Solar panel that unfurls automatically

A polymer sheet representing a solar cell opens from a folded form (inset) to a flat disk with 10 times the surface area. The process is autonomous, requiring no power source but just a rise in temperature.

Inspired by a toy known as Hoberman sphere, researchers led by Kristina Shea of the Swiss Federal Institute of Technology (ETH), Zurich, have developed an origami-like design for a solar panel that could be folded away compactly on a spacecraft and then expanded rapidly and deployed to harvest sunlight for power. Based on what is known as “shape-memory polymer” that responds to temperature changes, the structure can unfold to increase its surface area by ten times in just 40 seconds, without any power source to drive the process.

The concept could be used for creating sheet-like structures that undergo rapid changes in shape, from tents to roofing canopies. Deployable structures in the form of compact sheets and membranes that can unfurl have a variety of potential applications in architecture, energy generation and robotics.

Such structures are typically made from thin, flexible sheets, and the unfolding is effected through a mechanical framework. The engineers Kristina Shea of ETH and Chen, an ETH graduate student, had previously built mechanical structures that changed shape using shape-memory polymers. Since such a material becomes stiff below a particular temperature, it can be bent and locked into a specific structure by cooling. At higher temperatures, the material becomes soft and will return to its initial state. In fact, shape-memory polymers that respond to temperature changes have been used to make sheets that self-fold into boxes, pyramids and other shapes.

The device the team created is a circular frame holding a disk-shaped sheet. It has a rigid framework made from hinged joints which use a mechanism devised by the engineer Chuck Hoberman for use in shape-changing architecture. The toy Hoberman sphere is a compact ball when closed but opens into a much larger spherical shell. Much like the petals of a flower, the sheet folds into a compact spiral using the so-called “flasher” origami principle. By computationally optimising the folding pattern to best fit within the framework of 20 Hoberman hinges, the Kristina Shea-Chen team was able to achieve an efficiently packed and quickly unfolding structure.

The structure could be compacted by increasing the temperature beyond the polymer’s transition temperature of 35ºC to 40ºC and could be locked in by cooling it to room temperature. Warming the structure in 40ºC water opened it up again. What was originally a 25-cm diameter opened into a 79 cm diameter sheet in less than 40 seconds. In a solar-cell application, the shape-memory polymer sheet could be coated with a photovoltaic material, although it is conceivable that shape-memory materials might be devised with photovoltaic properties, said Kristina Shea.

If one used a shape-memory polymer whose transition temperature is what you get when the material gets warmed by solar radiation, the unfolding transition could be triggered, she said.