Imma Perfetto
Cosmos science journalist
Solar panels will play a major role in future sustainable energy production but they work best when sunlight hits them directly. This can be a problem when sunlight is diffused by cloud cover or as the sun passes above throughout the day.
Many solar arrays actively rotate towards the sun to capture as much energy as possible. This makes them more expensive and complicated to build and maintain than those that remain stationary.
But moving solar panels may not be necessary in the future, because an engineering researcher has designed a device that can capture 90% of the light that falls on it – regardless of its angle or frequency – and concentrate it to be three times brighter.
The research has been published in Microsystems & Nanoengineering.
“It’s a completely passive system – it doesn’t need energy to track the source or have any moving parts,” says first author Dr Nina Vaidya, who completed the research as a doctoral student at Standford University, US, and is now an assistant professor in astronautics and spacecraft engineering at the University of Southampton, UK.
“Without optical focus that moves positions or need for tracking systems, concentrating light becomes much simpler.”
AGILE – Axially Graded Index Lens
The device is called AGILE, which stands for axially graded index lens, and it looks like an upside down glass pyramid with the point cut off.
It works a bit like a how a magnifying glass can focus sunlight into a smaller, brighter point on a sunny day. But a magnifying glass’s focal point moves as the sun does, which isn’t helpful when you want to concentrate sunlight to a specific area of a photovoltaic cell throughout the entire day.
With AGILE, light enters the wide, square top from all angles and is funnelled down to concentrate at the same position at the bottom – creating a brighter spot at the narrow base which sits on top of a photovoltaic cell.
“We wanted to create something that takes in light and concentrates it at the same position, even as the source changes direction,” explains Vaidya. “We don’t want to have to keep moving our detector or solar cell, or moving the system to face the source.
Senior author Olav Solgaard, professor of electrical engineering at Stanford and Vaidya’s doctoral adviser, says: “An ideal AGILE has, at the very front of it, the same refractive index as the air and it gradually gets higher – the light bends in a perfectly smooth curve.
“But in a practical situation, you’re not going to have that ideal AGILE.”
Instead, the prototype AGILE is made of what’s known as a graded index material, which is comprised of different layers of glasses and polymers that bend light to different degrees. These layers change the incoming light’s direction in steps until it comes in almost vertically towards the output.
Taking AGILE from theory to reality
One of the biggest challenges in creating the AGILE prototype was finding and creating the right commercially available materials which could let in a broad spectrum of light, allow it to pass through, and bend it increasingly towards the output – all while being compatible with each other.
For instance, if one glass expanded in response to heat at a different rate than another, the whole device could crack. The materials also needed to be robust enough to be machined into shape and remain durable.
The sides of the prototypes are also mirrored to bounce any light going in the wrong direction back towards the base.
These AGILE devices could be installed in a layer on top of solar cells, replacing the existing encapsulation that protects solar arrays, and would even create more space for cooling and circuitry to run between the narrowing pyramids of the individual devices.
“To be able to use these new materials, these new fabrication techniques and this new AGILE concept to create better solar concentrators has been very rewarding,” concludes Vaidya.
“Abundant and affordable clean energy is a vital part of addressing the urgent climate and sustainability challenges, and we need to catalyse engineering solutions to make that a reality.”
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