperature gradient, the fluid inlet direction, and the working temperature difference between the inlet and initial ground. The results show that the changes (±10%) of basic thermal conductivity will decrease the heat exchange efficiency by 4.5% and increase by 4.1%, respectively. With an increase in the ground's non-uniformity (from 0.2 to 0.6 W/m·K), the heat exchange efficiency of the CBHEs will decrease (from 0.4% to 1.4%). Compared to the results without considering the temperature gradient, the heat exchange efficiency of the CBHEs increased by 0.3%, 0.9%, and 1.3% (corresponding to 1, 2, and 3 °C temperature gradients at a 200 m depth). The CBHEs with the annular region as the inlet presented better heat exchange efficiency (by about 0.3%) than CBHEs with the round region as the inlet. the increase in the working temperature difference between the inlet and initial ground was found to greatly improve the heat exchange efficiency of the CBHEs. Moreover, according to the experimental and design cases, the necessity of considering soil non-uniformity has been proved. This study will provide a useful analytical simulation tool and an important guide to optimize CBHEs design considering the ground non-uniformity in the horizontal and vertical direction.
... Thermal shunt becomes an important parameter for deep BHEs as the stretch over which the two legs of the BHE can thermally influence one another is longer. Expressions of * are given by Hellström [15] and Fang et al. [18] for a uniform borehole wall temperature. These expressions are based on networks of thermal resistances as shown in Figure 5. ...
... An estimation of thermal conductivity could nevertheless be performed using the undisturbed profile obtained through DTS data. Estimating a value for the geothermal heat flux, one can indeed calculate the thermal conductivity using Fourier's law; that is, assuming that the heat conduction is the dominant heat transfer mechanism in the establishment of the temperature gradient (18) where is the geothermal heat flux, is the thermal conductivity and is the temperature gradient along the depth. According to Näslund et al. [36] and the International Heat Flow Commission [37], a geothermal heat flux between 0.05 and 0.06 W/m² is a realistic assessment for Stockholm region. ...
http://urn.kb.se/resolve?urn=urn%3Anbn%3Ase%3Akth%3Adiva-239937
This report presents the obtained results and performed tasks during the project Deep Boreholes for Ground-Source Heat Pumps, within the framework of the research program Effsys Expand.
A price model for the investment of GSHP system with deep Borehole Heat Exchangers (BHEs) is derived from a survey submitted to Swedish drillers. Notably, it is shown that the price increases with the borehole depth in a cubic fashion. Up to 300 m depth, the model shows a good match with a linear correlation having a slope of 275
SEK/m, a figure that is close to commonly used estimates for the total installation price of a single BHE. For larger depths, however, the installation price becomes non-linear and deviates from this linear tendency. Examples of total installation prices, including heat pumps and BHEs installation, are given.
Measurements performed in three different installations with deep boreholes are reported. The first tests are performed in a 800 m deep borehole equipped with a coaxial collector. Five Distributed Thermal Response Tests (DTRTs) are performed in this BHE of which four were heat-extraction DTRTs. It is shown that heat flux inversion happens along the depth of the boreholes, that is heat is extracted at the bottom of the borehole but lost at the top. The flow rate is shown to have a significant effect on the thermal shunt effect and the coaxial BHE is shown to have significantly lower pressure drops that more traditional BHE (e.g. U-pipes). The pressure drop vs. flow rate relation
is experimentally characterized through a hydraulic step test. An effective borehole resistance of 0.21 m∙K/W was found. This value is relatively high and is explained by a limited flow rate and the large depth. More investigations as regards the measurement technique (DTS with fiber optic cables) are needed before performing further in-depth analysis.
In another installation, four 510 m boreholes are measured to deviate about 30% from the vertical direction, highlighting the importance of drilling precision for deep boreholes, more particularly in urban environment. The GSHP system, using 50mm Upipe
BHEs is monitored over a year and it is found that pumping energy consumption in the boreholes could be as high as 22% of the total energy consumption of the system (compressors and circulation pumps). The relevance of pressure drops and control strategies for the circulation pumps in the borehole loop is emphasized. The temperature profile with depth confirms the existence of stored heat in the top part of the ground in urban environment.
The results of two DTRTs performed in the same borehole (335 m) are reported, the latter being first water-filled before being grouted. The obtained thermal conductivities differ from one case to another, possibly highlighting the effect of the filling material on the results. Several other explanations are proposed although none can be fully
verified.
The design and construction phases of a laboratory-scale borehole storage model are reported. The design phase mainly focused on deriving analytical scaling laws and finding a suitable size for such a model. Through the design analysis, an explanation to the discrepancy observed in the only previous attempt to validate long-term thermal
behavior of boreholes is proposed.
Investigations as regards the KTH heat pump system, optimum flow rates in GSHP systems with deep BHEs and quantification of thermal influence between neighboring boreholes are discussed although the work could not be fully completed within the timeframe of the project.
The dissemination of knowledge through different activity is reported.
... In this study, we explored the DBHE heat transfer with these important features taken into account. An analytical model and a numerical model presented in our recent publications (Fang et al. 2017 and2018) are compared here, and their respective merits and aplicabilities are disscussed. ...
