Micro-concentrating photovoltaic (micro-CPV) technology has the potential to contribute to the energy transition, facilitating the shift toward more sustainable and renewable energy sources by combining minimal carbon footprint and energy demand with low levelized cost of electricity. Micro-CPV modules utilize direct normal irradiance to convert sunlight into electrical power, necessitating precise solar tracking. The performance of these modules is influenced by their alignment toward the sun and prevailing outdoor conditions during outdoor operation. The spectral conditions, along with the ambient temperature, irradiance, and wind speed, influence the current-voltage characteristics of multijunction solar cells and the optical behavior of the lens. We have developed a novel micro-CPV module concept, which is based on low-cost and high-throughput manufacturing processes. In this work, we present a prototype module in a 10 x 6 array configuration (205-cm(2) aperture area, submodule class). We discuss outdoor measurements recorded over one year and the influences of various outdoor conditions. In an IEC62670-3 power rating, efficiencies of 36.0 +/- 0.4% and 33.0 +/- 0.4% at concentrator standard test conditions and concentrator standard operating conditions, respectively, are determined. Highest efficiencies, about 0.4% higher than at standard conditions, were attained at a more red-rich spectrum, namely at a spectral matching ratio SMR12 of 0.94 +/- 0.03. Using measurements at different temperatures, we show that the planoconvex silicone-on-glass primary lens has a negligible temperature dependence. Changes in the module performance over the course of one year are discussed. Despite employing commercially available low-cost components and high-throughput processes, no significant degradation was observed during the first year of operation.

In micro concentrating photovoltaics (CPV) minimized components are used and promise significant cost reduction due to massive manufacturing parallelization. At Fraunhofer ISE, we have developed a micro-CPV module based on a circuit board on glass, 5-junction micro concentrator solar cells, spherical ball lenses and a silicone-on-glass primary lens optics. Various prototypes from single cell-lens units to panel-sized 24“×18” modules were fabricated and tested. Due to the small size of the components, process tolerances are of special relevance and can significantly affect module performance. We present a novel modeling approach to study this influence quantitatively based on a nested Monte Carlo optical and electrical simulation. Furthermore, we present experimental results of prototype modules measured outdoors. For a 60-cell submodule-class prototype a power rating according to IEC 62670–3 was performed and CSTC and CSOC efficiencies of 36.5% and 33.0% are demonstrated.

With miniaturization of dimensions, new manufacturing methods e.g. from microelectronics may lead to cost reduction of CPV. In this work, we focus on the thermal, electrical and optical boundaries of the micro-CPV module technology with respect to cell and lens miniaturization. Open-circuit voltage losses due to recombination at the solar cell perimeter and dark area as well as high operating temperature are investigated. The smaller the cell the higher the losses due to increased perimeter-to-area ratio. On the other hand, lower optical power per cell leads to lower temperatures and thus increased voltage. Based on the electrical and thermal simulations, an optical configuration for a two-stage optics with piano-convex primary optics and a glass sphere as secondary optical element is presented. Besides optical efficiency, also acceptance angle and flux distribution are considered. Simplified simulations where only two wavelengths are considered compare well with detailed full spectrum simulations. We identify two regions of high optical efficiency. Detailed simulations showed that only in one of the regions, the flux distribution on the solar cell is acceptable. The results are the basis for a module configuration that will be realized as a prototype in future work.

An imaging method for the parallel in-situ measurement of heliostat aim points during operation in central receiver systems is assessed. In a SFERAII-access campaign, the HelioControl prototype system was integrated into the heliostat control system Hyperviseur at the Themis facility. The method, determining aim points from the cumulative flux distribution at the receiver, uses a signal of periodic movement, modulated by means of the heliostat drives. The functionality was implemented into a group of heliostats and practically assessed. For few heliostats, exemplary parallel aim point measurements at a flux target - using the methodology - show very small differences compared to a reference. Further experiments with multiple overlapping focal spots have been conducted using the small experimental cavity receiver of the system. The highly variable reflectance of the receiver area is challenging for the imaging method evaluating the reflection of the incident flux. A simplified receiver model obtained from experiments with single heliostats, however, allows for a correction and preliminary evaluation. The promising results obtained from measurements with deviations below 0.7 mrad in exemplary evaluations display the potential of the method and motivate for further and comprehensive validation in the context of commercial scale central receiver systems.

