The ancient Japanese art of kirigami, or paper cutting, has been used by researchers in the US to improve the efficiency of solar-panel tracking systems. The researchers cut a pattern in thin-film gallium-arsenide solar cells, which causes the cells to tilt when stretched. The system is an improvement on existing solar-tracking equipment, which is bulky, expensive and generally beyond the reach of household solar arrays. The team says that its new design could easily be deployed on individual houses as well as in larger arrays, and, as an added bonus, also improves the optical and mechanical properties of the solar cell.
Flat-panelled solar-cell arrays are most effective when sunlight is directly incident on their surface. Solar trackers are used to orient such arrays, along one or two axes, allowing them to follow the Sun as its position in the sky changes during the course of a day and throughout the year.
Cumbersome trackers
Depending on the geographic location of a solar array, and whether it has one or two tracking axes, a conventional tracker could boost yearly energy generation by 20–40%, compared with a static array. But despite these promising figures, such systems have not been widely implemented because of the high costs, added weight and additional space that they require. Indeed, the additional components required for tracking account for nearly 12% of the total cost of the system, and while this number increases by about 1% annually, the price of actual solar cells is dropping. Also, thanks to the tracker’s size, they cannot be used on the roofs of most homes.
To overcome these problems, Max Shtein and colleagues at the University of Michigan in Ann Arbor used a laser to cut a 2D pattern into gallium-arsenide solar cells. By stretching these patterned cells, the researchers can produce tilted solar-cell arrays in 3D. While the cell panel remains flat, the array elements pop up when stretched.
“All in all, we’re getting about a 30% improvement in the amount of energy harvested across the course of a simulated day, say in Arizona, for a given amount of semiconductor used, compared with stationary panels,” says Shtein. “It basically matches what conventional trackers can do in boosting energy production, but with considerably less bulk.”
Optimal harvest
While the cuts do reduce the area of the array available for sunlight harvesting, it is by a very tiny amount, and Shtein explains that the corners of the cuts are also rounded off to reduce stress in the structure, further reducing the area. By adjusting the strain on the stretched solar cells, the team was able to optimize the cells’ optical and mechanical properties. The researchers found that longer cuts that are spaced closer together made for less pulling effort, and that the degree of tilt is proportional to the amount of pull. From a practical point-of-view, these kirigami-enhanced cells could be placed within a double-pane enclosure to make them more weatherproof, and could be reinforced via tensioned support cables in large arrays to prevent them from sagging.
Although the team’s technique is still in the design phase and further research is needed, it offers a lightweight, scalable and cheap alternative to solar tracking, thereby maximizing the efficiency of such solar cells. Shtein told physicsworld.com that the kirigami approach could be extended to other thin-film or flexible solar cells. “That’s not to say there won’t be integration challenges – plenty of development to do there – but the basic idea should be the same,” he says. According to the researchers, their design opens up new markets for solar tracking, including widespread rooftop, mobile and space-borne installations.
The research is published in Nature Communications.
