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Thermal Response Tests for Shallow Geothermal Systems

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A special issue of Energies (ISSN 1996-1073). This special issue belongs to the section "H: Geo-Energy".

Deadline for manuscript submissions: closed (31 March 2023) | Viewed by 19707

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Prof. Dr. Diego González-Aguilera

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Cartographic and Land Engineering Department, Higher Polytechnic School of Avila, University of Salamanca, Hornos Caleros, 50, 05003 Avila, Spain
Interests: photogrammetry; laser scanning; 3D modeling; topography; cartography
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Dr. Cristina Sáez Blázquez

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Department of Cartographic and Terrain Engineering, Universidad de Salamanca, 37008 Salamanca, Spain
Interests: renewable systems; geothermal energy; vertical closed-loop systems; U-tube heat exchangers; helical-shape pipe; grouting materials; heat carrier fluid; borehole heat exchanger (BHE)
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Dr. Ignacio Martín Nieto

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Department of Cartographic and Terrain Engineering, Universidad de Salamanca, 37008 Salamanca, Spain
Interests: geothermal energy; renewable resources; heat pumps; geophysics
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Geothermal energy has become essential in the current energy sector, as it is the only renewable source independent of solar radiation and/or the gravitational attraction of the sun and moon. This energy is especially important in the heating and cooling sector by use of low enthalpy geothermal resources. However, without an effective thermal characterization of the ground, the correct and continuous operation of the geothermal system is difficult to achieve. In this context, thermal response tests (TRTs) are commonly used in the determination of the surrounding ground thermal conductivity. The implementation of these tests means an important improvement of the global system operation, at the same time involving a high increase of the initial investment.

This Special Issue aims to collect original research or review articles on different solutions and devices focused on the thermal characterization of the ground. Different variations of the traditional thermal response tests will be considered for publication in this Special Issue.

Prof. Dr. Diego Gonzalez-Aguilera
Dr. Cristina Sáez Blázquez
Dr. Ignacio Martín Nieto
Guest Editors

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Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

Ground thermal characterization
Thermal conductivity
Thermal ground tests
Thermal response tests
Mathematical and numerical models
Statistic and economic studies

Published Papers (9 papers)

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Research

26 pages, 4748 KiB

Open AccessArticle

Evaluation of Subsurface Heat Capacity through Oscillatory Thermal Response Tests

by Nicolò Giordano, Louis Lamarche and Jasmin Raymond

Energies 2021, 14(18), 5791; https://doi.org/10.3390/en14185791 - 14 Sep 2021

Cited by 7 | Viewed by 1830

Abstract

Two methods are currently available to estimate in a relatively short time span the subsurface heat capacity: (1) laboratory analysis of rock/soil samples; (2) measure the heat diffusion with temperature sensors in an observation well. Since the first may not be representative of in-situ conditions, and the second imply economical and logistical issues, a third option might be possible by means of so-called oscillatory thermal response tests (OTRT). The aim of the study was to evaluate the effectiveness of an OTRT as a tool to infer the subsurface heat capacity without the need of an observation well. To achieve this goal, an OTRT was carried out in a borehole heat exchanger (BHE). The total duration of injection was 6 days, with oscillation period of 12 h and amplitude of 10 W m⁻¹. The results of the proposed methodology were compared 3-D numerical simulations and to a TRT with a constant heat injection rate with temperature response monitored from a nearby observation well. Results show that the OTRT succeeded to infer the expected subsurface heat capacity, but uncertainty is about 15% and the radial depth of penetration is only 12 cm. The parameters having most impact on the results are the subsurface thermal conductivity and the borehole thermal resistance. The OTRT performed and analyzed in this study also allowed to evaluate the thermal conductivity with similar accuracy compared to conventional TRTs (3%). On the other hand, it returned borehole thermal resistance with high uncertainty (15%), in particular due to the duration of the test. The final range of heat capacity is wide, highlighting challenges to currently use OTRT in the scope of ground-coupled heat pump system design. OTRT appears a promising tool to evaluate the heat capacity, but more field testing and mathematical interpretation of the sinusoidal response is needed to better isolate the subsurface from the BHE contribution and reduce the uncertainty. Full article

(This article belongs to the Special Issue Thermal Response Tests for Shallow Geothermal Systems)

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18 pages, 4222 KiB

Open AccessArticle

A New Approach for Characterizing Pile Heat Exchangers Using Thermal Response Tests

by Charles Maragna and Fleur Loveridge

Energies 2021, 14(12), 3375; https://doi.org/10.3390/en14123375 - 08 Jun 2021

Cited by 2 | Viewed by 1530

Abstract

Pile heat exchangers offer a cost effective route to implementation of ground-source heat pump systems for many large commercial buildings compared with traditional boreholes. Such projects typically use thermal response tests to determine the key input parameters for system design, namely soil thermal conductivity and heat exchanger thermal resistance. However, this brings challenges for pile heat exchanger based systems, where in situ thermal response tests are known to be less reliable due to the large thermal capacity of the pile. This paper presents a new “black box” resistance capacitive model for applications to pile thermal response tests. The approach is tested against case study data and shown to perform well. Additional test duration savings are shown to be possible if a novel combination of borehole and pile thermal response tests is applied together to determine design parameters. Full article

