Solar Air Receiver Research Articles

A solar air receiver with porous ceramic structures for process heat at above 1000 °C – Heat transfer analysis

Abstract Concentrated solar energy can be used as the source of heat at above 1000 °C for driving key energy-intensive industrial processes, such as cement manufacturing and metallurgical extraction, contributing to their decarbonization. The cornerstone technology is the solar receiver mounted on top of the solar tower, which absorbs the incident high-flux radiation and heats a heat transfer fluid. The proposed high-temperature solar receiver concept consists of a cavity containing a reticulated porous ceramic (RPC) structure for volumetric absorption of concentrated solar radiation entering through an open (windowless) aperture, which also serves for the access of ambient air used as the heat transfer fluid flowing across the RPC structure. A heat transfer analysis of the solar receiver is performed by means of two coupled models: a Monte Carlo (MC) ray-tracing model to solve the 3D radiative exchange and a computational fluid dynamics (CFD) model to solve the 2D convective and conductive heat transfer. Temperature distributions computed by the iteratively coupled models were compared with experimental data obtained by testing a lab-scale 5 kW receiver prototype with a silicon carbide RPC structure exposed to 3230 suns flux irradiation. The receiver model is applied to optimize its dimensions for maximum efficiency and to scale up for a 5 MW solar tower.

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.

Thermal stress durability and optical characteristics of a promising solar air receiver based black alumina ceramics

In solar thermal technology, solar receiver materials with high solar absorptivity and photocatalytic efficiency have recently become a mandatory requirement for promoting and maximizing the released thermal and electrical energy. This paper presents a detailed study of the optical performance and thermal stress durability of the promising solar receiver material, black Al2O3/CuO ceramics. Different Al2O3/CuO ceramics with different CuO content (10–40 wt%) were obtained by the pressureless sintering method. Optical properties such as solar absorbance and reflectance, band gap energy and photoluminescence are inclusively investigated. The thermal stress resistance of the obtained ceramic receivers is simulated using the finite element modeling (FEM) method at different temperatures. Results indicated that adding and increasing the content of CuO to Al2O3 has a significant role in transforming alumina from a non-solar light-absorbed material to a solar absorber material with high absorptivity in the Ultraviolet–Visible-Near Infrared (UV-VIS-NIR) spectrum. Composite with 40 wt% CuO has recorded the maximum absorbance of 75% in the visible light region. Moreover, Al2O3/CuO ceramics gave multiple graded band-gaps in the range of (1.6–5 eV). Composites with high wt% of CuO have the most increased photocatalytic activity and light emissivity efficiency. Thermal stress analysis of the different ceramics showed outstanding stress durability with uniform heat distribution. Hence, black Al2O3/CuO ceramics can be considered ideal solar absorber materials with high sustainability and durability at high temperatures.

Numerical and experimental studies on a pressurized hybrid tubular and cavity solar air receiver using a Scheffler reflector

• Coupled optical and thermal model of a pressurized tubular and cavity solar air receiver. • Modelled the dynamic variation of concentrated solar radiation input to the receiver cavity surface. • Three-dimensional CFD analysis is performed on the hybrid receiver model. • Validation with on-sun experimental results using a Scheffler dish. • Natural convection and radiation heat loss estimation. In this study, a pressurized tubular and cavity solar air receiver is analyzed numerically, and the results are validated experimentally using a Scheffler fixed focus concentrator. Transient numerical analyses for periods similar to experimental testing duration allows realistic performance prediction of new receiver designs and help in understanding the dominant heat loss mechanisms, thus mitigating or reducing them. A coupled optical and thermal model is developed for analyzing the receiver with a novel method of capturing the spatial and temporal variation in the heat input to the receiver. The concentrated heat flux input to the receiver is defined as the product of two separate functions. The first function defines the incident radiation to the receiver as a spatially resolved flux profile on the cavity inside surface, obtained from the ray tracing algorithm. The second function captures the transience in heat input by curve-fitting the measured direct normal irradiation variation with time corresponding to the experimental testing period. Receiver heat loss modelling involves natural convection heat loss from the inside cavity surface estimated using an existing correlation, while radiation heat loss is estimated by prescribing the surface emissivity and computing the ambient view factor. A three-dimensional transient CFD analysis is performed on the hybrid receiver model under identical heating conditions as experiments to predict the flow heat transfer. The result from the numerical model is subsequently compared with on-sun experimental results obtained using a test rig equipped with a Scheffler dish for heat input and supply of compressed air as the heat transfer fluid through the receiver. The deviation between numerical and experimental results are less than 16 °C for air outlet temperature and less than 9% for the receiver thermal efficiency, thus proving the effectiveness of the coupled optical and thermal model. To account for the transient nature of the receiver heating during the on-sun experiments, the receiver thermal efficiency definition has been modified to include the thermal inertia of the receiver material. It is also observed that natural convection is the dominant heat loss mechanism for the present configuration and can significantly reduce the overall thermal efficiency of a receiver. The present numerical model incorporating an optical model of the solar field and as-measured variation of solar radiation input can be effectively used for optimizing the design of any generic concentrator-receiver system with high-pressure heat transfer fluid, with the objective of minimizing thermal losses.

