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.
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 n2 - 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.
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.
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°C, while the spectral absorptivity was measured at room temperature and 500°C. Emissivity at wavelengths of 0.4–1.7 μm was evaluated indirectly using multiple measurements of directional reflectivity. Emissivity at wavelengths 2–14 μm 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.
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.
We report the first direct measurement of the spatial coherence of solar beam radiation. Although often perceived as
incoherent, direct sunlight exhibits spatial coherence at a sufficiently small scale. These dimensions were recently
derived theoretically to be around two orders of magnitude greater than the wavelength. The partial coherence of
sunlight raises tantalizing prospects for a new paradigm for solar power conversion via the antenna effect exploited so
successfully in radio-frequency and microwave technologies (albeit at frequencies of order 1 PHz for solar). After
reviewing the equal-time mutual coherence function of sunlight, we explain the particular suitability of a lateral cyclicshearing interferometer wherein the solar beam is split into two parts that are subsequently recombined with a relative
lateral displacement. The method is relatively uncomplicated, inexpensive and obviates the problem of component
dispersion (potentially problematic for a light source as broadband as sunlight). The experimental results are in good
agreement with the recent theoretical predictions.
KEYWORDS: Mirrors, Solar concentrators, Tolerancing, Solar cells, Geometrical optics, Dielectrics, Manufacturing, Optics manufacturing, Optical alignment, Chemical elements
Dual-mirror aplanatic optics - recently developed for concentrator photovoltaics - can offer efficient, ultra-compact,
high-irradiance solar concentration. However, intrinsic limitations site the focus inside the concentrator and hence
engender a dielectric terminal concentrator to deliver the concentrated radiation to the photovoltaic cell outside the optic,
with the associated requirement of an optical bond to the cell. Can a modification in the design strategy place the focus
outside the optic - and hence obviate the need for a terminal concentrator and optical bond - without compromising
compactness, low shading losses, or even the practical virtue of the primary and secondary mirrors being coplanar
toward easing optical component alignment? We show how nested dual-mirror aplanats can satisfy all these goals,
supported by raytrace performance evaluations.
A high irradiance solar furnace geared toward elucidating the distinctive physics of concentrator photovoltaics and
driving high-temperature reactors for the generation of novel nanostructures is described, with a target irradiance up to
12 W/mm2. The opto-mechanical design permits real-sun flash illumination at a millisecond time scale so that solar cells
can be characterized with only insubstantial increases in cell temperature even at irradiance levels of thousands of suns.
KEYWORDS: Solar concentrators, Thermodynamics, Mirrors, Photovoltaics, Reflectors, Thermography, Image quality, Optical spheres, Space telescopes, Near field optics
Originally developed to ameliorate image quality, aplanatic optics were only recently analyzed as maximumperformance
light-transfer systems with concentration approaching the fundamental limit. A basic categorization scheme
is presented that appears to cover the full spectrum of aplanatic designs, illustrated for far-field dual-mirror concentrators and motivated by high-irradiance solar applications. Several previously unrecognized classes of concentrators are identified.
Some current high-concentration photovoltaic designs are based on folded mirror optics, i.e., the optical path is reversed
via reflection in order to achieve minimal aspect ratio. One apparently overlooked class of aplanats is the unfolded dualmirror
aplanat explored in this presentation. The confluence of practical constraints presented by many highconcentration
photovoltaic systems creates a niche for unfolded aplanatic optics that permit the elimination of optical
bonds between dielectric elements and solar cells while achieving an angular tolerance near the fundamental limit. The
long-term material integrity of the optical bonds - especially at high irradiance - remains to be established and has been
viewed as potentially precarious. Furthermore, the unfolded optic and the target can be decoupled. The disadvantage of
unfolded optics is depth: concentrator aspect ratios far larger than the fundamental compactness bound realizable with
folded optics. However, with high-concentration photovoltaics evolving to millimeter-scale cells, the corresponding
depth of the concentrator would be only a few centimeters: compatible with precision large-volume fabrication
techniques. We also show how a lens at the center of the aplanat's glazing permits a noticeable reduction in concentrator
depth. A fringe benefit is the paucity of hot spots on the optical elements, creating the option of coated polymeric
reflectors, which reduces mass and cost.
