Porous Volumetric Solar Receiver Research Articles

Comparison of Direct Pore-Scale and Volume-Averaging Methods for the Performance Evaluation of Porous Volumetric Solar Receiver

Comparison of Direct Pore-Scale and Volume-Averaging Methods for the Performance Evaluation of Porous Volumetric Solar Receiver

Volumetric solar carbon dioxide receiver designs with geometric parameters optimized in combination

Porous volumetric solar receivers with carbon dioxide as the working fluid can simultaneously harvest solar energy and utilize carbon dioxide. This means they hold promise for next-generation solar thermal power applications. The optimization of porous receivers has been performed in many works with their geometric parameters optimized using an exhaustive method. Such methods are computationally intensive and their deduced design guidelines are not consistent. This work presents a distinct design method where geometric parameters (porosity, pore size, and length) are combined into three heat transfer parameters (volumetric convection coefficient, effective solid-phase thermal conductivity and optical thickness), so that they are optimized in combination. The optimization objective is the combination of a large volumetric convection coefficient, a large effective solid-phase thermal conductivity and an optimal optical thickness. The corresponding combination of parameters is a small pore size, a small length and a moderate porosity. Guided by this criterion, the best combination is 0.4 mm pore size, 4 mm length and 0.85 porosity, which result in a corresponding thermal efficiency of 71.5%. This combination is consistent with that from the exhaustive method and the optimization complexity for the developed method is reduced by two dimensions.

Implementation of Deep Neural Networks for performance prediction and optimization of a porous volumetric solar receiver considering mechanical safety

The volumetric solar receiver is a major component of concentrating solar thermal systems. Proper selection of design and operating parameters is necessary to obtain the best receiver performance. The current work models and optimizes the hydraulic, thermal, and mechanical performance of a porous volumetric solar receiver using the Deep Neural Network (DNN) technique. The dataset for DNN training is generated by numerical simulations and porosity and pore size of foam structure, inlet fluid velocity and absorber length are selected as the input parameters. The pressure drop, outlet fluid temperature and maximum failure index are selected as the output variables. The trained model demonstrates high accuracy in performance prediction for any combination of input variables in the trained range. Further, the present work proposes the integration of the DNN technique with Genetic Algorithm for the constrained multi-objective optimization of the absorber performance with a novel constraint of maximum failure index to select configurations preventing mechanical failure. Such integration is found to be nearly 524 times faster than the conventional direct approach, demonstrating a significant decrease in the time needed for optimization. Furthermore, the potential of the Technique for Order of Preference by Similarity to Ideal Solution in selecting the desired configuration from the Pareto optimal solutions as per individual requirements is also investigated. The optimization results indicate that the optimum values for porosity, pore size, and absorber length are in the range of 0.9–0.93, 2–4 mm, and 0.01–0.06 m, respectively, when all variables are optimized simultaneously. Furthermore, the optimal input velocity is observed to be close to the lower bound of 0.4 m/s while the corresponding maximum failure index varies from 0.346 to 0.836, indicating mechanically safe configurations. The high accuracy and faster optimization results obtained in the present work demonstrate the ability of the proposed approach in implementing quick and computationally cheap optimization studies for improved and mechanically safe absorber designs.

Tomography based pore-level structural optimization for reducing pressure drop of porous volumetric solar receiver

Tomography based pore-level structural optimization for reducing pressure drop of porous volumetric solar receiver

Analysis on the Effects of Different Receiver Structures and Porous Parameters on the Volumetric Effects and Heat Transfer Performance of Porous Volumetric Solar Receiver

The volumetric solar receiver is an important heat transfer component of the concentrated solar power (CSP) system. Moreover, in order to improve the absorption of concentrated solar radiation, the porous media are widely used in volumetric solar receiver. In recent years, many studies were concerned with the effects of porosity, pore number density and size, Reynolds number, and Darcy number on heat and flow performance in volumetric solar receiver. However, there are few studies on the effects of structure type and geometric parameters on the volumetric effects and the radiation characteristics of a porous volumetric solar receiver with nonuniform heat flux boundary condition. In this contribution, in order to analyze the effects of structures on the volumetric effects and heat transfer performance, we design six types of porous volumetric receiver structures, whereas the volume of different structures keeps consistent. Then, we apply the local non-thermal-equilibrium model and also consider the effect of structure shape on the concentrating solar radiation transport characteristic. Furthermore, we apply the Gaussian distribution model (GDM) to simulate the actual nonuniform heat flux boundary condition. The result shows that drum-type structure (RT-III) has the best heat transfer performance among the six structures; e.g., the outlet average temperature of fluid is up to 851 K when the pore size and porosity of porous media are 2 mm and 0.7, respectively, which is 41 K increased than the scaled cone-type structure. In addition, the pore diameter has less influence on the outlet temperature of fluid, but it has greater influence on the pressure drop. The contribution can provide a reference for this type of solar receiver design and reconstruction.

