scholarly journals Method of Averaging the Effective Thermal Conductivity Based on Thermal Response Tests of Borehole Heat Exchangers

Energies ◽  
2020 ◽  
Vol 13 (14) ◽  
pp. 3737
Author(s):  
Aneta Sapińska-Śliwa ◽  
Tomasz Sliwa ◽  
Kazimierz Twardowski ◽  
Krzysztof Szymski ◽  
Andrzej Gonet ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
Heat Exchanger ◽  
Effective Thermal Conductivity ◽  
Heat Exchangers ◽  
Thermal Response ◽  
Method Of Averaging ◽  
Thermal Response Test ◽  
Borehole Heat Exchanger ◽  
Measurement Results ◽  
Borehole Heat Exchangers

This work concerns borehole heat exchangers and their testing using apparatus for thermal response tests. In the theoretical part of the article, an equation was derived from the known equation of heat flow, on which the interpretation of the thermal response test was based. The practical part presents the results of several measurements taken in the AGH Laboratory of Geoenergetics. They were aimed at examining the potential heat exchange capacity between the heat carrier and rock mass. Measurement results in the form of graphs are shown in relation to the examined, briefly described wells. Result analysis made it possible to draw conclusions regarding the interpretation of the thermal response test. The method of averaging the measurement results was subjected to further study. The measuring apparatus recorded data at a frequency of one second, however such accuracy was too large to be analyzed efficiently. Therefore, an average of every 1 min, every 10 min, and every 60 min was proposed. The conclusions stemming from the differences in the values of effective thermal conductivity in the borehole heat exchanger, resulting from different data averaging, were described. In the case of three borehole heat exchangers, ground properties were identical. The effective thermal conductivity λeff was shown to depend on various borehole heat exchanger (BHE) designs, heat carrier flow geometry, and grout parameters. It is important to consider the position of the pipes relative to each other. As shown in the charts, the best (the highest) effective thermal conductivity λeff occurred in BHE-1 with a coaxial construction. At the same time, this value was closest to the theoretical value of thermal conductivity of rocks λ, determined on the basis of literature. The standard deviation and the coefficient of variation confirmed that the effective thermal conductivity λeff, calculated for different time intervals, showed little variation in value. The values of effective thermal conductivity λeff for each time interval for the same borehole exchanger were similar in value. The lowest values of effective thermal conductivity λeff most often appeared for analysis with averaging every 60 min, and the highest—for analysis with averaging every 1 min. For safety reasons, when designing (number of BHEs), safer values should be taken for analysis, i.e., lower, averaging every 60 min.

scholarly journals A New Method Based on Thermal Response Tests for Determining Effective Thermal Conductivity and Borehole Resistivity for Borehole Heat Exchangers

Energies ◽  
2019 ◽  
Vol 12 (6) ◽  
pp. 1072 ◽  
Author(s):  
Aneta Sapińska-Sliwa ◽  
Marc Rosen ◽  
Andrzej Gonet ◽  
Joanna Kowalczyk ◽  
Tomasz Sliwa
Keyword(s):  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Thermal Energy ◽  
Heat Exchangers ◽  
Thermal Response ◽  
New Method ◽  
Thermal Resistivity ◽  
Thermal Response Test ◽  
Response Test ◽  
Borehole Heat Exchangers

Research on borehole heat exchangers is described on the development of a method for the determination, based on thermal response tests, of the effective thermal conductivity and the thermal resistivity for borehole heat exchangers. This advance is important, because underground thermal energy storage increasingly consists of systems with a large number of borehole heat exchangers, and their effective thermal conductivities and thermal resistivities are significant parameters in the performance of the system (whether it contains a single borehole or a field of boreholes). Borehole thermal energy storages provide a particularly beneficial method for using ground energy as a clean thermal energy supply. This benefit is especially relevant in cities with significant smog in winter. Here, the authors describe, in detail, the development of a formula that is a basis for the thermal response test that is derived from Fourier’s Law, utilizing a new way of describing the basic parameters of the thermal response test, i.e., the effective thermal conductivity and the thermal resistivity. The new method is based on the resistivity equation, for which a solution giving a linear regression with zero directional coefficient is found. Experimental tests were performed and analyzed in support of the theory, with an emphasis on the interpretation differences that stem from the scope of the test.