... Zeng et al. (2003) have developed the methodology and studied effective borehole resistance of double U-tubes in detail. A recent study has been reported by the authors on the effective borehole resistance of coaxial tube boreholes (Fang et al. 2017). An improved version of the study is briefly presented below. ...
... Borehole Thermal Resistance. The effective borehole resistance of the coaxial tube boreholes can be determined according to the analytical solutions of fluid temperature profiles (Diao & Fang 2006;Fang et al. 2017), which takes the expression ...
... Meanwhile, insulation between the upmost section of the outer pipe and the borehole wall may be usually considered necessary, especially for high temperature Borehole Thermal Energy Storage (BTES), because of the low temperature of surrounding soil in the upper layers of ground. Besides, there is thermal short-circuiting between the downwardand upward-flowing streams in the borehole, as seen in the shallow BHEs with U-tubes [5,6] or coaxial tubes [7] . Even greater losses in the DBHEs may be resulted from such thermal shortcircuiting than those in shallow BHEs because of a much longer passage and greater temperature difference between the two circuits involved here. ...
... The main objective of this analysis is to determine the inlet and outlet temperatures of the circulating fluid according to the borehole wall temperature and the heat exchange rate of the BHE. Detailed analyses on single U-tube [2] , double U-tube [5] and coaxial tube [7] boreholes have been available. The two separate regions must be linked on the borehole wall where a uniform temperature distribution is usually assumed. ...
The present study aims to analyze the thermal performance of the water-based nanofluids inside the wellbore heat exchangers to extract geothermal energy from abandoned oil and gas wells (AOGW). Consequently, the study has considered Al2O3/water, Cu/water and CuO/water nanofluids as well as the Al2O3–Cu/water hybrid nanofluid with various particle volume concentrations ranging from 1% to 4% inside the coaxial wellbore heat exchanger (CWHE). For this purpose, a conjugate heat transfer module with a well-known k-epsilon (k-ε) turbulent model of COMSOL Multi-physics has been employed to simulate the fluid flow and heat transfer of the
CWHE installed in AOGWs. Additionally, a sensitivity analysis has also been performed to determine the impact of individual operational parameters such as injection fluid mass flow rate and temperature on the system’s thermal performance. It has been noted that the outlet fluid temperature enhanced by 25% with the addition of 4% of Cu nanoparticles to the water. Moreover, the maximum coefficient of performance of CWHE has been observed around 1.83 using Cu/water nanofluid with 4% volumetric fractions at an injection fluid temperature of 20 ◦C and a mass flow rate of 5 kg/s. Finally, the Cu/water nanofluid provided the best thermal performance followed by Al2O3-Cu –/water, CuO/water and Al2O3/water nanofluids. It is expected that the findings of this study could motivate the researchers to work on the potential application of nanofluids/hybrid nanofluids to extract geothermal energy from AOGWs.
An accurate prediction for deep-buried ground heat exchangers (DBGHEs) is the premise for efficient utilization of geothermal energy. Due to the complexity of the geological composition spanning thousands of meters, the configuration of boundary conditions plays a critical role in evaluating DBGHE thermal performance. This paper proposed a three-dimensional model of full-scale DBGHE involving both conductive and convective heat transfer in aquifuge and aquifer layers. The constant inlet temperature and constant heating power boundaries in the DBGHE domain, and the surface–bottom temperature and heat flux boundaries in the rock-soil domain were examined. It was found that the differences in the performance prediction caused by different DBGHE boundary conditions were closely related to the system’s operating time. The relative differences in heat extraction amount and average borehole temperature of 2000 m DBGHE caused by the two inlet boundaries on the 30th day were, respectively, 19.5% and 18.3%, while these differences on the 120th day were decreased to 8.4% and 9.9%, respectively. It was found that the constant inlet temperature boundary was more appropriate than the constant heating power condition for estimating aquifer effects on the performance of DBGHE. For the rock-soil domain, the results showed that the heat extraction amount of DBGHE under the heat flux boundary was 12.6%–13.6% higher than that under the surface–bottom temperature boundary. Particularly, when considering the velocity change of groundwater in the aquifer, the relative difference in heat extraction amount increments caused by the two types of rock-soil boundaries can reach 26.6% on the 120th day. It was also found that the thermal influence radius at the end of a heating season was hardly affected by either the DBGHE inlet or rock-soil domain boundary conditions.
To understand the heat transfer performance of the middle-deep U-bend geothermal well, the 2500 m deep U-bend geothermal well was taken as the research object at the Hebei University of Engineering. This study established a mathematical and physical model of heat transfer in a U-bend geothermal, and the influence of geothermal gradient and lithologic changes on heat transfer performance was considered. The established model was verified based on the experimental data. Numerical simulation results show that a reasonable reduction of inlet temperature can increase thermal power. The thermal power of the geothermal well will increase by about 40 kW for every 1°C reduction of inlet temperature. The thermal power can be increased with a reasonable increase in the flow rate of the circulating fluid, but the increment of the thermal power decreases gradually. The simulation results show that the thermal power and outlet temperature of a deep U-bend geothermal well can be effectively improved by appropriately reducing the inlet temperature, increasing the flow rate, increasing the collection length, increasing the well depth, increasing the insulation length of the vertical well section and using low thermal conductivity insulation materials. The research results from this study could provide a reference for the heat transfer system optimization of the middle-deep U-bend geothermal well.
