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Wind-induced tracking deviations of heliostats can be analyzed through numerical simulations or experimental studies on a full-scale heliostat. However, these methods are relatively costly and often, simpler estimations are necessary and sufficient. One simpler approach is to use an analytical model that describes the wind-induced tracking deviations and which requires only a few input parameters that can be determined through straightforward measurements at the full-scale heliostat. Therefore, this paper presents the derivation of an analytical model and describes the development process in a detailed way to clarify the necessary assumptions and simplifications. For verification purposes, the developed model is furthermore applied to measurement data of a field study. It is shown that the results of the model application agree well with the expectations and that the measured heliostat response matches the predicted response well, provided the underlying assumptions of the model fully apply to the investigated heliostat. Overall, no unexplainable inconsistencies are identified and the results support the model well. Thus, a method is provided which allows the estimation and prediction of wind-induced tracking deviations with comparatively little effort. In addition, the developed model helps to identify and analyze those parameters that have the greatest impact on the wind-induced tracking deviations of different types of heliostats. • Estimation of wind-induced tracking deviations of heliostats with little effort. • Only few and easy-to-determine input parameters for analytical model required. • Model developed for T-type and Stellio type heliostats, further applications possible. • Model confirmed through measurement data of a full-scale field study. [ABSTRACT FROM AUTHOR]

Wind loads on heliostats are often investigated through experimental wind tunnel studies or numerical simulations. However, these approaches cannot consider the full-scale conditions but require a reproduction and simulation of both the heliostat structure and the wind conditions. Complementary full-scale investigations are therefore crucial but are at the same time relatively costly, as it is cumbersome to set up large heliostats with measurement devices. This paper introduces a new pressure measurement system which is easy-to-apply to a real-scale heliostat and enables full-scale investigations with minimized effort. The results of a first field study are presented during which wind and pressure measurements were taken simultaneously over periods of 10 min at a 48.5 m2 pentagonal Stellio heliostat. The non-dimensional pressure distributions are investigated and it is shown that the mean c p -values are very consistent amongst the measurement periods and reach values between 2.2 and 2.6. These values, in turn, are found to agree well with results of a wind tunnel study. Moreover, the behavior of the load coefficients with varying angles of attack is studied and the aerodynamic admittance functions are evaluated which give in-depth insight into the generation of aerodynamic loads. The admittance functions are found to start decreasing at a non-dimensional frequency of approximately 0.2. This finding indicates that eddies which are about the size of the heliostat or larger are fully effective in generating aerodynamic loads while the effectiveness of smaller eddies is reduced. • New pressure measurement system developed with maximum sampling frequency of 50 Hz. • Full-scale wind and pressure measurements conducted over several 10-minute periods. • Mean pressure distributions found to reach consistent cp-values between 2.2 and 2.6. • Aerodynamic admittance gives insight into wind load generation. • Eddies smaller than the heliostat are not fully effective in generating loads. [ABSTRACT FROM AUTHOR]

Hybrid concentrated solar thermal power (CSP) and photovoltaic (PV) plants are gaining relevance because they combine their advantages: easy installation and low cost of PV plus dispatchability of CSP. This paper presents results of a techno-economic modelling of this hybrid approach for sites in Jordan, Tunisia and Algeria. Local boundary conditions such as meteorology, cost and electricity demand have been considered to determine the best configurations for these three sites. Different CSP technologies with thermal energy storage have been selected. Hybridization with natural gas has also been included. The optimization is done towards minimizing the LCOE while covering the electrical demand 24/7. Results are presented for different CO2 emissions ranges, as the use of fossil fuel has a strong impact on the LCOE and for environmental reasons, it may be preferred to be kept to a minimum. For most of the cases analyzed, the fraction of energy from PV that leads to minimum LCOE is lower than the energy from CSP. It is shown that for countries with a high fuel price, the use of natural gas reduces the LCOE until a share from this source of about 20%. A higher integration of fossil fuel for sites rich in solar irradiation is considered not advantageous if the price of natural gas is above EUR 40/MWh. [ABSTRACT FROM AUTHOR]

