Optical properties of some luminescent solar concentrators

Luminescent solar concentrator (LSC) samples with polymethyl methacrylate (PMMA) as the matrix substrate have been prepared by a casting method using the organic laser dyes: Rh.6G, Rh.B and K1. Optical absorption measurements have been carried out in the wavelength region 200–900 nm at room temperature before and after the samples have been exposed to sunlight for up to 95 days. The obtained spectra in the visible region are interpreted in terms of photodegradation of the dye molecules. On the other hand, the u.v. spectra and the optical absorption edge are analysed in the light of approaches delivered by Urbach and by Davis and Mott.     

Efficiency of luminescence in luminescent solar concentrators

The power efficiency of luminescence excited by solar radiation in luminescent solar collectors is calculated for a glass sheet doped with Cr3+. The achievable chemical potential for an optically thick absorber irradiated by diluted blackbody radiation as a function of Cr3+ concentration, sheet thickness, sunlight dilution, and luminescence quantum yield leads directly to overall conversion efficiency of solar power to luminescence power.     

Optical performance of axisymmetric edge-ray concentrators and illuminators

The optical performance of axisymmetric radiation concentrators and illuminators that are derived when two-dimensional edge-ray designs are rotated about their optic axis is investigated. Of particular interest are devices with spherical and cylindrical absorbers or light sources, for which the inherent ray rejection can be substantial. From the principle of etendue (phase-space) conservation, a lower bound for ray rejection can be established. With computer ray tracing, we demonstrate that this bound underestimates the actual ray rejection by only a few percent at most. Hence, to a good approximation, it can be used as an equality in analytic predictions of characteristic efficiency-concentration curves. By designing for absorbers or sources with a bald spot, the full range of efficiency and flux concentration values can be realizedand the trade-off between them can be quantified. The optical performance of these edge-ray designs is also compared against fundamental upper bounds on the flux concentration and efficiency of axisymmetric devices.     

Influence of waveguide material on light emission in luminescent solar concentrators

Five different substrates were tested for their suitability for use as waveguides for luminescent solar concentrators for solar energy applications. The function of such a waveguide is to collect light and to transport it towards a solar cell. In this study the influence of the material of the waveguide on the light emission of the system was investigated. It was determined that the material chosen for the waveguide was not as important as the presence or absence of additives in the waveguide itself. This indicates that economic factors (material cost) and secondary properties such as refractive index, outdoor lifetime and weight are at least as important in the waveguide selection as the nature of the materials used.     

Exciton recycling via InP quantum dot funnels for luminescent solar concentrators

Luminescent solar concentrators (LSC) absorb large-area solar radiation and guide down-converted emission to solar cells for electricity production. Quantum dots (QDs) have been widely engineered at device and quantum dot levels for LSCs. Here, we demonstrate cascaded energy transfer and exciton recycling at nanoassembly level for LSCs. The graded structure composed of different sized toxic-heavy-metal-free InP/ZnS core/shell QDs incorporated on copper doped InP QDs, facilitating exciton routing toward narrow band gap QDs at a high nonradiative energy transfer efficiency of 66%. At the final stage of non-radiative energy transfer, the photogenerated holes make ultrafast electronic transitions to copper-induced mid-gap states for radiative recombination in the near-infrared. The exciton recycling facilitates a photoluminescence quantum yield increase of 34% and 61% in comparison with semi-graded and ungraded energy profiles, respectively. Thanks to the suppressed reabsorption and enhanced photoluminescence quantum yield, the graded LSC achieved an optical quantum efficiency of 22.2%. Hence, engineering at nanoassembly level combined with nonradiative energy transfer and exciton funneling offer promise for efficient solar energy harvesting.     

Light transport in planar luminescent solar concentrators: the role of DCM self-absorption

The influence of self-absorption in a 4-dicyano-methylene-2-methyl-6-p-dimethyl amino-styrl-4H-pyran (DCM) doped polymethylmethacrylate (PMMA) optical waveguide on the light transport efficiency has been evaluated. A Monte Carlo technique was used to simulate intermolecular energy transfer and calculate the energy emission profile of an active waveguide. The calculated and measured edge emission profiles were found to be in excellent agreement. The edge emission spectra for various distances of excitation from the edge were used to estimate the DCM self-absorption cross section.     

Reactive self-tracking solar concentrators: concept, design, and initial materials characterization

étendue limits angular acceptance of high-concentration photovoltaic systems and imposes precise two-axis mechanical tracking. We show how a planar micro-optic solar concentrator incorporating a waveguide cladding with a nonlinear optical response to sunlight can reduce mechanical tracking requirements. Optical system designs quantify the required response: a large, slow, and localized increase in index of refraction. We describe one candidate materials system: a suspension of high-index particles in a low-index fluid combined with a localized space-charge field to increase particle density and average index. Preliminary experiments demonstrate an index change of aqueous polystyrene nanoparticles in response to a low voltage signal and imply larger responses with optimized nanofluidic materials.     

Optical designs for ultrahigh-flux infrared and solar energy collection: monolithic dielectric tailored edge-ray concentrators

A new class of optical designs is developed for attaining ultrahigh flux in infrared and solar energy concentrators. These concentrators are required to satisfy simultaneously three criteria: (1) being monolithic, i.e., comprising a single piece of dielectric such that no mirrored surfaces or air spaces between concentrator elements are introduced; (2) attaining at least 90% of the thermodynamic limit to concentration; and (3) being relatively compact, e.g., aspect ratios of the order of unity or less. Our inventions are rooted in the recently developed formalism of tailored edge-ray concentrators.     

Overturning some assumptions about the effects of evaluation systems on publication performance

In 1989 the Spanish Government established an individual retrospective research evaluation system (RES) for public researchers. Policy makers have associated the establishment of this evaluation system with the significant increase in the volume of scientific publications attributed to Spain over the last decades. In a similar vein to the analyses of other country cases, some scholars have also claimed that the growth of Spain’s international scientific publications is a result of the establishment of the new evaluation system. In this paper, we provide a methodological revision of the validity threats in previous research, including some interrupted time-series analyses and control groups to investigate the effects of this policy instrument on the number of papers produced by Spanish authors. In the years following the establishment of the evaluation system, the results indicate a considerable increase in the number of papers attributed to Spanish authors among those eligible for evaluation (the “treated” group), but also in the control groups. After testing various alternative explanations, we conclude that the growth in Spanish publications cannot be attributed indisputably to the effect of the establishment of the RES, but rather to the increase of expenditure and number of researchers in the Spanish R&D system along with some maturation effects. We take this case as an example of the need to improve and refine methodologies and to be more cautious when attributing effects to research evaluation mechanisms at the national level.     

Optical assessment of nonimaging concentrators

An optical measurement method for nonimaging radiation concentrators is proposed. A Lambertian light source is placed in the exit aperture of the concentrator. Looking into the concentrator's entrance aperture from a remote position, one can photograph the transmission patterns. The patterns show the transmission of radiation through the concentrator with the full resolution of the four-dimensional phase space of geometric optics. By matching ray-tracing simulations to the measurement, one can achieve detailed and accurate information about the geometry of the concentrator. This is a remote, noncontact measurement and can be performed in situ for installed concentrators. Additional information regarding small-scale reflector waviness and surface reflectivity can also be obtained from the same measurement with additional analysis.