Sunflowers inspire more efficient solar power system

August 16, 2012 by Mark Riechers
UW-Madison engineering professor Hongrui Jiang looked to sunflowers to help find more efficient ways to harvest solar energy.

( -- A field of young sunflowers will slowly rotate from east to west during the course of a sunny day, each leaf seeking out as much sunlight as possible as the sun moves across the sky through an adaptation called heliotropism.

It's a clever bit of natural engineering that inspired imitation from a UW-Madison electrical and computer engineer, who has found a way to mimic the passive heliotropism seen in sunflowers for use in the next crop of .

Unlike other "active" solar systems that track the sun's position with GPS and reposition panels with motors, electrical and computer engineering professor Hongrui Jiang's concept leverages the properties of unique materials in concert to create a passive method of re-orienting in the direction of the most direct sunlight.

His design, published Aug. 1 in and recently highlighted in Nature, employs a combination of liquid crystalline elastomer (LCE), which goes through a and contracts in the presence of heat, with carbon nanotubes, which can absorb a wide range of light wavelengths.

"Carbon nanotubes have a very wide range of absorption, all the way to infrared," says Jiang. "That is something we can take advantage of, since it is possible to use sunlight to drive it directly."

Video of a proof-of-concept of Jiang's design in action.

Direct sunlight hits a mirror beneath the solar panel, focused onto one of multiple actuators composed of LCE laced with carbon nanotubes. The carbon nanotubes heat up as they absorb light, and the heat differential between the environment and inside the actuator causes the LCE to shrink.

This causes the entire assembly to bow in the direction of the strongest sunlight. As the sun moves across the sky, the actuators will cool and re-expand, and new ones will shrink, re-positioning the panel over the 180 degrees of sky that the sun covers in the course of the day.

Artificial heliotropism in action.

"The idea is that wherever the sun goes, it will follow," says Jiang.

In Jiang's tests, the system improved the efficiency of solar panels by 10 percent, an enormous increase considering material improvements in the solar panels themselves only net increases of a few percent on average. And a passive system means there are no motors and circuits to eat into increased energy harvest.

"The whole point of solar tracking is to increase the electricity output of the system," says Jiang.

The materials driving Jiang's design have only been available in the past few years, so for now, he and his team are researching ways to refine them for use driving larger solar panels, where the net energy gain from his system will be the greatest.

But eventually, Jiang hopes to see huge industrial solar farms where fields of photovoltaic solar panels shift effortlessly along with the sunflowers that inspired him.

"This is exactly what nature does," says Jiang.

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1 / 5 (1) Aug 16, 2012
What mechanism(s)re-orient both Flowers and Panels back to the position of the rising Sun???
1 / 5 (1) Aug 17, 2012
I also wonder what happens when there are clouds passing over the sun? My reasoning goes that when there is cloud coverage, all cells will cool down, turning the panel to night position. Then when clouds pass by, the panel will reorient towards the sun, missing some energy while reorienting. GPS oriented panels always keep the right direction despite the cloud coverage.
not rated yet Aug 17, 2012
What mechanism(s)re-orient both Flowers and Panels back to the position of the rising Sun?

The same one. When the sun (or a panel with this meachnism) goes down the heat on both sides even out (making the flower/panel go into a 'neutral' position, which is optimalley right between east and west facing). When the sun risies one side gets heated more and the flower/panel starts turnnig towards the sun.

I also wonder what happens when there are clouds passing over the sun?

Not much since infrared still penetrates somewhat. It's the relative difference of energy that determines the direction - and that relative difference is the same with or without clouds.
then again: Panels output is severly decreased when clouds pass - so: meh.

I think this is a pretty nifty idea as there's nothing to break down/sevice (and some joker can't come along and spoof the GPS signal - which would actually be a neat April-Fools gag)
not rated yet Aug 19, 2012
Mr jiang's idea is a prime example of why it is a missmatch to use a complex system to do a simple job. Cost and reliability are also benefits of doing things the simple way.
not rated yet Sep 02, 2012
Can someone explain to me why GPS would be necessary here? A tracking solar panel needs just one thing, a clock/calendar, to know where to point. And I'm not buying the notion that a lot of energy is required to swing a solar array 15 degrees an hour.

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