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Philippe Pasquier - One of the best experts on this subject based on the ideXlab platform.
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stochastic interpretation of thermal response test with trt sinterp
Computers & Geosciences, 2015Co-Authors: Philippe PasquierAbstract:A program designed to analyze thermal response tests by deterministic or stochastic inversion is presented. In its current state, the program treats variable heating power signals and emulates a borehole heat exchanger by a finite Line-Source Model or a thermal resistance and capacity Model. The possibly unknown parameters identified may comprise the thermal conductivity and volumetric heat capacity of the ground or grout, as well as the pipes spacing and initial ground temperature. If the thermal resistance and capacity Model is used as the interpretation Model, it is possible to integrate to the inversion the temperature measurements made at various depths in the fluid and grout and to take into account the thermal capacity of the underground components and the fluid flow rate. The program is tested under real field conditions by using the temperature measurements recorded by 18 probes installed at various depths in a borehole heat exchanger during a thermal response test. The test results indicate a relative insensitivity of the fluid temperature to the ground volumetric heat capacity and suggest that it is currently illusive to try identifying its real value from a conventional thermal response test.
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unit response function for ground heat exchanger with parallel series or mixed borehole arrangement
Renewable Energy, 2014Co-Authors: Denis Marcotte, Philippe PasquierAbstract:A novel approach is presented that allows to predict fluid temperatures entering a Ground Heat Exchanger (GHE) for parallel, series and mixed arrangements of boreholes. The method determines at each time step the heat transfer rates occurring at each borehole so as to reproduce the fluid temperature at the GHE inlet for a specific borehole arrangement. The analytical finite line source Model is used to compute the borehole wall temperatures, whereas the fluid temperatures are assumed to vary linearly along the pipes. The method requires to solve a linear system of equations at a small number of time steps. The different systems of equations for each arrangement are determined. A comprehensive 3D finite element numerical Model shows good agreement with the computed fluid temperatures. The proposed approach is computationally very efficient. The fluid temperature unit response function can be convolved with any desired heat load to estimate fluid temperatures at the GHE inlet for a wide variety of scenarios.
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unit response function for ground heat exchanger with parallel series or mixed borehole arrangement
Renewable Energy, 2014Co-Authors: Denis Marcotte, Philippe PasquierAbstract:A novel approach is presented that allows to predict fluid temperatures entering a Ground Heat Exchanger (GHE) for parallel, series and mixed arrangements of boreholes. The method determines at each time step the heat transfer rates occurring at each borehole so as to reproduce the fluid temperature at the GHE inlet for a specific borehole arrangement. The analytical finite line source Model is used to compute the borehole wall temperatures, whereas the fluid temperatures are assumed to vary linearly along the pipes. The method requires to solve a linear system of equations at a small number of time steps. The different systems of equations for each arrangement are determined. A comprehensive 3D finite element numerical Model shows good agreement with the computed fluid temperatures. The proposed approach is computationally very efficient. The fluid temperature unit response function can be convolved with any desired heat load to estimate fluid temperatures at the GHE inlet for a wide variety of scenarios.
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fast fluid and ground temperature computation for geothermal ground loop heat exchanger systems
Geothermics, 2008Co-Authors: Denis Marcotte, Philippe PasquierAbstract:Heat pumps (HPs) coupled to ground-loop heat exchangers (GLHEs) have become increasingly popular for heating and cooling purposes in the context of growing energy costs. Precise design of GLHEs requires the computation of hourly fluid and ground temperatures, especially when the geothermal system is coupled to another system (e.g. boiler, cooling tower). However, because of the computational burden, hourly computation is often simplified in actual designs by rules of thumb or approximations that can cause over- or under-design of the GLHE system. The hourly temperature computation can be seen as a convolution in the time domain that is most efficiently evaluated by fast Fourier transform (FFT). An additional substantial reduction in computing time is obtained by subsampling the analytical function at a few selected times according to a geometric sequence and then using a good quality interpolant such as the cubic spline. This combined “FFT-S approach” enables one to obtain a 30-year hourly simulation in less than a second on a standard laptop computer, even for the computationally intensive finite Line-Source Model. This reduction of one to two orders of magnitude in computing time compared to time-domain approaches with load aggregation should help promote the use of hourly temperature simulation for GLHE design purposes.
