1,720,962 research outputs found
Modelling and validation of a new hybrid scheme for predicting the performance of U-pipe borehole heat exchangers during distributed thermal response test experiments
No full textThe reliable design of Ground Coupled Heat Pumps (GCHP) requires accurate models for both predicting the long term behavior of the borehole (BHE) field and their sub-hourly response during the thermal response test experiments. Several literature models have been proposed in the past to cope with this goal: resistance/capacitance (RC) models, finite difference (FD) thermal descriptions of both inner BHE and surrounding ground, FEM models based on commercial software packages. The present model, as Fortran code, is a hybrid approach that employs an RC scheme (to single or double U pipes) BHEs and solves the full Fourier equation in the (even layered) ground volume. The fluid vertical energy transfers are treated with an upwind scheme. New thermal parameters have been introduced and used as indicators of the physical and geometrical conditions of the U-BHE. Crucial of the RC part is the proper estimation of local resistances and the correct position of the inner BHE thermal capacitance, to be located somewhere in between the pipe surface and the BHE periphery. The extensive validation with experimental TRT data allowed the model to be refined in order to provide very close agreement among predictions and measurements (root mean square error within 0.17 °C)
On the ground thermal conductivity estimation with coaxial borehole heat exchangers according to different undisturbed ground temperature profiles (vol 173, 115198, 2020)
No full textThis corrigendum fixes the errors that appear in the previously published original research paper [1]. This corrigendum deals with the rearrangement of Table 3, Table 4, Table 5, the slope values shown in Fig. 8, and the related text reported in [1]. The kgr estimations reported in Table 3, Table 4, Table 5 of [1] were affected by a systematic error due to computational issues. The corrected parts of the text and the corrected values related to Table 3, Table 4, Table 5, Fig. 8 are reported in the present document. The main conclusions in [1] remain unchanged with some clarifications detailed below. The authors regret that the printed version of the above article contained this systematic error. The correct and final version follows. The authors would like to apologize for any inconvenience caused
Study on the best heat transfer rate in thermal response test experiments with coaxial and U-pipe borehole heat exchangers
No full textThis paper concerns the modeling of vertical Borehole Heat Exchangers (BHEs) for Ground Source Heat Pump (GSHP) applications. Focus is devoted to the analyses of Thermal Response Test (TRT) simulations aimed at understanding the main factors that influence the ground thermal conductivity and the effective borehole thermal resistance estimations. The conventional infinite line-source (ILS) model does not include any influence of the external heat transfer rate on the BHE/ground property evaluation. Analyses of numerically simulated TRTs show this omission can sometimes produce an error in the estimate of the ground thermal conductivity. The error may be between ±10% and ±22% for coaxial boreholes (800 m depth), if the ground has a significant geothermal gradient. On the other hand, for single and double U-pipe BHEs the error is less than ±5% under similar conditions. The parameter qratio is identified as an indicator of when the error is significant. This parameter is equal to the external heat rate (injection or extraction) divided by a natural heat rate that is related to the geothermal gradient. Errors greater than ±10% tend to occur for coaxial boreholes with a center-pipe fluid inlet when |qratio |<1. Under the same conditions but with the annulus as the fluid inlet, the error is less than ±6%
On the ground thermal conductivity estimation with coaxial borehole heat exchangers according to different undisturbed ground temperature profiles
No full textThis paper concerns the modeling of vertical coaxial heat exchangers for Ground Source Heat Pump (GSHP) applications. Vertical coaxial borehole heat exchangers (CBHEs) can be buried at depths which are even higher than the conventional ones. In this case they are referred as Deep Borehole Heat Exchangers (DBHEs). As it is known, there is indeed a strong recent tendency, especially in the Scandinavian regions, to use high depth (500–1000 m) underground heat exchangers for the GSHP applications. This study is aimed at the analysis of the BHE behaviour in the early period, say for Fourier numbers typical of the Thermal Response Test (TRT) measurements. The novelty of the present numerical results is related to the applicability of standard TRT methods when referred to DBHEs and different geothermal gradients can be found. To this aim a Fortran code has been developed for describing a 2D transient conduction and convection