Deformations and permeability variations in fine sediments induced by freezing-thawing cycles caused by borehole heat exchangers

2021 ◽  
Author(s):  
Giorgia Dalla Santa ◽  
Simonetta Cola ◽  
Antonio Galgaro
Keyword(s):  
Heat Exchanger ◽  
Pore Size ◽  
Heat Exchangers ◽  
Heat Extraction ◽  
Water Salinity ◽  
Cohesive Sediments ◽  
Borehole Heat Exchanger ◽  
Freeze Thaw ◽  
Vertical Permeability ◽  
Borehole Heat Exchangers

<p>In closed-loop Ground Source Heat Pump system, the circulation of a heat-carrier fluid into the heat exchanger provides the thermal exchange with the underground.</p><p>In order to improve the heat extraction from the ground, the fluid temperature is often lowered down to subzero temperatures; as a consequence, the thermal alteration induced in the ground is more intense and can cause freezing processes in the surroundings. In sediments with significant clay fraction, the inner structure and the pore size distribution are irreversibly altered by freezing-thawing cycles.</p><p>A wide laboratory program has been performed in order to measure the induced deformations and the permeability variations under different conditions of mechanical loads/depth [1], interstitial water salinity [2] and soil plasticity [3]. In addition, vertical deformations and permeability variations induced by freeze-thaw cycles have been measured also in Over-Consolidated silty clays at different OCR [4].</p><p>The results suggest that, despite the induced frozen condition is quite confined close to the borehole [5], in Normal-Consolidated silty clay layers the freezing-thawing-cycles induce an irreversible settlement up to 16%, gathered cycle-after cycle depending on sediment plasticity, pore fluid salinity and applied load. In addition, despite the overall contraction of the soil, the vertical hydraulic conductivity may increase by about 8 times due to a remarkable modification of the soil fabric with increases in pore size, pores connectivity and orientation [6].</p><p>The OC silty-clays show an opposite behavior. Experimental results point out that, in case of OC deposits, higher the OCR lower the freeze-thaw induced settlement. In case of OCR > 15, the settlement turns to a slight expansion. Conversely, the observed augment in vertical permeability increases with the OCR degree [4].</p><p>These occurrences are significant and irreversible and could affect the functionality of the system as well as lead to environmental effects such as local settlements, negative friction on the borehole heat exchangers or interconnection among aquifers in the probe surroundings.</p><ul><li>[1]. Dalla Santa G*, Galgaro A, Tateo F, Cola S (2016). Modified compressibility of cohesive sediments induced by thermal anomalies due to a borehole heat exchanger. <strong>Engineering Geology</strong> 202, 143-152.</li> <li>[2]. Dalla Santa G*, Galgaro A, Tateo F, Cola S (2016). Induced thermal compaction in cohesive sediments around a borehole heat exchanger: laboratory tests on the effect of pore water salinity. <strong>Environmental Earth Sciences</strong>, 75(3), 1-11.</li> <li>[3]. Cola S, Dalla Santa G, Galgaro A (2020). Geotechnical hazards caused by freezing-thawing processes induced by borehole heat exchangers. <strong>Lecture Notes in Civil Engineering</strong>, 40, pp. 529–536</li> <li>[4]. Dalla Santa G, Cola S, Galgaro A (2021). Deformation and Vertical Permeability Variations Induced by Freeze-Thaw Cycles in Over-Consolidated Silty Clays. <strong>Challenges and Innovations in Geomechanics</strong>, 117</li> <li>[5]. Dalla Santa G*, Farina Z, Anbergen H, Rühaak W, Galgaro A (2019). A Comparative Study on the Relevance of Computing Freeze-Thaw Effects for Borehole Heat Exchanger Modelling. <strong>Geothermics</strong> 79, 164-175.</li> <li>[6]. Dalla Santa G*, Cola S, Secco M, Tateo F, Sassi R, Galgaro A (2019). Multiscale analysis of freeze-thaw effects induced by ground heat exchangers on permeability of silty-clays. <strong>Geotechnique</strong> 2019, 69(2).</li> </ul>