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 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.

Flexible electricity dispatch for CSP plant using un-fired closed air Brayton cycle with particles based thermal energy storage system

This paper presents a novel power block concept for flexible electricity dispatch in a Concentrating Solar Power (CSP) plant. The power block is based on intercooled – unfired regenerative closed air Brayton cycle that is connected to a pressurized solar air receiver. The Closed Brayton cycle uses a mass flow regulation system centered on the pressure regulation (auxiliary compressor and bleed valve) in order to control the Turbine Inlet Temperature (TIT). Doing so, the system is able to modulate turbine electricity production according to variations in the solar resource and changes in power electric demand. It has been found that the proposed power block is able to fully cover the electricity demand curve for those days with high solar resource. In case of integrating particles-based high temperature Thermal Energy Storage (TES) system, the power block can extend its production till the next day following the electricity curve demand during summer period. During winter period, the power plant can extend its production for a few hours due to the lower solar resource and the higher electric curve demand load.

Experimental evaluation of a novel solar receiver for a micro gas-turbine based solar dish system in the KTH high-flux solar simulator

This work presents the experimental evaluation of a novel pressurized high-temperature solar air receiver for the integration into a micro gas-turbine solar dish system reaching an air outlet temperature of 800 °C. The experiments are conducted in the controlled environment of the KTH high-flux solar simulator with well-defined radiative boundary conditions. Special focus is placed on providing detailed information to enable the validation of numerical models. The solar receiver performance is evaluated for a range of operating points and monitored using multiple point measurements. The porous absorber front surface temperature is measured continuously as it is one of the most critical components for the receiver performance and model validation. Additionally, pyrometer line measurements of the absorber and glass window are taken for each operating point. The experiments highlight the feasibility of volumetric solar receivers for micro gas-turbine based solar dish systems and no major hurdles were found. A receiver efficiency of 84.8% was reached for an air outlet temperature of 749 °C. When using a lower mass flow, an air outlet temperature of 800 °C is achieved with a receiver efficiency of 69.3%. At the same time, all material temperatures remain below permissible limits and no deterioration of the porous absorber is found.

Scaling effects of a novel solar receiver for a micro gas-turbine based solar dish system

Laboratory-scale component testing in dedicated high-flux solar simulators is a crucial step in the development and scale-up of concentrating solar power plants. Due to different radiative boundary conditions available in high-flux solar simulators and full-scale power plants the temperature and stress profiles inside the investigated receivers differ between these two testing platforms. The main objective of this work is to present a systematic scaling methodology for solar receivers to guarantee that experiments performed in the controlled environment of high-flux solar simulators yield representative results when compared to full-scale tests. In this work the effects of scaling a solar air receiver from the integration into the OMSoP full-scale micro gas-turbine based solar dish system to the KTH high-flux solar simulator are investigated. Therefore, Monte Carlo ray-tracing routines of the solar dish concentrator and the solar simulator are developed and validated against experimental characterization results. The thermo-mechanical analysis of the solar receiver is based around a coupled CFD/FEM-analysis linked with stochastic heat source calculations in combination with ray-tracing routines. A genetic multi-objective optimization is performed to identify suitable receiver configurations for testing in the solar simulator which yield representative results compared to full-scale tests. The scaling quality is evaluated using a set of performance and scaling indicators. Based on the results a suitable receiver configuration is selected for further investigation and experimental evaluation in the KTH high-flux solar simulator.