Dynamic Performance Characteristics of a Porous Volumetric Solar Receiver Under Transient Flux Conditions

Abstract The solar flux incident on a volumetric receiver is inherently unsteady, resulting in high thermal stresses, fatigue failures, and reduced component life. The knowledge of transient response characteristics of a porous volumetric receiver used in concentrating solar technologies is cardinal for its reliable and safe working. The dynamic controlling of the solar-to-thermal conversion process is also possible with the prior prediction of the output variations. The present study aims to investigate the transient behavior of a porous volumetric receiver subjected to flux variations approximations occurring in real working scenarios with the help of a coupled transient model. The solid and fluid temperature fields, output fluid temperature, and pressure drop variations are determined for transient flux conditions during start-up, shut-down, clear sky, and cloud passage. The results are used to analyze the thermal response of the receiver during various operating conditions. In addition, the effects of structural parameters of the porous absorber are also investigated. The results indicate that the receiver transient performance is comparatively more affected by the variation in porosity than in pore size for all conditions. Smaller porosities and pore sizes show slower thermal response to transient fluctuations and less temperature changes during cloud passage. Conversely, higher values help in the faster restoration of the steady-state output conditions without dynamic control.

Thermo-mechanical analysis of a porous volumetric solar receiver subjected to concentrated solar radiation

The damage caused by high working and non-uniform temperature distributions is one of the major obstacles to the development of receivers exposed to concentrated solar radiation. The thermal and mechanical performance of a typical porous Silicon Carbide volumetric receiver with foam structure is investigated in the present study with a coupled thermo-mechanical model. The developed model includes coupling of fluid flow, thermal and mechanical models to determine flow field, temperature and stress distributions. The main objective is to investigate the effects of various geometric, structural and design parameters on the absorber's thermal and mechanical performance and to identify the failure-prone regions when the absorber works under steady-state conditions. The effects of porosity, pore size, inlet velocity, absorber's geometrical dimensions, and incident solar flux distribution on the receiver's performance are investigated in detail. It is observed that higher porosities, pore sizes and inlet velocities result in lesser thermal stresses in the porous absorber. The uniformity of the incident radiation flux also reduces thermal stresses and improves the absorber's performance. The near-wall region at the inlet is found to be most prone to mechanical failure. The results of the study provide important conclusions to help in the selection of the proper set of parameters that ensure the efficient, safe and reliable working of the receiver.

Effects of Volumetric Property Models on the Efficiency of a Porous Volumetric Solar Receiver

A porous volumetric receiver is the key component in concentrated solar power systems. In this paper, we investigate the effects of volumetric parameter models on the heat collection efficiency of the volumetric receiver by numerical simulations with the combination of local thermal non-equilibrium and discrete ordinate methods. Seven volumetric convective heat transfer coefficient models and three extinction coefficient models were investigated. The efficiencies calculated using these models were compared among each other. The results show that volumetric convective heat transfer coefficient models have significant effects with a maximum difference of 27.7% in receiver efficiency for these models. Extinction coefficient models have less effects on receiver efficiency with a maximum difference of 7.3%.

Effect of thickness on the thermo-hydraulic performance of porous volumetric solar receivers with different internal geometries

In this work, the effect of thickness on the thermal and hydrodynamic performance of porous volumetric solar receivers made of open-cell silicon carbide (SiC) ceramic foam is investigated using an in-house detailed numerical model. The model is based in a Computational Fluid Dynamics (CFD) technique to solve the volume averaged mass, momentum and energy conservation equations, including the exchange of thermal radiation inside the receiver. A Monte Carlo Ray Tracing (MCRT) method was developed and then used to model the solar radiation transport in the porous media. Two optimised internal geometries (porosity and pores size) of the receiver with adiabatic side-walls are investigated for different thicknesses. Results show that the optimal thickness depends on the porosity and pores size and there is a value from which the thermal efficiency is nearly constant and the pressure drop always increase. It was also found that the thickness should be approximately between 5 and 7 cm for porosity and pores diameter between 0.85 and 0.90 and 3.0 mm and 4.5 mm, respectively, aiming to maximise thermal efficiency by decreasing the transmission losses of solar radiation, and to keep low pressure drop.