Wind loads may decrease a heliostat's optical performance by affecting its shape and orientation. As small scale wind tunnel or numerical models cannot reflect a heliostat's dynamic behavior entirely, in this study we investigated a real scale Stellio heliostat installed on the DLR heliostat testing platform where it was exposed to the natural wind. The wind-induced response was captured with a dynamic photogrammetry system that combines a high spatial and high temporal resolution. As its application to a heliostat is novel, the first focus of this paper is to validate the applied setup and the accuracy of measured displacements. The second focus is to discuss the wind-induced Stellio behavior. A method is presented which allows for separating the total wind-induced behavior into a tracking- and a slope-relevant part. Based on this separation, the wind-induced tracking deviation (0.44mradRMS) during the investigated measurement period (mean wind speed ≈ 4.8m/s, mean turbulence intensity ≈ 26%) reached a level of approximately one third of a heliostat's typical total tracking deviation. Likewise, during the time step of largest deformations, the wind-induced slope deviation (0.75mradRMS) of the most affected facet reached a level of approximately one third of a Stellio facet's total slope deviation. However, wind-induced slope deviations occurred only locally and temporally. Furthermore, wind-excited eigenfrequencies were revealed to have a negligible impact on both the tracking and slope deviation in case of the Stellio. Oscillations and deformations related to frequencies below the eigenfrequencies were rather found to have a predominant impact on the optical performance. • Dynamic photogrammetry: highly resolved insights into heliostat dynamics. • Dynamic photogrammetry setup and accuracy of displacement measurements validated. • Separation of tracking- and mirror slope-relevant heliostat behavior. • Level of wind-induced tracking deviation reaches one third of total tracking deviation. • Level of wind-induced slope deviation reaches one third of total slope deviation. [ABSTRACT FROM AUTHOR]

Large scale central receiver systems typically deploy between thousands to more than a hundred thousand heliostats. During solar operation, each heliostat is aligned individually in such a way that the overall surface normal bisects the angle between the sun's position and the aim point coordinate on the receiver. Due to various tracking error sources, achieving accurate alignment ≤1 mrad for all the heliostats with respect to the aim points on the receiver without a calibration system can be regarded as unrealistic. Therefore, a calibration system is necessary not only to improve the aiming accuracy for achieving desired flux distributions but also to reduce or eliminate spillage. An overview of current larger-scale central receiver systems (CRS), tracking error sources and the basic requirements of an ideal calibration system is presented. Leading up to the main topic, a description of general and specific terms on the topics heliostat calibration and tracking control clarifies the terminology used in this work. Various figures illustrate the signal flows along various typical components as well as the corresponding monitoring or measuring devices that indicate or measure along the signal (or effect) chain. The numerous calibration systems are described in detail and classified in groups. Two tables allow the juxtaposition of the calibration methods for a better comparison. In an assessment, the advantages and disadvantages of individual calibration methods are presented. [ABSTRACT FROM AUTHOR]

The air return ratio is a key factor for the overall efficiency of the open volumetric receiver concept. Although first measurements of the air return ratio exist for smaller setups of the open volumetric receiver concept, so far no measurement of the air return ratio has been presented for the Solar Thermal Test and Demonstration Power Plant Jülich ( ≈ 1.5 MW, ≈10 (kg air)/s; ≈700 °C). This paper describes the application of a tracer gas method at the solar tower Jülich to determine this substantial ARR. As tracer gas the environmentally friendly helium has been chosen. The helium is injected dynamically into the circular air flow of the system and the helium mole fraction is measured using a mass spectrometer. The dynamic concentration response of the system is used to determine the air return ratio. This dynamic method only requires one location of measurement. First measurements with this dynamic method were conducted at the solar tower Jülich. The ARR of STJ was measured with and without irradiation of the main receiver with high accuracy. Under low-wind conditions and without irradiation of the main receiver the air return ratio was measured to be ( 67.7 ± 0.5 ) % for an air mass flow of ( 9.96 ± 0.04 ) kg / s . A slightly higher air return ratio of ( 68.6 ± 0.7 ) % was measured under irradiation with an air mass flow of ( 9.94 ± 0.04 ) kg / s . The air return ratio was sensitive to the air mass flow, showing significantly lower rates when moving further away from the 10 kg / s design air mass flow to 5 kg / s . [ABSTRACT FROM AUTHOR]

The ferrocene moiety with its well-defined redox behavior and its substitution potential is ideally suited for the assembly of multinuclear complexes. Utilizing the 'click' strategy, we present here mono- and disubstituted pyridyl-triazolyl-ferrocene units. The 'pyridyl-triazolyl' chelating pocket is then used to chelate to copper(I) centers that additionally contain dppf [dppf = 1,1′-(diphenylphosphino)ferrocene], which has an additional ferrocene moiety. This strategy delivers a heterotrinuclear (Fe-Cu-Fe) and a heteropentanuclear (Fe-Cu-Fe-Cu-Fe) complex. All complexes were characterized by 1H and 13C NMR spectroscopy and by mass spectrometry, and the heteropentanuclear complex was characterized by single-crystal X-ray diffraction. Electrochemistry and UV/Vis/NIR spectroelectrochemistry were used to investigate the redox properties of the complexes and to determine the site of electron transfer. The results presented here show the potential of 'click'-derived ligands in generating heteromultinuclear assemblies and the potential of ferrocene to function as a rigid, redox-active unit in these assemblies. [ABSTRACT FROM AUTHOR]