Anisotropy effects on convective heat transfer and pressure drop in Kelvin’s open-cell foams

Open-cell foams have the potential to offer heat transfer enhancement in many applications, such as heat exchangers, solar air receivers, porous burners, thanks to their high heat transfer surface to volume ratio and to tortuosity, that promotes the internal flow mixing. Their microstructural transport features are affected by foam anisotropy, and could play an important role not only because manufacturing processes can stretch the cells along a preferential direction, but also because modern techniques allow for the customization of the foam cell shape. However, structural anisotropy in open cell foams has been scarcely investigated. In this paper, anisotropy effects on convective heat transfer and pressure drop in Kelvin’s open-cell foams are analyzed. Since Kelvin’s model is geometrically regular, anisotropy has been investigated by stretching the foam along three orthogonal directions, at equal cell volume. Governing mass, momentum and energy equations have been solved using a finite element method. Results are presented for different fluid inlet velocities and cell sizes along three orthogonal stretching directions. They show that anisotropy affects the velocity and temperature fields, and, consequently, the permeability, inertial factor and volumetric heat transfer coefficients.

Coupled optical and CFD parametric analysis of an open volumetric air receiver of honeycomb type for central tower CSP plants

The performance of an open volumetric solar air receiver of honeycomb type (multiple parallel channels) for central tower CSP plants is evaluated numerically. A parametric study is conducted at the single channel level in order to investigate the influence of the main geometrical variables and of the air mass flow rate on the receiver performance. The adopted methodology consists of two steps: the first one is the optical analysis that is conducted using Tonatiuh, an open-source Monte-Carlo based ray-tracing software, providing the distribution of the absorbed heat flux on the channel inner surfaces, to be finally exploited as input data in the second step, i.e. the numerical evaluation of the thermal fluid dynamic performance of the channel. Using the commercial CFD software ANSYS Fluent, the convective heat transfer between the air flow and the absorber and the radiative heat transfer among the absorber inner walls and the channel aperture are simulated, computing the heat losses to the ambient. Different channel configurations are simulated, identifying the influence of the three key-parameters (the tilt angle with respect to the horizontal, the channel size and the air mass flow rate) on the receiver performance, in terms of solar-to-electricity efficiency.

Solugas – Comprehensive analysis of the solar hybrid Brayton plant

This article presents the first megawatt scale solar-hybrid plant with a solarized gas turbine. The works and improvements that made the Solugas project succeed during design, construction and long-term solar operation are described and explained.More than 1000 operation hours and receiver outlet temperatures of 800°C were achieved. The operation behaviour and performance of the key components – the solar pressurised air receiver and the gas turbine – are presented here as a proof of the high potential of the technology.

A tracer gas leak rate measurement method for circular air circuits

Qualitative measurements of leaks in air flows are frequently conducted in engineering. However, an environmentally friendly leak rate measurement technique which yields quantitative results and is applicable to hot, large-scale circular air flows (≈10kg/s; ≈ 700°C) with high leak rates has not been presented yet. This paper describes the development, test and validation of a stationary and a dynamic, helium based tracer gas method on a lab-scale model of a solar air receiver with partial air recirculation. Helium is chosen as tracer gas since it is environmentally friendly, stable under high temperatures and is also cheap for large-scale air flows. The tracer gas is injected either continuously or intermittently into the model system, its concentration is measured using a mass spectrometer and the static or dynamic concentration response of the system is used to determine the leak rate. The stationary method needs two measurement points upstream and downstream the leak, the dynamic method only one measurement point if applied to a circular air flow system. Since the dynamic method is time dependent the transfer function of the measurement setup was determined and the dynamic measurement error was considered. An extensive uncertainty analysis is presented for both the stationary and the dynamic method. Exemplary measurements were conducted at the model system with very good results. Both dynamic and static measurements yield the same result within their confidence intervals. The leak rate of the solar receiver model with a mass flow rate of ṁin=(0.247±0.008)/kgs was measured to be lstat=(36.1±2.3)% and ldyn=(34.5±3.6)% with the static and the dynamic method respectively.

Available online Efficiency potential of indirectly heated solar reforming with different types of solar air receivers

