Volumetric Solar Receiver Research Articles

Comparison of Aiming Point Strategy Algorithms and Their Assessment for Volumetric Solar Receivers

The present work summarizes the work done within the scope of CATION, CHLOE, and HECTOR projects related to the aiming point strategies optimization algorithms applied to volumetric solar receivers. Several optimization methods have been applied in the literature for optimizing the aiming strategy of solar power tower plants. However, the use of these algorithms for the requirements of volumetric solar receivers and solar fuel applications has not been accomplished. Herein, the problem is formulated as a constrained optimization problem whose objective function is a combination of the total power on the receiver surface and the flux homogeneity. A comparison of the results is presented in this work. It will be seen that TABU search and SA algorithms are the most efficient in terms of computation time and, in addition, the first one shows very good results in power and spillage. The rest of the mentioned algorithms are much slower and, in general, the results are slightly worse.

Efficient Computation of Radiative Heat Recovery from Porous Ceramic Monoliths for Efficient Solar Thermochemical Fuel Production

Solar thermochemical hydrogen (STCH) produced by heat-driven water-splitting is a promising route for producing green hydrogen and other zero-emission synfuels. However, the efficiency of STCH must be dramatically increased for it to make an impact on decarbonization efforts. We have previously presented a novel Reactor Train System (RTS) for significantly increasing the efficiency of STCH by employing heat recovery from the redox material and efficient gas exchange processes. In this paper we present a higher-fidelity model for the RTS that accommodates the slow heat diffusion through the STCH redox material. For this purpose, a novel method is introduced for transient modelling of radiative heat in participating media. This method, called GREENER: Generalized Radiation Exchange Factors and Net Radiation, combines the accuracy of Monte Carlo Ray Tracing with the low computational cost of the P1 or Rosseland diffusion approximations. Along with STCH, GREENER has application for modelling volumetric solar receivers, high temperature heat recovery systems like heat exchangers and regenerators, and packed bed reactors. Using the GREENER method, the RTS counterflow radiative heat exchanger is shown to achieve heat recovery effectiveness greater than 70%. The performance of non-uniform porous redox morphologies is evaluated, and high-performing configurations are identified.

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

Thermo-mechanical performance enhancement of volumetric solar receivers using graded porous absorbers

The potential of graded porous absorbers in enhancing the thermo-mechanical performance of a volumetric solar receiver is investigated. A coupled numerical model is developed and simulations are conducted on various configurations with porosity and pore size variations in axial, radial, and bidirectional arrangements. The performance of graded configurations is first compared with a representative uniform configuration, and then the optimal configurations within each arrangement are further analyzed to determine the overall best configuration. The parametric study reveals conflicting thermo-mechanical performances in majority of the arrangements. The configurations with a porosity range of 0.9–0.95 and a pore size range of 2–4 mm exhibit the best performance across all variations. Among the best configurations, the bidirectional configuration with radially decreasing porosity and axially increasing pore size demonstrates the best performance with an increase of 8.96 % in air outlet temperature and a decrease of 22.7 % and 4 % in Failure Index and pressure drop, respectively. Bidirectional configuration maintained superior performance over the receiver length range of 1–6 cm and velocity range of 0.4–2.4 m/s. The results underscore the suitability of certain graded arrangements for achieving improved thermal performance while ensuring higher mechanical safety.

Adjoint-based shape optimization for radiative transfer in porous structure for volumetric solar receiver

Due to their large surface areas, porous structures are often used for volumetric solar receivers for efficient utilization of the solar energy. However, the precise simulation and optimization of radiative heat transfer inside porous structures are still challenging due to their complex geometries. In the present study, an adjoint-based shape optimization framework for radiative transfer in porous structures is first proposed. Specifically, an arbitrary complex geometry of a porous structure is represented by a level-set function embedded in a Cartesian grid system. The volume penalization method combined with the discrete ordinate method is employed to simulate a forward radiative transfer problem in a porous structure. Then, a cost functional for an adjoint analysis is defined by the volume average of the divergence of the radiative heat flux, which corresponds to the total heat absorbed by the porous structure with a negative sign. The adjoint equations for minimizing the cost functional, and the corresponding shape update formula are derived and implemented in an open-source software, OpenFOAM. The developed adjoint-based shape optimization framework is applied to three types of initial porous structures, i.e., SC Kelvin, BCC Kelvin, and square honeycomb models. For all the initial porous models, the front surface commonly extends towards the direction of the incident radiation, and consequently notable improvements of the absorption efficiency can be achieved through the optimization. In addition, a systematic parametric survey for the initial shape of the SC Kelvin model is conducted to reveal the impacts of the number of unit cells, the solid emissivity, and the solid temperature on the optimal shapes and the resulting absorption efficiencies.

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.

