A new optical lens harvests and concentrates scattered light from multiple directions without any moving components, raising hopes that it could help make future solar cells more efficient. Designed by Nina Vaidya and Olav Solgaard at Stanford University, US, the lens relies solely on the increasing refractive indices of successive glass layers to redirect light. The success of a prototype suggests that it could be used as a tile-able surface on solar panels.
To improve the efficiency of solar cells, many researchers are working on techniques to concentrate incoming sunlight onto smaller areas. This can be done using a wide range of advanced optical setups – but for optimum efficiency, these devices must move to face the Sun at all times, which requires costly and complex tracking systems.
As an alternative, Vaidya and Solgaard designed a lens that collects scattered sunlight passively, over a broad range of incident angles, and concentrates it onto a single spot. Dubbed the Axially Graded Index Lens (AGILE) by its designers, the device is shaped like an inverted square pyramid with the apex cut off. It is composed of eight glass layers, with refractive indices that increase progressively towards the bottom.
Thanks to this arrangement, when a beam of light enters the larger square at the top of AGILE, its path curves downwards as it progresses through the pyramid. Regardless of the beam’s angle of incidence at the top, it will thus be almost vertical once it reaches the smaller square at the bottom. Vaidya and Solgaard also coated the sloping sides of their pyramid with a mirror, so light that might otherwise escape from the lens gets reflected inside.
Search for the right materials
To build a prototype version of AGILE, Vaidya and Solgaard carried out an extensive search of possible glass materials. These glasses would need to satisfy a stringent set of requirements, including the ability to transmit a broad range of wavelengths from the solar spectrum, which spans roughly 300 to 1200 nm. The materials would also need to display similar rates of thermal expansion, while still encompassing a broad range of refractive indices.
Once the duo identified a set of optical glasses that fulfilled these conditions, they fabricated a prototype by bonding the layers together into a vertical stack, before carving out the lens’ pyramid shape and coating it with reflective aluminium.
Kirigami solar cells follow the Sun
In their initial experiments, which they describe in Microsystems and Nanoengineering, the researchers showed that AGILE transmitted over 90% of incoming scattered light, concentrated on a spot a third of the size of the upper square surface. Based on this result, they suggest that solar panels could be coated with arrays of AGILE tiles, which would not only allow the panels to capture the Sun’s light passively throughout the day, but also permit them to harvest the diffuse light scattered by Earth’s atmosphere.
The duo report that the next step will be to show how AGILE could be manufactured on large scales, through techniques including spray coating, moulding and 3D printing.