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 (≈10 kg/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 m ̇ in = ( 0.247 ± 0.008 ) / kg s was measured to be l stat = ( 36.1 ± 2.3 ) % and l dyn = ( 34.5 ± 3.6 ) % with the static and the dynamic method respectively. [ABSTRACT FROM AUTHOR]

Flux density measurement applied to central receiver systems delivers the spatial distribution of the concentrated solar radiation on the receiver aperture, measures receiver input power, and monitors and might control heliostat aimpoints. Commercial solar tower plants have much larger aperture suifaces than the receiver prototypes tested in earlier research and development (R&D) projects. Existing methods to measure the solar flux density in the receiver aperture face new challenges regarding the receiver size. Also, the requirements regarding costs, accuracy, spatial resolution, and measuring speed are different. This paper summarizes existent concepts, presents recent research results for techniques that can be applied to large-scale receivers and assesses them against a catalog of requirements. Direct and indirect moving bar techniques offer high measurement accuracy, but also have the disadvantage of large moving parts on a solar tower. In the case of external receivers, measuring directly on receiver suifaces avoids moving parts and allows continuous measurement but may be not as precise. This promising technique requires proper scientific evaluation due to specific reflectance properties of current receiver materials. Measurement-supported simulation techniques can also he applied to cavity receivers without installing moving parts. They have reasonable uncertainties under ideal conditions and require comparatively low effort. [ABSTRACT FROM AUTHOR]

Solar thermal energy generation needs receiver technologies which can drive highly efficient turbines and decouple the collection of energy from its use by an economic storage technology. High-temperature solid particle receivers for solar tower systems with particle storage are one option. Important issues regarding high-temperature particle receivers are minimization of convective losses, no particle loss due to susceptibility to wind, and high efficiency also in part-load operation. A design approach facing these challenges is the face-down receiver using recirculation of particles. A screening performance analysis studying different recirculation patterns is presented. Using smart recirculation schemes, high receiver efficiencies can be maintained also at part-load operation (100% load ∼90%; 50% load ∼86%; 20% load ∼67%). Simulations of the face-down geometry yield total annual solar-to-electric efficiencies of 24% using a surround field. From the analyses, it can be concluded that solid particle receivers using smart recirculation patterns are a viable receiver option for storage and high-temperature high-efficiency turbine processes. [ABSTRACT FROM AUTHOR]

Photoelectrochemical water splitting is a promising pathway for solar-driven hydrogen production with a low environmental footprint. The utilization of solar concentrators to supply such water splitting devices with concentrated solar irradiation offers great potential to enhance the economic viability of water splitting at "sunny" site locations. In this work, we defined a set of functional requirements for solar concentrators to assess their suitability to power such water splitting devices, taking into account concentrator optical performance, device coupling efficiency, perceived system complexity, as well as technological costs and risks. We identified, classified and compared a broad range of existing solar concentrator design concepts. Our geometrical analysis, performed on a yearly basis with a one-minute time step, shows that two-axis tracking concentrators with water splitting devices positioned parallel to the optical aperture plane exhibit the highest potential, given the initial conditions applied for the device tilt constraints. Demanding an angle of at least 20° between horizontal and the front side of the water splitting device, allows the device to be operational for 97% of the daylight time in Seville, Spain. The relative loss with respect to the available direct normal irradiance is estimated to 6%. Results moderately depend on the location of application, but generally confirm that the consideration of tilt angle constraints is essential for a comprehensive performance assessment of photoelectrochemical water splitting driven by concentrated sunlight. [ABSTRACT FROM AUTHOR]

