Nonimaging Optics: Efficient Design for Illumination and Concentration XIX

Nonimaging optics is a broad field, with research groups across the globe generating hundreds of nonimaging optics-related publications each year. These publications cover everything from new design methods and new application areas to new manufacturing techniques tailored for nonimaging optics. In this presentation, we use data analysis and visualization to provide a birds-eye perspective on this literature. We identify and explore trends in the field and the research groups working on specific problems. We also provide a new overview of historical developments and share our thoughts on promising future directions for nonimaging optics.

In this presentation we will talk about how asymmetrical nonimaging optics can use flexible materials to accommodate different range of acceptance angles, resulting in tracking the seasonal changes of the sun, while integrate to the roof with a none-shading configuration.

In illumination optics, the goal is to modify a light source’s spatial distribution to achieve a specific irradiance target. By using freeform surfaces, the emitted light can be transformed into arbitrary irradiance patterns. Despite significant advances in freeform optics design, mostly for ideal light sources, current methods offer limited control over the surface shape, generally resulting in globally convex or concave surfaces. In an illumination context, smooth and oscillating freeform surfaces are possibly more interesting, but calculation methods are currently non-existent. This presentation introduces a deep learning approach for predicting such complex freeform topologies, capable of rapidly generating optics that transform a prescribed light source into arbitrary irradiance patterns. This allows for the creation of surfaces with convex, concave, and saddle regions, showcasing the potential of deep learning in accelerating illumination design.

The recent development of a new laser-based light source offers enhanced performance and efficiency across various domains, notably in light microscopy, medical procedures, advanced industrial applications, and far-field illumination. This innovative light source, with a directional and narrow angle beam, is characterized by its high luminous flux, an impressive luminous intensity exceeding 8000 cd and ability to localize its optical power outperforming any existing LED technology by an order of magnitude. The light source is constructed around a 5 W blue laser diode and unique single crystal conversion element and boasts a compact design, making it highly amenable to seamless system integration. Designed specifically for scientific, biotech, and machine vision applications, this product features a remarkably low étendue and narrow viewing angle of as low as 4.5°. This promises unparalleled system efficiency and has the potential to revolutionize optical setups.

Quantum dots technology enhances light emitters and becomes increasingly important in display applications for providing higher luminance and larger color gamut. When light passes through a film with quantum dots, the light is scattered or absorbed, and a portion of the absorbed light is re-emitted at a different wavelength. The photon events within quantum dots, on the macroscopic level, can be described probabilistically using rays with unconverted or converted wavelengths. When the rays exit the material, metrics can then be evaluated from the ray data to assess display performance. Therefore, by varying the parameters of the quantum dots, the performance of quantum dot displays can be optimized. In this paper, we discussed how scattering events are modeled in quantum dots, explain a few display performance metrics, and describe an example of quantum dot display design.

We discuss recent advances in both the fundamental science and technological application of radiative cooling. We will introduce a range of super-white paint-based strategies for daytime radiative cooling materials. We will next discuss a new domain of application for radiative cooling: vertical facades of buildings that experience asymmetric thermal radiative environments, highlighting the remarkable cooling benefits that infrared selective thermal emitters, including low-cost scalable ones made from existing polymers, can enable. Finally, we highlight new work on harnessing radiative cooling for water technologies, including passive freezing desalination as well as near-optimal condensation of dew through the development of multi-functional slippery hydrophilic radiative cooling materials.

Passive radiative cooling, a method for lowering the temperature of outdoor objects without the need for electrical power, has garnered interest for its potential in energy conservation. However, the high reflectivity of the radiative cooler in solar spectra typically results in a white or silver color, posing limitations for aesthetic and practical applications. Here we provide the method for calculating the minimum thermal load across the CIE 1931 xy chromaticity diagram. In addition, we introduce a spectral-conversion microphotonic thin film as the colored radiative cooler based on photoluminescence (PL) phosphor. By using the microstructure array, 89% of the light trapped by total internal reflection (TIR) can be extracted from the film, which lowers the amount of heat produced. It proves the potential of the colored radiative cooler based on PL phosphor and light-extraction microstructures.