Conceptual design of porous volumetric solar receiver using molten salt as heat transfer fluid

Thermal radiation loss hinders the high-efficient operation of solar receiver at high temperature. Porous volumetric solar receiver using molten salt instead of air as heat transfer fluid is proposed. A comprehensive literature review is done to investigate the optical and radiative properties of molten in solar and infrared spectra. Direct pore-scale numerical simulation method based on X-ray computed tomography technique is applied to compare the fluid flow and heat transfer performance of molten salt and air porous volumetric solar receiver. The results present that some types of molten salts behave to be transparent in solar spectrum and opaque in infrared spectrum, which allow the penetration of solar radiation while strongly absorb the infrared emission in the receiver. The thermal efficiency of Hitec porous volumetric solar receiver is higher than traditional air porous receiver especially at higher working temperature, and it is improved by 9.6% at receiver’s outlet temperature of 1000 K. Due to its large heat storage capability and efficient convective heat transfer, small mass flow rate is required in the molten salt receiver to absorb solar radiation, which leads to orders-magnitude smaller pressure drop for the Hitec porous receiver. Further research related to this type of porous volumetric solar receiver is outlined with respect to experiment verification and application of high temperature molten salt.

Experiment and optimization study on the radial graded porous volumetric solar receiver matching non-uniform solar flux distribution

Radial graded porous volumetric solar receiver is designed to match the non-uniform solar flux distribution. Based on the computed tomography and image-processing techniques, uniform and radial graded porous volumetric solar receivers are reconstructed. The 3D printing technique and suitable post processing are implemented to fabricate complex porous samples using super-alloy Inconel 718 as material. Both experimental and numerical studies are conducted to investigate the fluid flow and heat transfer processes in porous volumetric solar receivers. The results present that the 3D printed porous samples are suitable for solar thermal energy absorption and high temperature utilization. As for uniform porous receivers, porous media with small pore diameter has larger thermal efficiency because of enhanced convective heat transfer. Compared with the uniform porous receiver with highest thermal efficiency, the radial graded porous volumetric solar receiver with large pore diameter inside could further relatively increase the thermal efficiency by 4.1% while relatively decreases the flow resistance by 8.6%. The reasonable distribution of pore diameter of porous media could regulate the mass flow distribution and direct more air to the high heat flux region. Moreover, local overheating phenomenon is observed in the uniform porous receiver using air as heat transfer fluid. By applying the coupled optimization method, an optimum pore diameter distribution is determined for the radial graded porous volumetric solar receiver.

Experimental and numerical analysis of the hydraulic and thermal performances of the gradually-varied porous volumetric solar receiver

A gradually-varied porous structure is designed to increase the thermal performance of the porous volumetric solar receiver. Based on the replica method and multilayer recoating technique, the silicon carbide porous ceramic with linear-changed geometrical parameters is fabricated. The performances of the uniform and gradually-varied porous volumetric solar receivers are studied by both experiment and numerical simulation. An optimization method combining genetic algorithm and computational fluid dynamics analysis is applied to determine the optimum porosity distribution. The results present that porous volumetric solar receiver with linear-changed geometrical parameters exhibits better thermal performance than the uniform porous volumetric solar receivers, especially when the thickness of the receiver is small. Larger porosity in the front is beneficial for increasing the solar radiation penetration depth, which limits the reflectance and thermal radiative losses. Smaller porosity in the rear traps more solar radiation and increases the convective heat transfer. When the receiver’s thickness is larger, the performance of the gradually-varied volumetric solar receiver is nearly identical to that of the uniform receiver with largest porosity. The double-layer configuration is found to be the optimized structure of the gradually-varied porous volumetric solar receiver. The thermal efficiency could be further improved using genetic algorithm with an 11 K increase of the outlet temperature.

Experimental and numerical study on the reflectance losses of the porous volumetric solar receiver

