Aplanats refer to inherently imaging optics that wholly eliminate both spherical aberration and coma. They typically comprise two refractive and/or reflective surfaces. For radiative transfer (which is typically nonimaging in nature), aplanats can closely approach the thermodynamic bounds for collimation and concentration, especially significant for light-emitting diodes (LEDs), solar energy, and infrared applications. Recently, we identified previously unrecognized basic categories of aplanats and showed how they can offer powerful new possibilities for LED collimation and for concentrating sunlight. Here, we review and elaborate the full scope of aplanat classifications, with illustrative examples of maximum-performance practical optics for all possible combinations of reflective and refractive contours. These examples subsume the latest invention of faceted (Fresnel) aplanats toward achieving greater compactness and lower mass. We also show how hybrid aplanats that combine the basic categories can improve concentrator and collimator performance. (C) 2019 Optical Society of America.

The pragmatic virtues of ground-level receivers in solar towers have long been recognized, but the associated beam-down optics reduce concentration, resulting in higher heat loss and cost, or the need for an actively-cooled tertiary concentrator that incurs additional optical losses. Here, we describe the concept of an aplanatic beam-down solar tower, where concentration can be increased without the need for a tertiary concentrator, while retaining a ground-level receiver. The basis for satisfying aplanatism constitutes tailoring the contour of a stationary secondary mirror atop the tower with a distinct heliostat tracking strategy. Conflating the aplanatic solar tower with the option of multiple towers, where a heliostat can be aimed at different targets depending on solar geometry, can markedly reduce shading, blocking and cosine losses. Also, a system of many mini-towers each of which is only a few meters in height could enable systems that are more modular, efficient and easily-maintained.

Aplanatic optics were invented over a century ago, motivated principally to achieve high-fidelity imaging in telescopes, microscopes and cameras. Aplanats are designed to completely eliminate the two leading orders of geometric aberration spherical and comatic - and the simplest designs comprise two contours that can be reflective and/or refractive. Aplanats of high radiative efficiency can also approach the thermodynamic limit to flux concentration and light collimation - of particular value in nonimaging applications such as solar energy collection, light-emitting-diode collimation, and infrared technology. Recently, it was discovered that the original aplanatic mirrors and lenses cover only a small spot in a rich landscape of fundamental categories of optical devices, which opened a broad spectrum of powerful new designs. In this presentation we review these advances, and summarize the complete classification schemes that have now been elucidated for aplanats. They include examples of practical designs for achieving radiative transfer near the thermodynamic limit in flux concentration and irradiation applications, based on dual-mirror, dual-contour lens and lens-mirror combinations. The representative designs that are illustrated also include the most recent progress in Fresnel (faceted) aplanats, motivated by the quest for progressively more compact optical systems, as well as examples of hybrid designs - combining aplanats of different classifications for enhanced performance.

Herein, the use of highly concentrated sunlight for materials science research is reviewed. Specific research directions include: (1) the generation of inorganic nanostructures, some of which had eluded experimental realization with conventional synthetic processes, and (2) elucidating the processes governing the degradation of organic and perovskite-based photovoltaic materials and devices, along with accelerated assessment of their stability. Both approaches employ solar concentrators capable of producing flux densities exceeding those of terrestrial solar radiation by up to three orders of magnitude, and are geared toward either creating extensive ultrahot reactor conditions conducive to the rapid, safe synthesis of unusual nanomaterials or judiciously interrogating photovoltaic devices.

New types of core–shell nanoparticles are reported: Pb@GaS fullerene-like and nanotubular structures, achieved via the continuously high reactor temperatures and ultra-hot strong-gradient annealing environments created by highly concentrated sunlight. Structural and chemical characterizations suggest a formation mechanism where vaporized Pb condenses into nanoparticles that are stabilized as they become covered by molten GaS, the ensuing crystallization of which creates the outer layers. Hollow-core GaS fullerene-like nanoparticles and nanotubes were also observed among the products, demonstrating that a single solar procedure can generate a variety of core–shell and hollow nanostructures. The proposed formation mechanisms can account for their relative abundance and the characterization data.