Radiative cooling is a process that surfaces emit thermal energy into outer space. In this work, a method is proposed to consistently and fairly compare different radiative cooling materials under various and repeatable working conditions. A model that accurately replicates the radiative heat transfer in the actual surface–atmosphere–universe system using the surface–surface heat transfer process in a laboratory setup is developed. A novel ilmenite-pigmented polyethylene material that effectively represents the entire atmosphere in the natural radiative cooling phenomenon is created. With the material, it is further proved that the radiative heat transfer in the naturally occurring surface–atmosphere–universe system equals to the laboratory surface–surface heat transfer between the cooling surface and the newly created atmosphere-emulating material. An instrument is constructed accordingly and an experimental protocol is subsequently developed to standardize the assessment of all radiative cooling materials in a repeatable laboratory setting. These will contribute to the field by providing a standard assessing the performance of all developed radiative cooling materials.

Radiative cooling is a form of cooling that dissipates heat through thermal radiation, which can be done without any external power source. Herein, we present a simple, low-cost, and low-weight hollow microsphere-based white coating that can be fabricated through spray coating method. This work introduces ceramic (Silicon Dioxide, SiO2) hollow microspheres and an inorganic binder material (Potassium Bromide, KBr) to form a white coating to perform radiative cooling. The coating can stay approximately 5 °C cooler than that of a commercial TiO2-based white paint and 10 °C cooler than the ambient air under about 1000 W/m2 of direct sunlight. In addition to radiative cooling, this coating is also thermally stable at high temperatures that it can endure temperatures beyond 700 °C. Our coating has the potential to offer radiative cooling at high temperatures as well as for ambient temperature under direct sunlight, achieving sub-ambient cooling.

The escalating global surface and atmospheric temperatures necessitate increased cooling demands, urging the exploration of sustainable alternatives to conventional cooling systems. Radiative cooling emerges as a promising solution, leveraging the dissipation of thermal energy to deep space without external power input. This technique relies on materials capable of emitting long-wavelength infrared radiation while reflecting short-wavelength solar radiation, particularly within the atmospheric transparency window (8-13 μm). Numerous structures have been developed to optimize radiative cooling performance, including multilayered films, composite materials, and polymeric coatings. Despite significant progress, challenges remain in achieving scalability and practical applicability. This study builds upon utilizing a phase inversion technique to fabricate hierarchically porous poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) structures for efficient radiative cooling. Precise control over fabrication parameters yields materials with exceptional solar reflectivity (98.2%) and emissivity (98.5%), showcasing promising advancements towards sustainable cooling solutions.

This research investigates silicon-based absorbers to efficiently convert solar energy into heat in order to initiate chemical or physical reactions. We designed structures that utilize colloidal lithography to nanopattern silicon-based solar absorbers as a way to enhance light trapping in the visible to near-infrared range. Colloidal lithography is a scalable and cost-effective patterning technique that uses self-assembled colloidal arrays. The silicon-based surface absorber achieves excellent absorption in the wavelength range from 380 nm to 1500 nm. The versatility and simplicity of the absorbers make them potentially applicable to a wide range of research projects and industrial products.

The thermal spectra of hot and cool bodies are well-known and described by Planck’s law for blackbody emission. However, for many modern technologies and applications, it is desirable to have deviations from this law to achieve directional or wavelength-controlled emission. In this talk, we will discuss the control of thermal emission for various applications. We will first discuss our work on alternative power generation concepts to produce power after sunset by optically coupling to deep space. Then, we will describe an alternative geoengineering strategy to increase the Earth's radiative heat emission, potentially stabilizing or cooling the planet to help mitigate climate change by increasing the earth’s thermal emission by 1 W/m2.