• Flow calorimeter field measurement at real-scale parabolic trough at 330 °C. • Uncertainty analysis for cp (~±2.6% @330 °C) revealing influence of all measurands. • Validation results do not deviate more than 0.1% from water reference. • Flow calorimeter and lab results deviate uniformly from HTF manufacturer data. • Detailed descriptions of several technical improvements. Performance tests of concentrating solar collectors are based on energy balance studies which require accurate measurements of fluid mass flow rate, temperature difference over the collector length and the knowledge of the specific heat capacity of the used heat transfer fluid (HTF). Especially at operation temperatures in the range of 200–400 °C, the accuracy of HTF manufacturer data regarding the specific heat capacity of used HTFs is insufficient. This shortcoming contributes significantly to the overall measurement uncertainty of thermal collector assessments. In the present paper, the measurement results of a flow through calorimeter measuring the specific heat capacity of SYLTHERM 800 under field conditions at temperatures up to 330 °C are presented. The HTF measurement shows good agreement (deviation below 1.2%) with a Differential Scanning Calorimetry (DSC) measurement for temperatures below 270 °C. For higher temperatures, the deviation between flow through calorimeter and DSC measurement increases to 3.7% at 330 °C. Furthermore, several technical design and measurement improvements over the calorimeter design presented in 2012 are described in detail. A validation measurement using water reveals deviations below 0.1% from the water reference values at ambient temperatures. A stringent uncertainty analysis reveals the largest individual measurement uncertainty. The unexpectedly large uncertainty of the Coriolis mass flow sensors at temperatures above 270 °C is distinguished by a large individual uncertainty and the comparison to DSC. [ABSTRACT FROM AUTHOR]

• Review of the literature of the forced convective heat transfer of molten salts in enhanced tubes. • Measurement of the forced convective heat transfer of Solar Salt in a spirally grooved tube for 11000 < R e < 285000. • Comparison of the measured friction factor in a spirally grooved and a smooth tube to correlations from the literature. • Measurement of the Nusselt number at film temperatures above the chemical stability temperature limit of Solar Salt. The forced convective heat transfer and pressure difference of molten nitrate salt in a spirally grooved tube with a relative groove height e/2r i =0.017, a relative groove pitch p/2r i =0.913 and groove angle α=73.8° is experimentally investigated using a water-cooled induction heater setup. The data is provided for Reynolds numbers ranging between 11000 up to 285000 and Prandtl numbers from 3.7 to 10. The experiments are carried out for bulk salt temperatures ranging between 300 and 550 °C and for heating flux densities between 330 and 930 kW/m². Inner wall temperatures up to 633 °C are achieved with the setup, thus providing data on the behavior of the forced convective heat transfer as a function of inner wall temperature. The data is compared to the measurement results obtained with the same setup and a circular smooth tube as well as correlations and data from the literature in order to deduce relative Nusselt numbers and friction factors. [ABSTRACT FROM AUTHOR]

Advanced cleaning strategies for parabolic trough collectors at concentrated solar power plants maximize the yield and minimize the costs for cleaning activities. However, they require information about the current soiling level of each collector. In this work, a novel, data-driven method for soiling estimation with machine learning for parabolic trough collectors is developed using gloss values as a surrogate for soiling values. Operational data and meteorological data from the solar field Andasol-3 with changing time horizons are used together with various Machine Learning techniques to estimate the soiling of every collector in the field. The best results were achieved with a Decision Tree model, with a coefficient of determination of R 2 = 0. 77 from the maximum value of 1 and a mean squared error of M S E = 6. 14 for the determination of specific soiling values. A second metric to evaluate the quality of soiling predictions from the models classifies whether soiling is above or below a cleaning threshold was also investigated. Model results are compared to soiling measurements that indicate the need for cleanings. Cleaning recommendations are derived and compared with the current fixed-time cleaning schedule of Andasol-3. All models show an improvement over the cleaning schedule currently in use. The use of a Decision Tree model increases the detected necessary cleanings by 12.2 %, while the number of unnecessary cleanings are reduced by 14.3 %. This has the potential to reduce operational costs and increase the solar field yield. The dataset used in this work is made publicly available https://doi.org/10.5281/zenodo.7061913 , along with the code to reproduce all results, which can be found at https://doi.org/10.5281/zenodo.7554806. • Data-driven soiling model for parabolic troughs with process and meteo data. • Comparison of various machine learning models and dataset sizes. • Evaluation of model for soiling prediction and cleaning schedule optimization. • Investigation of the most important model inputs via permutation feature importance. • Code and dataset available at (https://doi.org/10.5281/zenodo.7554806 , https://doi.org/10.5281/zenodo.7061913). [ABSTRACT FROM AUTHOR]

Strong efforts are being made to drive heliostat cost down. These efforts are summarised to give an update on heliostat technology comprising: determination of wind loads, heliostat dimensioning, solutions for the different sub-functions of a heliostat, a review of commercially available and prototype heliostat designs, canting, manufacturing, qualification, heliostat field layout, and mirror cleaning. There is evidence that commercial heliostat costs have dropped significantly in the past few years, with commercial suppliers of heliostat technologies now claiming heliostat field costs around 100 USD/m 2 . With new approaches even target cost of 75$/m 2 seem to be realistic. [ABSTRACT FROM AUTHOR]

