Aplanatic solar concentrators for tubular absorbers.

A fundamentally new, to the best of our knowledge, class of linear (2D) dual-mirror aplanats tailored to tubular absorbers is developed for the types of solar concentrators used for thermal power. It is shown that prior investigation establishing this concept possesses unrecognized additional designs, as well as having missed high-performance configurations. It is shown that our line-focus solar concentrators can attain intercept factors exceeding 0.9 at concentration values as high as 55, with practical mirror contours and assemblies. Designed expressly for tubular absorbers, they represent improvements upon previous aplanatic concentrators that were tailored to flat one-sided absorbers but applied to tubular absorbers (as well as to conventional parabolic troughs).

Sine-limiting microcell solar concentrators for space.

Microcell concentrating photovoltaics (µCPV) have the potential to improve performance and reduce the cost of solar power in space. Here, we introduce an ultracompact V-cone tailored edge ray (V-TERC) concentrator, rooted in nonimaging optics, which enables operation near the sine limit. Relative to previous space µCPV implementations, this concentrator design enables an approximate four-fold increase in concentration ratio for a given acceptance angle and specific power. We validate the design through ray tracing simulations and construction of a proof-of-concept system that consists of a 650 × 650 µm2 triple-junction microcell bonded to a 3.1 mm-thick prototype V-TERC optic. In outdoor testing on a sunny day, the system achieves a power conversion efficiency of 30% at a geometric gain of 137× with a specific power of 90 W kg-1 and an acceptance angle of ±4.5°. This is a record combination for µCPV to date and represents an important step toward increasing efficiency and lowering the cost of solar power in space.

Enhanced Algal Photosynthetic Photon Efficiency by Pulsed Light.

We present experimental results demonstrating that, relative to continuous illumination, an increase of a factor of 3-10 in the photon efficiency of algal photosynthesis is attainable via the judicious application of pulsed light for light intensities of practical interest (e.g., average-to-peak solar irradiance). We also propose a simple model that can account for all the measurements. The model (1) reflects the essential rate-limiting elements in bioproductivity, (2) incorporates the impact of photon arrival-time statistics, and (3) accounts for how the enhancement in photon efficiency depends on the timescales of light pulsing and photon flux density. The key is avoiding "clogging" of the photosynthetic pathway by properly timing the light-dark cycles experienced by algal cells. We show how this can be realized with pulsed light sources, or by producing pulsed-light effects from continuous illumination via turbulent mixing in dense algal cultures in thin photobioreactors.

Design and demonstration of ultra-compact microcell concentrating photovoltaics for space.

Optical concentration can improve the efficiency and reduce the cost of photovoltaic power but has traditionally been too bulky, massive, and unreliable for use in space. Here, we explore a new ultra-compact and low-mass microcell concentrating photovoltaic (µCPV) paradigm for space based on the monolithic integration of transfer-printed microscale solar cells and molded microconcentrator optics. We derive basic bounds on the compactness as a function of geometric concentration ratio and angular acceptance, and show that a simple reflective parabolic concentrator provides the best combination of specific power, angular acceptance, and overall fabrication simplicity. This architecture is simulated in detail and validated experimentally with a µCPV prototype that is less than 1.7 mm thick and operates with six, 650 µm square triple-junction microcells at a geometric concentration ratio of 18.4×. In outdoor testing, the system achieves a terrestrial power conversion efficiency of 25.8 ± 0.2% over a ±9.5° angular range, resulting in a specific power of approximately 111 W/kg. These results lay the groundwork for future space µCPV systems and establish a realistic path to exceed 350 W/kg specific power at >33% power conversion efficiency by scaling down to even smaller microcells.

Expansive scope of aplanatic concentrators and collimators.

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.

Concentrated Sunlight for Materials Synthesis and Diagnostics.

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.

Comment on "Water harvesting from air with metal-organic frameworks powered by natural sunlight".

Kim et al (Reports, 28 April 2017, p. 430) presented results for the solar-driven harvesting of water from air via metal-organic frameworks as a prodigious potential advance toward remedying global water shortages. Basic thermodynamics and a survey of multiple off-the-shelf technologies show that their approach is vastly inferior in efficiency (and thereby in feasibility) to available alternatives.

Aplanatic Fresnel optics.

We provide a general formulation for the design of any dual-surface aplanatic Fresnel optic (including combinations of refractive and reflective surfaces), with categories of devices that had not previously been recognized. Raytrace simulations for representative Fresnel aplanats in collimation (illumination) mode reveal compact designs with radiative efficiencies close to those of their aplanatic continuous non-Fresnel counterparts, and optical performance approaching the thermodynamic limit for radiative transfer.