Thermal response test analysis for U-pipe vertical borehole heat exchangers under groundwater flow conditions

2021 ◽  
Vol 165 ◽  
pp. 391-404
Author(s):  
Teresa Magraner ◽  
Álvaro Montero ◽  
Antonio Cazorla-Marín ◽  
Carla Montagud-Montalvá ◽  
Julio Martos
Keyword(s):  
Groundwater Flow ◽  
Heat Exchangers ◽  
Thermal Response ◽  
Thermal Response Test ◽  
Flow Conditions ◽  
Test Analysis ◽  
Response Test ◽  
Borehole Heat Exchangers ◽  
Vertical Borehole

An Experimental Study on the Thermal Performance Evaluation of SCW Ground Heat Exchanger

2017 ◽  
Vol 25 (01) ◽  
pp. 1750006 ◽  
Author(s):  
Keun Sun Chang ◽  
Min Jun Kim ◽  
Young Jae Kim
Keyword(s):  
Thermal Conductivity ◽  
Performance Evaluation ◽  
Heat Exchanger ◽  
Effective Thermal Conductivity ◽  
Thermal Performance ◽  
Thermal Response ◽  
High Heat ◽  
Injection Rate ◽  
Operating Parameters ◽  
Ground Heat Exchanger

In recent years, application of the standing column well (SCW) ground heat exchanger (GHX) has been noticeably increased as a heat transfer mechanism of ground source heat pump (GSHP) systems with its high heat capacity and efficiency. Determination of the ground thermal properties is an important task for sizing and estimating cost of the GHX. In this study, an in situ thermal response test (TRT) is applied to the thermal performance evaluation of SCW. Two SCWs with different design configurations are installed in sequence to evaluate their effects on the thermal performance of SCW using a single borehole. A line source method is used to derive the effective thermal conductivity and borehole thermal resistance. Effects of operating parameters are also investigated including bleed, heat injection rate, flow rate and filler height. Results show that the effective thermal conductivity of top drawn SCW (Type A) is 11.7% higher than that of bottom drawn SCW (Type B) and of operating parameters tested bleed is the most significant one for the improvement of the thermal performance (40.4% enhanced in thermal conductivity with 10.9% bleed).

scholarly journals Analysis of Relaxation Time of Temperature in Thermal Response Test for Design of Borehole Size

Energies ◽  
2020 ◽  
Vol 13 (13) ◽  
pp. 3297
Author(s):  
Hobyung Chae ◽  
Katsunori Nagano ◽  
Yoshitaka Sakata ◽  
Takao Katsura ◽  
Ahmed A. Serageldin ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
Relaxation Time ◽  
Heat Exchange ◽  
Exchange Rates ◽  
Effective Thermal Conductivity ◽  
Thermal Response ◽  
Thermal Response Test ◽  
Vertical Temperature Profile ◽  
Response Test