KEYWORDS: Near field optics, Lamps, Mirrors, Near field, Monochromatic aberrations, Geometrical optics, Plasma, Solar concentrators, Radiative transfer, Reflectors
In the design of high-temperature chemical reactors and furnaces, as well as high-radiance light projection applications,
reconstituting the ultra-high radiance of short-arc discharge lamps at maximum radiative efficiency constitutes a
significant challenge. The difficulty is exacerbated by the high numerical aperture necessary at both the source and the
target. Separating the optic from both the light source and the target allows practical operation, control, monitoring,
diagnostics and maintenance. We present near-field unfolded aplanatic optics as a feasible solution. The concept is
illustrated with a design customized to a high-temperature chemical reactor for nano-material synthesis, driven by an
ultra-bright xenon short-arc discharge lamp, with near-unity numerical aperture for both light input and light output. We
report preliminary optical measurements for the first prototype, which constitutes a double-ellipsoid solution. We also
propose compound unfolded aplanats that collect the full angular extent of lamp emission (in lieu of light recycling optics) and additionally permit nearly full-circumference irradiation of the reactor.
Reactors driven by highly concentrated sunlight can create conditions well suited to the synthesis of inorganic
nanomaterials. We report the experimental realization of a broad range of closed-cage (fullerene-like) nanostructures,
nanotubes and/or nanowires for MoS2, SiO2 and Si, achieved via solar ablation. The solar technique generates the strong
temperature and radiative gradients - in addition to the extensive high-temperature annealing environment - conducive
to producing such nanostructures. The identity of the nanostructures was established with TEM, HRTEM and EDS. The
fullerene-like and nanotube MoS2 configurations achieved fundamentally minimum sizes predicted by molecular
structural theory. Furthermore, our experiments represent the first time SiO2 nanofibers and nanospheres have been
produced purely from quartz. The solar route is far less energy intensive than laser ablation and other high-temperature
chemical reactors, simpler and less costly.
The brightness of many lamps, and hence the attainable power density at the target application, can be enhanced by
recycling light back into the lamp's radiant zone. We report measurements of the effectiveness, spectral characteristics
and modified plasma brightness maps that result from light recycling with a specular hemispherical mirror in commercial
150 W ultra-bright Xenon short-arc discharge lamps. Lamp brightness can be increased by up to 70% for certain spectral
windows and plasma arc regions. However, lamp geometry reduces overall light recycling effectiveness to about half
this value. This study was motivated by biomedical and high-flux furnace applications where the full spectrum of lamp
emissions can be exploited, heightened brightness allows a broader range of procedures, and the design of the affiliated
optical systems is contingent upon how plasma radiometric characteristics are altered by photon regeneration.
The latest generations of ultra-bright Xenon short-arc discharge lamps have prodigious emissions outside the visible
spectrum, primarily in the near infrared. Their brightness distributions are spatially and angularly inhomogeneous due to
both the pronounced non-uniformities of the plasma arc and the substantial infrared radiation from the hot electrodes.
These characteristics are fortuitously favorable for applications in photonic surgery, biomedical diagnostics, high-temperature
chemical reactors and furnaces: cases where the full lamp spectrum is utilizable, and the key is
reconstituting the spectral power density of the optimal regions of the lamp's plasma at a remote target. The associated
optical systems must be tailored to lamp radiometric properties that are not extensively available and invariably are
restricted to visible light due to their widespread use in projection systems. We present experimental measurements for
the spectral, spatial and angular distributions of 150 W lamps of this genre, and relate to their ramifications for
broadband high-flux applications.
