Thermal Response Test Research Articles

New approach to reduce analysis time of thermal response test: Active thermal response test using linear heat optical fiber sensor

New approach to reduce analysis time of thermal response test: Active thermal response test using linear heat optical fiber sensor

Analysis of the Heat Transfer Performance of a Buried Pipe in the Heating Season Based on Field Testing

Ground source heat pump (GSHP) systems have been widely used in the field of shallow geothermal heating and cooling because of their high thermal efficiency and environmental friendliness. A borehole heat exchanger (BHE) is the key part of a ground source heat pump system, and its performance and investment cost have a direct and significant impact on the performance and cost of the whole system. The ground temperature gradient, air temperature, seepage flow rate, and injection flow rate affect the heat exchange performance of BHEs, but most of the research on BHEs lacks field test verification. Therefore, this study relied on the results of a field thermal response test (TRT) based on a distributed optical fiber temperature sensor (DOFTS) and site hydrological, geological, and geothermal data to establish a corrected numerical model of buried pipe heat transfer and carry out the heat transfer performance analysis of a buried pipe in the heating season. The results showed that the ground temperature gradient of the test site was about 3.0 °C/100 m, and the temperature of the constant-temperature layer was about 9.17 °C. Increasing the air temperature could improve the heat transfer performance. The temperature of the surrounding rock and soil mass of the single pipe spread uniformly, and the closer it was to the buried pipe, the lower the temperature. When there is groundwater seepage, the seepage carries the cold energy generated by a buried pipe’s heat transfer through heat convection to form a plume zone, which can effectively alleviate the phenomenon of cold accumulation. With an increase in seepage velocity, the heat transfer of the buried pipe increases nonlinearly. The heat transfer performance can be improved by appropriately reducing the temperature and velocity of the injected fluid. Selecting a backfill material with higher thermal conductivity than the ground body can improve the heat transfer performance. These research results can provide support for the optimization of the heat transfer performance of a buried tube heat exchanger.

Comparison between new enhanced thermal response test methods for underground heat exchanger sizing

For the efficient design and implementation of a Ground Source Heat Pump (GSHP) system, the local subsoil stands as the core element. Alongside the conventional Thermal Response Test (TRT), recent research has developed improved approaches that garner more detailed information about ground thermal properties. One such technique is the fiber optic-based distributed thermal sensing. It relies on copper wires to thermally stimulate the ground, while optical fibers collect temperature variations over time along the cable. Another pioneering technology, the enhanced GEOsniff (produced by enOware GmbH), enables high-resolution, spatially-distributed representation of subsoil thermal properties along the Borehole Heat Exchanger (BHE) via wireless data transmission. This study compares and discusses data acquired through these two innovative techniques at the new campus for the humanities of the University of Padova, situated in Northern Italy's Eastern Po river plain. The findings are further juxtaposed with conventional TRT results, in terms of thermal conductivity and borehole thermal resistance. The thermal conductivity vertical profiles are also compared with direct measurements conducted on samples. These advanced techniques show promise in aiding the optimization of borehole length design, particularly in geological settings of heightened complexity.

Performance of thermally enhanced backfill material in vertical ground heat exchangers under continuous operation in a cooling season

Backfill materials in vertical ground heat exchangers significantly affect the heat-transfer efficiency of ground-source heat pump systems. In this study, the use of graphite improving the thermal conductivity of backfill materials is investigated via laboratory tests, and its feasibility is verified via in-situ thermal response testing. Based on the results, the relationship between the coefficient of performance (COP) and thermally enhanced backfill materials is built through a numerical model. The variation of COP under different thermal-physical soil conditions and backfill materials with enhanced thermal conductivities is investigated; then a method for selecting backfill materials is proposed by matching thermal properties of backfill material and surrounding soil. The results indicate that when the soil thermal conductivity is at a relatively low level of 1.0 W/(m·K), the highest COP improvement ratios range between 7.99 % and 16.13 % during the 90 days operation However, the COP improvement ratio decreases gradually as the soil thermal conductivity increases, and the COP improvement ratios decrease to less than 1.0 % as the soil thermal conductivity increases to 5.0 W/(m·K). Parameter-design charts are made to intuitively illustrate the COP variation. The result of contributions of influencing parameters shows that high-velocity groundwater seepage contributes positively to COP improvement, followed by thermal conductivity of soil, and then that of backfill material during a cooling season.