In solar reforming, the heating value of natural gas is increased by utilization of concentrated solar radiation. Hence, it is a process for storing solar energy in a stable and transportable form that also permits further conversion into liquid fuels like methanol. This process has the potential to significantly decrease the natural gas consumption and the associated CO2-emissions of methanol production with only few open questions to be addressed prior to commercialization. In the medium and long term, it has the potential to generate methanol as an environmentally friendly fuel for both transport as well as flexible electricity production in combined cycle gas turbines, when biogas is used as reactant. In a previous study the high potential of indirectly heated solar reforming with solar air receivers was shown; however, the efficiency is limited when using state of the art open volumetric receivers. Therefore, different types of air receivers are implemented into an indirectly heated solar reforming process and the overall efficiency potential is assessed in the present study. The implemented receivers are an open volumetric cavity receiver, a closed volumetric cavity receiver and a tubular cavity receiver. The open volumetric cavity receiver and tubular cavity receiver achieve the best results due to their capability of operating efficiently at temperatures well above 700 °C. For these receivers peak efficiencies up to 29% and 27% respectively are predicted. As the utilization of an open volumetric cavity receiver constitutes an open heat transfer loop, the air return ratio together with air return temperature is identified as a major impact factor for overall efficiency. With an improved air return ratio, annual process efficiency of 18.4% is feasible. Considering its relative technical simplicity, this makes indirectly heated solar reforming a promising technology to efficiently provide renewable energy for methanol production and thereby diversify solar energy utilization.

Analysis of Radiative Effect under Local Thermal Non-Equilibrium Conditions in Porous Media-Application to a Solar Air Receiver

The effect of thermal radiation within a porous medium while incorporating local thermal nonequilibrium (LTNE) is investigated in this work, with specific application to solar air receivers. It is shown that the radiation effect is significant. The temperature distributions for both solid and fluid phases are affected by conduction-radiation parameter N, the ratio of solid to fluid phase conductivities ζ, and the interphase convection parameter H. The limiting interactions between conduction and thermal radiation incorporating convection are revealed. The impact of conduction and convection versus radiation is systematically analyzed in this work and displayed through a number of contour maps.

Materials and Processes for Renewable Energy Technologies

This collection of papers is dedicated to the topic ‘‘Materials and Processes for Renewable Energy Technologies’’. The focus is on concentrated solar energy, which provides a source of clean, non-polluting, high-temperature heat for power and fuels production. Current technologies are based on solar receivers that usually operate below 500 C, coupled to steambased Rankine cycles for power generation at 20% peak efficiency. The next generation of technologies will allow for surpassing 1000 C and enable higher efficiencies via Brayton–Rankine combined cycles. Olivares et al. present a thermogravimetric study on promising high-temperature-resistant alloys for application in solar air receivers coupled to gas turbines. Solar thermochemical approaches to splitting CO2 and H2O inherently operateat high temperatures and use the entire solar spectrum, and as such, they provide a thermodynamically favorable path to solar fuels production. Several authors investigate twostep redox cycles, encompassing the solar thermal reduction of a metal oxide followed by its re-oxidation with H2O and CO2 for H2 and CO generation. Allen et al. examine doped ferrite as redox material and analyze its reaction kinetics. Ceria emerges as another attractive redox candidate because of its high oxygen ion conductivity. Ganesan et al. determine the optical properties of ceria when exposed to concentrated solar radiation. Suter and Haussener present a tomography-based methodology to characterize and optimize porous reticulated ceramic materials used in solar power and fuel applications. Solar concentrating technologies offer round-theclock dispatchability of heat and power through the integration of thermal storage and/or hybridization. There is an opportunity for the minerals processing industry to take advantage of these developments and use the same solar concentrating infrastructure as a direct heat source at high temperatures. The candidate processes are the production of metals (e.g., zinc, magnesium, aluminum, and silicon) from their oxide ores by carbothermal reductions. These processes are highly endothermic and are characterized by their concomitant vast emissions of greenhouse gases (GHGs) and other pollutants. The use of solar process heat integrated with thermal storage offers unique potential advantages for GHG avoidance in extractive metallurgical processes. Eglinton et al. assess this potential for the Australian minerals industry. Vishnevetsky et al. describe exploratory experimental studies on the production of aluminum by carbothermal reduction of alumina at vacuum pressures, enabling its vacuum distillation while bypassing the formation of undesired by-products. Tzouganatos et al. experimentally demonstrate the recycling of zinc by clinkering of Waelz oxide and by its carbothermal reduction using a packed-bed reactor driven by concentrated solar energy. Finally, Perruchoud and Fischer describe the production of upgraded metallurgical silicon, which is the preferred feedstock for the preparation of solar grade silicon for photovoltaic cells. These original papers indicate favorable longterm potential of materials and processes for solar technologies, warranting further research, development, and large-scale demonstration.