Characterizing thin-film materials with high refractive index, high thermal stability and infrared transparency is important for designing thermophotovoltaic (TPV) selective emitters and thermal barrier coatings (TBCs). Here, we report spectroscopic ellipsometer measurements of several thin films in the wavelength range 210 nm to 2500 nm from room temperature to 1000 deg C. Our findings provide insights into the potential impacts of temperature change on the aforementioned applications, induced by the underlying changes in their electronic band structures. In the first step, we present the properties of magnesium oxide and strontium titanate substrates. Next, we consider layers of cerium oxide and barium zirconate deposited on top of these substrates. Finally, we apply these initial characterizations to understand data obtained from multilayer samples comprised of a combination of layers from all these materials, and project the potential performance for TPV and TBC applications.

Unitary control changes the optical absorption and emission of an object by transforming the external modes. We answer two basic questions: Given an object, what absorptivity, emissivity, and their difference are attainable via unitary control? How to obtain given absorptivity, emissivity, and their difference? We show that both questions can be answered using the mathematics of majorization. We further provide explicit algorithms for the practical implementation of unitary control.

This talk will consider the future of metal halide perovskite (MHP) photovoltaic (PV) technologies as photovoltaic deployment reaches the terawatt scale. The requirements for significantly increasing PV deployment beyond current rates and what the implications are for technologies attempting to meet this challenge will be addressed. In particular how issues of CO2 impacts and sustainability inform near and longer-term research development and deployment goals for MHP enabled PV will be discussed. To facilitate this, an overview of current state of the art results for MHP based single junction, and multi-junctions in all-perovskite or hybrid configurations with other PV technologies will be presented. This will also include examination of performance of MHP-PVs along both efficiency and reliability axes for not only cells but also modules placed in context of the success of technologies that are currently widely deployed.

The recent advent of robust, refractory (having a high melting point and chemical stability at temperatures above 2000°C) photonic materials such as plasmonic ceramics, specifically, transition metal nitrides (TMNs), MXenes and transparent conducting oxides (TCOs) is currently driving the development of durable, compact, chip-compatible devices for sustainable energy, harsh-environment sensing, defense and intelligence, information technology, aerospace, chemical and oil & gas industries. These materials offer high-temperature and chemical stability, great tailorability of their optical properties, strong plasmonic behavior, optical nonlinearities, and high photothermal conversion efficiencies. This lecture will discuss advanced machine-learning-assisted photonic designs, materials optimization, and fabrication approaches for the development of efficient thermophotovoltaic (TPV) systems, lightsail spacecrafts, and high-T sensors utilizing TMN metasurfaces. We also explore the potential of TMNs (titanium nitride, zirconium nitride) and TCOs for switchable photonics, high-harmonic-based XUV generation, refractory metasurfaces for energy conversion, high-power applications, photodynamic therapy and photochemistry/photocatalysis. The development of environmentally-friendly, large-scale fabrication techniques will be discussed, and the emphasis will be put on novel machine-learning-driven design frameworks that leverage the emerging quantum solvers for meta-device optimization and bridge the areas of materials engineering, photonic design, and quantum technologies.

We propose a new design principle for optimal concentration of light with small diffusivity based on the conservation of local brightness in passive optical transformations. A coordinate transformation is applied on the incoming rays to compensate for the variations in local brightness by the focusing stage. We apply this analytic design for a compact reflective configuration for ideal imaging concentration of diffuse light such as sunlight in one dimension on an elongated target with arbitrary cross-sectional shape at the thermodynamic limit. As illustrations, we present the design for two different target geometries and verify its validity using numerical ray-tracing simulations. The same configuration can be used in reverse as an ideal collimator of a finite diffuse source.

The recent surge in low temperature solar thermal process research (e.g. solar-driven evaporation and desalination) has renewed the need for low-cost stationary (non-tracking) solar concentrators. Stationary solar concentrators boost flux without tracking, but their design is challenged by the large acceptance angles required. Here, we introduce a generalized source/acceptance map matching method to design simple yet effective non-tracking concentrators for any latitude. We then explore several practical configurations based on generalized asymmetric theta-in/theta-out transformers. Notably, one configuration which makes use of a pivotable front wall to achieve a seasonally-adaptive design which surpasses the static concentration limit. We present the detailed design, construction, and experimental on-sun demonstration of a prototype of the seasonally-adaptive design. Experimental results confirm the theoretical predictions and pave the way for maximizing the solar concentration achieved by non-tracking collectors.