Accurate identification of soil thermal parameters and groundwater flow from thermal response tests

The soil heat transfer parameters constitute the key information required for designing ground source heat pump (GSHP). Due to the simplification of the common analytical models, traditional methods are difficult to achieve accurate identification of soil thermal parameters and seepage velocity simultaneously. In this work, a high-precision method to simultaneously identify soil thermal conductivity, volumetric heat capacity, and seepage velocity based on deep neural network is proposed. Through the inversed orthogonal method, the training and validation samples are obtained from a large number of thermal response tests (TRTs) on a full-scale simulation platform. The accuracy of this method was verified by comparing identification results with the true values. Meanwhile, the uncertainty of identification results under different noise conditions was quantified, and the impact of test duration was discussed. The results showed that when the maximum random noise is 0.1 °C, the identification errors of the thermal conductivity, volumetric heat capacity, and seepage velocity are only 1.14 %, −4.34 %, and −3.08 %, respectively. The identification reliability can be improved by obtaining the average value of the results under multiple tests and extending the test duration. When the test duration increased from 50 to 100 h, the uncertainty of the identified parameters reduced by 54.57 %, 48.41 %, and 65.70 %, respectively.

Thermal Properties of Cu–Al–Ge Alloy PCM for Improving Thermal Energy Storage of Current CSP

Metallic phase change material was proposed and examined at melting/eutectic temperatures less than 600 °C for latent heat storage in concentrated solar power. The PCM alloy can potentially charge/discharge thermal energy as a sensible/latent heat at the temperature range of 414-527 °C. The working temperatures of PCM alloy are in operation temperatures of current molten-salt based CSP plant. In this study, thermal response testing of PCM alloy and sensible/latent storage unit that PCM alloy was loaded in a ceramic honeycomb were prepared in a laboratory scale. The thermal charging/discharging performance of PCM alloy and sensible/latent storage unit were experimentally examined and compared. The storage unit provided thermal release/absorption behaviour at close to temperatures of the thermodynamic equilibrium state.

Forecasting regional in-situ thermal conductivity of soil based on tree-based ensemble learning

Borehole heat exchanger is essential component in ground-source heat pumps (GSHPs) for developing geothermal energy, where device efficiency depends on the thermal conductivity of soil. Although the thermal response test is generally used to measure this parameter, its time-consuming and susceptibility to test conditions can impact the accuracy of the test. To address this issue, the study conducted shallow geothermal surveys to establish a comprehensive dataset, enabling the construction of the in-situ thermal conductivity predictive model. By selecting seven input features, four classic tree-based ensemble learning models and stacking algorithms were used. Among all the post-tuned models examined, XGBoost-RF emerged as the standout performer, boasting an impressive R2 value of 0.9809 along with the lowest RMSE and MAE. Notably, it demonstrated generalization capabilities, with errors consistently staying within 5 % during the validation process. Additionally, the SHapley Additive exPlanations (SHAP) was used to enhance the interpretability of the model. Among these, the weighted thermal conductivity of the geotechnical was the most influential feature, while groundwater conditions such as permeability coefficient and aquifer thickness were also significant factors. This study provides a rapid and precise prediction of regional in-situ thermal conductivity. It leads to effective decision-making in the design stage of GSHPs.

A multi-parameter estimation of layered rock-soil thermal properties of borehole heat exchanger in a stratified subsurface

The high initial investment of borehole heat exchanger (BHE) prevents the popularization and application of ground-coupled heat pump system (GCHPs). As the premise of designing BHE, ground thermal properties are usually obtained using thermal response test (TRT), and parameter estimation method are applied based on homogeneous BHE model independent of geological stratification. In this paper, a multi-parameter estimation method using Bat algorithm (BA) is firstly proposed to obtain the layered rock-soil thermal properties based on the numerical layered BHE model, considering the characteristics of geological stratification. Then, a TRT experimental platform of three-layer BHE is developed to measure thermal responses of different stratified subsurface. Finally, the six thermal properties are obtained by the proposed multi-parameter estimation method based on BA, and their accuracy is validated using the experimental data in TRT. The five temperatures obtained from the numerical layered BHE model using the estimated results are compared with the corresponding experimental data, the results show that the maximum error of the inlet and outlet water temperatures of the BHE is respectively 3.32 % and 3.59 %, and it is 3.31 %, 2.38 % and 2.7 % for the coarse sand, fine sand and brown soil, respectively.