Heat losses from line focus solar collectors, such as parabolic troughs and Linear Fresnel Reflectors (LFR), are reduced by using selectively coated vacuum heat collector elements (HCE). The main parameters governing convective and radiative heat losses are the metal tube's outer surface radiative emittance and the vacuum level between tube and glass envelope. We investigate the effect of surface roughness on tube emittance as function of substrate roughness. Through a series of experiments, using several surface polishing techniques, such as grinding processes and electro polishing, a good correlation between tube surface roughness and heat loss was obtained. We show the limitations of representing the surface roughness by only using the roughness average (Ra) or the root mean square (RMS) roughness (Rq), especially when an electro polishing process is involved. A better metric is the roughness slope (1/Er) or RMS slope (RΔq) in addition to the roughness height (Rz) for which a good correlation to heat loss is demonstrated. While the electro polishing process is shown to reduce emittance and therefore heat losses, less costly and simpler changes in the grinding process can lead to a similar result. Recommendations for metal tube roughness yielding minimum heat loss are presented.

The full scope of solutions for dual-contour aplanatic lenses that can approach the basic limit for radiation transfer are identified, analyzed, and illustrated. Complementary solutions are shown to yield lenses that are either monolithic or have two refractive contours separated by an air gap. The performance of a promising representative design for LED collimation is presented.

We identify fundamentally new classes of aplanatic lenses where the focus resides inside the lens. These new aplanatic designs comprise a primary contoured dielectric entry, and a secondary contoured profile that, in general, is mirrored, but also admit solutions satisfying total internal reflection. We show that these aplanatic lenses engender 8 basic, distinct design categories, of which 6 yield physically admissible solutions. Flux concentration for far-field small-angle sources such as the sun and, conversely, narrow-field collimation of wide-angle emitting light sources such as LEDs can approach the thermodynamic limit. Losses due to chromatic aberration are smaller than in conventional lenses of comparable f-number, primarily due to the focus being in the lens. By the same token, exit numerical aperture can be increased by a factor of n (the dielectric's refractive index) - and hence flux concentration can be increased by a factor of n(2) - relative to common lenses where the focus resides outside the lens.

We identify and evaluate a variety of efficient and feasible micro-optics for confining the radiative emission of solar cells. The key criteria used for assessing viable designs are (1) high optical efficiency for both the transmission of impinging solar beam radiation and the external recycling of isotropic cell luminescent emission; (2) liberal optical tolerance; (3) compactness; and (4) being amenable to fabrication from existing materials and manufacturing processes. Both imaging and nonimaging candidate designs are presented, and their superiority to previous proposals is quantified. The strategy of angular confinement for boosting cell open-circuit voltage-thereby enhancing conversion efficiency-is limited to cells where radiative recombination is the dominant carrier recombination pathway. Optical systems that restrict the angular range for emission of cell luminescence must, by reciprocity, commensurately restrict the angular range for the collection of solar radiation. This, in turn, mandates the introduction of concentrators, but not for the objective of delivering concentrated flux onto the cell. Rather, the optical system must project an acceptably uniform spatial distribution of solar flux onto the cell surface at a nominal averaged irradiance of 1 sun. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)

We derive, illustrate, and analyze previously unrecognized basic categories of dual-contour reflective-refractive aplanats, evaluated for solar concentration.

We report the synthesis and supporting density-functional-theory computations for a closed-cage, misfit layered-compound superstructure from PbS-SnS2, generated by highly concentrated sunlight from a precursor mixture of Pb, SnS2, and graphite. The unique reactor conditions created in our solar furnace are found to be particularly conducive to the formation of these nanomaterials. Detailed structural and chemical characterization revealed a spontaneous inside-out formation mechanism, with a broad range of nonhollow fullerene-like structures starting at a diameter of ∼20 nm and a wall thickness of ∼5 layers. The computations also reveal a counterintuitive charge transfer pathway from the SnS2 layers to the PbS layers, which indicates that, in contrast to binary-layered compounds where it is principally van der Waals forces that hold the layers together, polar forces appear to be as important in stabilizing superstructures of misfit layered compounds.

We identify and evaluate new categories of dual-contour refractive-reflective aplanatic lenses, some of which can satisfy total internal reflection at the secondary surface. Raytrace simulations for a representative design in both solar concentrator and collimator (illumination) mode reveal high efficiency while approaching the thermodynamic limit for radiative transfer.

