Volumetric Solar Receiver Research Articles

Optical analysis of a semi-transparent packed bed of spheres for next-generation volumetric solar receivers

Volumetric solar absorbers are attractive for high-temperature concentrating solar applications because they—theoretically—have lower re-radiation losses than surface absorbers since concentrated radiation penetrates their volume. In practice, however, the incident radiation is predominantly absorbed in the top/frontal surfaces of porous and distributed absorber materials. Another challenge with high-temperature absorbers, generally, is the risk of creating hot spots resulting from a non-uniform temperature distribution. To overcome these limitations, this study reports on a semi-transparent absorber design, enabling the peak temperature to occur within the internal volume. Eight different ‘constant-transmittance’ and ‘variable-transmittance’ combinations of transparent/semi-transparent/opaque spheres were packed into a cylindrical cavity and exposed to a solar simulator for a layer-by-layer transmission test. A 3-D ray-tracing model was also developed to further elucidate the relative optical absorption of the cavity walls and spheres. An optimal variable-transmittance design enabled ∼40% of the rays to penetrate to >70% of the absorber's depth, and ∼56% of the total rays were absorbed within the inner volume. Therefore, the optimal design yields uniform absorption, potentially leading to a uniform temperature distribution and a minimisation of thermal emission losses. This study provides the first experimental proof that a semi-transparent packed bed can significantly outperform an opaque design.

Functionally-graded foams for volumetric solar receivers

The open volumetric receiver, one of the most important components of a Concentrated Solar Power (CSP) plant, is made up by a ceramic foam on which the concentrated solar radiation impinges. Ceramic foams are employed in volumetric solar receivers because of their high specific surfaces and their operating temperatures higher than those of metal foams. Thermo- fluid-dynamics in the graded ceramic foam of a volumetric solar air receiver for concentrated solar power is investigated numerically. Variable porosities and Pores Per Inch (PPI), according to different power laws, are accounted for. Governing equations are written with the Volume Averaging Technique (VAT) and are solved with the commercial software Comsol Multiphysics. The effects of different porosity and PPI laws, on the fluid velocity, pressure drop and temperatures, under different thermo-fluid-dynamic conditions, are highlighted.

Experimental study of innovative periodic cellular structures as air volumetric absorbers

This work presents an experimental thermal evaluation of new and innovative porous morphologies suitable for volumetric solar receivers. Five new morphologies, four of them based on the repetition of single unit cells: diamond, rotated cube, tetrakaidecahedron, cube shifted, and a random morphology following the Voronoi tessellation technique were tested. In addition, for emerging geometries, two different cell size configurations were evaluated: one with constant cell size dimension and one with cell's size changing with flux direction. All the lattices were analysed experimentally in a lab-scale solar simulator. The results were compared with the ones of the already established and more conventional silicon carbide honeycomb and random foams proving that a gain in performance is achievable with the Voronoi morphology, both with constant cell size and increasing cell size pattern.

In-line measurement of absorbed solar irradiance using nanofluids

In this paper a novel technique for the in-line evaluation of the absorption rate of solar radiation by nanofluids in a volumetric solar receiver is presented. This method allows to experimentally investigate the optical behaviour of a nanofluid when circulating in a volumetric solar receiver under non-concentrated solar irradiance and it is based on the combined use of pyranometers. This technique is used in the present work to study the absorption capability of a Single-Wall-Carbon-NanoHorns (SWCNHs) based nanofluid. From the experiments, it can be seen that after some hours of circulation, the absorption rate of the nanofluid decreases, due to a loss of nanoparticles in the suspension.

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.

Digital design and 3D printing of innovative SiC architectures for high temperature volumetric solar receivers

In order to demonstrate that digital material engineering methodology is able to address the design and optimisation of architectured ceramic materials, solar volumetric receivers employed in Solar Thermal Power Plants (STPP) have been studied. A digital design approach for obtaining new receivers, at the macroscopic structural scale, is proposed. This approach couples virtual structure generation, ray tracing and thermal simulations at the scale of the base structural components (microscopic scale). Then, a recently developed process for manufacturing silicon carbide (SiC) parts by binder jetting is used to elaborate three optimised structures which are tested on-sun at high temperature in a solar concentrator reproducing the STPP operation conditions. The results obtained with these structures, having original shapes, are promising: the average experimental outlet air temperature reaches a maximum of 1133 K, energy yields can reach 0.49 despite high experimental heat losses, and all the SiC structures, made with a new material based on 3D printing, withstood the high temperatures reached, up to 1500 K. Comparison between digital and experimental results shows that the approach presented in this paper paves the way to a new digital material engineering approach.

