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In the last years, it has been experimentally proven that the precursors of synthetic fuels can be produced exploiting the thermochemical water and CO2 splitting process, driven by concentrated solar radiation. This process requires large amount of thermal energy at very high temperature levels (1300-1400°C) far from being obtainable with conventional commercial receivers. For this reason, Synhelion SA is developing an innovative receiver design capable of operating at very high temperatures (in the order of 1500°C). To assist the design phase of the receiver, a CFD model was developed and exploited to run some CFD simulations campaigns. At first, an analysis on the effect of modeling the receiver behavior with different turbulence models (realizable k-ε and k-ω SST) was conducted with the aim of determining the most suitable for the cases under investigation. The CFD model was then applied to study the receiver behavior under different operating conditions. In particular, in terms of receiver operating pressure, two different cases were considered: ambient pressure and 10 bar. For each of them, a specific receiver design was described, and a relative simulations campaign, assuming different values of heat transfer fluid (HTF) mass flow rates, was performed not only to evaluate its thermo-fluid dynamics behavior but also to determine the most promising HTF inlet configuration between the two proposed of single and double inlets. Based upon the results obtained, the absorbing gas receiver resulted to have promising performance for all the working conditions evaluated. In the case of receiver operating at ambient pressure, the best resulting thermal efficiency values were in the range of 0.78 and 0.43, with relative HTF outflow temperature range of 1200°C and 1800°C. While, in the case of pressurized receiver, the best resulting efficiency values were in the range of 0.88 and 0.54, with relative HTF outflow temperature range of 1000°C and 1670°C. [ABSTRACT FROM AUTHOR]

Emerging directly irradiated particle receiver technologies have the capability of generating temperatures exceeding 800oC. This is important for industrial processes involving high temperatures, such as electricity generation and thermochemical processes, such as the Bayer alumina process. The utilization of particles in these devices is receiving growing attention due to their potential to withstand high temperatures and their effectiveness on solar radiation absorption. Among all particle receiver technologies being developed globally, the suspension flow receiver, better known as a vortex flow receiver, is receiving growing attention as a method to heat particles in suspension, and hence to heat both phases. In this study, a set of novel scale-up strategies was investigated for a 50-MW windowless directly irradiated vortex-based solar particle receiver to identify the strengths and limitations of each strategy. A receiver design was determined and chosen as the candidate for the current computational study. A systematic analysis was performed to assess the influence of particle mass loading and inlet air mass flowrate on the global performance of the receiver at scale (50-MW) under steady-state conditions with simulated solar flux, along with CARBO-CP used as the particle material. A fixed ratio of overventilation was applied to the induced draft fan to reduce particle egress. The numerical study was conducted using computational fluid dynamics (CFD) software (ANSYS/CFX 2020 R2). The result predicts that up to 60% thermal efficiency can be achieved under high air mass flowrates and particle loadings at scale for the operating conditions analyzed here. [ABSTRACT FROM AUTHOR]

Indonesia is a country that has abundant solar energy. Cooking with solar energy is one way to reduce dependence on fossil fuels. In this research, a Fresnel solar cooker (FSC) will be designed with a solar tracker. PMMA Fresnel lens is used to concentrate Direct Normal Irradiance (DNI) in a cooking pot with a conical cavity receiver cover. The thermal performance of the FSC was investigated by simulation and experiment. The simulation results show that the disorientation of the focal point causes a deceleration in the thermal distribution. The experiment results show that the focal point temperature of the Fresnel is influenced by Direct Normal Irradiance (DNI). The increase in solar radiation produces a focal temperature, exponentially. The test results show that DNI = 862.50 W/m2 can produce a focal temperature, Tf = 1064.00°C. [ABSTRACT FROM AUTHOR]

The article deals with the problem of synthesis of the control system of the power unit of a solar space gas turbine power unit. To ensure the uninterrupted operation of the turbine in the shadow region of the orbit, a thermal storage unit is used as an energy source, which prudently stores solar energy in the light region of the orbit. Operation of the turbine is provided by the system "receiver of solar radiation – thermal storage", which forms a single power unit. The problem of structural and parametric identification of its mathematical model is solved. Thermal storage efficiency is justified by maximum possible temperature of working fluid at turbine inlet. The criterion of optimality of choice of operating parameters of power plant is to ensure constancy of thermal storage characteristics at maximum thermodynamic efficiency of the system, which is equivalent to maximization of exergy efficiency of the system. The results of calculations have shown the possibility of using the proposed structural solutions to control the capacity of the power unit. [ABSTRACT FROM AUTHOR]

