Rahul Bose

Luminescent Solar Concentrators - A review of recent results

Luminescent solar concentrators (LSCs) generally consist of transparent polymer sheets doped with luminescent species. Incident sunlight is absorbed by the luminescent species and emitted with high quantum efficiency, such that emitted light is trapped in the sheet and travels to the edges where it can be collected by solar cells. LSCs offer potentially lower cost per Wp. This paper reviews results mainly obtained within the framework of the Full-spectrum project. Two modeling approaches are presented, i.e., a thermodynamic and a ray-trace one, as well as experimental results, with a focus on LSC stability.

Luminescent and geometric concentrators for building integrated photovoltaics

In developed countries 60% of the electricity consumed is attributable to commercial and public buildings. Even in the UK, the solar energy incident on buildings is more than 7× the electrical energy they consume. This represents a problem (the management of solar heat gain and glare) but also an opportunity that may be taken advantage of using complementary concentrator technologies. We are investigating conventional geometric and luminescent concentrators that may be combined to optimally harvest the direct and diffuse components of sunlight within a double glazed window unit. Initial results suggest that the combined system can achieve power conversion efficiencies approaching 20% under standard AM1.5g illumination at normal incidence.

Luminescent solar concentrators: Nanorods and raytrace modeling

Nanorods are a novel and promising component for luminescent solar concentrators (LSCs). In particular, their spectra suggest reduced re-absorption losses. We report the incorporation of core-shell nanorods in homogeneous and thin film LSCs. The rods in the solid host appear to retain their spectral features compared to their dissolved state. Short-circuit current measurements with a calibrated solar cell have been compared with the computational simulation of the LSCs. Our raytrace model was applied to fit the fundamental emission spectrum and extract the quantum efficiency (QE) of the nanorods in the concentrators. In the case of the homogeneous LSC, the extracted QE was (67±4)%, which is in good agreement with the quoted value of about 70% for rods in solution. The thin film samples showed noticeably worse performance, which was attributed to possible agglomeration of rods and to macroscopic defects in the film. Finally, the raytrace model was applied to compare the self-absorption between a typical quantum dot concentrator and a nanorod concentrator. The result supported the argument that nanorods exhibit a smaller spectral overlap and consequently less re-absorption losses.

Resonance energy transfer in luminescent solar concentrators

Luminescent solar concentrators (LSCs) harvest sunlight by absorption in a luminescent centre and re-emission in a red-shifted and narrow-band spectrum within a waveguide. Fluorescence resonance energy transfer (FRET) can allow for more efficient LSCs. Our research goal is to produce an LSC technology based on the biological properties of phycobilisomes which are antennae proteins from algae that naturally exhibit FRET, absorbing blue-green light and emitting red. In this paper, we present our initial results of incorporating phycobilisomes into waveguides and coupling these waveguides to solar cells to form a proof of concept for a phycobilisome-based LSC (phyco-LSC). We present initial experimental data and modeling results that establish the feasibility of the concept and make projections of ultimate system performance. Our modeling suggests that with optimized phycobilisomes, photon concentration ratios close to ten could be achieved for practically sized LSCs. This is an international collaboration bringing together the PV expertise of NRL and Imperial College London with the bioscience expertise of Columbia Biosciences and the waveguide fabrication expertise of the Fraunhofer IAP.

Classical behaviour of output light emitted by the edge of a luminescent solar concentrator

A straightforward technique for measuring the output light from one edge of a luminescent solar concentrator (LSC) was recently developed. A set of measurements was performed both on conventional LSCs and thin-film composite LSCs, showing interesting differences between two types of devices. In order to investigate the origin of this behaviour, various models have been developed by our groups. In this work we present the experimental setup and main results, together with a simple three-dimensional Monte Carlo code, modelling light re-emission by a LSC and distribution of the output light from the edge under examination. Light is described by means of classical non-interacting free particles undergoing classical optics laws at glass-to-air interfaces. Simulation results agree well with the experimental data and explain the output light distribution close to the edge under examination.