The receiver at the optical-to-thermo-hydraulic interface of a Solar Tower plant needs careful optimization in the design stage to guarantee maximum yield during operation. Due to the highly transient behavior of a Solar Tower plant, evaluation of the receiver performance based on annual yield as a figure of merit is important. In this study, a novel method for the optimization of Solar Tower molten salt cavity receivers is introduced, which integrates optical, thermal, hydraulic and operational aspects. The receiver geometry, the hydraulic layout and the aiming strategy are optimized simultaneously. The optimization objective function is based on a validated simulation model that integrates a sky discretization approach for optical assessment and an Artificial Neural Network for fast system simulation. This approach allows to accelerate the transient annual assessment such that annual thermal yield can be used as figure of merit in the iterative optimization. For the latter, an Evolutionary Algorithm adapted to the problem has been applied, which allows for identifying optimized receiver configurations with reasonable computational effort. The methodology is demonstrated by means of a 55 MWth receiver and a given Heliostat Field in southern Spain. For this example, the evolution of receiver parameters during the course of the optimization and the break-down of different loss contributions are discussed. The optimized receiver configuration delivers more than 4% higher annual yield, as compared to the reference configuration based on static design considerations. The approach's applicability to different systems - external receivers, other heat transfer fluids, commercial scale system sizes - is discussed.

In this study, an optimization approach for solar tower receivers is presented. At its core is a detailed optical and thermo-hydraulic simulation model, built on ray tracing and a spatially resolved heat and pressure loss model. This detailed physical model is incorporated in an approach for dynamic system simulation, that uses a sky discretization and flux level interpolation approach for fast optical assessment and accelerates the annual performance assessment by means of an artificial neural network. Using an objective function based on the introduced modeling approaches, the receiver is optimized for effective annual thermal gain. The entire optimization methodology is called EvoRec. The developed methodology is demonstrated for a reference system, which has been modeled based on data from literature to resemble the Gemasolar plant in Spain. By optimizing the receiver in terms of six degrees of freedom, a relative increase of annual yield by 12% compared to the reference setup is reached.

We present measurements of the Bidirectional Reflectance Distribution Function (BRDF) of the state-of-the art solar receiver coating, Pyromark (R) 2500, and show how the data can be accurately described with physical models. In the conducted measurements of the receiver coating, a peak shift of the radiation away from the direction of ideal specular reflection was observed. Both the Cook-Torrance and the He-Torrance models reproduce this behaviour. For the application of the former model, an empirical angular dependence of the parameter describing the surface roughness has to be assumed in order to obtain an adequate fit to the data. The latter model is less exact in a statistical sense, but it is physically based, with six meaningful fit parameters. As we found that the He-Torrance model with its original form of the specular reflection yielded physically incorrect results for incidence angles larger than 80 degrees, a modification is proposed to overcome this deficiency. Overall, the fitted models are able to provide a good description of the BRDF of the receiver coating over three orders of magnitude. Our results can play an important role in facilitating camera-based flux control or optical simulations of solar tower systems. Moreover, we think that our findings can serve as a sound starting point for the physical modelling of other absorber coatings currently being developed.