CSP feasibility analysis, bankability assessment and plant design are generally based in one-year meteorological series representative of "typical" o extreme conditions. There is a growing interest in accounting on the inherent variability of the performance of solar plants on their input variables in the plant model. In particular, for generating the probability distributions of annual energy yield, simulation tools require as input a large number of plausible meteorological years (PMYs). In this article we generate 100 synthetic years of DNI in the 1-min resolution for the location of Seville, Spain, and we estimate and evaluate the power produced by two CSP plants with similar configuration as two well-known parabolic trough and central receiver solar plants (Andasol 3 and Gemasolar) using System Advisor Model (SAM). We estimate that there is an almost linear relation between the Probabilities of Exceedance (PoE) of the DNI and the gross electricity generation of the solar plants but that for a given annual value that corresponds to a PoE of the DNI, there is a range of possible production values depending mainly on the monthly distribution of the solar radiation datasets. [ABSTRACT FROM AUTHOR]

The use of hybrid solar thermal devices, which harness the energy from both concentrated solar radiation and combustion, is receiving growing attention due to their potential to provide a firm and dispatchable thermal energy supply while lowering the costs of energy systems and assisting the penetration of renewable energy. Direct hybrids, which integrate the functions of both a solar receiver and a combustor into a single device, feature the greatest potential in terms of high efficiency, cost-reduction and flexibility in operation, among all the hybrid design proposed to date. In this study, we propose a novel concept of direct hybrid, which integrate the functions of the radiant burner, RB, technology into a billboard-type solar receiver. RB technology was selected here given its potential to provide high radiative heat transfer and heat fluxes similar to those of concentrated solar radiation. First-of-a-kind, systematic measurements of the performance of a 20-kW laboratory-scale Hybrid Solar Radiant Burner Receiver, HSRBR, unit are reported here under simulated solar conditions and using natural gas as energy source for combustion and mixed operations. It was found that the device can achieve thermal efficiency of up to 80% and provide firm supply of hot air of up to 700 °C. Also, it was found that the RB can efficiently manage transients with a fast response time, so that the device can provide both a steady thermal output and a constant outlet temperature of the heat transfer fluid. The location of the RB within the solar receiver (either in the front or back of the billboard-like heat exchanger) was found to have only a small influence on the performance of the device, indicating a great design flexibility. [ABSTRACT FROM AUTHOR]

This work presents the first experimental results dealing with the direct absorption of concentrated solar energy by a body of glass contained in a solar cavity receiver. Container glass of various colors was adopted as working medium in a setup tested in the ETH’s High Flux Solar Simulator. Maximum radiative fluxes of 1.2 MWm−2 and power input of 1.5 kW were directly absorbed by the glass, which reached temperatures up to 1300 °C. Such conditions were maintained for 5 to 10 hours and no technical problems were encountered with the containment of the hot glass. These preliminary results demonstrate that solid or molten glass acts as an effective volumetric absorber of solar radiation at temperatures exceeding 1000 °C. [ABSTRACT FROM AUTHOR]

The SUN-to-LIQUID project aims to demonstrate a pre-commercial scale liquid solar fuels production facility in Mostoles, Spain. Development of the solar reactor and power delivery technology are critical to the success of the project. Ultra-modular heliostats are densely packed in front of a 15 m high solar tower and can deliver concentrated solar radiation in excess of 3,000 kW/m². Two solar receiver vessels are install on the tower; the east installation contains a water calorimeter, and the west installation contains the solar reactor, comprised primarily of the ceria reticulated porous ceramic (RPC) structures. The ceria RPC reactor has been successfully scaled up from 4 kW and commissioning tests were performed in a high flux solar simulator. In the solar tower, a flux measurement system combined with a water calorimeter are used to calibrate the method to determine solar power entering the aperture of the solar reactor. Here we report on progress in the commissioning of the two primary sub-systems of the facility, the heliostat field and the solar reactor. [ABSTRACT FROM AUTHOR]