Essentially loss-less all-dielectric micro-fabricated optics can be tailored to completely eliminate the shadowing losses metallization grids create on the surface of concentrator solar cells. The nonimaging micro-concentrator exploits total internal reflection to redistribute the elevated flux from available macro-concentrators, rather than increasing overall concentration. The optical designs permit widening the metal fingers toward lessening series resistance losses, which can also finesse the need for the intricate metallization patterns of some high-flux cells. Realistic net efficiency gains of ~15% (relative) are achievable in a wide variety of concentrator cells.
KEYWORDS: Near field optics, Dielectrics, Mirrors, Near field, Chromatic aberrations, Lamps, Solar concentrators, Spherical lenses, Radiative transfer, Optical components
Aplanatic optics crafted from transparent dielectrics can approach the etendue limit for radiative transfer in pragmatic
near-field systems. Illustrations are presented for the more demanding realm of high numerical aperture (NA) at the
source and/or target. These light couplers can alleviate difficulties in aligning system components, and can achieve the
fundamental compactness limit for optical devices that satisfy Fermat's principle.
The macroscopic parameters that characterize photovoltaic (PV) performance, including their spatial dependence,
especially at high flux, are determined with extensive localized solar measurements on high-efficiency concentrator solar
cells. We present two studies that explore (a) the impact of nonuniform flux distribution on PV behavior, (b) how PV parameters vary across the cell surface (of particular interest in many high-concentration optical systems) and (c) the
sensitivity of PV parameters to the spatial variation of series resistance Rs that stems from irregular cell metallization. In
so doing, we identify current-voltage trends unique to strongly inhomogeneous illumination and to Rs losses at high flux.
We report the first realization of interstitial surgery by ultrabright lamp light, on the kidneys and livers of live animals. A high-flux optic concentrates lamp emissions into an optical fiber for power delivery inside the body. The trials reveal surgical efficacy comparable to corresponding laser fiber optic treatments, as well as pronounced delayed tissue death.
We report the rapid high-yield generation of inorganic fullerene-like cesium oxide (IF-Cs2O) nanoparticles, activated by
highly concentrated sunlight. The solar process represents an alternative to the only reported method for synthesizing
IF-Cs2O nanostructures: laser ablation. IF-Cs2O formed at solar irradiation greater-than or equal to 6W, confirmed by high resolution
transmission electron microscopy. These closed-cage Cs2O nanostructures are stable under electron microscope conditions, and also when exposed temporarily to air - of significance for their use in a variety of photonic devices.
KEYWORDS: Near field optics, Dielectrics, Thermodynamics, Near field, Mirrors, Solar concentrators, Chromatic aberrations, Spherical lenses, Lamps, Thermography
Practical aplanatic optics crafted from transparent dielectrics can approach the thermodynamic limit for radiative transfer in near-field systems. Designs are presented for the particularly challenging realm of high numerical aperture (NA) at the source and/or target. These light couplers can alleviate difficulties in aligning system components and can achieve the fundamental compactness limit for optical devices that satisfy Fermat's principle. Examples and performance estimates based on ray-trace simulations are presented.
We report pronounced delayed tissue death in photothermal surgery performed on the livers of live healthy rats with highly concentrated sunlight (ultrabright noncoherent light). Exposure times and power levels were selected to produce immediate necroses of the order of hundreds of cubic millimeters. Pathology reveals that lesion volumes increase by up to a factor of 5 within approximately 24 h after surgery, and then stabilize. Islands of viable cells can persist within damaged tissue, in the immediate vicinity of blood vessels, but also necrose within about 48 h.
KEYWORDS: Near field optics, Lamps, Mirrors, Near field, Fiber couplers, Solar concentrators, Optical fibers, Thermodynamics, Optical spheres, Light sources
Compact aplanatic concentrators can reconstitute the flux of a near-field ultrabright incoherent light source and couple it into an optical fiber. Performance near the thermodynamic limit for light transfer can be realized, even at high numerical aperture. The prospect of arc-discharge lamps as alternatives to lasers for many surgical fiber-optic procedures motivate this investigation. LED-fiber and fiber-fiber coupling constitute additional applications. The contours of these achromatic mirrored systems are analytic functions, which facilitates rapid surveying of a wide range of design options.