Soiling impacts energy harvesting of solar thermal power plants and operation and maintenance cost. Our research showed that reflectance of soiled glass reflectors diminishes significantly with increasing angles of incidence. The incidence angle dependent attenuation due to dust on the reflector surface can be modelled based on the Lambert-Beer law. In this study we show the effect of incidence angle dependent reflectance on the expected yield of exemplary power plants by integrating the model into ray tracing. Results illustrate the additional reduction of the solar yield in dependence of the mean incidence angles on the mirrors of a heliostat field, a parabolic trough field and a linear Fresnel collector array, depending on the power plants site and seasonal effect. Results show mean hourly incidence angles and optical performance for every hour of the year. In conclusion, not considering angle dependent reflectance can lead to on overestimation of the annual plant performance of up to 2%. For line focusing collectors the effect is dominant in winter, in the northern hemisphere, underlining the importance of cleaning during this season.

We describe the angular distribution of the reflected radiation for a coating used on central receivers: Our aim is to provide a model which is able to describe the bidirectional reflectance distribution function (BRDF) obtained with a 3 dimensional scanning photo-goniometer. Two different reflection models from computer graphics were tested for their ability to tit the measurements, and we found that the Cook-Torrance model and a modified version of the He-Torrance model are suitable choices. The first model is simpler but partly empirical, the latter gives a good description of the data, using a set of 6 physically interpretable parameters. The deeper understanding of the dependence of reflectance on angles and surface properties can help to improve numerical receiver simulations and thus optimization of receiver geometry and plant design and operation, for instance applying camera-based flux monitoring and control. We expect that our findings can serve as a sound starting point for the physical modelling of other absorber coatings currently being developed.

A novel methodology for the design of heliostat fields is presented, based on the selection of heliostats from an oversized field by means of a polygon. To obtain the ideal field shape, the polygon vertices are optimized with an evolutionary algorithm. The objective function calculates a weighted tradeoff between annual optical efficiency and ground usage and is applied to the entire field instead of individual heliostats. Various other figures of merit could be readily integrated. To be able to deal with complex shaped land available for the Solar Tower plant, area boundaries are taken into account during the optimization phase. The application of the methodology is demonstrated by means of a reference scenario and multiple variations of parameters and area boundaries. The polygon selection creates smooth, coherent heliostat fields with high performance regarding the objectives, while solving several practical issues in the heliostat field design phase at the same time.

Soiling impacts energy harvesting of CSP Plants as well as operation and maintenance cost. The identification of the proneness of a potential solar site to soiling would reduce risk and improve the performance and cost prediction. To allow for medium and long term monitoring of soiling rates, precise, replicable and fully automated measuring methods are needed, similar to today's solar resource assessment. Furthermore, the analysis of dust and its adhesion to solar reflectors are of interest. Tackling both requests, we developed a new instrument for automated online monitoring of soiling. The AVUS measurement set-up directly measures reflectance. Furthermore, the soiled samples are available for retrospective assessment of dust and its adhesion to solar reflectors. We present measurement results from a one-year monitoring campaign at a solar thermal power plant located in the south of Spain. Additionally we show a comparison of the AVUS sensor results and results from PV reference cells. We assess the advantage of soiling monitoring for financial risk reduction and for performance prediction. Detailed cost and performance models are applied to a case study of a parabolic trough plant.

In this paper, we present our experience in designing FLATCON (R) concentrator modules. The FLATCON (R) technology uses silicone-on-glass (SoG) Fresnel lenses and solar cells mounted to a metal heat distributor on a glass rear plate. During the past two decades, various FLATCON (R) module designs have been investigated. The first FLATCON (R) modules consisted of lenses with 16 cm(2) and GaAs single-junction solar cells with a diameter of 2 to 4 mm. In 2007, the first triple-junction solar cells with a diameter of 2.3 mm were integrated into FLATCON-type modules. The results of the test module ISE049T are discussed in this paper in detail. It was measured on the test tracker in Freiburg for more than 10 years. Moreover, the latest FLATCON (R) module design is presented together with the major manufacturing steps.