This research fully characterized a semi-transparent CPV module with microtracking optically, thermally, and electrically under the harsh, semi-arid climate of Abu Dhabi, UAE which included the design and construction of a test-rig. The peak electrical efficiency recorded is 26.13% coupled with a transmittance ratio of 26% that predominantly depends on the diffuse irradiance across the day rather than variations in the incidence angle. In situations prioritizing light transmittance over electrical output, utilizing the maximum light transmission mode, transmittance efficiency reaches a maximum of 71.61% which corresponds to a daily maximum of 1.8379×109 lumens/m2 and 29.69 PAR/m2 making it suitable for moderate daylighting applications and crops requiring high light intensity. On the thermal front, the temperature within the test-rig compartment surpasses the ambient temperature by a range of 10-15°C, with the disparity between the ambient environment and the solar cells being 27.3°C. Soiling characteristics and their effect on performance are also highlighted. A semi-empirical model was developed to evaluate the performance of the technology and for economic analysis.

This presentation will be on the optical section of the micro-concentrator photovoltaics (micro-CPV) review article. We'll briefly explain the attractiveness of micro-CPV and the synergies with other industries it enables. Then, go into detail into the freedom of optical designs and manufacturing methods the technology allows. We'll proceed with, the current optical approaches together with their performance and viability for industrial scalability. And conclude, with the most viable optics for this technology.

We demonstrate a new versatile platform for designing non-imaging metaoptics to use with incoherent sources. We provide a phase-design method for one-dimensional beam shaping, which allows us to compute a phase profile that efficiently redistributes a collimated beam with an arbitrary intensity profile to a desired output intensity. We verify the behavior of different metasurface designs using the opensource beam propagator diffractsim as well as full-field COMSOL simulations and find the results to be in good agreement with our expectations. We believe our work successfully bridges the fields of non-imaging optics and metaoptics and constitutes a new fast and versatile tool for metasurface design.

Traditional imaging optics aim to bring the light rays received from all object points within a desired field of view to their corresponding image points. Optical design optimization can be considered as the process of finding an optimal solution of an extremely overdetermined problem, by which a group of optical elements and their spatial relationships are optimized to minimize optical aberrations and yield an acceptable image quality over a desired field of view. This usually leads to complex stacks of optical elements due to the overdetermined nature of optical aberration correction. Unlike traditional imaging optics, nonimaging optics is concerned with the optimal transfer of light radiation between a source and a target. A good question to ask is “can a nonimaging optic be used instead of an imaging optic to simplify an optical imaging system?”. In this talk, we will introduce an imaging modality that achieves diffraction-limited imaging with a nonimaging spatial information transfer lens. This imaging modality is low-cost, simple in configuration, and flexible in arrangement.

Nonimaging optics, guided by the constructal law, revolutionizes solar concentrator design. This study explores constructal law application to devise a new truncation criterion for Compound Parabolic Concentrators (CPC). We aim to demonstrate the constructal law's effectiveness in optimizing CPC design. Formalizing its application introduces a leading truncation criterion. By evaluating geometrical, energy, and entropy-related parameters, we establish a constructal truncation criterion aligned with Rincon's, incorporating novel dimensions like the "Mora number" linking entropy and étendue scattering. Validation involves ray-tracing and experimental alignment of solar cooker designs. Constructal optimization reduces CPC height, decreases étendue scattering, minimizes entropy generation, and enhances energy transfer. These outcomes are validated through rigorous ray-tracing, signaling significant advancements in solar concentrator efficiency. Our work offers a comprehensive constructal perspective on nonimaging optics, promising heightened efficiency, reduced manufacturing costs, and improved optical uniformity for solar energy applications.

In this presentation we will discuss the possibility of utilizing flow line optics to concentrate or redirect high energy radiation such as X-ray, which tends to only be reflected at large incident angles.

