James Mitchell Crow
The ancient Japanese paper-cutting art of kirigami has inspired a simple rooftop solar cell design that can track the Sun across the sky – and capture much more sunlight than conventional panels. The cells developed by Max Shtein and his team at the University of Michigan even look pretty too. The design was published in Nature Communications in September.
Solar cells are most efficient when they face squarely to the Sun. Conventional rooftop solar panels are usually installed to catch the bright midday sunlight – their performance drops off before mid-morning and after mid-afternoon. It’s possible to mount them on to a rotating base that tracks the Sun all day but these cumbersome, heavy, costly systems can only be installed on the ground or on flat rooftops.
Shtein’s kirigami cells are a simple, lightweight alternative. They are made of a thin, flexible material called gallium arsenide – a promising solar material that captures sunlight more efficiently than silicon. To make them he cut a series of slits into a panel of the material and tugged one end gently. The slits formed loops tilting up from the flat surface. The stronger the tug, the steeper the tilt.
By continually adjusting the tension on the kirigami cell, the team could track the Sun all day. This simple trick makes the cell up to 40% more efficient than a flat panel cell made of the same material, says Shtein.
“I think it’s very clever,” says David Jones, who develops flexible electronics and solar cells at the University of Melbourne. “It really reinforces what we’ve been saying about these new thin-film solar technologies – that you have to be creative, go out and work with design people to say, can we use it in a different way?”
Shtein came up with his new take on thin film solar cells while talking to long-time friend and collaborator Matthew Shlian, from Michigan’s School of Art and Design. Part-artist, part-paper-engineer, Shlian uses kirigami – a form of origami in which the paper is cut as well as folded – to sculpt three dimensional paper forms. While the pair were discussing Shlian’s work, Shtein was suddenly struck by their potential for solar tracking. “I’ve been working on the area of solar cells for a long long time – I know the problem space,” Shtein says.
The team began to apply kirigami techniques to their flexible solar cells. They tried several cut patterns but found that a simple pattern performed best. One of the biggest advantages of the final design is that the harder you pull on it, the further apart the loops move. “Because you have this motion where they are moving out of each other’s way, the pieces aren’t shading each other,” Shtein says.
This simple trick makes the cell up to 40% more efficient
Shtein pictures encasing the somewhat delicate kirigami solar material, and the simple electronics and motors needed to drive it, within a thin double-pane enclosure. This would protect the cell from the weather, and it would be as easy to install on a roof as a conventional solar panel.
The team now needs to test whether the cells will withstand constant stretching and bending without cracking. So far, the team has shown the cells hold up well to more than 300 cycles. “There’s still a lot of thinking and refining to do,” says Shtein. “but already we can say we can get away with a lot less [light-capturing] semiconductor material for the same amount of energy collected over the course of a day.”
Gallium arsenide is a more efficient light capturing material than silicon, but it is also more expensive – especially now silicon is produced at such scale. Shtein says prices are beginning to fall, but even if gallium arsenide itself remains too expensive for commercial kirigami cells, there are plenty of cheaper flexible thin film solar materials in development.
Jones suggests kirigami solar cells could even be made from silicon itself – a thin, flexible form known as amorphous silicon. It is a bit thicker than gallium arsenide, but by adjusting the spacing between the cuts, “it might be possible”, he says.