• A novel experimental approach using an induction heater for the evaluation of the forced convective heat transfer in tubes is proposed and validated • The forced convective heat transfer of a molten nitrate salt (Solar Salt) in a circular tube is deduced experimentally for Reynolds numbers up to 220000 and for film temperatures up to 633°C • A review of the literature of the thermophysical properties of Solar and the forced convective heat transfer of molten salts in circular smooth tubes is provided The forced convective heat transfer of molten nitrate salt in a circular smooth tube is experimentally investigated using a water-cooled induction heater. This novel setup is first validated using water as the main fluid, as its heat transfer is well-known. To enable an accurate deduction of the Nusselt number, a detailed review on the state-of-the-art of the thermophysical properties of Solar Salt is provided. The final experiments are carried out with "Solar Salt" at fluid bulk temperature ranging between 300°C and 550°C. The salt mass flow is varied in order to cover Reynolds numbers ranging from 14000 up to 220000. Furthermore, the experiments are conducted for different heat fluxes ranging from 330 kW/m² up to 920 kW/m². This paper provides data on the behavior of the forced convective heat transfer of molten salt for Reynolds numbers larger than 100000. To investigate the impact of local overheating of the salt above its chemical stability limit (i.e. 600°C), the mean Nusselt number is evaluated for inner wall temperatures up to 633°C. Mean Nusselt numbers are reported and compared to the well-known Gnielinski correlation. [ABSTRACT FROM AUTHOR]

This paper describes a transient thermography method to measure the heat loss of parabolic trough receivers and separate their heat loss mechanisms. This method is complementary to existing stationary techniques, which use either energy balances or glass envelope temperature measurements to derive overall heat losses. It is shown that the receiver heat loss can be calculated by applying a thermal excitation on the absorber tube and measuring both absorber tube and glass envelope temperature signals. Additionally, the emittance of the absorber selective coating and the vacuum quality of the annulus can be derived. The benefits and the limits of the transient method are presented and compared to the established stationary method based on glass envelope temperature measurements. Simulation studies and first validation experiments are described. A simulation based uncertainty analysis indicates that an uncertainty level of approximately 5% could be achieved on heat loss measurements for the transient method introduced in this paper, whereas for a conventional stationary field measurement technique, the uncertainty is estimated to 17-19%. [ABSTRACT FROM AUTHOR]

Current measurement techniques do not allow the visualization of the return air flow of open volumetric receivers in solar tower power plants. The reason is that the region of interest is irradiated by concentrated solar radiation and is located on top of a tower. Therefore, a novel method of measurement, the Induced Infrared Thermography (IIT) is introduced within this paper. With this method the return air can easily be observed with an infrared camera. As air has a very low emissivity in the infrared region the activity has to be induced by the addition of an infrared-active component, here carbon dioxide. The temperature of the infrared-active component has the greatest influence on the signal strength but the mole fraction of the component and the distance to the infrared camera are also important. Due to temperature restrictions, the measured signal to noise ratio is low and therefore several post-processing steps have to be conducted to visualize the return air. The most important step for the visualization is subtracting a background image. Furthermore, a video filter is employed in order to remove noise. A time series of ITT images can be used to obtain information on the velocity fields of the flow. The process, called Infrared Image Velocimetry (IRIV) here, is similar to Particle Image Velocimetry (PIV) and is applied in this paper to external fluid flows of the open volumetric receiver. As IRIV is in an early stage of development the depicted results are treated as qualitative vector fields. [ABSTRACT FROM AUTHOR]

This paper describes a method for the efficiency determination of a cavity receiver using the example of the solar hybrid gas turbine system SOLUGAS. Major focus is given on the improvement of a new approach of the solar flux density determination based on a measurement-supported simulation technique where an acceptable uncertainty of the solar input power of −1.3%…+6.3% is achieved. For the thermal evaluation an uncertainty of 2.4% is determined that leads to an overall uncertainty of the thermal receiver efficiency of −2.8%…+7.7%. Detailed uncertainty propagation is presented and conclusions discussed. [ABSTRACT FROM AUTHOR]

