Stanford engineers’ optical concentrator could

image: Nina Vaidya measuring the experimental performance of optical concentrators under a solar simulator which acts as an artificial sun.
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Credit: Courtesy of Nina Vaidya

Even with the impressive and continuing advances in solar technologies, the question remains: how can we efficiently harvest sunlight energy from varying angles from sunrise to sunset?

Solar panels work best when sunlight hits them directly. To capture as much energy as possible, many solar panels actively rotate towards the sun as it moves across the sky. This makes them more efficient, but also more expensive and complicated to build and maintain than a stationary system.

These active systems may no longer be needed in the future. At Stanford University, engineering researcher Nina Vaidya has designed an elegant device that can effectively collect and focus light falling on it, regardless of the angle and frequency of that light. An article describing the performance of the system and the theory behind it is the cover story of the July issue of Microsystems & Nanoengineering, written by Vaidya and her thesis supervisor Olav Solgaard, professor of electrical engineering at Stanford.

“It’s a completely passive system – it doesn’t need power to track the source or have any moving parts,” said Vaidya, who is now an assistant professor at the University of Southampton, UK. “Without optical focus that shifts positions or the need for tracking systems, focusing light becomes much simpler.”

The device, which the researchers call AGILE — an acronym for Axially Graded Index Lens — is deceptively simple. It looks like an upside down pyramid with the tip cut off. Light enters the square top that can be tiled from any angle and is channeled downward to create a brighter spot as it exits.

In their prototypes, the researchers were able to capture more than 90% of the light that hits the surface and create spots at the exit three times brighter than the incoming light. Installed in a layer above the solar cells, they could make solar panels more efficient and capture not only direct sunlight, but also diffuse light that has been scattered by the atmosphere, weather conditions and seasons of Earth.

A top layer of AGILE could replace existing encapsulation that protects solar panels, eliminate the need to track the sun, create space for cooling and circuitry between the shrunken pyramids of individual devices, and most importantly, reduce the amount of solar energy. cellular surface needed to produce energy – and therefore reduce costs. And the uses are not limited to terrestrial solar installations: if applied to solar panels sent into space, an AGILE layer could both concentrate light without solar tracking and provide the necessary protection against radiation.

The basic principle behind AGILE is similar to using a magnifying glass to burn spots onto leaves on a sunny day. The magnifier lens concentrates the sun’s rays into a smaller, brighter spot. But with a magnifying glass, the focal point moves like the sun does. Vaidya and Solgaard found a way to create a lens that catches rays from all angles but still focuses the light at the same exit position.

“We wanted to create something that captures light and focuses it in the same place, even when the source changes direction,” Vaidya said. “We don’t want to have to constantly move our detector or our solar cell or move the system to face the source.”

Vaidya and Solgaard determined that, theoretically, it would be possible to collect and focus scattered light using an engineered material that gradually increases the index of refraction – a property that describes the speed at which light travels. through a material – causing light to bend and bend towards a focal point. On the surface of the material, the light would hardly bend. By the time it reached the other side, it would be nearly vertical and focused.

“The best solutions are often the simplest ideas. An ideal AGILE has, at the very front, the same refractive index as air and it gradually increases – the light bends into a perfectly smooth curve,” Solgaard said. “But in a practical situation, you’re not going to have that ideal AGILE.”

From theory to reality

For the prototypes, the researchers layered different glasses and polymers that bend light to different degrees, creating what’s called a graded-index material. The layers change the direction of the light in steps instead of a smooth curve, which the researchers found to be a good approximation of the AGILE ideal. The sides of the prototypes are mirrored, so any light going in the wrong direction is bounced back out.

One of the biggest challenges has been finding and creating the right materials, Vaidya says. The layers of material in the AGILE prototype allow a broad spectrum of light to pass through, from near ultraviolet to infrared, and bend that light increasingly outward with a wide range of refractive indices, which is not seen in nature or current optics. industry. These materials used also had to be compatible with each other – if one glass expanded in response to heat at a different rate than another, the whole device could crack – and strong enough to be machined into shape and remain sustainable.

“It’s one of those ‘moonshot’ engineering adventures, going from theory to actual prototypes,” Vaidya said. “There are many theoretical papers and good ideas, but it is difficult to bring them to life with real designs and real materials pushing the limits of what was considered impossible before.”

After exploring many materials, creating new manufacturing techniques, and testing several prototypes, the researchers came across AGILE designs that performed well using commercially available polymers and glasses. AGILE was also fabricated using 3D printing in the authors’ previous work that created lightweight, flexible polymer lenses with nanoscale surface roughness. Vaidya hopes AGILE designs can be used in the solar industry and other fields as well. AGILE has several potential applications in areas such as laser coupling, display technologies, and lighting – such as solid state lighting, which is more energy efficient than older lighting methods.

“Using our efforts and knowledge to create meaningful engineering systems has been my driving force, even when some trials didn’t work,” Vaidya said. “Being able to use these new materials, these new manufacturing techniques and this new AGILE concept to create better solar concentrators has been very rewarding. Abundant and affordable clean energy is a critical component to addressing pressing climate and sustainability challenges, and we must catalyze engineered solutions to make it a reality.

Solgaard is the director of the Edward L. Ginzton Laboratory; fellow of Stanford Bio-X, Stanford Cancer Institute and Wu Tsai Neurosciences Institute; and an affiliate of the Precourt Institute for Energy and the Stanford Woods Institute for the Environment.

This work was funded by the Global Climate and Energy Project and the Diversifying Academia Doctoral Fellowship Program, Recruiting Excellence. Thanks to Thomas E. Carver (Flexible Cleanroom) and Tim Brand (Ginzton Crystal Shop) for manufacturing support, and Reinhold Dauskardt, Professor of Materials Science and Engineering, for advice on materials science. Thanks to Xuan Wu for the AGILE video and Alan Truong for his help with the graphic images.

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