The reflectance losses are the predominate energy losses in solar radiation transfer process for the porous volumetric solar receiver. However, few studies pay attention to the influence of geometrical parameters of porous media on the reflectance losses, and the data of reflectance losses of porous volumetric solar receiver in solar spectrum is not complete. In this paper, both experimental and numerical simulation methods are applied to comprehensively investigate the reflectance losses of the porous volumetric solar receiver. A series of silicon carbide reticulated porous ceramics (SiC RPCs) is fabricated by replica method and recoating technique. The reflectance losses are measured based on UV-VIS-NIR spectrophotometer. Meanwhile, porous models with different geometrical parameters are artificially reconstructed. Monte Carlo Ray Tracing method is used to calculate the total reflectance. The results present that the SiC RPCs exhibit small reflectance losses in the solar spectrum with a peak at about 420 nm. The geometrical parameters, such as porosity and pore diameter do not change the spectral behavior but only influence the magnitude of the reflectance. Larger porosity and larger pore diameter are beneficial for reducing the reflectance losses. The correlation of the reflectance losses as functions of pore density and porosity has been developed. Furthermore, the influence of the incident angle of radiation on reflectance losses is studied. Relatively large increase is observed as the incident angle is larger than 30°. This phenomenon becomes more obvious for the porous media with larger porosity or larger pore diameter.

Parametric analysis and optimisation of porous volumetric solar receivers made of open-cell SiC ceramic foam

In recent years, research on the usage of porous materials as volumetric thermal receivers in concentrated solar power plants is growing significantly. This interest is due to their capabilities to improve the efficiency of solar radiation conversion into thermal energy. In this work, detailed parametric analysis and optimisation of the thermal and hydrodynamic performance of porous volumetric receivers are performed using a detailed numerical model. The transport and absorption of solar radiation is modelled through a Monte Carlo Ray Tracing algorithm, while the governing equations of fluid flow and heat transfer are solved using a Computational Fluid Dynamics model. The mean temperature of the fluid at the outlet, thermal efficiency and pressure drop across the receiver are investigated through a parametric analysis and optimisation for different values of porosity, pores size and inlet fluid velocity. The receiver of choice should have high thermal efficiency without greatly increasing the pressure drop and not decreasing the mean fluid temperature at the outlet. Results show that these conditions are achieved for receivers with high porosity (0.8−0.9) and pores size (4.5−3 mm). For a given receiver porosity, there is a pores size value that maximises thermal efficiency, being this value lower for high porosity.

Flow and heat transfer simulation in porous volumetric solar receivers by non-orthogonal multiple-relaxation time lattice Boltzmann method

Flow behavior and heat transfer (FBHT) process within porous volumetric solar receivers (VSR) were studied numerically by the non-orthogonal Multiple-Relaxation Time Lattice Boltzmann Method (NO-MRT-LBM). Three types of porous structures within VSR were distributed and studied. A non-uniform Gaussian heat influx distribution was adopted to approximate the actual conditions. The effects of pore structure (types 1-3) and heat flux distribution (w) on flow behavior and heat transfer process were studied. It was found that the effect of heat flux distribution on the internal heat transfer process of VSR with different porous structure types was mainly determined by the inlet distribution of solid particles. In type 1 and type 2, the instantaneous temperature growth rate of solid particles increased with decreasing w at the inlet area of the model, thus the maximum temperature, the average temperature of the solid particles within the receiver and the air at the outlet increased rapidly. The average temperature of solid particles and air at the outlet in type 3 both increased with the increase of w. Compared with type 2 and type 3, type 1 showed the best effect of heat transfer when heat flux distribution tended to be uneven, which, however, needs to be reduced to avoid the local temperature becoming excessively large. When heat flux distribution tended to be uniform, type 3 showed better heat transfer performance where any excessive local temperature was eliminated.

Combined experimental and numerical determination of the asymmetry factor of scattering phase functions in porous volumetric solar receivers

Modelling of solar radiation propagation and absorption in porous media is a crucial part in the modelling of porous volumetric receivers in concentrated solar power (CSP) plants. The radiative properties of the porous media should be known in detail for accurate receiver modelling. In this work, an experimental study and a numerical model are combined aiming to estimate the asymmetry factor of Henyey-Greenstein (HG) scattering phase function in the visible spectral range for porous volumetric receivers made of open-cell silicon carbide (SiC) ceramic foam. For the experimental, the hemispherical diffuse reflectance of five different samples is measured using a collimated light source and an integrating sphere. For the numerical modelling, an algorithm based on a three-dimensional Monte Carlo Ray Tracing (MCRT) method was developed to simulate radiation propagation and absorption in porous media for the same conditions of the experimental apparatus. The asymmetry factor is determined by adjusting its value in the numerical model in order to minimize the difference between measured and simulated values. Results show that the solar radiation scattering in open-cell SiC ceramic foams is slightly backwards, being the optimum asymmetry factor of the Henyey-Greenstein phase function approximately −0.25, with a mean bias error of 0.0045% and a root mean square difference of 0.2926% between modelled and measured values of diffuse reflectance. Experimental results were also compared with the phase function for packing of spheres and with the isotropic scattering phase function. The first overpredicts the diffuse reflectance, while the second function underpredicts it.