Fully coupled transient modeling of ceramic foam volumetric solar air receiver

Ceramic foam is a promising material for the absorber of volumetric solar air receiver in concentrated solar thermal power (CSP) plant. The transient behaviors of volumetric solar air receiver are crucial to the receiver’s controllability, and to some extent, the plant’s safety. This study numerically analyzes the transient behaviors of volumetric solar air receiver under various working conditions. A fully coupled transientmodel of the volumetric solar air receiver is developed in this paper. The pressure drop of the absorber, the interfacial heat transfer between the flowing fluid and solid, and the radiative heat transfer due to concentrated solar radiation absorption by ceramic foam and the radiation transport inside the media were included together in this transientmodel. In addition, the temperature fields of the fluid and solid phases were obtained by using the local thermal non-equilibrium model. A comparison of the computed results with experimental data shows that this coupled transient model can be used to predict the performance of volumetric solar air receiver. Based on this model, the transient behaviors of the solar air receiver under a sudden heat flux, a sudden loss of heat flux, and a step change of heat flux were studied. The results of this study are very helpful in designing and controlling volumetric solar air receivers.

타워형 태양열 발전용 공기흡수기의 열전달 성능해석

In this study, a heat-transfer performance analysis is carried out for a multi-channel volumetric air receiver for a solar power tower. On the basis of a series of reviews regarding the relevant literature, a calculation process is proposed for the prediction of the wall- and air- temperature distributions of a single channel at given geometric and input conditions. Furthermore, a unique mathematical model of the receiver effectiveness is presented through analysis of the temperature profile. The receiver is made of silicon carbide. A total of 225 square straight channels per module are molded to induce the air flow, and each channel has the dimensions of . The heat-transfer rate, temperature distribution and effectiveness are presented according to the variation of the channel and module number under uniform irradiation and mass flow rate. The available air outlet temperature applied to the solar power tower should be over . This numerical model was actually used in the design of a 200 kW-level commercial solar air receiver, and the required number of modules satisfying the thermal performance could be obtained for the specified geometric and input conditions.

Coupled radiation and flow modeling in ceramic foam volumetric solar air receivers

► A model for the performances of volumetric solar air receiver was developed. ► Sensitivity studies show the thermal non-equilibrium phenomena are distinct locally. ► The mean cell size has a dominant effect on the performances of solar air receiver. ► The solid thermal conductivity of absorber is not important to the air receiver. ► The desired temperature distribution of the absorber is realizable. Ceramic foams are promising materials for the absorber of volumetric solar air receivers in concentrated solar thermal power (CSP) receivers. The macroscopic temperature distribution in the volumetric solar air receiver is crucial to guarantee that volumetric solar air receivers work steadily, safely and above all, efficiently. This study analyzes the temperature distribution of the fluid and solid phases in volumetric solar air receivers. The pressure drop in the ceramic foams and the interfacial heat transfer between the flowing fluid and solid are included in the model. The radiative heat transfers due to concentrated solar radiation absorption by the ceramic foam and the radiation transport in the media were modeled with the P 1 approximation. The energy fields of the fluid and solid phases were obtained using the local thermal non-equilibrium model (LTNE). Comparison of the macroscopic model with experimental results shows that the macroscopic model can be used to predict the performance of solar air receivers. Sensitivity studies were conducted to analyze the effects of velocity, porosity, mean cell size and the thermal conductivity of the solid phase on the temperature fields. The results illustrate that the thermal non-equilibrium phenomena are locally important, and the mean cell size has a dominant effect on the temperature field.

Numerical simulation of convective heat transfer between air flow and ceramic foams to optimise volumetric solar air receiver performances

Porous ceramic foams are used to achieve high performance in solar heat recovery systems. Understanding the convective heat transfer between the air flow and the ceramic foam is of great importance when optimising the volumetric air receiver. In this work, the convective heat transfer was numerically studied. The present approach was designed to compute the local convective heat transfer coefficient between the air flow and a porous ceramic foam. For that purpose, the energy balance and the flow inside the porous ceramic foam were solved. In addition, a detailed geometry of the porous ceramic foam was considered. The ceramic foams were represented by idealised packed tetrakaidecahedron structures. The numerical simulations were based on the three dimensional Reynolds-averaged Navier–Stokes (RANS) equations. A sensitivity study on the heat transfer coefficient was conducted with the porosity, velocity and mean cell size as parameters. Based on the numerical simulation results, a correlation for the volumetric local convective heat transfer coefficient between air and ceramic foams was developed. The resulting correlation covers a wide range of porosities, velocities, cell sizes and temperatures. The correlation results were compared with experimental data from the literature, and the comparison shows good agreement. The correlation is intended to be used in the design of volumetric solar air receivers.