Stanford engineers’ optical concentrator can help solar panels capture more light even on a cloudy day without tracking the sun

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

Even with the impressive and ongoing advances in solar technologies, the question remains: how can we efficiently collect energy from sunlight coming from different angles from dawn to dusk?

Solar panels work best when sunlight hits them directly. To capture as much energy as possible, many solar panels actively rotate toward the sun as it moves through 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 not be needed in the future. At Stanford University, technical researcher Nina Vaidya designed an elegant device that can efficiently collect and concentrate the light that falls on it, regardless of the angle and frequency of that light. A paper a description of the system’s performance, and the theory behind it, is the lead article in the July issue of Microsystems and nanoengineering, written by Vaidya and her doctoral advisor Olav Solgaardprofessor of electrical engineering at Stanford.

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

The device, which the researchers call AGILE — an acronym for Axially Graded Index Lens — is deceptively simple. It looks like an inverted pyramid with the tip cut off. The light enters the square tileable top from any number of angles and is directed downwards to create a brighter spot at the exit.

In their prototypes, the researchers were able to capture more than 90% of the light that fell on the surface and create spots at the exit that were three times brighter than the incoming light. Installed in a layer on top of solar cells, they can make solar panels more efficient, capturing not only direct sunlight but diffused light scattered by Earth’s atmosphere, weather and seasons.

A top coat of AGILE could replace the existing encapsulation that protects solar panels, remove the need to track the sun, create space for cooling and circuitry to run between the narrowing pyramids of the individual devices, and most importantly, reduce the amount of solar -energy reducing cell surface area needed to produce energy – thereby lowering costs. And its use is not limited to terrestrial solar installations: when applied to solar panels sent into space, an AGILE layer can both concentrate light without solar tracking and provide the necessary protection against radiation.

The premise behind AGILE is similar to using a magnifying glass to burn spots on leaves on a sunny day. The lens of the magnifying glass focuses the sun’s rays into a smaller, brighter point. But with a magnifying glass, the focal point moves just like the sun. Vaidya and Solgaard have found a way to create a lens that catches rays from all angles, but always focuses the light on the same starting position.

“We wanted to create something that takes light and focuses it in the same position even if the source changes direction,” says Vaidya. “We don’t want to have to keep moving our detector or solar cell or moving the system to the source.”

Vaidya and Solgaard determined that it would theoretically be possible to collect and focus scattered light using an engineered material that gradually increases in refractive index — a property that describes how fast light travels through a material — causing it to bend and bend light toward a focal point. At the surface of the material, the light would hardly bend. By the time it reached the other side, it would be almost vertical and focused.

“The best solutions are often the simplest ideas. An ideal AGILE has the same refractive index as the air all the way up front and gradually gets higher — the light bends in a perfectly smooth curve,” Solgaard said. “But in practice, you won’t have that ideal AGILE.”

From theory to reality

For the prototypes, the researchers put together different glasses and polymers that bend light to varying degrees, creating what is known as a graded index material. The layers change the direction of the light incrementally instead of a smooth curve, which the researchers say is a good approximation of the ideal AGILE. The sides of the prototypes are mirrored, so light going the wrong way will be reflected back to the exit.

One of the biggest challenges was finding and making the right materials, says Vaidya. The material layers in the AGILE prototype allow a broad spectrum of light, from near-ultraviolet to infrared, to pass through and increasingly bend that light toward the exit 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 expands in response to heat at a different rate than another, the entire device could crack — and robust enough to be machined to shape and remain durable.

“It’s one of these ‘moonshot’ engineering adventures, from theory to real-life prototypes,” Vaidya said. “There are a lot of theory papers and great ideas, but it’s hard to turn them into reality with real designs and real materials that push the boundaries of what was previously thought impossible.”

After exploring many materials, creating new fabrication techniques and testing multiple prototypes, the researchers stumbled upon AGILE designs that performed well using commercially available polymers and glasses. AGILE is also fabricated using 3D printing in the earlier work who created lightweight and design-flexible polymer lenses with nanometer-scale surface roughness. Vaidya hopes that the AGILE designs can also be used in the solar industry and other fields. AGILE has several potential applications in areas such as laser coupling, display technologies and lighting – such as semiconductor lighting, which is more energy efficient than older lighting methods.

“Using our efforts and knowledge to create meaningful engineering systems has been what drives me, even when some trials didn’t work,” said Vaidya. “It was very rewarding to be able to use these new materials, these new manufacturing techniques and this new AGILE concept to create better solar concentrators. Abundant and affordable clean energy is an essential part of tackling the pressing climate and sustainability challenges, and we need to catalyze engineering solutions to make that happen.”

Solgaard is the director of the Edward L. Ginzton Laboratory; a member of Stanford Bio-Xthe Stanford Cancer Instituteand the Wu Tsai Neuroscience Institute† and a branch of the Precourt Institute of Energy and the Stanford Woods Institute for the Environment

This work was funded by the Global Climate and Energy Project and the Diversifying Academia, Recruiting Excellence doctoral fellowship program. Acknowledgments 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 help with the graphics.

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