Denis Marcotte - One of the best experts on this subject based on the ideXlab platform.
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unit response function for ground heat exchanger with parallel series or mixed borehole arrangement
Renewable Energy, 2014Co-Authors: Denis Marcotte, Philippe PasquierAbstract:A novel approach is presented that allows to predict fluid temperatures entering a Ground Heat Exchanger (GHE) for parallel, series and mixed arrangements of boreholes. The method determines at each time step the heat transfer rates occurring at each borehole so as to reproduce the fluid temperature at the GHE inlet for a specific borehole arrangement. The analytical finite line source Model is used to compute the borehole wall temperatures, whereas the fluid temperatures are assumed to vary linearly along the pipes. The method requires to solve a linear system of equations at a small number of time steps. The different systems of equations for each arrangement are determined. A comprehensive 3D finite element numerical Model shows good agreement with the computed fluid temperatures. The proposed approach is computationally very efficient. The fluid temperature unit response function can be convolved with any desired heat load to estimate fluid temperatures at the GHE inlet for a wide variety of scenarios.
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unit response function for ground heat exchanger with parallel series or mixed borehole arrangement
Renewable Energy, 2014Co-Authors: Denis Marcotte, Philippe PasquierAbstract:A novel approach is presented that allows to predict fluid temperatures entering a Ground Heat Exchanger (GHE) for parallel, series and mixed arrangements of boreholes. The method determines at each time step the heat transfer rates occurring at each borehole so as to reproduce the fluid temperature at the GHE inlet for a specific borehole arrangement. The analytical finite line source Model is used to compute the borehole wall temperatures, whereas the fluid temperatures are assumed to vary linearly along the pipes. The method requires to solve a linear system of equations at a small number of time steps. The different systems of equations for each arrangement are determined. A comprehensive 3D finite element numerical Model shows good agreement with the computed fluid temperatures. The proposed approach is computationally very efficient. The fluid temperature unit response function can be convolved with any desired heat load to estimate fluid temperatures at the GHE inlet for a wide variety of scenarios.
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fast fluid and ground temperature computation for geothermal ground loop heat exchanger systems
Geothermics, 2008Co-Authors: Denis Marcotte, Philippe PasquierAbstract:Heat pumps (HPs) coupled to ground-loop heat exchangers (GLHEs) have become increasingly popular for heating and cooling purposes in the context of growing energy costs. Precise design of GLHEs requires the computation of hourly fluid and ground temperatures, especially when the geothermal system is coupled to another system (e.g. boiler, cooling tower). However, because of the computational burden, hourly computation is often simplified in actual designs by rules of thumb or approximations that can cause over- or under-design of the GLHE system. The hourly temperature computation can be seen as a convolution in the time domain that is most efficiently evaluated by fast Fourier transform (FFT). An additional substantial reduction in computing time is obtained by subsampling the analytical function at a few selected times according to a geometric sequence and then using a good quality interpolant such as the cubic spline. This combined “FFT-S approach” enables one to obtain a 30-year hourly simulation in less than a second on a standard laptop computer, even for the computationally intensive finite Line-Source Model. This reduction of one to two orders of magnitude in computing time compared to time-domain approaches with load aggregation should help promote the use of hourly temperature simulation for GLHE design purposes.
Ryozo Ooka - One of the best experts on this subject based on the ideXlab platform.
