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Proof of principle demonstration of a self-tracking concentrator

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We present to the best of our knowledge the first successful demonstration of a planar, self-tracking solar concentrator system capable of a 2-dimensional angular acceptance of over 40°. The light responsive mechanism allows for efficient waveguide coupling and light concentration independently of the angle of incidence within the angular range. A coupling feature is created at the focal spot of the optical system by locally melting a phase change material which acts as an actuator due to the large thermal expansion. A dichroic prism membrane reflects the visible light so that it is efficiently coupled into a waveguide at the point of the created coupling feature. We show simulation results for concentration and efficiency, validated by an experimental proof of concept demonstration of a self-tracking concentrator array element. Simulations show that a system based on this approach can achieve 150X effective concentration by scaling the system collecting area to reasonable dimensions (40 x 10 cm2).

© 2014 Optical Society of America

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Supplementary Material (2)

Media 1: AVI (3468 KB)     
Media 2: AVI (3865 KB)     

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Figures (11)

Fig. 1
Fig. 1 The self-tracking planar concentrator device concept. a) and b) show the focused light for different incoming angles. The coupling feature shifts due to the paraffin actuator reacting to the shifted focal spot. Inset: Long wavelength light (red) is transmitted through a dichroic facet array to heat up the phase-change actuator below (black). The expanded actuator presses the membrane against the waveguide (grey) allowing short wavelength light (bright yellow) to be coupled into the waveguide. The lens array is shown with a largely reduced scale for this conceptual drawing.
Fig. 2
Fig. 2 a) The optical system using two off-the-shelf aspheric lenses to create a flat Petzval field curvature over ± 23° incoming angle. b) The spot size only changes marginally over the set of incoming angles for the two aspheric lens system compared to two plano-convex lenses or a single lens. c,d) Change of the spot size for a single plano-convex lens and the two aspheric lenses. The different colors represent different parts of the optical spectrum (blue = 400nm, green = 550nm, red = 800nm, yellow = 1200nm).
Fig. 3
Fig. 3 Waveguide (transparent vertical rectangle) and dichroic prism membrane (white square layer in the center) fixed in the holder. The phase-change actuator is square and located below the membrane.
Fig. 4
Fig. 4 The solar spectrum (blue) is divided by the dichroic membrane into long (yellow) and short (green) wavelength parts at roughly 750nm. The long wavelength portion is then used to drive the actuator while the short wavelength part is coupled into the waveguide to the PV cell.
Fig. 5
Fig. 5 Paraffin wax is mixed with carbon black (a) and filled into the honeycomb hole array (b). PDMS is spincoated on top to create a flexible membrane and the actuator is sealed on the back with a glass slide(c). d) Top view onto the steel actuator (bright) filled with paraffin wax/carbon black.
Fig. 6
Fig. 6 The effective concentration factor (c) is the product of the geometric concentration factor (a) and the coupling efficiency (b). Reasonable dimensions (40 x 10 cm2, 2mm waveguide) allow an effective concentration of 150X.
Fig. 7
Fig. 7 Schematic of the experimental setup. A single lens-pair was used to couple light into a small waveguide (length y, thickness t).
Fig. 8
Fig. 8 The assembled actuator and the waveguide were tested with a 1 sun solar simulator. A rotations stage (background) will set an angle and the thermal sensor (red) will measure the power output at the waveguide edge as a function of time.
Fig. 9
Fig. 9 a) The dynamic curve shows the behavior of actuator as a function of time for sudden onset, high intensity conditions (0-20 s) and sudden onset, low intensity conditions (21-40 s) for different angles in absolute power values. b) The dynamics of different angles are compared in normalized values. Smaller angles reach 90% of the maximum faster (<4 s) than larger angles (6-10 s). This is a slightly faster than the 8-12 s the heat needs to dissipate and the coupling value reaches value lower than 10%.
Fig. 10
Fig. 10 a) The actuator allows for actuation from −20° to + 20°. The blue curve shows the minimum amount of sunlight at for every angle whereas the yellow curve shows the maximum amount of coupling. b) Comparison of simulated optical efficiencies and the measured experimental result.
Fig. 11
Fig. 11 a) Screenshot from a video made in the lab showing the actuation for different angles. The angular speed is set to exceed the actuation speed. The exit facet therefore darkens after the system being turned until the actuation occurs again. See Media 1. b) Screenshot from a video made in the lab showing the actuation for different angles. The angular speed is set to match the actuation speed. The exit facet is continuously lit throughout the angular range. See Media 2.

Equations (3)

Equations on this page are rendered with MathJax. Learn more.

C C max = 1 sin 2 ( θ max,in ) .
CF= A in A out = x array y array x waveguide t waveguide = y array t waveguide .
CF= A in A out = 12.5 2 m m 2 π 25mm1mm 50.


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