problem able to provide the fluid and ground temperature evolution as a function of a series of boundary conditions, including the initial and far field ground temperature distribution along the depth. The application of the present model is related to coaxial BHEs for the assessment of the effects of the undisturbed ground temperature profile and the direction of the carrier fluid on the ground thermal conductivity estimation in TRT experiments. It is here demonstrated that different BHE depths (ranging from 150 to 800 m) and different undisturbed temperature profiles (including zero and positive geothermal gradients) can severely affect the TRT ground conductivity estimation (errors up to 25%) if the flow direction is based on the annular pipe or the central pipe inlets
Low-Cost Distributed Thermal Response Test for the Estimation of Thermal Ground and Grout Conductivities in Geothermal Heat Pump Applications
No full textThe design process of a borehole heat exchanger (BHE) requires knowledge of building thermal loads, the expected heat pump's COP and the ground's thermophysical properties. The thermal response test (TRT) is a common experimental technique for estimating the ground's thermal conductivity and borehole thermal resistance. In classic TRT, a constant heat transfer rate is provided above ground to the carrier fluid that circulates continuously inside a pilot BHE. The average fluid temperature is measured, and from its time-dependent evolution, it is possible to infer both the thermal resistance of the BHE and the thermal conductivity of the ground. The present paper investigates the possibility of a new approach for TRT with the continuous injection of heat directly into the BHE's grouting by means of electrical resistance imparted along the entire BHE's length, while local (along the depth) temperature measurements are acquired. This DTRT (distributed TRT) approach has seldom been applied and, in most applications, circulating hot fluid and optical fibers are used to infer depth-related temperatures. The distributed measurements allow the detection of thermal ground anomalies along the heat exchanger and even the presence of aquifer layers. The present paper investigates the new EDDTRT (electric depth-distributed TRT, under patenting) approach based on traditional instruments (e.g., RTD) or one-wire digital sensors. The accuracy of the proposed method is numerically assessed by Comsol Multiphysics simulations. The analysis of the data obtained from the "virtual" EDDTRT confirms the possibility of estimating within 10% accuracy both thermal ground and grout conductivities
Reduced scale experimental modelling of distributed thermal response tests for the estimation of the ground thermal conductivity
No full textThe knowledge of the ground thermal properties, and in particular the ground thermal conductivity is fundamental for the correct sizing of the Ground Coupled Heat Pump (GCHP) plant. The Thermal Response Test (TRT) is the most used experimental technique for estimating the ground thermal conductivity. This paper presents an experimental setup aimed to realise a suitable scale prototype of the real borehole heat exchanger (BHE) and the surrounding ground for reduced scale TRT experiments. The scaled ground volume is realised with a slate block. Numerical analyses were carried out to correctly determine suitable geometric and operational parameters for the present setup. The scaled heat exchanger, inserted into the block, is created with additive technology (3D printer) and equipped with a central electrical heater along its entire depth and with temperature sensors at different radial distances and depths. Present measurements highlight the possibility to reliably perform a TRT experiment and to estimate the slate/ground thermal conductivity with an agreement of about +12% with respect to measurements provided by a standard commercial conductivity meter on proper cylindrical samples of the same material and onto 10 different portions of the slate block
Comparison of 10- and 25-year horizon designs for vertical borehole heat exchangers in geothermal heat pump applications
No full textThe precise design of Borehole Heat Exchanger (BHE) fields in ground-coupled heat pump (GCHP) systems is essential for maintaining optimal long-term performance. Conventional sizing methodologies, such as the ASHRAE method, typically account for a 10-year operational horizon. This study seeks to expand the applicability of the ASHRAE-Tp8 method to a 25-year operational period and to compare the design results related to different time horizon strategies. To this aim, the dimensionless temperature penalties have been compared against reference results obtained from g-functions pertaining to actual borehole field geometries. New optimized constants have been derived specifically for the ASHRAE-Tp8 method, facilitating its adaptation to the longer time frame of 25 years. The accuracy and reliability of this enhanced method have been extensively validated (at a 25-year time horizon, the average error is 3.8 % respect to the reference code). The findings of this study underscore the potential errors in total borefield length and borehole depth that could arise when applying the conventional 10-year design methodology to a 25-year horizon. On the other hand, the present study leads to deducing that the drilling costs due to a necessary higher borehole depth for a correct design, proper to a 25-year horizon, relatively increase (overall length can increase up to +16.8 %). The proposed methodology demonstrates wide applicability, providing a robust framework adaptable to various operational time frames