Lorentz Geometry has been successfully applied to the study of geometrical vector flux J. Recently, has been shown that using Lorentz geometry it is possible to obtain irradiance patterns produced by optical systems with refractive elements. In this paper we advance in the application of Lorentz geometry to nonimaging optics. We provide a solution for the coupled systems of partial differential equations, obtained by the application of Lorentz geometry to a 3D refractive optical system. We also analyze the role of eigenvalues of Gram matrix G obtained by the application of Lorentzian formalism.

Fresnel lenses are commonly designed by optimizing a sag function, similar to the approach for traditional thick lenses. This method limits the degrees of freedom by constraining the vector field of surface normals to be integrable. We introduce a method that removes the integrability constraint, while still ensuring continuous long facets. The presented method initializes a facet at a starting point and follows it along the line of zero z gradient. The starting points are manually placed depending on the shape of the vector field, and new facets are automatically added when the distance between the facets is greater than a given threshold. Finally, we show how this design can be used to realize an étendue squeezing line-focus solar concentrator that could not have been achieved using conventional thick lenses.

Manipulating flux transportation of optical fields holds great promise across various kinds of applications, encompassing laser micro-processing, optical trapping, microscopic imaging, and illumination engineering. In recent decades, freeform optics, lauded for its capacity to efficiently control the wavefronts of optical beams with high design freedom, has garnered significant attention as a potent tool for shaping light. Nevertheless, the constraints imposed by geometric optics pose a further limitation on FOEs, particularly in their ability to sculpt coherent light, such as laser beams, as these are particularly susceptible to diffraction effects. Within the context of this study, we propose a design paradigm that harnesses freeform optics to craft light trajectories in three dimensions. This approach inherently gives rise to caustics, which are singularitie which are singularities within the realm of flux transfer in geometric optics. Crucially, our proposed method yields the capability to generate sharply defined light patterns, empirically giving better results for mitigating diffraction effects in contrast to previous design methodologies, which is observed by experiments.

Techniques are discussed for characterising and manipulating light sources using 3D numerical flowline analysis. Hyperbolic curve fitting from flowline measurements is proposed as a means of extrapolating flowlines to the far-field for calculation of etendue. The concept of flowlines as edge-rays is broken down to demonstrate that it is possible to measure etendue of non-Lambertian light sources directly from flowlines, using a light field sampling approach. Practical applications for concentrator design are given for Lambertian and asymmetric non-Lambertian light sources.

We introduce a new type of secondary concentrator for solar towers, utilizing a freeform reflector in a beam-down setup to significantly enhance concentration ratios. By optimizing the reflector's shape and the heliostat aiming strategy, our designs approach aplanatic conditions, doubling to tripling concentration ratios while maintaining a significant gap between concentrator and receiver unlike existing CPC secondary concentrators. The compact design allows a multi-MW solar field to share a single secondary concentrator and receiver aperture. We show simulations of and example design for the 2.5MWth SSPS-CRS heliostat field in Almeria, Spain, demonstrating a secondary concentrator not larger than the existing tower. Furthermore, we discuss implications this type of concentrator has for heliostat field design, both in terms of heliostat size and field layout.

Rapid growth in the number of Earth-orbiting satellites with electric propulsion as well as plans for colonizing the Moon will dramatically increase the demand for solar photovoltaic (PV) power in space. Most of these missions will be commercially-driven, heightening the need for PV systems that are more compact, lower-mass, more efficient, reliable, and affordable than ever before. In this talk, I will describe how microscale PV cells integrated with ultracompact concentrating optics offer a new opportunity to improve performance and reduce cost without sacrificing reliability. I will overview the unique constraints imposed on nonimaging concentrator design by operation in space and describe an experimental prototype <2 mm thick that achieves 26% power conversion efficiency at a geometric gain of 18x with a specific power >100 W/kg and an acceptance angle of nearly ±10°.

Nonimaging micro-concentrator arrays are presented that are fabricated using femtosecond laser irradiation followed by selective etching in fused silica. A seven-port concentrator array is demonstrated through simulations and fabrication. Applications include coupling to multimode fibers, small photodetectors and LED arrays.