Radiation propagation in a hierarchical solar volumetric absorber: Results of single-photon avalanche diode measurements and Monte Carlo ray tracing analysis

Volumetric solar receivers are essential components in high-temperature concentrated solar power plants. Their optical design is crucial for achieving efficient photon-thermal energy conversion; however, their three-dimensional geometry complicates a reliable and accurate optical characterization. This work proposes a new methodology for the 3D optical-performance analysis of the volumetric absorber having an open hierarchical structure. Firstly, the solar absorber is optically characterized using a test bench that mainly comprises a solar simulator and a customized instrument equipped with single-photon avalanche diodes, which holds the absorber and measures light flux on its external surface. Then experimental measurements are compared with a Monte Carlo ray-tracing numerical model. The results are consequently employed to understand light propagation in the absorber. This procedure is successfully applied to characterize a complex three-dimensional self-similar structure manufactured by Selective Laser Melting. The proposed experimental technique is a promising candidate for becoming a robust in operando method to characterize the radiation propagation within the complex porous structures employed for volumetric receivers.

Optical Analysis of a Semi-Transparent Packed Bed of Spheres for Next-Generation Volumetric Solar Receivers

Volumetric solar absorbers are attractive for high-temperature concentrating solar applications because they—theoretically—have lower re-radiation losses than surface absorbers since concentrated radiation penetrates their volume. In practice, however, the incident radiation is predominantly absorbed in the top/frontal surfaces of porous and distributed absorber materials. Another challenge with high-temperature absorbers, generally, is the risk of creating hot spots resulting from a non-uniform temperature distribution. To overcome these limitations, this study reports on a semi-transparent absorber design, enabling the peak temperature to occur within the internal volume. Eight different 'constant-transmittance' and 'variable-transmittance' combinations of transparent/semi-transparent/opaque spheres were packed into a cylindrical cavity and exposed to a solar simulator for a layer-by-layer transmission test. A 3-D ray-tracing model was also developed to further elucidate the relative optical absorption of the cavity walls and the spheres. An optimal variable-transmittance design enabled ~40% of the rays to penetrate to >70% of the absorber's depth, and ~56% of the total rays were absorbed within the inner volume. Therefore, the optimal design yields uniform absorption, leading to a uniform temperature distribution and a minimisation of thermal emission losses. This study provides the first experimental proof that a semi-transparent packed bed can significantly outperform an opaque design.

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.

A new design with preheating and layered porous ceramic for hydrogen production through methane steam reforming process

The solar-assisted methane steam reforming (SAMSR) process is an efficient hydrogen production technology. Volumetric solar receivers with an internal porous absorber can be used to concentrate the solar energy to provide proper conditions for MSR reactions. In this study, first, a sensitivity analysis is performed on effective design parameters of a conventional SAMSR reactor, including the inlet gas direction, internal porous medium properties, and use of a porous medium in the preheating conduit. Next, a new design is proposed – using outlet hot gas of reactor to preheat the feed gas – and its effects on the hydrogen production rate are analyzed to find the best configuration with the highest hydrogen production rate. Finally, the effect of feed gas flow rate on hydrogen production is studied in the optimal case and the results are compared with those of a conventional design. Results show that preheating improves the hydrogen production rate by about 20% in the direct case. Also, the optimal arrangement of the internal porous media in the direct case could improve the hydrogen production rate by 116%. The final case provides an extra 20% improvement in hydrogen production by the proper configuration of porous media in the preheating channel.

An approach to determine radiative properties of solid struts of open-cell foams by pore scale identification from macro scale measurements

Open-cell foams consisting of interconnected struts are widely used in thermal applications such as volumetric solar receivers. The thermal performance of foams strongly relies on their radiative behaviors which intrinsically depends on the radiative properties of struts. This study proposed a feasible approach to determine the radiative properties of solid struts by a pore scale identification from a macro scale measurement. First, the normal-hemispherical transmittance and reflectance spectra of two nickel foams were measured by an experimental system. Second, a pore scale forward calculation was conducted by a combination of Monte Carlo Ray-tracing method and real foam structures obtained from micro-computed tomography technique. Last, an Adaptive Particle Swarm Optimization algorithm was adopted to retrieve the spectral reflectivity and specularity of nickel struts from the measured spectra. The results show that it is unreasonable to assume the reflection behavior of strut surface simply as either specular or diffused, and also, it is unreliable to assign the reflectivity of strut surface directly by Fresnel's law. Within the investigated wavebands 0.4–2.2 μm, the average reflectivity of nickel struts is 12% lower than that of a nickel mirror. The proportion of specular reflection at nickel strut surface is between 37%–52%, almost increasing with wavelengths.