Theoretical analysis of using multiple borehole heat exchangers for production of heating and cooling energy in shallow geothermal reservoirs with underground water flow

In this work the feasibility of using the double-U borehole heat exchangers (BHE) in multiple arrangements for production of heating and cooling energy has been investigated.Theoretical case studies are given for a few hypothetical lithological cases corresponding to the lithology that can be found in City of Zagreb. The results show interference between the borehole heat exchangers and limits of use of shallow reservoir with respect to slow convection and loss of accumulated energy in the reservoir. Special emphasis is placed on the description of the groundwater flow determined by the parameters of the shallow geothermal reservoir, such as permeability, water saturation, thermal conductivity, specific heat capacity and soil density, topography, precipitation, etc. Simulations have been performed in FeFlow coupled with Python user-scripts for individual control of the BHE’s, and the surface equipment is modelled using RES2GEO. To assess the accuracy of numerical predictions in simulating thermal responses, FeFlow software was utilized to conduct Thermal Response Tests (TRT) for individual Borehole Heat Exchangers (BHEs). These numerical results were then juxtaposed with empirical TRT data collected from a site near Velika Gorica, a town within the City of Zagreb. For each case simulation is done for one year with one hour resolution. The temperature distribution analysis highlights notable differences between scenarios, where the temperature field with lower permeability experiences significant variations, particularly near the BHE. Here, temperatures can soar to 20 °C during cooling periods and plummet to 4 °C in heating seasons. These fluctuations cause overheating and subcooling of the reservoir, impairing the heat pump’s efficiency and necessitating increased electricity consumption. By using Multivariate linear regression and Multi-layer perceptron regressor method, the main parameter for determination of COP of ground source heat pump. The analysis reveals that the majority of errors fall within the range of ± 5 %, although the MLA prediction method exhibits slightly more frequent errors. Additionally, the R2 scores demonstrate strong performance: the MLA procedure achieves 0.95 for heating and 0.99 for cooling, while the MLP procedure attains 0.99 for both heating and cooling phases.

Thermal response tests on boreholes in multi-layered ground with geothermal gradient

The design of ground source heat pump systems requires knowledge of the properties of the ground and boreholes, which are often measured through thermal response tests (TRTs). Conventional TRT analysis methods apply for uniform ground properties and undisturbed ground temperature. These assumptions become less valid for deeper boreholes, which penetrate more ground layers while the geothermal gradient becomes more important. In multi-layered ground without a geothermal gradient, conventional methods estimate the effective ground thermal conductivity kgr near the thickness-weighted average, but a geothermal gradient adds complications. In the present study simulated TRTs with quasi-steady-state and fully transient models demonstrate that the required test duration for conventional methods may become impractically large as the borehole depth increases. In general, this occurs when the dimensionless parameter |qratio| approaches 1 or less. This parameter represents the ratio of the externally imposed heat injection/extraction rate to the natural heat rate associated with the geothermal gradient. In these cases, the conventional methods do not estimate the thickness-weighted average value of kgr. On the other hand, a fully transient multi-layered ground model can determine the thickness-weighted average value of kgr. This model requires measured fluid vertical temperature profiles to estimate individual layer values of kgr within reasonable uncertainties.

Scale modeling of thermo-structural fire tests of multi-orientation wood laminates

The stacking sequence of laminated wood significantly impacts the composite mechanical behavior of the material, especially when scaling down thermo-mechanical tests on plywood. In previous research, we developed a scaling methodology for thermo-structural tests on samples with similar cross sections, however this paper focused on testing plywood samples with different stacking sequences between the scales. Plywood samples at ½-scale and ¼-scale were subjected to combined bending and thermal loading, with the loading scaled to have the same initial static bending stresses. While the ¼-scale 4-layer [0°/90°]s laminate and the ½-scale 8-layer [0°/90°/90°/0°]s laminate had an equal number of 0° and 90° layers, as the char front progresses, the sections behave differently. Thus, modeling becomes essential to extrapolating the data from the smaller ¼-scale test to predict the behavior of the larger ½-scale test. Reduced cross-sectional area models (RCAM) incorporating classical laminated plate theory were used to predict the mechanical response of the composite samples as the char front increased. Three methods were proposed for calibrating the RCAM models: Fourier number scaling, from detailed kinetics-based pyrolysis GPyro models, and fitting to data from fire exposure thermal response tests. The models calibrated with the experimental char measurements produced the most accurate predictions. The experimental char models validated to predict the behavior of the ¼-scale tests within 2.5%, were then able to predict the ½-scale test behavior within 4.5%.