This presentation describes and discusses the various parts and components of a prototype that can handle 1 kW of solar power, in view of using the concentrated solar radiation and the solar-generated heat, for the processing of materials (i.e. solid substances). It is an advanced optical system - that collects, concentrates, controls and directs the solar radiation to the site of solar power utilization - to efficiently apply the output solar beam to the specific target. The principle of “double paraboloid reflection” recently proposed in our recent publications is used for the first time in this prototype. The motivation for the construction of the prototype is the validation of the work of Takashi Nakamura et al. presented at SPIE Optical Engineering + Applications events and proceedings in 2009 and 2011, in which silica optical fibers were used for passing high-flux solar energy.

Non-imaging concentrator is an important solar energy utilization mode. The conventional static non-imaging concentrator has small geometric concentrator, short effective working time and low heat collection temperature, which limits its large-scale industrial application. Therefore, the present research constructs a high-power concentrating solar energy CPC with intermittent tracking work, and explores the characteristics of its tracking strategy on solar radiation aggregation due to the CPC (Compound Parabolic Concentrator) without light escape. The experimental results demonstrate that the intermittent tracking strategy, based on the edge light theory, enables continuous, efficient, and reliable operation of CPC throughout the day. Moreover, it significantly enhances solar radiation collection performance compared to static CPC. The results show that compared with the standard CPC and the CPC without light escape of the same concentration ratio, the radiation collection amount is increased by 252.9 MJ/m2 and 272.4 MJ/m2 respectively, which has a potential engineering application prospect.

Ancient Persians harnessed radiative cooling to freeze thin layers of water in shallow pools on clear nights, even when the ambient temperature remained above the freezing point. However, due to solar heating, daytime ice-making without electricity has yet to be achieved even now. Inspired by passive daytime radiative cooling, sub-ambient cooling during the day can be attained by minimizing solar absorption and maximizing thermal emission. Intriguingly, water exhibits desirable optical properties for that purpose. Here, we demonstrated that liquid water can be utilized to construct a high-performance daytime radiative cooler, i.e., the water cooler. The water cooler's thermal emission and solar absorption can be controlled by adjusting the water’s thickness, as determined by theoretical calculations and experimental measurements. We observed a 6 °C sub-ambient cooling of the water cooler in the field test in Xi’an on 1st Dec. 2023 at noon. Daytime ice-making without electricity was also showcased using the same setup. Based on the results, the water was cooled below the freezing point for most of the day when the ambient temperature was above 0 °C.

This research evaluates the efficiency of working fluids in direct adsorption solar collectors by incorporating magnetite nanoparticles. Samples with Fe3O4 concentrations ranging from 0 to 1% were evaluated under direct solar exposure conditions. It was determined that the nanofluids exhibit higher thermal efficiency than pure ethylene glycol, indicating that magnetite enhances solar radiation absorption. However, higher nanoparticle concentration was observed to decrease the specific absorption rate (SAR), likely due to lower radiation penetration. These results suggest that SAR could be a useful selection criterion for formulating nanofluids in DASC collector applications.

The optical design of hybrid concentrating system is presented for solar-powered lasers to excite the electrons from a lower energy state to a higher energy state. The on-axis configuration is considered to minimize the axis aberration effect. The design parameters of the parabolic mirror are kept identical for both heliostats. Based on the comparative analysis, the achieved output power from the rectangular shaped heliostat is higher than the circular shaped heliostat because of the close packing of rectangular array. Although, due to the circular segments of the heliostat, some space is void between the mirrors.

Radiative cooling has been proven as a highly promising and scalable technology towards carbon neutrality and energy saving in buildings. However, in many climate zones where ambient temperature varies significantly throughout the year, it is often desirable to switch off the radiative cooling and instead switch on the heating (e.g., during winter). In this study, we have developed a smart, scalable nanophotonic system with the capability of switching between solar heating and radiative cooling. Through optical and photonic simulation and experimentation, the detailed structure of our smart thermal management system is tested and optimized for tunability in both the solar spectrum and MIR range. We believe that our design will be a valuable reference to studies on smart thermal management, as well as a practicable and scalable solution to energy-efficient buildings, pathing the way towards sustainability.