Enhancing solar cell conversion efficiency by angular confinement of radiative emission (photoluminescence) requires a combination of (1) high external luminescent efficiency, and (2) optics that can substantially and efficiently limit the angular range of cell luminescence. After covering the basic principles and recent proposals for suitable micro-optics, we investigate an assortment of alternative micro-optical designs that can improve device compactness considerably, which would reduce the amount of material required and would ease micro-fabrication, while offering liberal optical tolerance and high collection efficiency.

Although one approach for improving the performance of ultra-efficient solar cells is through optical concentration (principally by increasing open-circuit voltage, VOC), an equally potent alternative is decreasing the cell’s recombination current – an aim that can be realized by externally recycling cell photon emission. While the theory for the potential benefit of photon recycling has recently been elucidated, experimental proof-of-concept had proven elusive requiring a photovoltaic device possessing a high external luminescent efficiency (Qe) combined with efficient light recycling optics. The associated prospect of approaching – and even surpassing – the nominally fundamental (Shockley-Queisser) limit for solar cell conversion efficiency [1] is tantalizing and, in principle, achievable. Here, we report experimental evidence of enhancing the performance of today’s champion singlejunction commercial GaAs cells [2] by external recycling of photon emission from the cell’s front surface [3]. It is equivalent to restricting the angular range of photon emission, and can only be effective in photovoltaics with high external luminescent efficiency.

We present experimental evidence for improving the open-circuit voltage - and thereby efficiency - of photovoltaics via the external recycling of photon emission. This strategy is equivalent to limiting the angular extent of photon emission - effective only in photovoltaics with high external luminescent efficiency. This is why the effect has not been observed in current solar cell technologies. It is attainable with the latest generation of ultra-efficient single-junction non-concentrator thin-film GaAs cells. The findings are explained in terms of basic photovoltaic thermodynamics.

Spectrally selective coatings are common in low and medium temperature solar applications from solar water heating collectors to parabolic trough absorber tubes. They are also an essential element for high efficiency in higher temperature Concentrating Solar Power (CSP) systems. Selective coatings for CSP are usually prepared using advanced expensive methods such as sputtering and vapor deposition. In this work, coatings were prepared using low-cost wet-chemistry methods. Solutions based on Alumina and Silica sol gel were prepared and then dispersed with black spinel pigments. The black dispersions were applied by spray/roll coating methods on stainless steel plates. The spectral emissivity of sample coatings was measured in the temperature range between 200 and 500 degrees C, while the spectral absorptivity was measured at room temperature and 500 degrees C. Emissivity at wavelengths of 0.4-1.7 mu m was evaluated indirectly using multiple measurements of directional reflectivity. Emissivity at wavelengths 2-14 mu m was measured directly using a broadband IR camera that acquires the radiation emitted from the sample, and a range of spectral filters. Emissivity measurement results for a range of coated samples will be presented, and the impact of coating thickness, pigment loading, and surface preparation will be discussed.

The flow of water in pipes has been measured by placing thermistors at two points along a pipe's surface and measuring the temperature variations (on the order of 30 sec duration) as the fluid passes. Cross correlation of the two sets of temperature versus time data is used to determine the time delay from the upstream to the downstream sensor. The distance between the two sensors is then used to calculate an estimate of the bulk fluid velocity. This technique was first used in the laboratory on a half-inch copper pipe. The temperature variations were introduced with a propane torch applied to the pipe upstream from the sensors and data were taken using a microcomputer with an analog input module. Thé flow rate calculated using cross correlation was lower than that obtained by timing the flow into a graduated cylinder, especially at the higher rates tested. Significantly better agreement was obtained in tests where the temperature variations were introduced using valves to vary the mix of co Id and hot water. For these tests, flow rates calculated were less than 20% below the actual values. The observed consistent underestimation was hypothesized to be an effect of either the boundary layer or the heat capacities of the fluid and the pipe. A simpIe plug f~ow simulation of the thermal interaction between the fluid and the pipe gave accurate flow rates thus indicating that the consistent underestimation was caused by the fluid boundary layer. The technique was tested on a two-inch copper pipe of the domestic hot water system in the boiler room of a 62 unit apartment building which had also been instrumented with in-line flow meters. This test showed that sufficient temperature variations exist in the normal operation of the system to deduce flow rates with an uncertainty of less than 20%. The method appears to be a useful diagnostic tooI. More experimentation and testing is suggested in order for this technique to be used with confidence on pipes of various sizes and compositions and in various flow regimes.