Numerical modelling of a parabolic trough solar collector

Concentrated Solar Power (CSP) technologies are gaining increasing interest in electricity generation due to the good potential for scaling up renewable energy at the utility level. Parabolic trough solar collector (PTC) is economically the most proven and advanced of the various CSP technologies. The modelling of these devices is a key aspect in the improvement of their design and performances which can represent a considerable increase of the overall efficiency of solar power plants. In the subject of modelling and improving the performances of PTCs and their heat collector elements (HCEs), the thermal, optical and aerodynamic study of the fluid flow and heat transfer is a powerful tool for optimising the solar field output and increase the solar plant performance. This thesis is focused on the implementation of a general methodology able to simulate the thermal, optical and aerodynamic behaviour of PTCs. The methodology followed for the thermal modelling of a PTC, taking into account the realistic non-uniform solar heat flux in the azimuthal direction is presented. Although ab initio, the finite volume method (FVM) for solving the radiative transfer equation was considered, it has been later discarded among other reasons due to its high computational cost and the unsuitability of the method for treating the finite angular size of the Sun. To overcome these issues, a new optical model has been proposed. The new model, which is based on both the FVMand ray tracing techniques, uses a numerical-geometrical approach for considering the optic cone. The effect of different factors, such as: incident angle, geometric concentration and rim angle, on the solar heat flux distribution is addressed. The accuracy of the new model is verified and better results than the Monte Carlo Ray Tracing (MCRT) model for the conditions under study are shown. Furthermore, the thermal behaviour of the PTC taking into account the nonuniform distribution of solar flux in the azimuthal direction is analysed. A general performance model based on an energy balance about the HCE is developed. Heat losses and thermal performances are determined and validated with Sandia Laboratories tests. The similarity between the temperature profile of both absorber and glass envelope and the solar flux distribution is also shown. In addition, the convection heat losses to the ambient and the effect of wind flow on the aerodynamic forces acting on the PTC structure are considered. To do this, detailed numerical simulations based on Large Eddy simulations (LES) are carried out. Simulations are performed at two Reynolds numbers of ReW1 = 3.6 × 105 and ReW2 = 1 × 106. These values corresponds to working conditions similar to those encountered in solar power plants for an Eurotrough PTC. The study has also considered different pitch angles mimicking the actual conditions of the PTC tracking mechanism along the day. Aerodynamic loads, i.e. drag and lift coefficients, are calculated and validatedwith measurements performed in wind tunnels. The indepen-dence of the aerodynamic coefficients with Reynolds numbers in the studied range is shown. Regarding the convection heat transfer taking place around the receiver, averaged local Nusselt number for the different pitch angles and Reynolds numbers have been computed and the influence of the parabola in the heat losses has been analysed. Last but not the least, the detailed analysis of the unsteady forces acting on the PTC structure has been conducted by means of the power spectra of several probes. The analysis has led to detect an increase of instabilities when moving the PTC to intermediate pitch angles. At these positions, the shear-layers formed at the sharp corners of the parabola interact shedding vortices with a high level of coherence. The coherent turbulence produces vibrations and stresses on the PTC structure which increase with the Reynolds number and eventually, might lead to structural failure under certain conditions

Large-eddy simulations of fluid flow and heat transfer around a parabolic trough solar collector

This study reports on numerical simulations of a parabolic trough solar collector to predict the aerodynamic behaviour and the convection heat transfer from the heat collector element. In the study, the variation of fluid flow with different angles of attack has been taken into account. Calculations are performed using Large Eddy Simulations with a Variational Multiscale (VMS)approach for modelling the sub-grid scale stress tensor. The governing equations are discretised on a collocated unstructured grid arrangement by means of second-order spectro-consistent schemes. The numerical model is validated first with a cross flow around a horizontal cylinder. After that,aerodynamic coefficients at different angles of attack or pitch angles are calculated and compared to wind-tunnel experiments. It has been shown that, the orientation of the solar collector plays an important role in evaluating the aerodynamic performance and structural design criteria of the collector.Peer ReviewedPostprint (published version

On the CFD&HT of the flow around a parabolic trough solar collector under real working conditions

Parabolic trough solar collector is currently one of the most mature and prominent solar applications for production of electricity. These systems are usually located in open terrain where strong winds may occur and affect their stability and optical performance, as well as, the heat exchange between the solar receiver and the ambient air. In this context, a wind flow analysis around a parabolic trough solar collector under real working conditions is performed. A numerical aerodynamic and heat transfer study based on Large Eddy Simulations is carried out to characterize the wind loads and the heat transfer coefficients. Computations are performed for two Reynolds number ReW1=3.9×10^5 and ReW2=1×10^6 and various pitch angles. The effects of wind speed and pitch angle on the averaged and instantaneous flow have been assessed. The aerodynamic coefficients are calculated around the solar collectorand validated with measurements performed in wind tunnel tests. The variation of the heat transfer coefficient around the heat collector element with the Reynolds number is presented and compared to the circular cylinder in cross flow.Peer ReviewedPostprint (published version

Variability and associated uncertainty in image analysis for soiling characterization in solar energy systems