Achieving a decarbonized energy sector by 2050 will require the development of cost-effective technologies beyond today’s commercial concentrating solar-thermal power (CSP) technologies. Achieving the 2030 target will depend heavily on reducing the cost of heliostats, while improving technical performance. There are several opportunities for metrology, in addressing heliostat optical error, thus increasing the overall heliostat efficiency. In addition, opportunities in reducing the costs of manufacturing, assembly, calibration, or operations & maintenance exists.

Central Tower Concentrating Solar Power (CSP) harnesses heliostats, sun-tracking mirrors, to concentrate sunlight onto a receiver for thermal energy storage. Even slight surface errors in heliostats can lead to substantial performance losses. The National Renewable Energy Laboratory has developed two tools to characterize heliostat optomechanical errors. The first, Non-Intrusive Optical (NIO) technology, employs Uncrewed Aircraft Systems (UAS) to capture outdoor images of mirror reflections, estimating slope, canting, and tracking errors. The second, Reflected Target Non-Intrusive Assessment (ReTNA) System, uses an indoor automated rail system to photograph reflected printed targets with a coded chessboard pattern. Both methods' methodologies have been validated, and efforts are underway to automate and commercialize them, with data collected from commercial heliostats aiding in validation and demonstration.

Central Tower Concentrating Solar Power (CSP) employs heliostats, sun-tracking mirrors, to focus sunlight on a receiver for thermal energy storage. Heliostat metrology ensures performance quality, crucial for large-scale CSP implementation. CSP's precision and outdoor setting pose challenges for traditional optical measurement techniques. In this work, we conduct a scoping study to identify equipment and techniques for various heliostat measurements, including sun-shape, reflectance, surface shape, and opto-mechanical errors. Initially, available metrology technology was surveyed, listing tool names, suppliers, functions, cost, and accuracy data. Subsequently, tools were ranked based on cost, industry usage, and measurement impact on plant performance. Finally, a report was compiled with tool details and recommendations. This study aims to inform the development of a metrology platform for third-party heliostat evaluation, enhancing CSP efficiency and reliability.

Heliostat fields constitute one of the large-scale concentrating solar power (CSP) options for electricity production and/or thermal storage. In a typical field, several thousand heliostats, each consisting of one or more mirrors, are precisely oriented to track and reflect the sunlight onto a receiver atop a central tower. Any misorientation of the heliostats during operation - due to structural changes, tracking errors, etc. - must be detected and corrected to ensure maximum light collection essential for the economic viability of the plant. A number of metrology techniques for accurate determination of heliostats orientation during operation have been proposed and implemented. These are briefly reviewed in this paper followed by a detailed description of a novel ultrafast technique for the precise measurement and correction of the canting errors in a heliostat field.

This paper follows the pathway that is recommended in the HelioCon report, "Roadmap to Advance Heliostat Technologies for Concentrating Solar-Thermal Power" to address the Tier 1 gaps identified in metrology and standards. Most of these gaps are related to missing commercialized metrology to measure opto-mechanical errors of heliostats. Cross-validation of metrology techniques that are available or under development is needed. The authors evaluate the state of the art of those metrology techniques and outline a Round Robin Test plan based on the results. The goal is to initiate the next step in closing the Tier 1 gaps in metrology.

This work investigates the fundamental development of an extremum seeking control algorithm to improve tracking to reduce heliostat drift and other pointing issues. This work analyzes software logic non-linear pieces of the heliostat system, such as the power distribution and the gradient, to be developed into a linear gain, which is the inverse of the covariance matrix using log functions. The imposed feedback loop consists of a batch least squares (BLS) gradient algorithm as well as a controller to send signals to the heliostats to correct their locations. The subcomponents of this feedback algorithm contain a plant, estimator and controller, which are used to quickly allow heliostats to move to reduce the large gradient towards a defined centroid. Preliminary results have shown that beam characterization system (BCS) flux Gaussian distributions can be focused towards a centroid to within less than 10% rms error.