Validation and optimization of generalized porous media property relation for their use in volumetric solar receiver

Validation and optimization of generalized porous media property relation for their use in volumetric solar receiver

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.

Exploring and enhancing the volumetric behaviour in solar receivers through specular reflectivity and simplified design

One of the remaining challenges of point focusing concentrating solar power systems is the realization of a true volumetric receiver, one whose entire volume is utilized for the absorption of irradiance. Commercial state-of-the-art receivers (e.g., HiTRec-II and SolAir-200) have not demonstrated the volumetric effect, because of low radiation penetration within the absorber. Then, literature state-of-the-art receivers were able to demonstrate a volumetric behaviour (toward achieving the volumetric effect) through axially varied macro-scale porosity (void fraction), but at the cost of design complexity and manufacturability. This study presents the conceptual design and computational assessment of an improved volumetric receiver by employing varied reflectivity on the irradiated surfaces, thereby enhancing radiation penetration. This approach maintains the simplicity, manufacturability, and commercial viability of structured receivers. Three reflectivity distributions were initially investigated using a 3D heat transfer mathematical model, verified with literature experimental data, which incorporated a source term ascertained through Monte-Carlo Ray Tracing (MCRT). The findings from these three design cases revealed that while diffuse reflectivity distributions could improve thermal efficiency, they were unable to enhance radiation penetration. Consequently, an improved volumetric solar receiver was realized by integrating specular reflectivity, leading to a more uniform source term distribution and achieving a thermal efficiency of 96%, as opposed to the 85% thermal efficiency of the commercial state-of-the-art HiTRec-II receiver. The discovery of the specular receiver as a true volumetric receiver facilitated an exploration of quantitative definitions of the volumetric effect. The exploration concluded that volumetric effect metrics would only target designs that maximizing thermal efficiency when the receiver is exhibits a truly volumetric nature, characterized by a uniform source term distribution.

Pore-scale evaluation on a volumetric solar receiver with different optical property control strategies

Pore-scale flow and heat transfer model is built for a volumetric solar receiver subjected to highly concentrated irradiation, in order to provide insight into the detailed thermal and hydrodynamic performance. Direct three-dimensional simulations on the coupled radiation-convection-conduction are performed based on the Weaire-Phelan structure which is used to represent the open-cell ceramic foam absorber. The energy conversion characteristics with two ceramic materials (SiC and Al2O3) are firstly predicted and compared. And then several cases focused on the control approaches for the optical property (absorption/reflection/emission) of the solar absorber are introduced and investigated, including gradient surface absorptivity, spectral selectivity, and combination design of semi-transparent and opaque configuration. The results show that a moderate volumetric effect can be achieved at very low inlet fluid velocity for both ceramic materials, however presenting low conversion efficiency. Increasing the inlet velocity could enlarge the thermal non-equilibrium between solid and fluid phases and lower the outlet temperature. SiC absorber shows improved performance compared to Al2O3 absorber, and the efficiency changes obviously with the inlet velocity, while a variation of 24% is found. The optical property of the absorber front region significantly affects the overall performance. Decreasing absorptivity design and spectral selective design both show positive impact, especially for the Al2O3 absorber with an increment by 33% in efficiency relative to the basic one. Among the different strategies, spectral selective improvement shows the best effectiveness. Besides, adding porous fused silica as the front absorber layer can effectively move the high-temperature area inward at a cost of small decrement in efficiency, herein, honeycomb structure shows preponderance compared to the foam silica.

The volumetric effect indicator – A new dimensionless characteristic number for the optimum design and operation of volumetric solar receivers

This work introduces a new dimensionless number Z that should aid in the design phase of volumetric receivers. The number Z is able to indicate whether the volumetric effect is achievable for a specific combination of absorber geometry and thermal boundary conditions. It can be seen as an indicator of the slope of the solid-phase temperature profile in direction of gas flow. The volumetric effect is expected to occur at values of Z smaller than 1, when the convective plus radiative cooling (Pc) outweighs the absorbed solar power (Pa). An experimentally validated numerical model is used for a parametric simulation study, which sheds light on the absorber’s operating conditions (incident solar flux density and operating temperature) that favor the ideal temperature profile.