Receivers from solar power plants are subject to highly complex thermal loads that are very difficult to measure experimentally. The analysis of a complete receiver from one of those plants involves a great number of panels and tubes, increasing the complexity and the computational cost. In this work, a solar receiver has been studied during its daily operation time with a method that only considers one representative tube per panel, which reduces the computational time. Results show that the maximum stress is always located in the northern panels, where the circumferential temperature gradient is higher. Thus, stress has a high dependence on the circumferential temperature variation, and then, the most loaded panel could be found studying those temperature gradients. [ABSTRACT FROM AUTHOR]

An innovative experimental tool for in-situ investigation of thermo-mechanical damage in solar receiver materials is designed and tested. The major benefit of this technique lies in the possibility for the operator to locate and follow the real-time damage evolution of the sample during solar radiation cycling, and to classify the observed mechanisms. [ABSTRACT FROM AUTHOR]

The reflector is the first key component in the energy conversion process of concentrating solar technologies as any solar radiation that is not reflected by the mirror in the direction of the receiver is lost to the system. Due to the outdoor exposure, solar reflectors are subjected to constant soiling deposition (including dust, sand and/or dirtiness), which severely decreases the output power of the solar concentrating collectors, as it reduces the amount of radiation that can be reflected towards the receiver (associated to the absorption and scattering effects of light). Most of the Concentrating Solar Power (CSP) plants are installed in regions of high Direct Normal Irradiance (DNI) with arid and semi-arid climate, involving the existence of an important amount of dust and sand in the ambient. Collector cleanliness represents one of the most costly expenses of the solar-plant operation and maintenance (O&M). Consequently, minimizing this cost is an important issue for the solar-plant economic feasibility. Anti-soiling coatings deposited on the surface of silvered-glass reflectors are a powerful tool that can help in repelling all sort of dust, sand and particles. This technological solution presents the following benefits: (i) the maintenance cost is driven down by reducing washing times, (ii) the water consumption is drastically diminished and (iii) the efficiency is increased since the average reflectance of the solar field remains higher. This work presents the behavior of a new anti-soiling coating with improved durability and high efficiency. [ABSTRACT FROM AUTHOR]

In the process of characterize the attenuation of solar radiation by aerosols between the heliostats and the receiver in a central tower solar plant, it is important to know both the values of the aerosol extinction coefficient and its fluctuations in the lowest atmospheric layers. This objective, in the scope of the PRESOL project, can be achieved through the use of a ceilometer and a scattermeter, taking into account the ceilometer minimum usable height and the appropriate corrections to be made by the presence of water vapor and air molecules. The height of 100 m has been chosen because it is characteristic of the location of the receiver at the central tower plants. The developed methodology is based on combining measurements of a Vaisala CL51 ceilometer, a Biral SWS250 forward scattermeter and meteorological data, and simulations carried out with the SMARTS2 code to evaluate the contribution of the water vapor on the backscattered beam. The methodology has been applied in the case of an intense episode of Saharan dust intrusion that took place from 21nd to 27th February 2017 over large regions of the Iberian Peninsula. It has been possible to observe very high dust precipitation rates, which usually happen at night. As a verification of the type of aerosols, we have used data from the AERONET network at PSA. Results show the time evolution of the water vapor and aerosols extinction coefficients at 910 nm at 100 m height, throughout the episode. [ABSTRACT FROM AUTHOR]

Because of forward scattered radiation in the atmosphere, the circumsolar region closely surrounding the solar disk looks very bright. The radiation coming from this region, the circumsolar radiation, is in large part included in direct normal irradiance (DNI) measurements at the usual 2.5° pyrheliometer opening half angle, but only partially intercepted by the receivers of focusing solar collectors. Therefore, circumsolar radiation measurements are recommended to be included in solar resource assessment. Circumsolar radiation can be characterized using the radial angular distribution of the radiance around the center of the sun - the so-called sunshape. Several sunshape measurement methods have been developed recently. Most approaches use cameras or pyrheliometers with different apertures which require daily maintenance. The Rotating Shadowband Irradiometer (RSI) based method discussed here uses a conventional RSI without any hardware modification to enable sunshape measurements without affecting the RSI's fundamental function as a DNI measurement device. Thus, it allows to measure circumsolar radiation without any additional hardware and with significantly lower maintenance requirements. The presented RSI-based sunshape measurement algorithm has been validated with four RSIs and more than two years of data. After a short description of the method to derive the circumsolar contribution, the validation results are shown. Then the required calibration method is discussed, followed by the results from the measurement campaigns at four sites in Spain, India and Morocco. It was found that no individual calibration is required per instrument and the algorithm can be used for automatic data processing so that common RSI stations can measure the sunshape with comparably low extra effort. Furthermore, we explain how to derive sunshapes from the RSI measurements. [ABSTRACT FROM AUTHOR]