(This article belongs to the Special Issue Thermal Response Tests for Shallow Geothermal Systems)

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17 pages, 3167 KiB

Open AccessArticle

Evaluating Variability of Ground Thermal Conductivity within a Steep Site by History Matching Underground Distributed Temperatures from Thermal Response Tests

by Yoshitaka Sakata, Takao Katsura, Ahmed A. Serageldin, Katsunori Nagano and Motoaki Ooe

Energies 2021, 14(7), 1872; https://doi.org/10.3390/en14071872 - 28 Mar 2021

Cited by 8 | Viewed by 1871

Abstract

The variability of ground thermal conductivity, based on underground conditions, is often ignored during the design of ground-source heat pump systems. This study shows a field evidence of such site-scale variations through thermal response tests in eight borehole heat exchangers aligned at a site on a terrace along the foothills of mountains in northern Japan. Conventional analysis of the overall ground thermal conductivity along the total installation length finds that the value at one borehole heat exchanger is 2.5 times that at the other seven boreholes. History matching analysis of underground distributed temperature measurements generates vertical partial ground thermal conductivity data for four depth layers. Based on the moving line heat source theory, the partial values are generally within a narrow range expected for gravel deposits. Darcy velocities of groundwater are estimated to be 74–204 m/y at the borehole with high conductivity, increasing in the shallow layers above a depth of 41 m. In contrast, the velocities at the other seven boreholes are one-to-two orders of magnitude smaller with no trend. These high and low velocity values are considered for the topography and permeability. However, the relatively slow groundwater velocities might not apparently increase the partial conductivity. Full article

(This article belongs to the Special Issue Thermal Response Tests for Shallow Geothermal Systems)

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18 pages, 5333 KiB

Open AccessArticle

Innovative Solutions for Improving the Heat Exchange in Closed-Loop Shallow Geothermal Systems

by Giovanni Floridia, Federica Blandini, Salvatore Iuculano, Giuseppe M. Belfiore and Marco Viccaro

Energies 2021, 14(1), 108; https://doi.org/10.3390/en14010108 - 28 Dec 2020

Cited by 1 | Viewed by 2071

Abstract

Thermal conductivity, hydraulics properties and potential use in low-enthalpy geothermal applications of single and double U geothermal probes enhanced with carbon fibre are discussed in this work. Although the efficiency of a shallow geothermal installation is chiefly based on chemical and physical characteristics of rocks and hydrogeological aspects of the subsurface, the total heat extracted from the subsoil also depends on the intrinsic thermal characteristics of probes. New configurations and solutions aimed at enhancing the performance of components are therefore of considerable interest in this field of research. As a consequence of the economic and versatility advantages of the components, geothermal probes have been generally developed with materials like polyethylene, which presents, however, isolating behaviour that does not allow ideal heat exchange in ground source heat pump systems (GSHP). Innovative combinations of different materials are therefore necessary in order to improve thermal conductivity and to preserve the exceptional workability and commercial advantages of the finest elements available on the market. This work presents results coming from experimental tests involving standard polyethylene geothermal probes integrated with radial rings of polyacrylonitrile-based carbon fibre (PAN). Our evaluations are aimed at finding the best solutions for thermal exchange and adaptability with respect to traditional systems. Hydraulic and thermal performances and the response in a geo-exchange system have been verified. The new solutions appear to be highly suitable as geothermal exchangers in shallow geothermal systems and contribute to significantly reduce the total costs pertaining to the drilling operations. Full article

(This article belongs to the Special Issue Thermal Response Tests for Shallow Geothermal Systems)

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22 pages, 15288 KiB

Open AccessArticle

Geophysical Prospecting for Geothermal Resources in the South of the Duero Basin (Spain)

by Ignacio Martín Nieto, Pedro Carrasco García, Cristina Sáez Blázquez, Arturo Farfán Martín, Diego González-Aguilera and Javier Carrasco García

Energies 2020, 13(20), 5397; https://doi.org/10.3390/en13205397 - 15 Oct 2020

Cited by 11 | Viewed by 2113

Abstract

The geothermal resources in Spain have been a source of deep research in recent years and are, in general, well-defined. However, there are some areas where the records from the National Institute for Geology and Mining show thermal activity from different sources despite no geothermal resources being registered there. This is the case of the area in the south of the Duero basin where this research was carried out. Seizing the opportunity of a deep borehole being drilled in the location, some geophysical resources were used to gather information about the geothermal properties of the area. The employed geophysical methods were time-domain electromagnetics (TDEM) and borehole logging; the first provided information about the depth of the bedrock and the general geological structure, whereas the second one gave more detail on the geological composition of the different layers and a temperature record across the whole sounding. The results allowed us to establish the geothermal gradient of the area and to discern the depth of the bedrock. Using the first 200 m of the borehole logging, the thermal conductivity of the ground for shallow geothermal systems was estimated. Full article