In concentrating solar power, high-temperature solar receivers can provide heat to highly efficient cycles for electricity or chemical production. Excessive heating of the fused-silica window and the resulting recrystallization are major problems of high-temperature receivers using windows. Excessive window temperatures can be avoided by applying an infrared-reflective solar-transparent coating on the fused-silica window inside. Both glass temperatures and receiver losses can be reduced. An ideal coating reflects part of the thermal spectrum (λ2.5 μm) of the hot absorber (1100°C) back onto it without reducing solar transmittance. Extensive radiation simulations were done to screen different filter types. The examined transparent conductive oxides involve a high solar absorptance, inhibiting their use in high-concentration solar systems. Although conventional dielectric interference filters have a low solar absorption, the reflection of solar radiation, which comes from various directions, is too high. It was found that only rugate filters fulfill the requirements for operation under high-flux solar radiation with different incident angles. A thermodynamic qualification simulation of the rugate coating on a window of a flat-plate receiver showed a reduction of almost 175 K in mean window temperature and 11% in receiver losses compared with an uncoated window. For the configuration of a pressurized receiver (REFOS type), the temperature could be reduced by 65 K with slightly reduced receiver losses. Finally, a 25 μm thick rugate filter was manufactured and optically characterized. The measured spectra fitted approximately the design spectra, except for two absorption peaks, which can be avoided in future depositions by changing the deposition geometry and by using in situ monitoring. The issue of this paper is to share the work done on the choice of filter type, filter design, thermodynamic evaluation, and deposition experiments. [ABSTRACT FROM AUTHOR]

The construction of solar thermal power plants with several thousand m² of collector area requires quality control measures for components, subsystems, and the entire collector rows. While quality control has a significant potential to increase the solar field efficiency, the main objective is to assure high-quality standards for the whole solar field. Quality control, assembly documentation, and performance measurements are required by the investors. Based on previous R&D work in collector development and prototype qualification, measurement systems have been developed for use in solar field construction and operation supervision. In particular, close-range photogrammetry can be used to measure the geometry of collector steel structures. The measurement system consists of a digital camera, which moves around the structure automatically while shooting photos of the concentrator structure from various positions. The photos are evaluated with photogrammetry software to check the assembly quality. The whole measurement and evaluation procedure is computer controlled and is fast enough to be integrated in a solar collector production line. This paper deals with the required measurement accuracy and shows ways to reach, maintain, and control this accuracy in the rough environment of an on-site production line. [ABSTRACT FROM AUTHOR]

Secondary concentrators are used in solar concentrating systems to redirect solar beams reflected by the primary concentrators to the focal point or line. These components allow to increase the concentrated solar flux density and hence to lower thermal radiation losses. Solar reflectors for secondary concentrators are permanently exposed to environmental conditions, high radiation fluxes and elevated temperatures that potentially cause stress and degradation throughout the time. Therefore, analyzing solar reflectors of secondary concentrators by simulating these conditions is crucial. No previous research works about the durability of solar reflector materials for secondary concentrators have been reported. The present work is focused on studying the degradation of the reflector materials by simulating accelerated aging, caused by several ambient parameters and the effect of concentrated radiation. Both cooled and uncooled systems for secondary concentrators are included in this study. According to results obtained, aluminum reflectors and thin silvered-glass reflectors glued to an aluminum structure showed minimum reflectance losses and structural degradation under the operation conditions of cooled 3D secondary concentrators (tower systems). Following critical aspects to avoid reflector degradation were identified: to select a suitable adhesive material to glue the thin silvered-glass reflector to the support aluminum structure, to properly protect reflectors edges, to design a suitable cooling system and to avoid the combination of high radiation fluxes with mechanical stress. In addition, laminated silvered-glass reflectors have shown to be suitable for uncooled 2D secondary concentrators (Fresnel collectors). Furthermore, a comparison with naturally aged secondary concentrators using silvered-glass reflectors glued to an aluminum structure revealed that the simulated degradation under accelerated conditions performed in this work did reproduce the most frequent degradation patterns suffered in real operating conditions. [ABSTRACT FROM AUTHOR]

The presented review gives reliable information about the currently used measurement instrumentation in parabolic trough fields and recent monitoring approaches. The usually built-in measurement equipment in the solar field, clamp-on systems for flexible measurements of temperature and flow, solar irradiance measurements, standard meteorological equipment, laboratory devices for heat transfer fluid analyses and instruments related to the tracking of solar collector assemblies are presented in detail. The measurement systems are reported with their measurement uncertainty, approximate costs and usual installation location for the built-in instrumentation. Specific findings related to the installation and operation of the measurement devices are presented. The usually installed instrumentation delivers a lot of measurements all over the field at the expense of measurement accuracy, compared to special test facility equipment. Recently introduced measurement approaches can improve the standard instrumentation in terms of accuracy, frequency, spatial distribution or can even extend the amount of measurands. The information about available measurands is the basis for future operation and maintenance solutions based on data-driven approaches. [ABSTRACT FROM AUTHOR]