This work presents a new design of a solar particle receiver. Opposite to current technologies, the receiver is placed at the ground level, and thus, no tower is needed. To concentrate the solar radiation on the solar receiver, Fresnel and Beam-down optics are employed. The receiver consists on a multistage fluidized bed in which the gas flows consecutively through the different fluidized beds, which are connected horizontally. The receiver presents modular design, which allows the installation of several units in series configuration to achieve a gradual increase of the gas and particles temperature. As an example of the proposed design, a Fresnel field of heliostats and a beam-down reflector are sized following the common dimensions of Fresnel technology. A well-mixed fluidized bed model is used to estimate the temperatures of gas and particles. The temperature of gas and particles reach T ~ 800 ºC for a reflective Fresnel area of 9072 m², a beam-down mirror placed 18 m above the Fresnel reflectors and total length of Ltotal = 140 m. [ABSTRACT FROM AUTHOR]

Visibility has become a key input to determine the transmission losses of solar radiation propagating between heliostats and the receiver of solar tower power (STP) plants. Recent studies suggest that haze can reduce visibility and increase these losses up to 25% compared to clear conditions. Monitoring visibility would thus be needed for proper design and operation of STPs, but this is usually not done at all potential sites. In this work, the dependence of visibility's magnitude on relative humidity (RH) and aerosol optical depth (AOD) at three different wavelengths is analyzed. To that effect, 1-min observations from a visibilimeter located in Huelva (southwestern Spain) are analyzed during the winter season. RH is linearly correlated with visibility and explains 46% of its variability. A complex non-linear relationship between visibility and AOD is observed with also dependence on RH. Artificial neural networks (ANN) are thus investigated here for mapping the complex and non-linear relationships between visibility, RH and AOD at multiple wavelengths. This improves results significantly, increasing the explained visibility variability up to 68% and reducing RMSD from 30% to 22% with almost zero bias. The ANN analysis shows that the visibility-AOD relationship is not sensitive to the specific wavelength at which AOD is measured. These findings show that, using ANN, visibility can be estimated from local observations of RH and AOD at only a single wavelength. [ABSTRACT FROM AUTHOR]

Fluidized beds may be considered a promising option to collection and storage of thermal energy of solar radiation in Concentrated Solar Power (CSP) systems thanks to their excellent thermal properties in terms of bed-to-wall heat transfer coefficient and thermal diffusivity and to the possibility to operate at much higher temperature. A novel concept of solar receiver for combined heat and power (CHP) generation consisting of a compartmented dense gas fluidized bed has been proposed to effectively accomplish three complementary tasks: collection of incident solar radiation, heat transfer to the working fluid of the thermodynamic cycle and thermal energy storage. A dynamical model of the system laid the basis for optimizing collection of incident radiative power, heat transfer to the steam cycle, storage of energy as sensible heat of bed solids providing the ground for the basic design of a 700kWth demonstration CSP plant. [ABSTRACT FROM AUTHOR]

