Free-Space Diffused Light Collimation and Concentration

Abstract

Collimating and concentrating broad-band diffused light can increase the yield, decrease the cost, and open new opportunities for solar-generated electricity. Adherence to the second law of thermodynamics requires that collimation, and therefore the reduction of étendue or entropy, of diffused sunlight, i.e., light scattered by clouds or the atmosphere, can only occur if the photons lose energy during the process. This principle has been demonstrated in luminescent solar concentrators; solar photons are energetically down-shifted by a luminophore and the emitted photons are trapped within a transparent matrix and guided toward an edge lining solar cell. However, this process suffers from low efficiency as the photons are trapped within the waveguide for a long time, encountering many instances of accumulating loss mechanisms. Here, we theoretically describe and experimentally demonstrate the first free-space diffused light collimation system which overcomes these efficiency losses. The high photon energy solar spectrum is allowed to enter the system from all angles, whereas the re-emitted luminescent photons can only escape under a desired emission cone. We achieved this through doping a polymethylmetacrylate waveguide with Lumogen Red dye, which we cover on one side with a Lambertian reflector for photon recycling and induced randomization and on the top face with a complex multilayer dielectric nanophotonic coating stack. We experimentally found an angular concentration of 118% within the designed escape cone, where isotropic emission corresponds to 100%, thereby verifying the reduction of étendue in free space experimentally. Such free-space collimation systems will enable efficient redirection of sunlight toward solar panels, thereby increasing yield, decreasing heating through the emission of low energy photons, and expanding the range of available surfaces from which sunlight can be harvested.