An intercomparison of opto-thermal spectral measurements has been performed for some relevant receiver materials in concentrating solar thermal applications, from room temperature up to 800°C. Five European laboratories performed spectral measurements at room temperature, while two laboratories performed infrared spectral measurements at operating temperature up to 800 °C. Relevant materials include Haynes 230 (oxidized, Pyromark 2500 and industrial black coating) and silicon carbide. Two key figures of merit were analyzed: i) solar absorptance α sol at room temperature, over the spectral range [0.3 – 2.5] μm, ii) thermal emittance ε th (T), over the common spectral range [2-14] μm, derived from spectral measurements performed from room temperature up to 800 °C. Oxidized H230 reached an α sol value of 90.9±1.0%. Pyromark 2500 reached an α sol value of 96.3±0.5%, while the industrial black coating achieved an α sol value of 97.0±0.4%. Silicon carbide reached an α sol value of 93.5±1.1%. Low standard deviations in α sol indicate reproducible measurements at room temperature. For oxidized H230, the ε th,calc (T) value varied from 55% at room temperature up to 81% at 800 °C. For Pyromark 2500 and the industrial black coating, ε th,calc (T) fluctuated between 90% and 95%, with a weak temperature dependence. For silicon carbide, ε th,calc (T) varied from 70% at room temperature up to 86% at 800 °C. The typical standard deviation among participating laboratories is about 3%. ε th,meas (T) values derived from spectral measurements at operating temperature were consistent within a few percentage points in comparison to ε th,calc (T) values derived from spectral measurements at room temperature. • Spectral measurements of receiver materials for Concentrating Solar Power are performed from room temperature up to 800 °C. • Material substrates include H230 and SiC. Three different surface finishes are investigated for H230 (oxide, black coatings). • Solar absorptance is analyzed at room temperature, while thermal emittance is analyzed from room temperature up to 800°C. [ABSTRACT FROM AUTHOR]

Solar reflectors for Concentrating Solar Technologies require a high reflectance in the terrestrial solar spectrum (280–4000 nm). Besides the wavelength, the reflectance of solar mirror materials is also dependent on the incidence angle of the incoming sunlight. The commonly used measurement equipment measures the reflectance at fixed near-normal incidence angles, typically between 8° and 15°. In this work, the annual incidence angle frequency distribution has been calculated for a LS3/Eurotrough-type parabolic-trough collector located at different sites, and for the heliostat field of the solar tower system CESA-1 located at the Plataforma Solar de Almería in Tabernas, southern Spain. It was found that the most frequent incidence angles registered in the solar field are quite higher than the ones at which reflectance is measured with state of the art instruments, obtaining mean incidence angles in the range of 28–35° depending on the type and location of the solar field. A methodology to predict the off-normal reflectance of silvered-glass mirrors based on near-normal reflectance and transmittance measurements of the uncoated glass is presented. The complex refractive index of 2, 4 and 5 mm thick solar glass and the deposited silver was determined and used to model the solar weighted reflectance of silvered-glass mirrors at different incidence angles. The model was compared to experimental measurements. For this purpose, the Spectral Specular Reflectometer (S2R) has been improved and updated with a polarizer crystal to measure reflectance at perpendicular (s-pol) and parallel (p-pol) polarizations up to incidence angles of θ = 70°. Eight solar mirror materials (three silvered-glass mirrors of different glass thicknesses, two anti-soiling coated glass mirrors, two enhanced aluminum reflectors and a silvered polymer film) have been measured over a broad range of incidence angles and the results have been weighted with the annual incidence angle frequency distribution. The obtained incidence angle-weighted reflectance is a suited parameter to compare the efficiency of solar mirror materials taking into account their use in a specific collector type and location. [ABSTRACT FROM AUTHOR]

Solar thermal absorber coatings play an important role in the opto-thermal efficiency of receivers in Concentrated Solar Power (CSP). Two standard figures of merit are the solar absorptance α sol and thermal emittance ε th , derived from spectral directional hemispherical reflectance measurements at room temperature. These two figures of merit allow comparing coating formulations in terms of performance and durability. In this study, a black coating and a solar selective coating are optically characterized by different laboratories to compare spectral datasets, solar absorptance α sol and thermal emittance ε th calculations. The comparison includes various benchtop spectrophotometers operating in the UV-VIS-NIR and Infrared spectral ranges as well as three commercial portable reflectometers/emissometers. A good agreement is found between the nine parties participating in this intercomparison campaign. The black coating α sol value is 96.6 ± 0.2%, while the solar selective coating α sol value is 94.5 ± 0.4%. For the thermal emittance, spectral data is concatenated and integrated from 0.3 to 16 μm. The black coating ε th value calculated at 650 °C is 80.8 ± 3.8%, while the solar selective coating ε th value calculated at 650 °C is 25.0 ± 0.5%. • Spectral measurements of absorber coatings are compared at room temperature. • Spectral mismatch from 1.5 to 2.5 μm is analyzed for spectrophotometers. • Solar absorptance and thermal emittance calculations are homogenized. • The uncertainty propagation on the coating opto-thermal efficiency is evaluated. [ABSTRACT FROM AUTHOR]