Black zirconia composites with enhanced thermal, optical and mechanical performance for solar energy applications

This work discusses developing new black zirconia (ZrO2) composites with high efficiency and optimal properties as a volumetric solar receiver via low cost processing. The major aim is optimizing a new high temperature material with high durability, optical and thermal properties for solar energy applications. This was achieved by blackening of zirconia and producing black zirconia composites through addition of Fe2O3 and MnO2 instead of using coating. Fe2O3 is added to the white zirconia with different content (0–30 wt%) to promote its dark color and enhance its properties. 5 wt% MnO2 is added to assist the blackening and the sintering processes. The different ZrO2/Fe2O3 (0–30 wt%)/MnO2 composites are produced by pressureless sintering method at a temperature of 1700 °C for 2 hrs. Optical, thermal and mechanical performances of the obtained composites are optimized to ensure its suitability and sustainability as solar absorber material. Results indicated that highly dense composites with homogenous microstructural features are attained. Increment of Fe2O3 content has a substantial effect on promoting the thermal and solar to thermal efficiencies of the obtained composites. Composite with 30 wt % Fe2O3 showed the highest solar absorbance in the UV, visible and near IR regions. It recorded the best light emission efficiency with the highest photoluminescence performance. Black ZrO2/Fe2O3/MnO2 composites could be a promising candidate as solar receiver material for thermal solar plants than other reported carbide and nitride materials.

Optimization of volumetric solar receiver geometry and porous media specifications

Concentrated Solar power plants are one of the promising methods for converting solar energy into usable energy. Volumetric Solar receivers are one of the key components of these power plants that have received a lot of attention from researchers in recent years. In the present study, to optimize the important parameters in the performance of the solar receiver, average cell diameter (Φ [mm] ), the porosity of absorber (εp), absorber diameter Dabs and the distance of glass window from the absorber surface Lcavitywere selected. The parametric and optimization study presented in this research has been done by the response surface and experimental design method. The objective function in optimization is a minimization of the pressure drop in the receiver and at the same time maximization of the output temperature of the receiver. In this study, it has been emphasized on the simultaneous effect of parameters on problem answers because, in practical conditions, the effect of the parameters on the receiver performance is simultaneous. In optimization studies, first, the most effective parameters on the problem solutions are identified, then the optimal values of each of them are introduced and discussed.

Non-uniform and volumetric effect on the hydrodynamic and thermal characteristic in a unit solar absorber

Experimental and numerical analyses on the thermal and hydrodynamic performance of volumetric solar receivers were conducted. Silicon carbide (SiC) absorbers with various pore structures were systematically investigated on a laboratory-scale test platform. A three-dimensional theoretical model coupling the fluid flow with internal heat transfer was presented. The major characteristics of the thermal and hydrodynamic behavior combining radiation, thermal conduction, and interphase convection in the absorber were presented. In addition, the key design parameters were systematically analyzed. The experimental results revealed that a small pore diameter significantly enhances the volumetric heat-transfer coefficient, which in turn increases the fluid temperature and subsequently leads to a high thermal efficiency. Under high local thermal non-equilibrium state, the non-uniform hydrodynamic characteristic significantly influenced due to the increase in the fluid viscosity at the hot spot, especially in the case of a small-pore-sized absorber, thereby increasing the risk of overheating. For absorbers with similar porosities (ϕ ∼ 0.85), dp decreased from 5.83 mm to 2.22 mm, and the thermal efficiency η increased by nearly 3%. Moreover, the simulation results revealed that the absorber unit with dp of 2.22 mm and ϕ of 0.95 exhibits the maximum thermal efficiency of 72.48%.

Performance analysis on a volumetric solar receiver with an annular inner window

Large temperature gradient and local hot spot in solar receiver seriously impact the operation safety and thermal performance. To improve the temperature uniformity of porous foam absorber in a volumetric solar receiver, this work presents a receiver with an annular inner window to reshape the fluid field in the receiver. To examine the improvement effect of the proposed receiver, one numerical model is developed to analyze the coupled heat transfer in the receiver. The model takes the propagation of concentrated solar irradiation and thermal radiation in the consideration by using the Monte-Carlo ray tracing method. The effects of the inner window on the fluid flow and temperature distribution of the absorber as well as the efficiency of the receiver are analyzed. The results show that the standard difference of temperature in absorber of the proposed receiver is 35–158.6 K lower than that of the single windowed one, and the thermal efficiency is increased by 2.86%–7.18%, within the inner radius of the annular window range of 15–45 mm and flow rate of 4–7 g/s. It is demonstrated that installing an annular window is an effective approach to improve the uniformity of absorber temperature while increasing the thermal performance.