We explore compact, imaging (aplanatic) designs capable of reconstituting the power density of ultra-bright lamps at a remote target, with specific application to optical fiber light transport. The solutions presented here, for concentrating incoherent light from an extended near-field source, closely approach the thermodynamic limit for optical performance at high collection efficiency, as confirmed with raytrace simulations. Our investigations are motivated by the prospect of arc-discharge lamps as effective alternatives to lasers for many surgical procedures where high intensity heating rather than monochromaticity is needed. Additional applications include LED-fiber and fiber-fiber coupling, as well as projection systems. Consideration is restricted to pure reflective (mirrored) systems since refractive elements can introduce inadmissibly large chromatic aberrations. The tailored aplanatic mirror contours constitute monotonic functions that can be solved analytically, which facilitates rapid surveying of a wide range of design options. Several near-field designs suitable for coupling high numerical aperture light sources into optical fibers are presented.
The materialization of a recent conceptual advance in high-flux photovoltaic concentrators into first-generation prototypes is reported. Our design strategy includes a tailored imaging dual-mirror (aplanatic) system, with a tapered glass rod that enhances concentration and accommodates larger optical errors. Designs were severely constrained by the need for ultra-compact (minimal aspect ratio) modules, simple passive heat rejection, liberal optical tolerances, incorporating off-the-shelf commercial solar cells, and pragmatic considerations of affordable fabrication technologies. Each unit has a geometric concentration of 625 and irradiates a single square 100 mm2 triple-junction high-efficiency solar cell at a net flux concentration of 500.
We report results of ultra-high-flux experiments on tandem and triple-junction solar cells, with a real-sun probe predicated on mini-dish fiber-optic concentrators. We focus on the sensitivity of cell efficiency to a wide range of flux levels and distributions. Our experiments also revealed pronounced reversible photovoltaic hysteresis at high concentration, and provide a non-destructive method for assessing tunnel diode characteristics.
We report pronounced delayed tissue death in photothermal surgery performed with highly concentrated sunlight on the livers of healthy live rats. Pathology reveals that lesion volumes increase by up to a factor of 5 within approximately 24 h after surgery, and then stabilize. Islands of viable cells can persist within damaged tissue, in the immediate vicinity of blood vessels, but also necrose within about 48 h. Delayed cell death is an unambiguously non-thermal process, apparently linked solely to biochemical messengers. The dramatic enlargement of the affected region appears to have been essentially overlooked in laser surgery studies. The ramifications include (a) proper gauging of the required scale of tissue damage during surgery, toward averting excessive destruction of untargeted surrounding tissue; and (b) avoiding false positives from the substantial amount of tissue that appears viable immediately after surgery but will necrose within 24 h. The comparable performance of high-flux solar and concentrated laser light for hyperthermic treatments permits effective surgery and the probing of tissue death dynamics with a solar energy system that is simpler and markedly less expensive than surgical lasers.
Purely imaging strategies can offer ultra-compact concentrators and illuminators both of which approach the thermodynamic limit to optical performance. The tailored optical surfaces are monotonic functions that can be expressed analytically, which can facilitate optimization studies as well as practical fabrication. Also referred to as aplanatic designs, devices with two smoothly contoured surfaces can compete with, and even exceed, the performance of high-flux nonimaging systems. We develop and analyze tailored imagin devices for two-stage reflector systems of varying numerical aperture, motivated by applications in solar concentration as well as light collimation.
Experimental results generated with novel miniature fiber-optic concentrators and commercial tandem III-V concentrator solar cells are reported, including (1) measured power densities up to 10,000 suns, (2) solar cell efficiencies in excess of 30% and (3) totally passive cooling. Mini-dish concentrators (a) generate uniform and individualized cell illumination, (b) allow assembly from readily available elements, and (c) are devoid of chromatic aberration. Measurements include the sensitivity of conversion efficiency to (i) power input, (ii) extreme flux inhomogeneities and (iii) the modified spectrum from fiber-optic concentrators. The weak sensitivity of cell performance to acute non-uniformities in flux map is addressed with a relatively simple model that regards the cell as an effective parallel connection of its uniformly irradiated areal elements. Our findings bode favorably for the feasibility of such concentrator designs at concentration levels as high as thousands of suns.