(This article belongs to the Special Issue Thermal Response Tests for Shallow Geothermal Systems)

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14 pages, 6374 KiB

Open AccessArticle

A Novel Approach to the Analysis of Thermal Response Test (TRT) with Interrupted Power Input

by Jin Luo, Yuhao Zhang, Jiasheng Tuo, Wei Xue, Joachim Rohn and Sebastian Baumgärtel

Energies 2020, 13(19), 5033; https://doi.org/10.3390/en13195033 - 24 Sep 2020

Cited by 3 | Viewed by 1716

Abstract

The quality of measuring datasets of the thermal response test (TRT) significantly influences the interpretation of borehole thermal parameters (BTP). A thermal response test with an unstable power input may induce an unacceptable error in the estimation of the borehole thermal parameters. This paper proposes a novel approach to treat the dataset with interrupted power input. In this approach, the test records were segmented into several subsections with a constant time interval of 100 min, 60 min, and 30 min, separately. The quality of each data section was assessed and analyzed. Then, two algorithms, including the continuous algorithm and semi-superposition algorithm, were developed. The results estimated by the linear source model (LSM) were compared with one Thermal response test datasets with a stable power input at the same testing site. It shows that the effects of power interruption during the test can be effectively mitigated by deploying both the continuous and semi-superposition methods. The lowest deviation of the calculated thermal conductivity to a thermal response test with stable power input was 2.8% in the continuous method and 0.9% using the semi-superposition method. Thus, the proposed approaches are effective measures to mitigate the effects of interrupted power input on the interpretation of the thermal properties of the ground. Full article

(This article belongs to the Special Issue Thermal Response Tests for Shallow Geothermal Systems)

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Graphical abstract

20 pages, 5192 KiB

Open AccessArticle

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

by Aneta Sapińska-Śliwa, Tomasz Sliwa, Kazimierz Twardowski, Krzysztof Szymski, Andrzej Gonet and Paweł Żuk

Energies 2020, 13(14), 3737; https://doi.org/10.3390/en13143737 - 20 Jul 2020

Cited by 6 | Viewed by 2114

Abstract

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 λ_eff 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 λ_eff 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 λ_eff, calculated for different time intervals, showed little variation in value. The values of effective thermal conductivity λ_eff for each time interval for the same borehole exchanger were similar in value. The lowest values of effective thermal conductivity λ_eff 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. Full article

(This article belongs to the Special Issue Thermal Response Tests for Shallow Geothermal Systems)

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20 pages, 9539 KiB

Open AccessArticle

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

by Hobyung Chae, Katsunori Nagano, Yoshitaka Sakata, Takao Katsura, Ahmed A. Serageldin and Takeshi Kondo

Energies 2020, 13(13), 3297; https://doi.org/10.3390/en13133297 - 27 Jun 2020

Cited by 4 | Viewed by 2181

Abstract

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. Full article

(This article belongs to the Special Issue Thermal Response Tests for Shallow Geothermal Systems)

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22 pages, 6920 KiB

Open AccessFeature PaperArticle

Novel Experimental Device to Monitor the Ground Thermal Exchange in a Borehole Heat Exchanger

by Cristina Sáez Blázquez, Laura Piedelobo, Jesús Fernández-Hernández, Ignacio Martín Nieto, Arturo Farfán Martín, Susana Lagüela and Diego González-Aguilera

Energies 2020, 13(5), 1270; https://doi.org/10.3390/en13051270 - 09 Mar 2020

Cited by 7 | Viewed by 2803

Abstract

Ground source heat pump (GSHP) systems are becoming popular in space heating and cooling applications. Despite this fact, in most countries, the role of this energy is not as important as it should be nowadays according to its capabilities for energy generation without CO₂ emissions, mainly due to the lack of technical knowledge about GSHP performance. The analysis of the physical processes that take part in the geothermal exchanges is necessary to allow the optimal exploitation of the geothermal resources. For all the above, an experimental geothermal device was built in the laboratory to control the phenomena that take place in a borehole heat exchanger (BHE). A 1-m high single-U heat exchanger was inserted in the center of a polyethylene container which also included granular material (surrounding ground) and the grouting material. Temperature sensors were situated in different positions of the experimental setup. Physical processes are evaluated to finally validate the model. Numerous applications can be developed from the experimental BHE. In this research, the determination of the thermal conductivity of the material used as medium was carried out. Results of this parameter were also compared with the ones obtained from the use of the KD2 Pro device. Full article

(This article belongs to the Special Issue Thermal Response Tests for Shallow Geothermal Systems)

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