Investigation of Nanofluids Circulating in a Volumetric Solar Receiver

Abstract Single-wall carbon nanohorn (SWCNHs)-based nanofluids have been proven to be promising media for the direct absorption of solar radiation due to their favorable optical properties and potential low cost. Still their stability in real working conditions is an open issue because they have been studied mainly under stagnant conditions, while limited information is available on the performance of these nanofluids during circulation in real systems. In the present work, the optical behavior of SWCNH-based nanofluids has been investigated with the aim of detecting possible effects of circulation and exposure to radiation, avoiding other effects such as thermal instability. An ad hoc experimental apparatus has been realized to check the stability of the circulating fluids in situ using a novel approach based on the use of pyranometers. Three suspensions are tested, two are surfactant stabilized and one is based on preoxidized and functionalized SWCNHs. Efficiency values higher than 90% were measured for more than 65 h of circulation. The effects of fluid circulation and exposure to solar radiation have been addressed, finding that the absorption efficiency decreases during tests due to the degradation of the optical properties and the nanofluid circulation is the main responsible for such degradation.

Thermal and Flow Characteristics of a Volumetric Solar Receiver; Including Turbulent Flow

Thermal and Flow Characteristics of a Volumetric Solar Receiver; Including Turbulent Flow

Thermophysical properties of porous Ti2AlC and Ti3SiC2 produced by powder metallurgy

The physicochemical properties of porous Ti2AlC and Ti3SiC2 MAX phase compounds with controlled porosity and grain size obtained by powder metallurgy techniques was studied in depth in order to access their suitability of applications such as catalytic devices on vehicles, heat exchangers or impact resistant structures. The study was performed on isostatic consolidated samples with different amount (20–60 vol%) and size of space holder (250–1000 µm) and in samples without space holder. Oxidation tests were performed at different temperatures for each material depending on their maximum service temperature. In order to understand the oxidation mechanism, oxidation kinetics were analysed to determine the influence of size and amount of porosity in each case. In addition, the microstructure and composition of the oxide layers formed after the tests were analysed by scanning electron microscopy (SEM). Electrical and thermal conductivity where studied at room temperature and at temperature up to 1000 °C. The effect of pore size and cell wall thickness is discussed. Permeability of foams was also measured. The effect of micro porosity and macro porosity on permeability is discussed. The coefficient of thermal expiation was also measured for all foams produced. It is established that these porous MAX phases have suitable properties for their use as catalytic substrates, heat exchanges, high temperature filters or volumetric solar receivers.

Experimental Study of the Convective Heat Transfer and Local Thermal Equilibrium in Ceramic Foam

Foam materials have been widely used in various industrial applications, where higher and higher heat and mass transfer performances are pursued. However, the mechanism of many factors on the heat transfer performances is still unclear. The main purpose of this article is to investigate how the porous properties, porosity, cell size and the sample thickness affect the volumetric convective heat transfer. In this study, the single-blow method is used to determine the volumetric heat transfer coefficient of ceramics foam in the temperature range from 283 K to 323 K. In particular, sensitivity analysis of the foam porosity, cell size, velocity and the sample thickness on the volumetric heat transfer coefficient within the ceramics foam were all conducted. The results indicate that the sample thickness has a significant effect on the volumetric heat transfer coefficient which decreases with the sample thickness. In addition, the local thermal equilibrium phenomenon is verified and its influence on the volumetric heat transfer coefficient discussed. Based on the experimental data, a new correlation is proposed that includes sample thickness, porosity, superficial velocity and fluid properties. This study is crucial to the theory of the convective heat transfer inside the porous media, and can be used to guide the design and optimization of volumetric solar air receivers, compact heat exchangers, heat sinks, heat regenerators, packed bed reactors and so on.

Numerical investigation of single-blow transient testing technique

The single-blow transient testing technique has been used for more than 50 years, and many correlations have been proposed from this technique. However, these correlations differ significantly from one another and the behind reasons are still unknown. In this study, we used numerical simulation to detect the detailed temperature evolution of an entire porous matrix during the single-blow experiment. The numerical simulation was performed on a three-dimensional geometry with idealized packed tetrakaidecahedron structures, and the numerical method was based on three-dimensional RANS equations. The initial and boundary conditions of the simulation were the same as those of the single-blow transient testing technique. The detailed temperature evolution of the sample and air stream was obtained. Computational results indicated a local thermal equilibrium region inside the porous matrix during the single-blow transient experiment. The local thermal equilibrium is affected by the sample thickness, superficial velocity, and thermal capacity. The “local thermal equilibrium” inside the porous matrix during the single-blow transient test contradicts the prerequisites of this measurement method. This finding is validated by experimental data and is crucial to heat transfer inside porous media, as well as to the design and optimization of heat exchangers, volumetric solar receivers, and thermal storage systems.