The accumulation of soiling on photovoltaic modules and on the mirrors of concentrating solar power systems causes non-negligible energy losses with economic consequences. These challenges can be mitigated, or even prevented, through appropriate actions if the magnitude of soiling is known. Particle counting analysis is a common procedure to characterize soiling, as it can be easily performed on micrographs of glass coupons or solar devices that have been exposed to the environment. Particle counting does not, however, yield invariant results across institutions. The particle size distribution analysis is affected by the operator of the image analysis software and the methodology utilized. The results of a round-robin study are presented in this work to explore and elucidate the uncertainty related to particle counting and its effect on the characterization of the soiling of glass surfaces used in solar energy conversion systems. An international group of soiling experts analysed the same 8 micrographs using the same open-source ImageJ software package. The variation in the particle analyses results were investigated to identify specimen characteristics with the lowest coefficient of variation (CV) and the least uncertainty among the various operators. The mean particle diameter showed the lowest CV among the investigated characteristics, whereas the number of particles exhibited the largest CV. Additional parameters, such as the fractional area coverage by particles and parameters related to the distribution's shape yielded intermediate CV values. These results can provide insights on the magnitude inter-lab variability and uncertainty for optical and microscope-based soiling monitoring and characterization

Wind speed effect on the flow field and heat transfer around a parabolic trough solar collector

Parabolic trough solar collectors are currently one of the most mature and prominent solar technology for the production of electricity. These systems are usually located in an open terrain where strong winds may be found, and could affect their stability and optical performance, as well as the heat exchange between the solar receiver and the ambient air. In this context, a wind flow analysis around a parabolic trough solar collector under real working conditions is performed. A numerical aerodynamic and heat transfer study based on Large Eddy Simulations is carried out to characterise the wind loads and heat transfer coefficients. After the study carried out by the authors in an earlier work (Hachicha et al. 2013) at ReW1=3.9e5, computations are performed at a higher Reynolds number of ReW2=1e6, and for various pitch angles. The effects of wind speed and pitch angle on the averaged and instantaneous flow are assessed. The aerodynamic coefficients are calculated around the solar collector and validated with measurements performed in wind tunnel tests. The variation of the heat transfer coefficient around the heat collector element with the Reynolds number is presented and compared to the circular cylinder in cross-flow. The unsteady flow is studied for three pitch angles: 0 ; 45 and 90 and different structures and recirculation regions are identified. A spectral analysis around the parabola and its receiver is also carried out in order to detect the most relevant frequencies related to the vortex shedding mechanism which affects the stability of the collector.Peer Reviewe

Large-eddy simulations of fluid flow and heat transfer around a parabolic trough solar collector

This study reports on numerical simulations of a parabolic trough solar collector to predict the aerodynamic behaviour and the convection heat transfer from the heat collector element. In the study, the variation of fluid flow with different angles of attack has been taken into account. Calculations are performed using Large Eddy Simulations with a Variational Multiscale (VMS)approach for modelling the sub-grid scale stress tensor. The governing equations are discretised on a collocated unstructured grid arrangement by means of second-order spectro-consistent schemes. The numerical model is validated first with a cross flow around a horizontal cylinder. After that,aerodynamic coefficients at different angles of attack or pitch angles are calculated and compared to wind-tunnel experiments. It has been shown that, the orientation of the solar collector plays an important role in evaluating the aerodynamic performance and structural design criteria of the collector.Peer Reviewe

Wind speed effect on the flow field and heat transfer around a parabolic trough solar collector

Parabolic trough solar collectors are currently one of the most mature and prominent solar technology for the production of electricity. These systems are usually located in an open terrain where strong winds may be found, and could affect their stability and optical performance, as well as the heat exchange between the solar receiver and the ambient air. In this context, a wind flow analysis around a parabolic trough solar collector under real working conditions is performed. A numerical aerodynamic and heat transfer study based on Large Eddy Simulations is carried out to characterise the wind loads and heat transfer coefficients. After the study carried out by the authors in an earlier work (Hachicha et al. 2013) at ReW1=3.9e5, computations are performed at a higher Reynolds number of ReW2=1e6, and for various pitch angles. The effects of wind speed and pitch angle on the averaged and instantaneous flow are assessed. The aerodynamic coefficients are calculated around the solar collector and validated with measurements performed in wind tunnel tests. The variation of the heat transfer coefficient around the heat collector element with the Reynolds number is presented and compared to the circular cylinder in cross-flow. The unsteady flow is studied for three pitch angles: 0 ; 45 and 90 and different structures and recirculation regions are identified. A spectral analysis around the parabola and its receiver is also carried out in order to detect the most relevant frequencies related to the vortex shedding mechanism which affects the stability of the collector.Peer Reviewe

Numerical simulation of a parabolic trough solar collector considering the concentrated energy flux distribution

Peer ReviewedPostprint (published version

Recent developments in ports data technology

Includes indexAvailable from British Library Document Supply Centre- DSC:02/41148 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

Numerical simulation of wind flow around a parabolic trough solar collector

The use of parabolic trough solar technology in solar power plants has been increased in recent years. Such devices are located in open terrain and can be the subject of strong winds. As a result, the stability of these devices to track accurately the sun and the convection heat transfer from the receiver tube could be affected. In this paper, a detailed numerical aerodynamic and heat transfer model based on Large Eddy Simulations (LES) modelling for these equipments is presented. First, the model is verified on a circular cylinder in a cross-flow. The drag forces and the heat transfer coefficients are then validated with available experimental measurements. After that, simulations are performed on an Eurotrough solar collector to study the fluid flow and heat transfer around the solar collector and its receiver. Computations are carried out for a Reynolds number of Re W = 3.6 x 10(5) (based on the aperture) and for various pitch angles (h=0,45,90, 135, 80, 270). The aerodynamic coefficients are calculated around the solar collector and validated with measurements performed in wind tunnel tests. Instantaneous velocity field is also studied and compared to aerodynamic coefficients for different pitch angles. The time-averaged flow is characterised by the formation of several recirculation regions around the solar collector and the receiver tube depending on the pitch angle. The study also presents a comparative study of the heat transfer coefficients around the heat collector element with the circular cylinder in a cross-flow and the effect of the pitch angle on the Nusselt number.Peer Reviewe