The Implementation and Comparison of Conventional and Enhanced Borehole Thermal Response Tests: A Case Study

The Implementation and Comparison of Conventional and Enhanced Borehole Thermal Response Tests: A Case Study

Thermal performance enhancement of borehole heat exchanger by thermally induced groundwater convection in fractured rock

Thermal performance enhancement of Borehole Heat Exchanger (BHE) was a key to sustain the high effectiveness of this technology. Many measures have been taken to improve heat transfer efficiency of BHE in the past decades. However, all these changes leaded to slight thermal improvement due to restriction of heat conduction dominated heat transfer process in subsurface. To achieve an alternatively higher heat transfer mode of BHE, this study proposes a novel approach to induce groundwater thermal convection in fractured rock. A borehole was backfilled by gravels to sustain free groundwater flow in fracture network. A series of Thermal Response Tests (TRTs) show the fluid temperature of a gravel-filled BHE is 2.0–––2.4 °C lower than a sand-bentonite mixture grouted BHE, indicating the higher cooling efficiency. Two main factors including fracture dip angle and aperture, determine the flow behavior of a fracture network. The flow speed increases linearly with enlarging dip angle and shows a cubic polynomial relation with fracture aperture. Physical model tests show that the BHE induces groundwater convection flow by forming a circuit which couples with heat convection, mitigates the thermal stagnation around a borehole. As a result, thermal performance of a BHE was improved effectively.

Definition of a thermal conductivity map for geothermal purposes

The use of geothermal energy is spreading globally due to its many advantages, especially for heating and cooling. The correct design of a geothermal system requires knowledge of the parameters of the subsoil rocks, and particularly the thermal conductivity (k), which is the intrinsic ability of a material to transfer thermal energy as a result of a temperature gradient. A thermal conductivity map of the geological formations is time-consuming to produce, but can be of great help when selecting the location of a low-enthalpy geothermal installation, resulting in significant savings and an increase in the efficiency of that installation. The preferred option for determining k is an in situ thermal response test, but laboratory methods may be an alternative if it is not available or affordable. In this work, the needle thermal probe method has been used to measure the k of representative outcropping rocks in Oviedo (NW Spain), since it allows to obtain a rapid determination, its cost is comparatively low and it can be implemented in a portable device. 162 measurements have been carried out on a total of 27 samples, ranging from 0.2 (clay) to 5.4 W m−1 K−1 (quartzite). A relationship has been found between the k of the rocks and their characteristics, such as mineralogy, anisotropy or geological age and a thermal conductivity map was created.

Integrated Underground Analyses as a Key for Seasonal Heat Storage and Smart Urban Areas

The design and performance of a shallow geothermal system is influenced by the geological and hydrogeological context, environmental conditions and thermal demand loads. In order to preserve the natural thermal resource, it is crucial to have a balance between the supply and the demand for the renewable energy. In this context, this article presents a case study where an innovative system is created for the storage of seasonal solar thermal energy underground, exploiting geotechnical micropiles technology. The new geoprobes system (energy micropile; EmP) consists of the installation of coaxial geothermal probes within existing micropiles realized for the seismic requalification of buildings. The underground geothermal system has been realized, starting from the basement of an existing holiday home Condominium, and was installed in dry subsoil, 20 m-deep below the parking floor. The building consists of 140 apartments, with a total area of 5553 m2, and is located at an altitude of about 1490 m above sea level. Within the framework of a circular economy, energy saving and the use of renewable sources, the design of the geothermal system was based on geological, hydrogeological and thermophysical analytical studies, in situ measurements (e.g., Lefranc and Lugeon test during drilling; Rock Quality Designation index; thermal response tests; acquisition of temperature data along the borehole), numerical modelling and long-term simulations. Due to the strong energy imbalance of the demand from the building (heating only), and in order to optimize the underground annual balance, both solar thermal storage and geothermal heat extraction/injection to/from a field of 380 EmPs, with a relative distance varying from 1 to 2 m, were adopted. The integrated solution, resulting from this investigation, allowed us to overcome the standard barriers of similar geological settings, such as the lack of groundwater for shallow geothermal energy exploitation, the lack of space for borehole heat exchanger drilling, the waste of solar heat during the warm season, etc., and it can pave the way for similar renewable and low carbon emission hybrid applications as well as contribute to the creation of smart buildings/urban areas.

A novel oscillatory thermal response test method for efficient characterization of ground thermal properties: Methodology and data analysis