A new practical method for thermal response test (TRT) is proposed herein to estimate the groundwater velocity and effective thermal conductivity of geological zones. The relaxation time of temperature (RTT) is applied to determine the depths of the zones. The RTT is the moment when the temperature in the borehole recovers to a certain level compared with that when the heating is stopped. The heat exchange rates of the zones are calculated from the vertical temperature profile measured by the optical-fiber distributed temperature sensors located in the supply and return sides of a U-tube. Finally, the temperature increments at the end time of the TRT are calculated according to the groundwater velocities and the effective thermal conductivity using the moving line source theory applied to the calculated heat exchange rates. These results are compared with the average temperature increment data measured from each zone, and the best-fitting value yields the groundwater velocities for each zone. Results show that the groundwater velocities for each zone are 2750, 58, and 0 m/y, whereas the effective thermal conductivities are 2.4, 2.4, and 2.1 W/(m∙K), respectively. The proposed methodology is evaluated by comparing it with the realistic long-term operation data of a ground source heat pump (GSHP) system in Kazuno City, Japan. The temperature error between the calculated results and measured data is 6.4% for two years. Therefore, the proposed methodology is effective for estimating the long-term performance analysis of GSHP systems.

Comparison between traditional and enhanced Thermal Response Test for ground thermal properties estimation

2021 ◽  
Author(s):  
Antonio Galgaro ◽  
Alberto Carrera ◽  
Eloisa Di Sipio
Keyword(s):  
Thermal Conductivity ◽  
Thermal Properties ◽  
Heat Exchangers ◽  
Thermal Response ◽  
Physical Parameters ◽  
Thermal Response Test ◽  
Wireless Data ◽  
Stratigraphic Sequence ◽  
Response Test ◽  
Geological Unit

<p>For the design and implementation of an efficient Ground Source Heat Pump (GSHP) system, the local<br>subsoil represents the core element. Since the thermal performance of Borehole Heat Exchangers (BHEs) is<br>site-specific, its planning typically requires the knowledge of the thermal proprieties of the ground, which<br>are influenced by the local stratigraphic sequence and the hydrogeological conditions. The evaluation of<br>the variations of the ground thermal conductivity (TC) along the depth, as well as its undisturbed<br>temperature, are essential to correctly plan the BHEs field and improve the performance of the ground<br>heat exchangers themselves.<br>Thermal Response Test (TRT) is a well-known experimental procedure that allows to obtain the thermal<br>properties of the ground. However, the traditional method provides a single value of the equivalent TC and<br>the undisturbed temperature, which can be associated with the average value over the entire BHE length,<br>with no chance to detect the thermo-physical parameters variations with depth and to discriminate the<br>contributions of the different geological levels crossed by the geothermal exchange probe. Indeed,<br>different layers within a stratigraphic sequence, may have different thermal properties, according to the<br>presence and to the flow rate of groundwater, as well as to granulometry and mineralogical composition,<br>density, and porosity of the lithologies. The identification of the different contributions to the thermal<br>exchange provided by each geological unit, in practice, can further support BHE design, helping to<br>determine the most suitable borehole length and number, achieving the highest heat exchange capability<br>at the lower initial cost of implementing of the entire geothermal plant.<br>In the last years, new improved approaches to execute an enhanced thermal response test have been<br>developed, as the pioneer wireless data transmission GEOsniff technology (enOware GmbH) tested in this<br>study. This measurement method is characterized by its sensors, 20mm-diameter marbles equipped by<br>pressure and temperature transducers combined with a system of data storing and wireless data<br>transmission. Released at regular intervals down the testing BHE, infilled with water, each marble freely<br>floats allowing the measurement of the water temperature variations over time at different depths, in<br>order to identify areas with particular values of thermal conductivity related to distinctive hydrogeological<br>conditions or lithological assessment. This way, the GEOsniff technology allows a high-resolution spatially-<br>distributed representation of the subsoil thermal properties along the BHE.<br>In this work, we present the test outputs acquired at the new humanistic campus of the University of<br>Padova, located in the Eastern Po river plain (Northern Italy). The thermal conductivity data obtained by<br>the GEOsniff method have been compared and discussed, by considering the standard TRT outputs. This<br>innovative technique looks promising to support the optimization of the borehole length in the design<br>phase, even more where the complexity of the treated geological setting increases.</p>

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