New strategies for the efficient use of concentrated sunlight to synthesize carbon nanomaterials are described - approaches that offer a potentially far less expensive production facility that is also amenable to being scaled up, in contrast to the conventional costly technologies of laser ablation furnaces and plasma discharge chambers. Our designs employ solar fiber-optic mini-concentrators that completely decouple the collection and remote indoor delivery of solar radiation into a high-temperature nanomaterial reactor. High flux on the target graphite rod is produced by the overlap of low numerical aperture concentrator units - a strategy that also acommodates the sizable gap required between the target inside the reactor and the distal fiber tips on the reactor exterior. The reactor incorporates a nonimaging photon regenerator that traps thermal radiation emitted from the target. This in turn allows a dramatic reduction in solar input relative to earlier solar nanomaterial furnaces. Designs and performance estimates are provided for a systemwith target temperatures in excess of 3000 K, and hence with significant nanomaterial yields.
KEYWORDS: Optical fibers, Cladding, Signal attenuation, Solar concentrators, Refractive index, Light sources and illumination, Systems modeling, Interfaces, Manufacturing, Reflectivity
The optical performance of remote lighting systems and recent innovations in solar fiber-optic concentrators is acutely sensitive to transmission losses in their optical fibers. Typically, these multi-mode fibers are expected to incur small losses over long distances for broad-spectrum light sources. Experimental results reveal substantial light leakage within the nominal numerical aperture of optical fibers that have been deemed suitable for these applications. The same fibers exhibit negligible attenuation in their core. Of particular interest is the dependence of this leakage on: (a) incidence angle, (b) the optical properties of the core and the cladding, and (c) fiber length. We present laboratory measurements of fiber angular transmission, along with a theoretical model.
A new approach for concentrating photovoltaic systems that can easily attain the maximum flux levels commensurate with solar cell technology is proposed. The collection unit is a miniature paraboloidal dish which concentrates sunlight into a short glass rod. The flux distribution of the transported light is homogenized in a miniature glass kaleidoscope that is optically coupled to a small high-efficiency solar cell. The cell resides behind the dish and can be cooled adequately with a passive heat sink. These nominally independent collection units can be assembled into modules and arrays that produce almost any prescribed power level. All system elements are predicated on existing technologies.
A new approach to the efficient collection and remote delivery of concentrated solar energy is proposed. The system's building block is a miniature (e.g., 0.2 m diameter) dish which concentrates sunlight into a single optical fiber. A number of mini-dishes comprise a module from which the optical fibers transport bundled power to a remote receiver. A second-stage nonimaging concentrator can boost flux levels to those approaching the thermodynamic limit and can be performed either in each individual dish or collectively in one or more larger devices at the entrance to the remote receiver. There are substantial advantages in efficiency, compactness, reduced mechanical loads, and ease of fabrication and installation relative to conventional solar designs. The design exploits the availability of low- attenuation optical fibers of high numerical aperture, as well as the practical advantages of mass producing highly accurate very small parabolic dishes. Designs for maximum efficiency attaining collection efficiencies as high as 80%, and maximum-concentration designs realizing flux levels of 30,000 suns, are achievable.
We develop a range of practical nonimaging devices for optical fiber applications where rays emerging from a fiber over a restricted angular range (small numerical aperture) must illuminate a small near-field detector at maximum radiative efficiency. These designs range from pure reflector (all-mirror), to pure dielectric (refractive and based on total internal reflection), to lens-mirror combinations. Sample designs are presented for a specific infrared fiberoptic irradiation problem of practical interest. Optical performance is checked with computer 3D raytracing. Compared to conventional imaging solutions, our new nonimaging units offer considerable practical advantages in compactness and ease of alignment, as well as noticeably superior radiative efficiency.
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