Aplanatic lenses revisited: the full landscape.

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.

Identifying Fundamental Limitations in Halide Perovskite Solar Cells.

The temperature dependence of the principal photovoltaic parameters of perovskite photovoltaics is studied. The recombination activation energy is in good agreement with the perovskite's bandgap energy, thereby placing an upper bound on the open-circuit voltage. The photocurrent increases moderately with temperature and remains high at low temperature, reinforcing that the cells are not hindered by insufficient thermally activated mobility or carrier trapping by deep defects.

New types of refractive-reflective aplanats for maximal flux concentration and collimation.

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.

Basic categories of dual-contour reflective-refractive aplanats.

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

Solar Synthesis of PbS-SnS2 Superstructure Nanoparticles.

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.

Basic limit for the efficiency of coherence-limited solar power conversion.

A basic upper bound for the efficiency of solar power conversion (generally, from any blackbody source) is derived, generalizing the Landsberg limit to arbitrary solar and sky view factors (e.g., arbitrary concentration or angular confinement), and to coherence-limited devices such as rectifying aperture antennas.

High-yield synthesis of silicon carbide nanowires by solar and lamp ablation.

We report a reasonably high yield (~50%) synthesis of silicon carbide (SiC) nanowires from silicon oxides and carbon in vacuum, by novel solar and lamp photothermal ablation methods that obviate the need for catalysis, and allow relatively short reaction times (~10 min) in a nominally one-step process that does not involve toxic reagents. The one-dimensional core/shell ß-SiC/SiOx nanostructures-characterized by SEM, TEM, HRTEM, SAED, XRD and EDS-are typically several microns long, with core and outer diameters of about 10 and 30 nm, respectively. HRTEM revealed additional distinctive nanoscale structures that also shed light on the formation pathways.

Exact analytic flux distributions for two-dimensional solar concentrators.

A new approach for representing and evaluating the flux density distribution on the absorbers of two-dimensional imaging solar concentrators is presented. The formalism accommodates any realistic solar radiance and concentrator optical error distribution. The solutions obviate the need for raytracing, and are physically transparent. Examples illustrating the method's versatility are presented for parabolic trough mirrors with both planar and tubular absorbers, Fresnel reflectors with tubular absorbers, and V-trough mirrors with planar absorbers.

First direct measurement of the spatial coherence of sunlight.

Direct sunlight is often deemed incoherent, hence unsuitable for antenna power conversion. However, all radiation exhibits spatial coherence when detected on a sufficiently small scale. We report the first direct measurement of the spatial coherence of solar beam radiation, achieved with a customized tabletop cyclic-shearing interferometer. Good agreement is found between experiment and theory, with promising ramifications for solar aperture antennas.

New high-temperature Pb-catalyzed synthesis of inorganic nanotubes.

A new procedure for the synthesis of MoS(2) nanotubes is reported, and additionally demonstrated for MoSe(2), WS(2), and WSe(2). Highly concentrated sunlight creates continuous high temperatures, strong temperature gradients, and extended hot annealing regions, which, together with a metallic (Pb) catalyst, are conducive to the formation of different inorganic nanotubes. Structural characterization (including atomic resolution images) reveals a three-step reaction mechanism. In the first step, MoS(2) platelets react with water-air residues, decompose by intense solar irradiation, and are converted to molybdenum oxide. Subsequently, the hot annealing environment leads to the growth of Pb-stabilized MoO(3-x) nanowhiskers. Shortly afterward, the surface of the MoO(3-x) starts to react with the sulfur vapor supplied by the decomposition of nearby MoS(2) platelets and becomes enveloped by MoS(2) layers. Finally, the molybdenum oxide core is gradually transformed into MoS(2) nanotubes. These findings augur well for similar syntheses of as yet unattained nanotubes from other metal chalcogenides.

Realizable planar gradient-index solar lenses.

The design of single element planar hemispherical gradient-index solar lenses that can accommodate the constraints of realistic materials and fabrication techniques are presented, and simulated with an extended and polychromatic solar source for concentrator photovoltaics at flux concentration values exceeding 1000 suns. The planar hemispherical far-field lens is created from a near-field unit magnification spherical gradient-index design, and illustrated with an f/1.40 square solar lens that allows lossless packing within a concentrator module.

Gradient-index lenses for near-ideal imaging and concentration with realistic materials.

Fundamentally new classes of spherical gradient-index lenses with imaging and concentration properties that approach the fundamental limits are derived. These analytic solutions admit severely constrained maximum and minimum refractive indices commensurate with existing manufacturable materials, for realistic optical and solar lenses.