Weiya Zhang

Pushing concentration of stationary solar concentrators to the limit

We give the theoretical limit of concentration allowed by nonimaging optics for stationary solar concentrators after reviewing sun- earth geometry in direction cosine space. We then discuss the design principles that we follow to approach the maximum concentration along with examples including a hollow CPC trough, a dielectric CPC trough, and a 3D dielectric stationary solar concentrator which concentrates sun light four times (4x), eight hours per day year around.

Novel aplanatic designs.

We have discovered an aplanatic design that contributes to a celebrated problem in classical optics in a novel way. In so doing new devices are envisioned with applications to illumination, concentration, and imaging.

Efficiency Improvement by Near Infrared Quantum Dots for Luminescent Solar Concentrators

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.

Size- and structure-dependent efficiency enhancement for luminescent solar concentrators

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 LSCs 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.

Performance of Organic Luminescent Solar Concentrator Photovoltaic Systems

Organic luminescent solar concentrator (LSC) photovoltaic (PV) systems are investigated as low concentration systems in harvesting solar energy. The prototypes are fabricated by embedding red and green organic dyes into PMMA plastic sheets. High efficiency mc‐Si PV cells are attached at the edges of the fabricated concentrators. The performance of the fabricated system is characterized based on the spectral properties and the outdoor electrical gain of the system. Properties of single‐ layer LSCs are compared with properties of stacked LSCs. The largest prototype LSC PV system we fabricated yields the concentration factor of 4.3. The output power is improved up to 2× by using optical optimization methods. The tested results for the prototypes as “smart” windows show that LSCs can perform very well for concentrating both direct and diffuse light. These results can be applied for further optimal design of LSC PV systems.

Simple Köhler Homogenizers for Image‐forming Solar Concentrators

We demonstrate that the Kohler illumination technique can be applied to the image‐forming solar concentrators to solve the problem of “hot” spot and to generate the square irradiance pattern. The Kohler homogenizer can be simply a single aspheric lens optimized following a few guidelines. Two examples are given including a Fresnel lens based concentrator and a two‐mirror aplanatic system.

Beating the optical Liouville theorem: How does geometrical optics know the second law of thermodynamics?

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.

Simple Köhler homogenizers for image-forming solar concentrators

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 system.

Optical enhancement for luminescent solar concentrators

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.

Publisher's Note: Journal of Photonics for Energy, Volume 1

This PDF file contains the errata for “JPE Vol. 1 Issue 1 Paper 3567199” for JPE Vol. 1 Issue 1