 Proceedings of Engineering and Technology Innovation , vol. 3, 2016, pp. 10 - 12 10 Optimization of Coupler Inlet for Planar Solar Concentrator Tien-Hsiang Yu, Tun-Chien Teng * Department of Mechatronic Engineering, National Taiwan Normal University, Taipei, Taiwan. Received 25 February 2016; received in revised form 25 March 2016; accept ed 13 April 2016 Abstract In this study, we imp le mented optimization for the coupler inlets of the planar solar co n- centrator, wh ich was proposed in our prior work. The planar solar concentrator had a waveguide slab to carry an array of light-collecting ele - ments thereon. The light entering the elements is focused on the coupler in lets and guided into the slab with propagating therein through total in- ternal re flection (T IR). Thus, all the light fro m the elements is guided to the ends of the slab and highly concentrated. Because the coupler in lets couple light into the slab and also decouple the light propagating in the slab in subsequent in- teractions, they must be optimized to balance between coupling effic iency and decoupling loss. The optimized arrangements of coupler in lets included three types here: uniform-thic kness platform, stepped-thickness platform, and stepped-width platform. In the simulat ion, the three types have the same area o f the inlet to keep the concentrator with the same toleran ce of the incident angle of light. We analyzed both the optical efficiency and range of the vertical an- gular distribution of the light reaching the outlet end of the slab; then made a co mparison for the three inlet arrange ments. The simulation results demonstrate the stepped-thickness and stepped-width platforms provide higher effi- ciency and concentrated irradiance. Ke ywor ds : planar concentrator, coupler, solar energy, illumination design 1. Introduction Traditionally, the planar concentrator is equivalent to a device that divides a single large concentrated element into an array of optics (lens) coupled to a co mmon slab waveguide. The light entering each lens is focused on localized coupling mic rostructures embedded on the bot- tom of the waveguide and reflected at predicted angles, thereby propagating in the waveguide through total internal reflection (T IR), so the light fro m a ll the lenses is guided to the ends of the waveguide [1]. The advantages of the planar concentrator include a compact volume and a light we ight. Because the coupling mic rostru c- tures on the waveguide surface couple light into the waveguide and also decouple the light guided in the waveguide in subsequent interac- tions, the decoupling loss increases with the waveguide length. To manage the decoupling loss, various designs of the waveguide and co u- pler geo metries we re p roposed to reduce or eliminate interactions with the coupling mic ro- structure [1-4]. However, these designs require an accurate align ment of the lens array and the coupling mic rostructure, which is adverse to assemble a large product. Thus, we proposed a planar solar concen- trator in our prior work, which had a waveguide slab to carry an array of T IR light-collecting ele ments thereon without requiring a lignment [5]. The light entering the e le ments is focused on the coupler inlets and guided into the slab with propagating therein through TIR. Thus, all the light fro m the ele ments is guided to the ends of the slab and highly concentrated. Because the coupler in lets couple light into the slab and a lso decouple the light propagating in the slab in subsequent interactions , we further optimized them to ba lance between coupling e ffic iency and decoupling loss in this study. 2. Design concept A coupler inlet is the junction of each T IR collector and the slab, through which the light focused by the TIR collector enters the slab below. Because the field angle of the sun itself is 0.52 o , the sunlight is focused onto a spot, rather than on a point; therefore, the coupler in let must have an area enough for the focused light to pass through. Moreover, the T IR co llector focuses the light within a larger spot area if t racking toler- ance is considered. Thus, a coupler inlet with a * Corresponding aut hor. Email: walt er.teng@ntnu.edu.tw Proceedings of Engineering and Technology Innovation , vol. 3, 2016, pp. 10 - 12 11 Copyright © TAETI larger a rea contributes to an increased coupling efficiency. However, a coupler inlet with a larger a rea a lso raises the probability of decou- pling the light propagating in the slab and thus reduces the propagation effic iency. Because the total optical e ffic iency depends on the product of the coupling effic iency and the propagation efficiency, securing a balance between th em is critical. Fig. 1 A TIR collector on a slab with a slope-facet inlet In our prior work, the tolerance of trac king was set to ±0.5° for the rotation angle of the x-a xis and the z-a xis (i.e.  and , respectively; referring to Fig. 1). The optima l a rea of the inlet is a trape zoid with two base widths of 0.9 mm and 0.3 mm, and a height of 1.05 mm. In this study, we optimized the