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.

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.

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.

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.

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.

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°.

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.

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 performance.

Terrestrial radiative cooling utilizes materials that allow solar radiation to pass through or reflect away while efficiently radiating heat across the atmospheric transmission window. Here we show that liquid water, when its thickness is in a proper range, is highly transparent to the solar spectrum and emits strongly in the infrared. By incorporating a solar reflector underneath, a thin water layer of proper thickness exhibits a solar reflectivity of 96.6% and a thermal emissivity of 0.94. During outdoor testing, the water-based radiative cooler achieved sub-ambient temperatures of 6 °C at noon and 11 °C around 18:00 when direct sunlight was absent. Notably, the water temperature remained below freezing for most of the day, despite ambient temperatures above 0 °C.