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Thermal phase change actuator for self-tracking solar concentration

E. J. Tremblay, D. Loterie, and C. Moser

Open Access Open Access

Abstract

We present a proof of principle demonstration of a reversible in-plane actuator activated by focused sunlight, and describe a concept for its use as a self-tracking mechanism in a planar solar concentrator. By actuating at the location of focused sunlight and splitting the solar spectrum for actuation energy, this phase change device aims to provide the adaptive mechanism necessary to efficiently couple concentrated solar light from a lens into a planar lightguide in a manner that is insensitive to incidence angle. As a preliminary demonstration we present a planar actuator array capable of in-plane deflections of >50μm when illuminated with focused light from a solar simulator and demonstrate solar light activated frustrated total internal reflection (FTIR) with the actuator array. We further propose how this solar induced FTIR effect can be modified using a dichroic facet array to self-adaptively couple and concentrate solar light into a planar lightguide.

©2012 Optical Society of America

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Supplementary Material (2)

Media 1: AVI (4711 KB)     
Media 2: AVI (4272 KB)     

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Figures (6)
Fig. 1
Fig. 1 Self-tracking planar solar concentrator concept. (a) Focused light from a single lens actuates a position dependent lightguide coupling feature. (b) Reversible phase change actuator concept: solar light is split by a dichroic mirror into its coupled (short wavelength) and heating (infrared) components. Transmitted Infrared light is absorbed, actuating a phase-change in-plane deflection which acts to press a deformable elastomeric layer against the lightguide to couple the reflected short wavelength light.

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Fig. 2
Fig. 2 Tuned-angle facet coupler design. (a) Focused normal incidence light reflected and coupled by 30°/30° facet array without shadowing. (b) Focused light at 25° incidence (in air) coupled by 18.6°/35.7° facet array without shadowing. In (a) and (b) the chief ray (blue) reflects parallel to the adjacent facet for broad-angle coupling centered around the chief ray angle. (c) Minimum lens F/# for complete lightguide coupling vs. field angle for a fixed 30°/30° facet array (blue) and a tuned-angle facet array (red). Tuned-angle facets allow for efficient coupling over a broad field of view with low F/# optics. PDMS (n = 1.43) was used as the coupling material.

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Fig. 3
Fig. 3 (a) A polycarbonate honeycomb array with 2.3mm cells filled with a paraffin/carbon black composite. (b) A steel honeycomb array with 0.75mm cells filled with a paraffin/carbon black composite. A 300μm thick PDMS layer covers both devices. (c) Photograph of 2.3mm cell device. (d) Photograph of 0.75mm cell device.

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Fig. 4
Fig. 4 (a) Device actuation and white-light interferometer measurement setup (b) White light interferometric profile of the 2.3mm cell device (red) and the 0.75mm cell device (blue) actuated by longpass filtered (>700nm) light from a solar simulator focused through a 30mm, F/2 lens.

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Fig. 5
Fig. 5 FTIR test. (a) Light passed through dove prism is reflected by TIR. (b) FTIR caused by device actuation using a 30 mm lens and long-pass (>700nm) light from a solar simulator. The middle green ray is coupled out of the prism by FTIR. (c) Theoretical relationship (Eq. (13)) between radius of FTIR (rFTIR) relative to the cell radius (ra) and the separating airspace for 750 μm actuator cells. From the visible contact area measured in the experiment (rFTIR/ra = 0.76), we estimate an air space of 22 μm using Eq. (13). Inset image shows the visible FTIR contact area viewed through the roof of the dove prism (coaxial with solar simulator light).

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Fig. 6
Fig. 6 Sequence of images demonstrating reversibility and tracking of FTIR. (a) FTIR reversibility for 0.75mm cell (Media 1). (b) Light tracking with 0.75mm cell (Media 2). (c) Reversibility with 2.3mm cell. (d) Light tracking with 2.3mm cell. FTIR actuated using long-pass (>700nm) light from a solar simulator focused by a 30mm lens. In all images, the FTIR regions appear non-circular due to the tilted camera observing the effect through the facets of the dove prism.

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Equations (13)

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θ C,elastomer =sin−1( 1 n elastomer )
θ i =60− θ r
θ in air max,30/30 = sin −1 ( n elastomer sin[ 60− sin −1 ( 1 n elastomer ) ] )
F/ # min,30/30 = 1 2NA = cos( θ f ) 2sin( θ in air max,30/30 - θ f )
α 1 =30− 2 3 θ i
α 2 =30− 1 3 θ i
θ in air min, facet1 = sin −1 ( n elastomer sin[ sin −1 ( 1/ n elastomer )−2 α 1 ] )
θ in air min, facet2 = sin −1 ( n elastomer sin[ 2 α 2 − sin −1 ( 1/ n elastomer ) ] )
F/ # min, tuned facets = 1 2NA = cos( θ f ) 2sin( θ f − θ in air min, facet1 ) ≅ cos( θ f ) 2sin( θ in air min, facet2 )
h(r)= h max ( 1− ( r r a ) 2 ) 2
V d ≈ 1 3 π h max r a 2
h max =3m t p
t air ≈ h max 1+ r FTIR r a + ( r FTIR r a ) 2
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