A transient simulation methodology for cavity receivers for Solar Tower Central Receiver Systems with molten salt as heat transfer fluid is described. Absorbed solar radiation is modeled with ray tracing and a sky discretization approach to reduce computational effort. Solar radiation re-distribution in the cavity as well as thermal radiation exchange are modeled based on view factors, which are also calculated with ray tracing. An analytical approach is used to represent convective heat transfer in the cavity. Heat transfer fluid flow is simulated with a discrete tube model, where the boundary conditions at the outer tube surface mainly depend on inputs from the previously mentioned modeling aspects. A specific focus is put on the integration of optical and thermo-hydraulic models. Furthermore, aiming point and control strategies are described, which are used during the transient performance assessment. Eventually, the developed simulation methodology is used for the optimization of the aperture opening size of a PS10-like reference scenario with cavity receiver and heliostat field. The objective function is based on the cumulative gain of one representative day. Results include optimized aperture opening size, transient receiver characteristics and benefits of the implemented aiming point strategy compared to a single aiming point approach. Future work will include annual simulations, cost assessment and optimization of a larger range of receiver parameters. [ABSTRACT FROM AUTHOR]

Analytical algorithms to compute flux distribution on central receiver are considered as a faster alternative to ray tracing. They have quite too many modifications, with HFLCAL and UNIZAR being the most recognized and verified. In this work, a generalized algorithm is presented which is valid for arbitrary sun shape of radial symmetry. Heliostat mirrors can have a nonrectangular profile, and the effects of shading and blocking, strong defocusing and astigmatism can be taken into account. The algorithm is suitable for parallel computing and can benefit from hardware acceleration of polygon texturing. [ABSTRACT FROM AUTHOR]

A 2-dimensional radiation analysis has been developed to analyse the radiative efficiency of an arrangement of heat transfer tubes distributed in layers but spaced apart to form a tubed, volumetric receiver. Such an arrangement could be suitable for incorporation into a cavity receiver. Much of the benefit of this volumetric approach is gained after using 5 layers although improvements do continue with further layers. The radiation analysis splits each tube into multiple segments in which each segment surface can absorb, reflect and radiate rays depending on its surface temperature. An iterative technique is used to calculate appropriate temperatures depending on the distribution of the net energy absorbed and assuming that the cool heat transfer fluid (molten salt) starts at the front layer and flows back through successive layers to the rear of the cavity. Modelling the finite diameter of each layer of tubes increases the ability of a layer to block radiation scattered at acute angles and this effect is shown to reduce radiation losses by nearly 25% compared to the earlier 1-d analysis. Optimum efficient designs tend to occur when the blockage factor is 0.2 plus the inverse of the number of tube layers. It is beneficial if the distance between successive layers is = 2 times the diameter of individual tubes and in this situation, if the incoming radiation is spread over a range of angles, the performance is insensitive to the degree of any tube positional offset or stagger between layers. [ABSTRACT FROM AUTHOR]

This paper describes the numerical evaluation of a tubular receiver used in a dish Brayton cycle. In previous work considering the use of Computational Fluid Dynamics (CFD) to perform the calculation of the absorbed radiation from the parabolic dish into the cavity as well as the resulting conjugate heat transfer, it was shown that an axi-symmetric model of the dish and receiver absorbing surfaces was useful in reducing the computational cost required for a full 3-D discrete ordinates solution, but concerns remained about its accuracy. To increase the accuracy, the Monte Carlo ray tracer SolTrace is used to perform the calculation of the absorbed radiation profile to be used in the conjugate heat transfer CFD simulation. The paper describes an approach for incorporating a complex geometry like a tubular receiver generated using CFD software into SolTrace. The results illustrate the variation of CFD mesh density that translates into the number of elements in SolTrace as well as the number of rays used in the Monte Carlo approach and their effect on obtaining a resolution-independent solution. The conjugate heat transfer CFD simulation illustrates the effect of applying the SolTrace surface heat flux profile solution as a volumetric heat source to heat up the air inside the tube. Heat losses due to convection and thermal re-radiation are also determined as a function of different tube absorptivities. [ABSTRACT FROM AUTHOR]

In Solar Thermal Electricity (STE) plants the thermal energy of solar radiation is absorbed by solar receiver tubes (HCEs) and it is transferred to a heat transfer fluid. Therefore, heat losses of receiver tubes have a direct influence on STE plants efficiency. A new test bench called HEATREC has been developed by Plataforma Solar de Almería (PSA) in order to determinate the heat losses of receiver tubes under laboratory conditions. The innovation of this test bench consists in the possibility to determine heat losses under controlled vacuum. [ABSTRACT FROM AUTHOR]

