Image-forming solar concentrators which project an image of the sun on the photovoltaic cell may have high thermodynamic efficiency, but suffer from non-uniform irradiance distribution and a nonsquare irradiance pattern. In this paper we discuss how to apply the Köhler illumination technique to solve these problems with an optimization approach. Two examples, a Fresnel lens-based concentrator and a two-mirror aplanatic system, are shown for demonstration. The uniformity of these systems is greatly improved by adding simple Köhler homogenizers in the form of aspheric lenses generated with our approach.
Size- and structure-dependent efficiency enhancement methods are studied for luminescent solar concentrators (LSCs) fabricated by casting organic laser dyes into PMMA matrixes. The enhancement are achieved mainly by attaching a white diffuser with an airgap at the bottom of the LSC and adding refractive index matched optical gel between the LSC's edges and the attached photovoltaic cells. The size-dependent efficiency enhancement is studied for a single layer by changing the size up to 120 cm. The results show that the enhancement from the white diffuser drops and then tends to plateau at a certain size of LSC. This also applies to multilayer LSCs. Together with optical gel, the efficiency enhancement is higher for multilayer structures than that for single layers. We also demonstrate the optimal length for the design of LSCs due to reabsorption of dyes. These results could be applied to optimize the design of other LSCs.
By adding simple Köhler homogenizers in the form of aspheric lenses generated with an optimization approach, we
solve the problems of non-uniform irradiance distribution and non-square irradiance pattern existing in some image-forming
solar concentrators. The homogenizers do not require optical bonding to the solar cells or total internal
reflection surface. Two examples are shown including a Fresnel lens based concentrator and a two-mirror aplanatic
A luminescent solar concentrator (LSC) generally is a sheet of highly transparent materials embedded with luminescent
materials. Incident sunlight is absorbed by the luminescent materials, and then emitted through down conversion process
at longer wavelengths. A large portion of the emitted light is trapped in the sheet and travels to the edges where
photovoltaic solar cells are attached. In this study, we investigate the optical enhancement methods for LSCs with
different sizes mainly by using optical gel and white diffuser. The largest tested LSC is up to 1.2m in length and with
geometrical gain 64. This is, as we know, the largest reported size. It yields electrical gain 3.9 by optical enhancements.
And the optical efficiency is still as large as 10%. The study shows that the enhancement by white diffuser is more
sensitive to the size of the LSCs than that of the optical gel. Such enhancement drops with the increase of the sizes of
LSC, but tends to plateau at certain size.
We discuss the theoretical limits of concentration for stationary solar concentrators allowed by the law of
thermodynamics. The principles to design systems that approach the theoretical limits are then presented followed by a
few examples, including the first 4x fixed solar concentrator.
Quantum dot (QD) luminescent solar concentrator (LSC) uses a sheet of highly transparent materials doped with
luminescent QDs materials. Sunlight is absorbed by these quantum dots and emitted through down conversion process.
The emitted light is trapped in the sheet and travels to the edges where it can be collected by photovoltaic solar cells. In
this study, we investigate the performance of LSCs fabricated with near infrared QDs (lead sulfide) and compared with
the performance of LSCs containing normal visible QDs (CdSe/ZnS), and LSCs containing organic dye (Rhodamine B).
Effects of materials concentrations (related to re-absorption) on the power conversion efficiency are also analyzed. The
results show that near infrared QDs LSCs can generate nearly twice as much as the output current from normal QDs and
organic dye LSCs. This is due to their broad absorption spectra. If stability of QDs is further improved, the near infrared
QDs will dramatically improve the efficiency of LSCs for solar energy conversion with lower cost per Wp.
It is well-known that conservation of phase-space volume or optical etendue leads to strict limits to concentration. Less
well- known is the connection between entropy and etendue. Entropy has a logarithmic dependence on etendue in
addition to the familiar linear dependence on heat. This trade-off permits in principle an exponential boost in
concentration. Optical systems that make use of this possibility will be discussed.
Aplanatic designs have interested scientists since the days of Galileo, Newton and Descartes. It is remarkable that an onaxis
condition which can be simply formulated ensures good off-axis performance. The condition is that rays parallel to
the axis intersect rays converging to the focus on the surface of a sphere. In this paper the authors have extended
aplanatic designs to refractive media in a non-trivial way, which yields highly compact and fast aplanatic singlets.
We show that an aplanatic imaging system can approach theoretical maximum concentration limit following a brief
review of Abbe Sine condition and aplanatism. We use a two-mirror case to demonstrate how to construct such
aplanatic systems using Luneburg method. The result is useful in designing high-performance concentrators.
We report holographic recording in photosensitive polymer optical fibers by guided beams. The fibers are made of poly(methyl methacrylate) doped with Disperse Red 1as photosensitive element. Holographic recordings are performed with parallel- and orthogonally-polarized writing beams. Recording of Fourier-transform images in these fibers is also demonstrated. The recording mechanism is photoinduced reorientation of Disperse Red 1 molecules.
The principle of mode-cut optical limiting in fibers is reviewed briefly, and a calculation method based on angular spectrum analysis is proposed. Experiments that show high efficiency holographic grating generation and self defocusing in disperse-red-1 (DR1) doped poly(methyl methacrylate) (DR1/PMMA) bulk material suggest that it is a good candidate to be used as a core material in polymer fibers to achieve mode-cut optical limiting. Such fibers are fabricated in our lab and its optical limiting effect is reported.
Optical phase conjugation (PC) by non-resonant degenerate four-wave mixing (DFWM) in thick media of poly(methyl methacrylate) (PMMA) with doped disperse red 1 (DR1) is reported. With vertically polarized counterpropagating pump waves, PC reflectivities of 43% and 37% were achieved respectively for a horizontally and vertically polarized probe wave, which is more than 50 times higher than the value reported on resonance. Reflectivities over 30% were achieved over a wide range of intensity for both polarization configurations. Photoinduced modulation of ordering of the DR1 chromophore is the main mechanism of the PC wave generation. Other mechanisms involved in the configuration of all vertical polarization waves are also examined. Influence of the squeezing process in making volume samples on the PC wave efficiency is significant.