Tomography-based determination of Nusselt number correlation for the porous volumetric solar receiver with different geometrical parameters

Pore-scale numerical models of the porous volumetric solar receiver are established in this paper. By using the X-ray computed tomography and the imaging processing techniques, models of porous media with different geometrical parameters are reconstructed. The conjugate heat transfer process in the porous volumetric solar receiver is solved based on the direct pore-scale numerical simulation. The turbulent effect of fluid flow inside porous geometry is considered by the Shear-Stress Transport k-ω model and the absorbed solar energy is simulated by following the Beer's law. The results present that the inlet velocity and the geometrical parameters influence the thermal performance of the porous volumetric solar receiver. Larger inlet velocity tends to enhance the convective heat transfer between fluid and solid phases meanwhile decreases noticeably the overall temperature. Receiver with larger porosity is preferred because it limits the reflection losses. The Nusselt number increases as the porosity becomes larger. As a result, the general correlation of Nusselt number for the porous volumetric solar receiver is derived as a function of porosity and Reynolds number. This correlation is applicable with the porosity ranging from 0.74 to 0.89 and the pore Reynolds number ranging from 3 to 233.

Multiple-Relaxation-Time Lattice Boltzmann Simulation of Flow and Heat Transfer in Porous Volumetric Solar Receivers

The flow and heat transfer (FHT) in porous volumetric solar receiver was investigated through a double-distributed thermally coupled multiple-relaxation-time (MRT) lattice Boltzmann model (LBM) in this study. The MRT-LBM model was first verified by simulating the FHT in Sierpinski carpet fractal porous media and compared with the results from computational fluid dynamics (CFD). Three typical porous structures in volumetric solar receivers were developed and constructed, and then the FHT in these three porous structures were investigated using the MRT-LBM model. The effects of pore structure, Reynolds (Re) number based on air velocity at inlet, the porosity, and the thermal diffusivity of solid matrix were discussed. It was found that type-III pore structure among the three typical porous structures has the best heat transfer performance because of its lowest maximum temperature of solid particles at the inlet and the highest average temperature of air at the outlet, under the same porosity and heat flux density. Furthermore, increasing the thermal diffusivity of solid particles will lead to higher averaged air temperature at the outlet. It is hoped that the simulation results will be beneficial to the solar thermal community when designing the solar receivers in concentrated solar power (CSP) applications.

Optimization method for the porous volumetric solar receiver coupling genetic algorithm and heat transfer analysis

The porous volumetric solar receiver shows advantages such as the volumetric absorption of solar radiation and the enhanced convective heat transfer. However, few studies were focused on the selection of the appropriate parameters of the receiver to improve its performance. In this contribution, an optimization method which couples the genetic algorithm and the heat transfer analysis of the porous volumetric solar receiver is proposed. The fluid flow and heat transfer in the receiver are evaluated by the volume-averaging simulation method based on the local thermal non-equilibrium model. By combining with the genetic algorithm, the solar receiver with high thermal efficiency and low flow resistance could be identified. The single-objective optimization results present that larger porosity and higher inlet velocity are preferable to improve the thermal efficiency of the porous volumetric solar receiver. The optimized pore size increases with the increase of the thickness of the receiver and the decrease of the inlet velocity. Meanwhile, the porosity and the pore size are optimized simultaneously through the multi-objective optimization. The Pareto front which indicates the receiver with relatively lower flow resistance and relatively higher thermal efficiency is derived.

Pore-scale numerical simulation of fully coupled heat transfer process in porous volumetric solar receiver

A fully coupled heat transfer model at pore-scale of volumetric solar receiver is established in this paper. The X-ray computed tomography technique is applied to reconstruct the porous structure. By generating the voxel mesh and coupling with the Monte Carlo Ray Tracing method, the energy source due to the solar radiation could be determined and then added to the unstructured CFD mesh. The governing equations are solved by the commercial CFD software FLUENT. The results show that the details of the fluid flow and heat transfer in volumetric solar receiver are successfully captured. The pressure drop correlation corresponds satisfactorily to the previous study. The local convective heat transfer coefficient varies in a small range along the inlet fluid flow direction inside volumetric solar receiver and the average Nusselt number could be correlated to a power function of the Reynolds number. The radiation transfer inside the porous media is visualized and thermal radiation loss is evident at the entrance of the solar receiver. The proportion taken by radiation in the total heat transfer is determined as a function of the average temperature of porous skeleton.