Novel designs to increase light trapping and thermal efficiency of concentrating solar receivers at multiple length scales have been conceived and tested. The fractal-like geometries and features are introduced at both macro (meters) and meso (millimeters to centimeters) scales. Advantages include increased solar absorptance, reduced thermal emittance, and increased thermal efficiency. Radial and linear structures at the meso (tube shape and geometry) and macro (total receiver geometry and configuration) scales redirect reflected solar radiation toward the interior of the receiver for increased absorptance. Hotter regions within the interior of the receiver can reduce thermal emittance due to reduced local view factors to the environment, and higher concentration ratios can be employed with similar surface irradiances to reduce the effective optical aperture, footprint, and thermal losses. Coupled optical/fluid/thermal models have been developed to evaluate the performance of these designs relative to conventional designs, and meso-scale tests have been performed. Results show that fractal-like structures and geometries can increase the thermal efficiency by several percentage points at both the meso and macro scales, depending on factors such as intrinsic absorptance. The impact was more pronounced for materials with lower intrinsic solar absorptances (<0.9). The goal of this work is to increase the effective solar absorptance of oxidized substrate materials from ~0.9 to 0.95 or greater using these fractallike geometries without the need for coatings. [ABSTRACT FROM AUTHOR]

A detailed 3-D radiation analysis of a dish-Stirling cavity receiver is carried out to estimate the cavity steadystate temperatures in order to assess the receiver integrity, lifetime and efficiency performance. For this purpose, a parabolic dish was modeled with 5.2 m focal length, 8.85 m aperture diameter and 2 mrad surface error. Three generic cavity shapes (cylindrical, diamond-shaped and reverse-conical) with three different emissivities (0.2, 0.4 and 0.7) are studied. Worst-case scenario heat generations (total absorbed radiation), maximum steady-state temperatures and energy balances of the cavities are calculated to evaluate the receiver performance. The results show that reverse-conical cavities can significantly reduce cavity wall peak temperatures (by 40-120 K), improve the temperature evenness and decrease the radiation losses by 4-5%. Regarding radiation properties, low reflectivities present lower steady-state temperatures even for low/moderate direct solar fluxes. Due to the lower temperatures, lower total thermal losses are also expected. [ABSTRACT FROM AUTHOR]

Typical plant configurations for Central Receiver Systems (CRS) are comprised of a large field of heliostats which concentrate solar irradiation onto the receiver, which is elevated hundreds of meters above the ground. Wind speed changes with altitude above ground, impacting on the receiver thermal efficiency due to variations of the convective heat losses. In addition, the physical properties of air vary at high altitudes to a significant degree, which should be considered in the thermal losses calculation. DNV GL has long-reaching experience in wind energy assessment with reliable methodologies to reduce the uncertainty of the determination of the wind regime. As a part of this study, DNV GL estimates the wind speed at high altitude for different sites using two methods, a detailed estimation applying the best practices used in the wind energy sector based on measurements from various wind sensors and a simplified estimation applying the power law (1, 2) using only one wind measurement and a representative value for the surface roughness. As a result of the study, a comparison of the wind speed estimation considering both methods is presented and the impact on the receiver performance for the evaluated case is estimated. [ABSTRACT FROM AUTHOR]

Volumetric absorbers constitute one of the key elements in order to achieve high thermal conversion efficiencies in concentrating solar power plants. Regardless of the working fluid or thermodynamic cycle employed, design trends towards higher absorber output temperatures are widespread, which lead to the general need of components of high solar absorptance, high conduction within the receiver material, high internal convection, low radiative and convective heat losses and high mechanical durability. In this context, the use of advanced manufacturing techniques, such as selective laser melting, has allowed for the fabrication of intricate geometries that are capable of fulfilling the previous requirements. This paper presents a parametric design and analysis of the optical performance of volumetric absorbers of variable porosity conducted by means of detailed numerical ray tracing simulations. Sections of variable macroscopic porosity along the absorber depth were constructed by the fractal growth of single-cell structures. Measures of performance analyzed include optical reflection losses from the absorber front and rear faces, penetration of radiation inside the absorber volume, and radiation absorption as a function of absorber depth. The effects of engineering design parameters such as absorber length and wall thickness, material reflectance and porosity distribution on the optical performance of absorbers are discussed, and general design guidelines are given. [ABSTRACT FROM AUTHOR]