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effect of natural convection on thermal response test conducted in saturated porous formation comparison of gravel backfilled and cement grouted borehole heat exchangers
Renewable Energy, 2016Co-Authors: Wonjun Choi, Ryozo OokaAbstract:Thermal response tests (TRTs) have been conducted to evaluate two design parameters of borehole heat exchangers (BHEs): effective thermal conductivity and borehole thermal resistance. The effect of natural convection on groundwater-filled BHE performance has been reported mainly from northern Europe. Even in a backfilled or grouted BHE, if the formation is saturated and composed of porous medium, the estimation may depend on the heat injection rate. In this study, we experimentally examined the effect of natural convection on TRTs conducted in saturated porous formation. TRTs were conducted with two BHEs having the same geometry but different backfill materials: one was cement-grouted and the other was gravel-backfilled. TRTs were conducted for each BHE at two different heat injection rates (approximately 45 W/m and 90 W/m). The TRT data were analyzed by a parameter estimation method using a temporal superposition-applied infinite line source Model. The results show that when the heat rate was almost doubled, the borehole thermal resistances of the gravel-backfilled and cement-grouted BHEs decreased by 9.8% and 8.7%, respectively. Based on the results, discussions on existing design methods related to typical practices in TRTs and advantages of backfilled BHEs from the perspectives of performance and constructability are presented.
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effect of disturbance on thermal response test part 2 numerical study of applicability and limitation of infinite line source Model for interpretation under disturbance from outdoor environment
Renewable Energy, 2016Co-Authors: Wonjun Choi, Ryozo OokaAbstract:The approximated infinite line source (ILS) Model is widely used to interpret thermal response tests (TRTs). It assumes a constant heat flux from the source. However, this assumption is violated in real field conditions by the heat exchange between the circulating fluid and the outdoor environment in an above-ground TRT setup. This results in a fluctuating behavior of sequential estimation and estimation error. In this study, we quantitatively examined the effect of disturbance from outdoor environment on TRTs, especially when TRTs are interpreted by the ILS Model, using numerical methods. An analytical Model that takes disturbance into account was incorporated as the boundary condition of a numerical Model. Using typical synthetic weather data of different seasons and 36 cases of measured weather data, numerical TRTs were conducted and interpreted. Some characteristic behavior of interpretation related to weather conditions was explained and changes in error range with testing duration were analyzed to clarify the applicability and limitation of the interpretation using the ILS Model. The results showed that at least 60 h of TRT is required to obtain results within the error range of ±5% compared with the reference case. Additionally, some practical suggestions regarding conducting and interpreting TRTs are provided.
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interpretation of disturbed data in thermal response tests using the infinite line source Model and numerical parameter estimation method
Applied Energy, 2015Co-Authors: Wonjun Choi, Ryozo OokaAbstract:Effective ground thermal conductivity and borehole thermal resistance, which are key parameters in the design of borehole heat exchangers (BHEs), are often determined on the basis of in-situ thermal response tests (TRTs). However, many disturbance factors can affect the accuracy of a TRT, e.g., voltage fluctuations from the power grid and oscillating external environments where a TRT rig is installed. Interpretation of TRT data is often done using the infinite line source (ILS) Model, combined with the sequential plot method, because it is not only simple but also provides additional information about the estimation behavior and convergence. However, estimation behavior using the sequential method tends to fluctuate over time because the constant heat flux assumption is always violated as a result of the disturbance factors. As an alternative, a temporal superposition applied analytical Model can be used in a recursive curve fitting manner, but this method cannot provide the additional information that sequential method can. In this study, as a solution for interpreting disturbed TRT data and to utilize additional information from the sequential plot method, we proposed an alternative method using a temporal superposition applied ILS Model combined with the quasi-Newton optimization method. To verify the effectiveness, the proposed method was applied to in-situ TRTs and the results were compared with those from the conventional method in terms of the estimation stability and convergence speed. The results showed that, compared to the conventional sequential method using the ILS Model, the proposed method yielded standard deviations for the effective thermal conductivity and borehole thermal resistance that were at least six times and four times lower, respectively. Moreover, the proposed method was able to achieve about four times faster convergence speeds.
Zhaohong Fang - One of the best experts on this subject based on the ideXlab platform.