A spectral method aimed at explaining the role of the heat transfer rate when the Infinite Line Source model is applied to Thermal Response Test analyses
No full textThe present paper introduces new concepts related to the modeling of vertical Borehole Heat Exchangers for Ground Coupled Heat Pump applications. A sensitivity analysis on how specific parameters affect the ground thermal conductivity kgr estimation when the Infinite Line Source model is used to interpret a Thermal Response Test has been performed. The study has been conducted considering shallow and deep BHEs, with and without geothermal gradient, and for homogeneous and stratified ground thermal conductivities. The qratio parameter scales the external heat rate to a natural heat rate associated with the geothermal gradient. The effect of qratio on the TRT analysis has been related to a specific dimensionless g-transfer function called g0, which incorporates the geothermal gradient. Three in-house built Fortran90 codes implementing the finite-difference models related to coaxial, single and double U-BHE geometries are exploited to evaluate the dimensionless g-transfer functions related to each fluid volume. A spectral method aimed to reconstruct the fluid temperature profiles by superposing two separated convolutions in the time domain exploiting the Fast Fourier Transform leads to considering qratio as the dominant parameter when the ILS model is used to estimate kgr. In the case of a single-layered subsurface, qratio >>1 guarantees the correct ILS-based kgr estimation for any BHE geometry. In the coaxial center-pipe inlet case with a single-layered subsurface and qratio<1, the ILS-based kgr estimation when the g0-function is taken into account can differ by -14% from the correct ILS-based kgr estimation without taking into account the g0-function. In the case of a multilayered subsurface, the qratio parameter indicates when the effective kgr estimated by the ILS model departs from the weighted-thickness average. A departure of 10% occurs for qratio between 2 and 2.5 for the coaxial center-pipe inlet cases considered and the departure increases with decreasing qratio
A Review and Comparative Analysis of Solar Tracking Systems
No full textThis review provides a comprehensive and multidisciplinary overview of recent advancements in solar tracking systems (STSs) aimed at improving the efficiency and adaptability of photovoltaic (PV) technologies. The study systematically classifies solar trackers based on tracking axes (fixed, single-axis, and dual-axis), drive mechanisms (active, passive, semi-passive, manual, and chronological), and control strategies (open-loop, closed-loop, hybrid, and AI-based). Fixed-tilt PV systems serve as a baseline, with single-axis trackers achieving 20-35% higher energy yield, and dual-axis trackers offering energy gains ranging from 30% to 45% depending on geographic and climatic conditions. In particular, dual-axis systems outperform others in high-latitude and equatorial regions due to their ability to follow both azimuth and elevation angles throughout the year. Sensor technologies such as LDRs, UV sensors, and fiber-optic sensors are compared in terms of precision and environmental adaptability, while microcontroller platforms-including Arduino, ATmega, and PLC-based controllers-are evaluated for their scalability and application scope. Intelligent tracking systems, especially those leveraging machine learning and predictive analytics, demonstrate additional energy gains up to 7.83% under cloudy conditions compared to conventional algorithms. The review also emphasizes adaptive tracking strategies for backtracking, high-latitude conditions, and cloudy weather, alongside emerging applications in agrivoltaics, where solar tracking not only enhances energy capture but also improves shading control, crop productivity, and rainwater distribution. The findings underscore the importance of selecting appropriate tracking strategies based on site-specific factors, economic constraints, and climatic conditions, while highlighting the central role of solar tracking technologies in achieving greater solar penetration and supporting global sustainability goals, particularly SDG 7 (Affordable and Clean Energy) and SDG 13 (Climate Action)
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