An advanced computational fluid dynamics (CFD) model was developed which allows detailed analysis of a direct absorption falling particle receiver with horizontal aperture (face down) for a solar tower plant. The CFD model includes all relevant effects: movement of the particle curtain and the air, solar radiation, thermal radiation transfer, mechanical and thermal interactions between the particles, the air and the walls and conduction through the walls. These are spatially resolved in three dimensions for the complete receiver including the surrounding air. The coupled equations for thermal radiation, conduction and convective heat transfer are solved iteratively. First results are compared with previous simulations with a simple Matlab model and the differences due to the improvements in the simulations are discussed. The receiver efficiency was determined to be 83% at the design point (400 MW solar input) using a single drop (no recirculation) to heat the particles from 300 °C to 800 °C. In contrast, by separating the circumferential particle curtain into 4 parts and recirculating the particles 3 times through all the sections in series the receiver efficiency increases to more than 92%. This is due to lower reflection losses from the higher particle mass flow and therefore higher opacity of the particle curtain. Wind effects have been studied for the cylindrical face down receiver for the 100% load case with only one recirculation. For 15 m/s horizontal wind speed the receiver efficiency reduces from 89% to 84%. [ABSTRACT FROM AUTHOR]

Solar thermal absorber coatings play a key role in the thermal efficiency of receivers for applications in the field of Concentrated Solar Power (CSP). The development of stable spectral selective coatings with a high solar absorptance α sol and a low thermal emittance ε th is often desired to reduce thermal losses. However, quantitative indicators describing selectivity and the trade-off between solar absorptance and thermal emittance is seldom discussed in the literature. In this review, relevant opto-thermal figures of merit are analyzed for the comparison of reference solar thermal absorber coatings, including real and ideal coatings, both black and spectral selective. The comparison is made for a temperature ranging from 25 to 1000 °C and for a concentration factor ranging from 20 to 1000, based on spectral data measured at room temperature from 0.25 to 20 μm. New figures of merit are introduced, i.e. a normalized selectivity ratio Si*, a trade-off factor Z trade-off , a normalized solar reflectance index SRI* and a peak efficiency temperature T peak,opt. These metrics are derived from existing figures of merit and adapted for CSP. The set of figures of merit analyzed in this review offer a complementary perspective for the detailed characterization of any coating opto-thermal performance. For solar thermal absorber coatings, thermal efficiency η thermal and peak efficiency temperature T peak,opt are respectively deemed more insightful than opto-thermal efficiency η opt-th and maximum steady-state temperature T SST,max , when comparing the relative opto-thermal performance of two coating formulations. • Opto-thermal figures of merit for solar thermal absorber coatings are analyzed. • Black and spectral selective reference coatings are chosen for the comparison. • Selectivity ratio and solar reflectance index are normalized for high temperature. • A trade-off factor between solar absorptance and thermal emittance is defined. [ABSTRACT FROM AUTHOR]

Abstract: As the optical efficiency of the solar field has a high impact on the overall performance and economics of a solar thermal power plant, qualification methods determining the geometrical accuracy of solar concentrator systems have gained high importance. However, it has not been possible yet to measure the geometrical accuracy of larger fractions of the solar field, resolving the relevant single characteristics like local mirror slope deviations, panel alignment and gravitational deformation. This paper describes the development and assessment of a measurement technique for the qualification of parabolic trough collector modules based on the distant observer method called TARMES (Trough Absorber Reflection Measurement System). Instead of a stationary camera at ground level taking pictures of a turning collector, the new approach called QFly makes use of an airborne camera vehicle which allows a completely automated and fast measurement of large numbers of collectors under relevant operating conditions. The new approach was validated against a stationary TARMES and photogrammetric measurement, supplemented by an extended uncertainty analysis. This analysis includes an assessment of the uncertainties of input parameters and their influence on measured local slope deviations. By applying a Monte Carlo approach, the effects on the RMS values of the local slope deviations of mirror panels and collector modules were investigated. Furthermore it includes an evaluation of the influence of the measurement sample rate. The results suggest an absolute measurement uncertainty for the RMS of slope deviations at module level of about ±0.1milliradians. [Copyright &y& Elsevier]