Retroreflectors ('retros') in front of the receiver provide feedback from multiple locations of each heliostat's radiant footprint thereon, simultaneously for all of them. The retros and their mounts are made of quartz glass to allow placement in the 'hot zone'. To discriminate samples of light returned by multiple retros from each other and from the brightly lit receiver, the retro reflectivities are modulated at unique frequencies by spinning them. The presentation will show component and system test results.

Optical performance of solar collectors is known to be sensitive to wind driven loads. This performance loss is typically quantified in the form of tracking and slope errors. Tracking error is defined as the angular offset of a collector away from the sun position whereas slope error is due to the deformation in the shape of the collectors’ mirror surfaces. Previous studies have explored the impact of tracking error on optical performance but have not fully addressed the impact of wind-driven loads on the tracking error. Further, there is a lack of long-term data characterizing spatial and temporal variations in tracking errors at an operational power plant. In this presentation, we will characterize tracking errors on parabolic troughs at the Nevada Solar One CSP plant and on heliostats at the Crescent Dunes power tower plant. This characterization of optical performance loss is generated using long-term field measurement of the collector orientation (elevation and azimuth) at multiple locations across the power plant. In addition to optical error quantification, we will also present observations on the influence of wind loading and other factors leading to loss of optical

The first automatic twisting heliostat, with 8 m2 reflector, was completed and tested on-sun in January 2024. It was set up on a target-oriented dual-axis mount, with the target-axis aimed at a target 113 m to the west. The shape-twisting is then purely automatic, made by a mechanical cam coupling to the cross-axis, which turns as the angle of incidence of the sun’s rays on the reflector. Three unsaturated images taken at different times of day were recorded of the sun on the target, reflected at angles of incidence of 5°, 49° and 68°. The measured FWHM of three images is very similar, about 1.09 m or 9.5 mrad, only slightly larger than an ideal solar disc, indicating that the different twisted reflector shapes are close to the ideal biconics needed to image the sun. The encircled energy measured for all three images was similar, 87% within ~1.30 m diameter.

Concentrated sunlight has the potential for manufacturing syngas and cement, and also for direct air capture of CO2. Small scale demonstrations have been made, mostly in Europe and Japan, of concentrating sunlight to reach the required reaction temperatures, as high as 1,500℃. However, for commercial viability, these high temperatures must be reachable in summer and winter, through the day, not just for high DNI and small angles of incidence. To address these requirements, Angel’s team has developed a new kind of heliostat to achieve the maximum possible concentration set by thermodynamics. It does this by having the reflector shape twist automatically to eliminate the astigmatic aberration that prevents fixed-shape heliostats from imaging the solar disc. We show different configurations of heliostat field and secondary reflector that allow high reaction temperatures to be maintained, even at 500 DNI and solar elevation of 26 degrees.

This paper describes a novel heliostat metrology system in which the image formed by a full-size reflector of a nearby large test screen is viewed by a distant camera. The reflector shape is obtained from measurements of the magnification and distortion of a fine-scale regular pattern on the screen. This method is especially valuable for setting and testing the different shapes of a reflector for use in concentrating solar-thermal power (CSP) systems targeting high concentration. Maintaining such concentration throughout the day requires an accurate shape deformation according to the changing of sun’s altitude and incidence angle to reflectors. The proposed method has been tested on the single sheet float glass reflector measuring 2.4 m × 3.3 m and bent to focus sunlight at 113 m distance. From images obtained from a camera at 50m distance, the reflector surface was measured to an accuracy of 0.1 mrad rms slope error at 25 mm spatial resolution.

With 140+ petabytes of historical data holdings, 3.8 million square kilometers of daily multi-spectral collection, integration of Synthetic Aperture Radar and newly launching assets every quarter, the opportunities to develop insight from sense making technologies at Maxar are ever growing. During this discussion, we will cover the challenges of collecting, organizing, and exploiting multi source electro-optical remote sensing systems at scale using modern machine learning architectures and techniques to derive actionable insights.