An exhaustive search optimization of heat transfer and pressure drop in Kelvin’s open cell foams

Thanks to their high effective thermal conductivity, specific surface area, and tortuosity, open-cell foams are well-known for their capability to enhance heat transfer in applications such as heat exchangers and volumetric solar air receivers. In the very recent years, innovative manufacturing techniques, including 3D designing and printing, have been looked very helpful to find foam morphologies that allow to maximize heat transfer and minimize pressure drop. Optimal foam structures can be obtained by means of pore-scale simulations, employing an exhaustive search with a bearable computational effort. A multi-objective optimization of convective heat transfer and pressure drop in Kelvin’s foams with air is presented in this paper. A pore-scale numerical model, with a uniform heat flux at the solid/fluid interface, is used to predict the interfacial convective heat transfer coefficient, hc , and pressure drop, Δp, in the foam. The cell size, porosity, cell anisotropy stretching factor, as well as the inlet velocity and the direction of the air, are assumed as the design variables for the optimization model, while the interfacial convective heat transfer coefficient and pressure drop are chosen as the objective functions to be maximized and minimized, respectively. Pareto fronts ranging from h = 110 W/m2 K and Δp = 0.77 Pa to hc = 460 W/m2 K and Δp = 51 Pa are predicted, within which the optimum point for the chosen foam morphology, air velocity and direction can be selected, according to the chosen criterion.

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.

Performance analysis of a concentrated direct absorption solar collector (DASC) with nanofluids using computational fluid dynamics and discrete ordinates radiation modelling (CFD-DORM)

This study utilises computational modelling to investigate the application of thermal nanofluids in direct absorption solar collectors (DASC), including the effect of optical and thermal properties on the prediction of thermal energy production. A single-phase, computational fluid dynamics (CFD) model has been used to understand the hydrothermal behaviour of nanofluids operated at high temperature. A non-gray discrete ordinate radiation model (DORM) was coupled to the CFD to capture the dependence of optical parameters on the bulk temperature variation. In order to increase the robustness of the numerical modelling approach, the extinction coefficient of the solar participating media, including the reflective and transmissive behaviour of the semitransparent wall, were taken into consideration. Results show that the exergy efficiency is lower when using fully diffusive conditions at the semitransparent wall as opposed to fully specular conditions. Overall, exergy efficiency decreases with the Nusselt number, Nu, (until Nu=10∼12), although it increases with non-dimensional irradiation. When modelling with temperature-dependent as opposed to temperature-independent properties of the working fluid, the Carnot efficiency was found to vary by 0.67% to 3.30%. Additionally, a correlation between the hydraulic entrance length and Reynolds number, Re, (in the range Re<1,000) has been established and this can be applied for the further hydrothermal analysis. This study serves as a basis for future numerical modelling of any volumetric solar receiver that takes into account the various key system performance indicators of the solar participating media.

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.

The Role of Computational Science in Wind and Solar Energy: A Critical Review

This paper concerns technology challenges for the wind and solar sectors and the role of computational science in addressing the above. Wind energy challenges include understanding the atmospheric flow physics, complex wakes and their interaction with wind turbines, aeroelastic effects and the associated impact on materials, and optimisation of wind farms. Concentrated solar power technologies require an optimal configuration of solar dish technology and porous absorber in the volumetric solar receiver for efficiency and durability and to minimise the convective heat losses in the receiver. Computational fluid dynamics and heat transfer have advanced in terms of numerical methods and physics-based models and their implementation in high-performance computing facilities. Despite this progress, computational science requires further advancement to address the technological challenges of designing complex systems accurately and efficiently, as well as forecasting the system’s performance. Machine Learning models and optimisation techniques can maximise the performance of simulations and quantify uncertainties in the wind and solar energy technologies. However, in a similar vein, these methods require further development to reduce their computational uncertainties. The need to address the global energy challenges requires further investment in developing and validating computational science methods and physics-based models for accurate and numerically efficient predictions at different scales.

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.

A novel approach for volumetric solar receiver performance assessments

A novel design and an efficient operation of solar volumetric receivers remain critical for successfully integrating such receivers into solar energy harvesting devices. The performance of the solar volumetric receiver is highly dependent on the geometric connectivity of the actual pore-sites in the SiC open-cell foams. Although the performance of SiC foam receivers has been investigated previously considering the homogeneous foam structure, further studies are needed to explore the influence of the actual path of the pores and their connectivity on the flow and thermal performance of the receiver. Hence, the present study considers the novel design of a solar volumetric absorber incorporating the actual SiC foam receiver geometric features. Accordingly, the thermal and flow behavior of the working fluid within the receiver is examined considering the 3D connectivity of the pore sites. Moreover, the performance of the solar volumetric receiver is evaluated for different Reynolds numbers, solar concentrations, and foam porosities. A 3D-dimensional numerical investigation is carried out incorporating the interactions between convection, conduction, and radiation heat transfer for which the discrete ordinate radiation model is used. Porous SiC foam is placed in a channel having an optically transparent wall at the top and air is used as the working fluid extracting heat from foam. The overall heat transfer coefficient, normalized Nusselt number, and thermal effectiveness are evaluated for various operating conditions. Findings revealed that the receiver thermal performance improves at high Reynolds numbers. The convection heat transfer is found to be the apparent heat transfer mode within the porous SiC foam. In addition, the pumping power to overcome the flow resistance in the channel is considerably less than that of the rate of heat transfer gain.

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%.