Asymmetric and symmetric light couplers of daylighting systems for direct indoor lighting

Daylighting systems, which collect and transport sunlight directly into building interiors for illumination, can reduce conversion losses between different forms of energy, and thus increase the efficiency of using solar energy. To collect enough sunlight for indoor illumination, a large number of sunlight concentrators and high-efficiency light couplers are needed. In the study, we developed theoretical models of the symmetric and asymmetric Y-couplers, and conducted numerical simulations of the light couplers of three coupling angles, 10 degrees, 30 degrees and 50 degrees using LightTools and Matlab. The coupling efficiencies of the asymmetric Y-couplers were 5% in average higher than those of the symmetric Y-couplers. The light couplers with a large coupling angle (50 degrees) achieved higher coupling efficiency for the light rays of large propagation angles, and the light couplers with small coupling angles achieved 100% coupling efficiency for the light rays of small propagation angles (< 24 degrees). Therefore, the f/# of the sunlight concentrators can be calculated from the coupler parameters to reduce the angles of the light rays and thus to increase the coupling efficiency. For the branch guide to have high efficiency, the largest coupling angle of the Y-couplers was equal to the critical propagation angle 46.77 degrees.