1,721,024 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
A novel approach for incidence angle modifier calculation of arbitrarily oriented linear Fresnel collectors: Theory, simulations and case studies
No full textThe easiest possibility to assess the combined opto-energy efficiency of Linear Fresnel Collectors (LFC) is to refer to the Incidence Angle Modifier (IAM) concept: for this kind of solar collector, IAM calculation is often performed according to the so-called factorization method. Unfortunately, the factored IAM fails in providing reliable results of the real LFC yield. In this investigation, hourly 3D ray tracing simulations have been carried out on two case-study plants located in Morocco and Italy: reference ray tracing calculated IAMs have been used to develop simple correlations for inferring the instantaneous IAM of arbitrarily aligned LFCs. Instantaneous IAM formulas resulted to be able to provide yield predictions with a Root Mean Square Error equal to 5.57 % of the yearly average IAM, much lower than the factorization method (19.42 %). Subsequently, a reduced 3-day dataset of ray tracing values has been considered to calibrate the regression constants, resulting in still high accuracy for orientations not exceeding 30°. Finally, the analysis of available power at the receiver allowed to further assess the robustness of the proposed approach regardless of the plant orientation angles
Calculation of the incidence angle modifier of a Linear Fresnel Collector: The proposed declination and zenith angle model compared to the biaxial factored approach
No full textSince their first appearance as a contribution by Professor Francia at the University of Genova, Italy, the Linear Fresnel Collectors (LFC) demonstrated to be an engineering efficient technology for medium to high temperature solar applications. The strength of the LFC concept is related to the simple mirror motion law, to the compactness of the mirror fields (power to land surface ratios), to the lowest resistance to wind, to the system intrinsic scalability. To perform reliable LCOE analyses, robust performance simulation tools are needed. The Authors developed to this aim a 3D ray-tracing model, able to account for shading, blocking, and end effects as a function of LFC geometry, including primary and secondary mirror curvatures. In this paper, a new approach is implemented to reduce huge yearly ray-tracing datasets and provide very compact analytical equations for fast hourly performance simulations. The present model introduces new Incidence Angle Modifier (IAM) correlations based on the declination and zenith angles. The new model demonstrated to fit subhourly 3D ray-tracing data all year long with an overall error lower than 1.5%, well below the best IAM factored models here compared as a general criticism to the biaxial factored approach related to Fresnel applications. (C) 2021 Elsevier Ltd. All rights reserved
Superposition of the single point source solution to generate temperature response factors for geothermal piles
No full textGeothermal piles are a very promising technique to exploit the low enthalpy resource for ground coupled heat pumps. In fact, they are heat exchangers integrated in the foundation structures of the buildings, with reduced need in term of ground surface availability and diminished drilling costs. Unfortunately, to evaluate the ground thermal response to their presence it is not possible to use classical analytical solutions due to their low aspect ratio and to the relevant effect of the heat capacity of the inner cylindrical volume. In addition, different shapes of the pipe arrangement are possible: helix around the foundation pile or a series of vertical pipes connected through U bends at top and bottom of the cylindrical volume. This study proposes a semi-analytical method to model ground heat exchangers with a great flexibility concerning their shape. The method, called Multiple Point Sources (MPS), applies the spatial superposition of the analytical solution for the Single Point Source. It has been validated by means of the comparison with literature analytical methods and FEM results for helix heat exchangers. Finally, it has been applied to find the temperature response factor for different shapes of heat exchanger in geothermal piles
Solar Fresnel modelling, geometry enhancement and 3D ray tracing analysis devoted to different energy efficiency definitions and applied to a real facility
No full textDespite their few installations, Linear Fresnel Collectors (LFC) represent a very promising technology for efficient solar energy exploitation at medium to high temperatures thanks to their lowest land area per electric energy ratio. Their first appearance was in the ’60, thanks to Professor Giovanni Francia realizations at the University of Genova, Italy. This research aims to determine the performance of a LFC and perform parametric studies through 3D ray-tracing simulations. The in-house developed code accounts for all geometrical parameters of the mirrors and receiver assembly, including mirror dimensions, curvature and distance, primary mirror optical errors, receiver aperture area and elevation, secondary mirror compound parabolic shape. The present study includes a detailed investigation on shading, blocking and end effect issues while introducing 6 different different optical and energy efficiency definitions. A parametric analysis is applied to the distance between mirrors and the receiver height. After the code validation against Tonatiuh, the calculations are performed to analyse in details the performance of a real LFC plant in Morocco. The peak optical efficiency of the test case plant has been estimated up to 87% but it is demonstrated the selection of the efficiency definition is crucial for performing successful geometry optimizations
Studio sulle caratteristiche di efflusso nelle giunzioni del sistema di ricambio della carica dei motori a combustione interna
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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
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