arrangements of the inlets to raise the optical effic iency. The ar- rangements for optimization included three types: uniform-thic kness platform (Case A), stepped-thickness platform (Case B), and stepped-width platform (Case C). In Case A, the slab was flat; all the inlets were lined on the same horizontal level. In Case B, the slab had a stepped thickness, and thus the location of the former inlet was higher than the later. In Case C, the slab had a stepped width, and the inlets were on the same horizontal level, but the slab was tiled at a s mall angle with the line of inlets. The purpose of the arrangements of Case B and C is to prevent the coupled light fro m lea king out of other inlets ahead. The slab of Case B was the thickest; that of Case C was widest; those of Case A and B had the same width; those of Case A and C had the s a me thic kness. Therefore, Case A had the smallest area of the outlet end of the slab and thus the largest geometric concentration ratio; Case B had the same area as Case C. However, Case B and Case C had the higher propagation efficiency. Moreover, the inlet end of the slab in Case B and C had a slope facet to reduce the range of the vertical angular distri- bution of the light reaching the outlet end of the slab, wh ich help the light be further concentrated by attaching an extra co mpound parabolic co l- lector (CPC) to the outlet end. Therefore, we analyzed both the optical effic iency and range of the vertical angular distribution for the three cases under diffe rent para meters of the slab; then made a co mparison to find the optimal condi- tions . 3. Results and Discussion In this study, we established an optical model that had a T IR collector on a narro w slab with a slope-facet inlet. Then, we varied the angle of the slope facet (slope angle ) and the thickness of the slab for conducting simu lations to find the optima l condit ions. Moreover, we made a co mparison for the three arrangements of coupler inlets under the optima l condition. In the optical model, the T IR co llector had a p itch of 10 mm, a height of 15 mm, and a width of 15 mm; the slab had a width of 1.0 mm, and a length of 60 mm; the T IR collector and slab were made of poly methyl methacrylate (PMMA). The used ray-tracing software was LightTools 8.3. Fig. 2 Slope angles vs. ranges of vertical an- gular distributions and efficiency Fig. 2 shows the effects of slope angles on both the optical efficiency and range of vertical angular distributions. The light is incident at an angle (=0.4, =0) with an angular range of 0.26° (half-angle ). The range of vertica l angular distributions substantially decreases as the slope angle decreases, but increases as the thickness of the slab decreases. Moreover, slope angles have litt le effects on the effic iency, but the efficiency dramat ically drops if the slope angle is below 4°. Furthermore, the effic iency is not affected by the Proceedings of Engineering and Technology Innovation , vol. 3, 2016, pp. 10 - 12 12 Copyright © TAETI thickness of the slab. Fig. 3 shows the effect of slope angles on the optical effic iency for the light at inc ident angles of (0, 0), (0.4, 0), (0.4, 0.4), and (0.8, 0). Except the incident angle (0, 0), the other incident angles have the simila r trend, dra matic drop in the effic iency when the slope angle below 4°. Fig. 3 Slope angels vs. optical efficiency Finally, we assembled five T IR collectors according to the three types of arrange ments for conducting simulat ion. Case A had a slab with a thickness and width of 1.25 mm and 1 mm, respectively; Case B had a slab with a thic kness and width of 2.5 mm and 1 mm, respectively; Case C had a slab with a thic kness and width of 0.5 mm and 5 mm, respectively. Case B and C had an inlet with a slope facet of 4°. The irra - diance of the outlet end of Case A is 1.9 times that of Case B or C. However, the range of the vertical angular distribution in Case A was 96°; those in Case B and C were 22°, which means Case B and C can be further concentrated 3.8 times Case A by attaching an extra CPC the outlet end of the slab. Overall, Case B and C can provide the concentrated irradiance 2 times Case A at most. 4. Conclusions In this paper, the arrangements of coupler inlets are ana lyzed and optimized. As compared with the uniform thickness platform, the stepped-thickness and stepped- width platforms with a slope-facet inlet end o f 4° provide higher efficiency and double concentrated irradiance. References [1] J. H. Karp, E. J. T re mb lay, and J. E. Ford, “Planar micro-optic solar concentrator,” Opt. Exp ress , vol. 18, no. 2, pp. 1122-1133, 2010. [2] Y. Liu, R. Huang, and C. K. Madsen, “De- sign of a lens -to-channel waveguide system as a solar concentrator structure,” Opt. Ex- press , vol. 22, no. S2, pp. A198-A204, 2014. [3] O. Selimog lu, and R. Turan, “ Exp loration of the horizontally staggered light guides for high concentration CPV applications,” Opt. Express , vol. 20, no. 17, pp. 19137-19147, 2012. [4] B. L. Unger, G. R. Sch midt, and D. T . 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