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Self-tracking solar concentrator with an acceptance angle of 32°

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Solar concentration has the potential to decrease the cost associated with solar cells by replacing the receiving surface aperture with cheaper optics that concentrate light onto a smaller cell aperture. However a mechanical tracker has to be added to the system to keep the concentrated light on the size reduced solar cell at all times. The tracking device itself uses energy to follow the sun’s position during the day. We have previously shown a mechanism for self-tracking that works by making use of the infrared energy of the solar spectrum, to activate a phase change material. In this paper, we show an implementation of a working 53 x 53 mm2 self-tracking system with an acceptance angle of 32° ( ± 16°). This paper describes the design optimizations and upscaling process to extend the proof-of-principle self-tracking mechanism to a working demonstration device including the incorporation of custom photodiodes for system characterization. The current version demonstrates an effective concentration of 3.5x (compared to 8x theoretical) over 80% of the desired acceptance angle. Further improvements are expected to increase the efficiency of the system and open the possibility to expand the device to concentrations as high as 200x (Cgeo = 400x, η = 50%, for a solar cell matched spectrum).

© 2014 Optical Society of America

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Figures (15)

Fig. 1
Fig. 1 There is a trade-off between concentration factor and acceptance angle [Eq. (1)]. For any given acceptance angle, there is an upper limit of possible concentration (orange area, n = 1.5). Due to this the field of CPV technologies is divided into three categories: High, medium, and low CPV. Our approach has a concentration factor that is to the right of the curve [Eq. (1)] due to its self-tracking mechanism, capable of reaching 300x geometric concentration with ± 16° acceptance angle (dark spot).
Fig. 2
Fig. 2 The three stages show the actuation and the self-tracking mechanism. In stage 1, the dichroic mirror splits the spectrum in two parts. The transmitted part (red; >750 nm) is transmitted and absorbed by the paraffin wax (black). In stage 2, the paraffin wax melts and expands upward, creating a coupling feature for the reflected light (yellow). As the sun moves throughout the day/season the focal spot changes and a different part of the actuator is activated (stage 3).
Fig. 3
Fig. 3 The combination of the two lenses yields a flat Petzval field curvature (blue) over the desired angular range in contrast to the use of a single plano-convex lens (a). The experimental results of the acceptance angle (b) agree with the simulation [Fig. 3(a)] corresponding to a reduction of the acceptance angle to from ± 23° to ± 16°.
Fig. 4
Fig. 4 (a) The lens arrays were created from one inch off-the-shelf lenses by milling the outer parts and leaving the center square. (b) The pair of lens arrays use a custom holder to keep them at the desired separation. Simulations indicate a reduction of the acceptance angle down to ± 16°.
Fig. 5
Fig. 5 The experimental measurements (red) of the beam size at different positions around the focus and at different angles, are similar to the simulation results (blue) and indicate a good agreement between the actual fabricated lens arrays and the virtual model in Zemax.
Fig. 6
Fig. 6 Left: relative efficiency curve. The baseline is given by the power detected by the photodiode positioned at the edge of the waveguide (see position (1) on right figure). The photodiodes can also be placed on top of the waveguide (pos. 2, pos. 3). A photodiode placed on the top of the waveguide, having a lateral dimension W > 2 mm shows a difference in collection efficiency less than 5%, with respect to a photodiode placed at the edge.
Fig. 7
Fig. 7 (a) Response of the short circuit current of the photodiode to different intensity levels has been experimentally verified to ensure the validity of the results. (b) Photograph of the 52x80 mm2 fused silica waveguide with Ag/Cr metallization at the edges and the front ZnO contact and µc-Si layers deposited by LPCVD and PECVD, respectively. Eight 2x5 mm2 photodiodes integrated at the extremity of the waveguide after ZnO back contact deposition, lift-off and RIE processes. (c) Spectral response of a reference photodiodes.
Fig. 8
Fig. 8 The actuator consists of the actuation array (a) filled with paraffin wax and the dichroic membrane on top that splits the spectrum in two parts (b).
Fig. 9
Fig. 9 The experimental device (b) was based on the simulation model (a). The simulation is then adapted to incorporate the same materials as used in the demonstration device for a full understanding of the performance. The top view shows the actuator unit numbering and photodiode numbering used during the experiments (c).
Fig. 10
Fig. 10 The experimental setup uses a motorized rotation and linear stage to record the short-circuit current response to any input. The incoming light on the device was changed in angle from −16° (Start) to + 16° (End). Every single unit (lens pair + actuator) was analyzed on its own and the response of all nine units summed up.
Fig. 11
Fig. 11 a) The maximum (actuated state) and minimum (non-actuated state) values of ISC for the unit 5 show actuation over an angular range of 16° (green area) indicated by a large difference of the two values. A perfect unit would show this behavior over the angular range ± 16°. b) The actuation dynamics show a rise in measured current after removing the hot mirror and a decline towards the previous value after inserting the hot mirror into the beam.
Fig. 12
Fig. 12 Measuring every unit on its own and adding the results up, shows an effective concentration just short of 4x (a). However, not all lenses participate actively. In comparison the added simulation results for the unit achieve close to 8x (b).
Fig. 13
Fig. 13 In contrast to Fig. 12 the erroneous terms due to constant coupling have been removed and the difference ISC,max - ISC,min plotted. Apart from unit 2, no unit performs over the desired angular range. However most perform well over a reduced angular ranges.
Fig. 14
Fig. 14 Photograph of the concentrator. The lens array is focuses on the actuator (not visible in the photograph) which couples light into the waveguide. Light hitting the scattering ZnO layer around the photodiodes is outcoupled and lost (reason this region is seen in the picture). Two micro-probes (front right) are used to measure the short-circuit current.
Fig. 15
Fig. 15 The five cases observable in the actuation results are displayed. Case 1-3 can be seen in Fig. 11(a) whereas case 5 only is visible in Fig. 11(b).

Equations (1)

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C C max = n 2 sin 2 ( θ max,in )


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