This study presents a novel thermal response testing (TRT) approach to minimize test duration, labor, and associated costs. The proposed method, Oscillatory Thermal Response Test (OTRT), employs an oscillating heat flux using a sinusoidal wave with specific frequency and amplitude characteristics. A custom-designed TRT apparatus was developed to regulate the heating injection pattern for precise control. In Sapporo City, Hokkaido Prefecture, and Kai City, Yamanashi Prefecture, Japan, two in-situ OTRTs were conducted. The temperature profiles of the fluid and undisturbed soil were measured through optical fiber cables integrated within the U-tube legs. A novel analytical method was devised to filter, smooth, and fit the recorded data, enabling the determination of the response temperature amplitudes at the borehole inlet and outlet. Also, the temperature time derivative method calculates the effective thermal conductivity in the early 3 h. Comparative analysis with Normal Thermal Response Tests (NTRTs) demonstrated the accuracy and validity of the OTRT approach. Significant conclusions from this study include a remarkable reduction in TRT duration by over 95 %, with a high accuracy of 92 % achieved using only the first 3 h of testing. The proposed method exhibited a Root Mean Square Error (RMSE) of 0.1 W/(m·K) compared to NTRTs. To establish a strong correlation between the Amplitude Ratio and Effective Thermal Conductivity, further experimental studies are essential to address the limitations of the current study. These studies should explore a variety of geological and hydrological conditions, borehole diameters, type of heat exchanger, diameter of U-tubem grouting material, combined with numerical analysis, to enhance our understanding of this relationship.

Effects by geothermal gradient on thermal response tests for boreholes

A thermal response test (TRT) on a borehole provides information needed in the design of ground heat exchangers for ground source heat pump systems. Conventional analysis of TRT data sets uses the late-time period to estimate the ground thermal conductivity and the effective borehole resistance. During this quasi-steady-state period, the transient average fluid temperature has a logarithmic linear trend, which is consistent with conventional heat transfer models that assume a uniform undisturbed ground temperature. As the depth of the borehole increases the undisturbed ground temperature increases with depth following the geothermal gradient. This paper focuses on how the geothermal gradient affects a TRT. As the geothermal gradient increases, the required time to reach the conventional logarithmic linear trend may increase beyond any practical testing duration in coaxial boreholes. Then, conventional analysis methods based on a quasi-steady-state period incorrectly estimate the ground thermal conductivity. The magnitude of this effect is controlled through a dimensionless parameter |qratio|, which includes the geothermal gradient and other test parameters. In place of conventional models, a transient model that includes the geothermal gradient and fluid thermal storage can estimate the ground thermal conductivity and local borehole resistance within the typical TRT testing duration.

Thermal-force response analysis and applicability evaluation of strata energy shaft in Shanghai

Energy underground structures, such as energy shafts and tunnels, are a class of energy-saving and environmentally-friendly underground structures. They can be used as shallow geothermal energy harvesting systems for heat exchanges with the ground. Theoretical research and practical applications of energy shafts are still in the initial stages, and related tests are still relatively few. This study used the Shanghai Taihe energy shaft test section as the research background. The results of the thermal force response test of the energy shaft was numerically analyzed using the COMSOL software and compared with that of Beijing Qinghuayuan energy tunnel for applicability evaluation and analysis. The results show that the Shanghai strata energy shaft has a better heat transfer capacity than the Beijing strata energy tunnel owing to the different geological environments and is more economical. Simultaneously, the temperature stress generated in the heat transfer process of the Shanghai Strata energy shaft was smaller than that of the Beijing strata energy tunnel, which is safer. Therefore, underground energy structures can be widely applied to the Shanghai strata, which can significantly improve economic applicability while ensuring safety.

Flow-controlled thermal response test and its comparison with the conventional test methods

A novel flow-controlled (FC) thermal response test (TRT) system is introduced to resolve the recently addressed inconsistency between the constant heat flux (CHF) and constant temperature (CT) TRTs. FC-TRT allows us to keep both inlet and outlet temperatures constant and improve the accuracy of CT-TRT. Using the FC-TRT system, four types of TRT experiments are performed, providing CT, CHF, and constant inlet temperature conditions, besides the novel one keeping both temperature and heat flux constant. Thermal conductivities from these TRT measurements are compared, and a good agreement is observed. FC-TRT offers higher accuracy and various TRT applications in one platform.

Fast segregation of thermal response functions in short-term for vertical ground heat exchangers

A detailed understanding of the short-term performance of the ground heat exchanger (GHE) is a major concern in the design of ground-source heat pumps (GSHP), as it has a significant impact in the efficiency and costs of the facility. The use of numerical models or artificial intelligence techniques may help in the short-term, although they are generally very time-consuming, becoming unpractical in numerous cases. The aim of this work is to obtain the thermal response factors according to the GHE, segregating the transient borehole behavior from the soil effect, based on a previous study which applies experimental data from thermal response tests (TRT) to the finite line source (FLS) model. The proposed method implies an extensive study of the fluid temperature as well as the development of new auxiliary thermal resistances related to the borehole and fluid, avoiding the implementation of numerical models. This procedure has been validated with the temperature outcomes of a 3D numerical model, obtaining low deviations, into a range of ±0.1 K, and mean absolute deviations below 0.01 K.