Tech Briefs Magazine - September 2022 - 13

Q & A
Focusing the Sun - A Big Gain for Solar Power Efficiency
Professor Nina Vaidya has developed a new
kind of optical concentrator - Axially Graded
Index Lens (AGILE) - that can passively focus
the sun onto a photovoltaic cell from any
angle to reduce the amount of photovoltaic
material needed for a given amount of power
generation.
Tech Briefs: What led you to this project?
Vaidya: While working on my PhD at Stanford, Professor
Olav Solgaard asked whether it was possible to design an optical
device that can take all the light from all angles and focus it
at the same point without moving it toward the source, and he
mentioned gradient index optics.
Tech Briefs: Was this just a theoretical idea of his - to
ask that question?
Vaidya: Yes, we started from that impossible theoretical
dream, then I worked on a feasibility study and showed that
it could work. Several people told me that to make this idea
as a real device with real materials could not be done - but I
thought it could. I built a funnel-shaped concentrator, with a
refractive index graded from low to high. The challenge was
that the range of the refractive index difference needed is very
large, from about 1.0 to about 3.5. And not only that, I wanted
to be able to use the whole broadband spectrum of sunlight.
Tech Briefs: What made it possible to cover these
wavelengths?
Vaidya: I did an extensive material search, basically characterizing
various materials and then matching them to be used as
the layers in AGILE.
Tech Briefs: How did you bond the layers?
Vaidya: By basically machining it in a way similar to glass
polishing, but at a much higher level, with the help of the
crystal shop at Stanford. We wanted to have the surfaces
almost nanometer smooth so we could bond them to each
other with anodic bonding. For the anodic bonding, I built
equipment to be able to heat the joint and pass electricity
through it at the flexible cleanroom at Stanford.
Tech Briefs: I'm puzzled as to now it actually can work.
When I was burning dry leaves with my magnifying
glass, I always had to change the angle of the glass as
the day wore on. So, I can't imagine how your device
can hold the same focal point as the sun moves across
the sky.
Tech Briefs, September 2022
www.techbriefs.com
Vaidya: Great question. If you have a magnifying glass and
then the sun, you know that the spot on the leaf or your hand
will move as the sun does. Now imagine instead of that lens, you
have a material with a graded refractive index that goes from
a low index to a high index gradually across the height of the
AGILE from a larger aperture to a smaller aperture, with reflective
sidewalls. Because of that gradient index, as light enters
our funnel, it slows down and curves toward the normal due to
refraction instead of travelling in straight lines. That way we
can collect light from all angles without solar tracking and can
even collect light diffused from cloud cover and atmosphere.
Tech Briefs: What excites you most about this project?
Vaidya: In today's energy and urgent climate crisis, AGILE's
potential to make solar arrays more effective and cheaper is
going to be very impactful.
Read the full transcript of this Q&A at www.techbriefs.com.
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Tech Briefs Magazine - September 2022

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