Objective of this article is to show and discuss the shape accuracy of solar reflector panels for linear Fresnel collectors. Systematic shape deviations due to torsion or orientation errors are responsible for severe optical losses and underperformance. This is why this article investigates systematic surface deviations beyond the standard quality parameters like SDx and FDy. We discuss typical characteristics of linear Fresnel collector reflector panels. Our measurement results show local surface slope deviations measured by deflectometry. In the second part of this study, the effect of systematic surface slope deviations is analyzed by use of a parametric model. We apply the model to detect systematic production errors, investigate optical losses and the impact on the focal line with ray tracing.

Central Receiver Systems use thousands of heliostats distributed in a field to reflect the solar radiation to a receiver installed at a tower. The large distances between the heliostats and their aim point require a high precision of the heliostat control and drive system to efficiently concentrate the solar radiation and to allow for a safe and reliable plant operation. Precise calibration procedures are needed for the commissioning of open loop systems. Furthermore, the need for cost reduction in the heliostat field motivates for precise and fast calibration systems allowing for savings in the design of the individual heliostats. This work introduces a method for the measurement of true aim points of heliostats in operation. A proposed application is the plug-in system HelioControl, to be used for parallel in-situ calibration of multiple heliostats in operation. The approach aims at extracting the focal spot position of individual heliostats on the receiver domain using image analysis and signal modulation. A centralized remote vision system, installed far from the harsh conditions at the focal area, takes image sequences of the receiver domain. During the measurement, a signal is modulated on different heliostats which is then extracted for the retrieval of the true aim points. This paper describes the theoretical background of the methodology and demonstrates the functionality based on two simulated cases showing the practical advantages introduced with this approach.

Project ALCHEMI is a Solar Era.Net funded European collaborative project, which is developing a low cost, high efficiency HCPV module using a previous design developed by Fullsun PV. In this project, the 625 sun Gen 1 module has been redesigned to operate at 1000 suns and further improved by the utilization of state of the art 3 junction solar cells from three different vendors. Through use of similar to 42% efficient cells, it is expected to achieve module efficiencies of >37%. This paper describes the cell and module test and assembly process and initial evaluation of how the next generation of module will be improved still further through another iteration of the design of the secondary optical element (SOE).

An imaging method for the parallel in-situ measurement of heliostat aim points during operation in central receiver systems is assessed. In a SFERAII-access campaign, the HelioControl prototype system was integrated into the heliostat control system Hyperviseur at the Themis facility. The method, determining aim points from the cumulative flux distribution at the receiver, uses a signal of periodic movement, modulated by means of the heliostat drives. The functionality was implemented into a group of heliostats and practically assessed. For few heliostats, exemplary parallel aim point measurements at a flux target – using the methodology – show very small differences compared to a reference. Further experiments with multiple overlapping focal spots have been conducted using the small experimental cavity receiver of the system. The highly variable reflectance of the receiver area is challenging for the imaging method evaluating the reflection of the incident flux. A simplified receiver model obtained from experiments with single heliostats, however, allows for a correction and preliminary evaluation. The promising results obtained from measurements with deviations below 0.7 mrad in exemplary evaluations display the potential of the method and motivate for further and comprehensive validation in the context of commercial scale central receiver systems.

Reflector materials directly affect the performance of concentrating solar applications such as solar thermal power plants or process heat installations. For concentrating collectors the specular reflectance and scattering of reflector materials in the near specular range is of special relevance for a good performance. In this article, we show characteristic bidirectional scatter and specular reflectance curves for solar materials, such as glass-based, polymer-based and aluminum-based reflectors, measured using the instrument VLABS, a commercial reflectometer and a spectrophotometer. The solar weighted specular reflectance is a relevant performance indicator, allowing the comparison of the different reflectors. However, usually it is not measured in solar laboratories. This is why this article presents a method for modelling of solar weighted specular reflectance based on hemispheric reflectance and single wavelength measurements. The model applies surface scatter theory and the concept of total integrated scatter. It allows to predict the specular reflectance for the solar spectral range. The application of the model to solar reflector materials with different surface roughness features and the impact of beam spread on the solar weighted specular reflectance is shown. The methodology allows for an improved evaluation and comparison of innovative reflector materials for solar applications.