(a) Coupling efficiency (ηC) versus source angle (φ) for a planar solar panel. The panel projected area decreases with the cosφ. (b) A kirigami tracking structure that, upon stretching, simultaneously changes the angle of the elements comprising the sheet. By incorporating thin-film solar cells into this structure, it may be used as a low-profile alternative to conventional single-axis solar tracking. (c) The direction of feature tilt (that is, clockwise or counter-clockwise with respect to the original plane) is controlled by lifting or lowering one end of the sheet (step 1) before the straining process (step 2).
Solar cells capture up to 40% more energy when they can track the sun, but conventional, motorized trackers are too heavy and bulky for pitched rooftops and vehicle surfaces. Now, by borrowing from kirigami, Japanese art of paper cutting, University of Michigan researchers have developed solar cells that can have it both ways.”The design takes what a large tracking solar panel does and condenses it into something that is essentially flat,” said Aaron Lamoureux.
Residential rooftops make up about 85% of solar panel installations in the U.S but these roofs would need significant reinforcing to support the weight of conventional sun-tracking systems. A team of engineers and an artist developed an array of small solar cells that can tilt within a larger panel, keeping their surfaces more perpendicular to the sun’s rays.
(a) Response of a Kapton kirigami structure to stretching in the axial direction (εA) is accompanied by a decrease in sample width (εT) and a change in feature angle (θ). Also shown are the geometric parameters that define the kirigami structure, namely the cut length (LC) and spacing between cuts in the transverse (x) and axial (y) directions, which can be expressed in terms of the dimensionless parameters, R1 and R2. (b) Schematics of four kirigami structures, where R1=R2=3, 5, 10 and 20, along with their corresponding units cells. (c) εT and θ versus εA for several kirigami structures where R1=R2=3, 5, 10 and 20 (b). Theoretical predictions per equations (1) and (2) are shown by solid lines, while the closed symbols represent experimental data from a 50 μm-thick Kapton sample of the appropriate geometry. While larger R1 and R2 enable increased axial strains and correspondingly larger transverse strains, the change in feature angle is independent of cut geometry.
“… inside, it would be doing something remarkable on a tiny scale: the solar cell would split into tiny segments that would follow the position of the sun in unison.” By designing an array that tilts and spreads apart when the sun’s rays are coming in at lower angles, they raise the effective area that is soaking up sunlight.
METHOD: The team of engineers worked with paper artist Matthew Shlian, U-M School of Art and Design lecturer. Shlian showed Lamoureux and Shtein how to create them in paper using a plotter cutter. Lamoureux then made more precise patterns in Kapton, a space-grade plastic, using CO2 laser. Simplest patterns worked best. With cuts like rows of dashes, the plastic pulled apart into a basic mesh. The interconnected strips of Kapton tilt in proportion to how much the mesh is stretched, to an accuracy of ~1 degree.
(a) Integrated thin-film, crystalline GaAs solar cells, mounted by cold weld bonding on a Kapton carrier substrate, as used for testing. Here, LC=15 mm, x=5 mm and y=5 mm (R1=R2=3). (b) Normalized solar cell short circuit current density JSC(φ)/JSC(φ=0) for two samples, where R1=R2=3 and R1=R2=5 (closed symbols). Also shown are the simulated data for coupling efficiency (ηC) obtained from equation (4) (solid lines, open symbols). The agreement between experimental and simulated results suggests that ηC is a direct measure of optical coupling, and that performance may be optimized by increasing R1 and R2. (c) Output electrical power density incident on the solar cell versus time of day for several kirigami cut structures, stationary panel and single-axis tracking systems in Phoenix, AZ (33.45° N, 112.07° W) during the summer solstice. Inset: Integration of the curves yields the associated energy densities, where kirigami-enabled tracking systems are capable of near single-axis performance.
~To make the solar array, Kyusang Lee built custom solar cells. He and Lamoureux attached them to an uncut piece of Kapton, leaving spaces for the cuts. Then, Lamoureux patterned the Kapton with the laser cutter. The design with the very best solar-tracking promise was impossible to make at U-M as the solar cells would be very long and narrow. The cells became too long to fit into the chambers used to make the prototypes on campus, so the team is looking into other options.
The optimized design is effective because it stretches easily, allowing a lot of tilt without losing much width. According to the team’s simulations of solar power generation during the summer solstice in Arizona, it is almost as good as a conventional single-axis tracker, offering a 36% improvement over a stationary panel. “It could ultimately reduce the cost of solar electricity.” http://www.nature.com/ncomms/2015/150908/ncomms9092/full/ncomms9092.html
http://www.eurekalert.org/pub_releases/2015-09/uom-iba090415.php
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