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a moving finite line source Model to simulate borehole heat exchangers with groundwater advection
International Journal of Thermal Sciences, 2011Co-Authors: Nelson Molinagiraldo, Philipp Blum, Peter Bayer, Zhaohong FangAbstract:Available analytical Models for the thermal analysis of ground source heat pumps (GSHPs) either neglect groundwater flow or axial effects. In the present study a new analytical approach which considers both effects is developed. Comparison with existing analytical solutions based on the finite and infinite line source theory is carried out. This study shows that in general the heat transfer at the borehole heat exchanger (BHE) is affected by groundwater flow and axial effects. The latter is even more important for long simulation times and short borehole lengths. At the borehole wall the influence of the axial effect is restricted to Peclet numbers lower than 10, assuming the BHE length as characteristic length. Moreover, the influence of groundwater flow is negligible for Peclet numbers lower than 1.2. As a result for Peclet numbers between 1.2 and 10 the combined effect of groundwater flow and axial effects has to be accounted for when evaluating the temperature response of a BHE at the borehole wall and thus the use of the moving finite line source Model is required.
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numerical analysis and experimental validation of heat transfer in ground heat exchangers in alternative operation modes
Energy and Buildings, 2008Co-Authors: Ping Cui, Hongxing Yang, Zhaohong FangAbstract:A finite element numerical Model has been developed for the simulation of the ground heat exchangers (GHEs) in alternative operation modes over a short time period for ground-coupled heat pump applications. Comparisons between the numerical and analytical results show that the finite Line-Source Model is not capable of Modeling the GHEs within a few hours because of the Line-Source assumption. On the other hand, the experiments with respect to the alternative cooling and heating modes have been undertaken during a short-time period. The comparisons show a reasonable agreement between the numerical and the measured data. The results illustrate that the finite element numerical Model can be used to simulate the heat transfer behavior of the GHEs in short time scales instead of the typical finite Line-Source Model. Finally, the variation of the U-tube pipe wall temperatures demonstrates that the discontinuous operation mode and the alternative cooling/heating modes can effectively alleviate the heat buildup in the surrounding soil.
Alvin C K Lai - One of the best experts on this subject based on the ideXlab platform.
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review of analytical Models for heat transfer by vertical ground heat exchangers ghes a perspective of time and space scales
Applied Energy, 2015Co-Authors: Alvin C K LaiAbstract:Ground (or geothermal) heat exchangers are attracting a great deal of attention as a way of using shallow geothermal energy. This paper provides not only a critical review but also a thorough introduction to the analysis of heat transfer by borehole and foundation pile ground heat exchangers, with an emphasis on different analytical Models. The literature is reviewed in a time-scale framework because of the diversity of the time and space scales involved in the thermal processes of ground heat exchangers. We summarize, discuss, and evaluate major advances in this field, including heat-source Models, short-time Models, Models for energy piles, in situ thermal-response tests, indoor sandbox experiments, and parameter estimation as an inverse problem. Of particular note is that the unit-step temperature response (i.e., G-function) of a ground heat exchanger with one U-shaped pipe is calculated; and six analytical Models are compared: an infinite cylinder-source Model, two infinite Line-Source Models, two finite Line-Source Models, and a composite-medium Line-Source Model. This paper closes by identifying several unsolved problems that require solutions.
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analytical Model for short time responses of ground heat exchangers with u shaped tubes Model development and validation
Applied Energy, 2013Co-Authors: Alvin C K LaiAbstract:Short-time responses of borehole ground heat exchangers (GHEs) are difficult to predict because they involve heat capacity effect and various arrangements of U-shaped tubes within boreholes. To meet this engineering challenge, this paper builds a composite-medium Line-Source Model. The central idea of the new Model is that the downward and upward channels of U-shaped tubes (not the borehole) are approximated as line sources or sinks of heat placed in a composite medium. This approach can account for not only the influence of the heat capacity of grouting material but also the difference between properties of soil and grout, whilst it is flexible enough to Model various U-pipe configurations. The new composite-medium Model is validated by using a reported laboratory experiment; it can yield predictions matching the experimental response profiles with acceptable accuracy for times as short as 5min. Several uncertainties which may contribute to discrepancies between the Model prediction and the laboratory data are analyzed, including heat input rate, thermal conductivities and heat capacities of the composite medium. The potential errors caused by these uncertainties are comparable to the observed discrepancies.