Passive solar tracking follows the sun
Like sun flowers following the sun, US researchers claim to have a passive solar tracking system for concentrator solar power generators.
Concentrator systems use curved mirrors or lenses to focus the suns rays onto a small solar cell.
Because the cell is small, it can be of exotic construction – triple layer and actively cooled for example – and therefore highly efficient.
This kind of system can be more cost effective than flat solar panels in locations where direct sunshine is common, like deserts.
However, motors and either sensors or computers are required to steer the focussing system to follow the sun.
At the University of Wisconsin Madison, engineer Professor Hongrui Jiang is steering using a combination of liquid crystal elastomer (LCE), which goes through a phase change 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, visible light all the way to infrared,” said Jiang. “That is something we can take advantage of, since it is possible to use sunlight to drive it directly.”
Direct sunlight hits a mirror beneath the solar panel, focused onto one of multiple actuators composed of LCE laced with carbon nanotubes.
The nanotubes heat up as they absorb light, and the heat differential between the environment and inside the actuator causes the LCE to shrink.
As it shrinks, it 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”, said UW-Madison. “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.”
Concentrators operate at gains up to 500x – where the mirror has 500 times the area of the solar cell.
Accurate steering is more critical at higher (more cost-effective) gains, so the proof of the pudding with Jiang’s system will be how accurately it can be made to steer over a large area of sky.
The design is described in Advanced Functional Materials (Aug. 1) and recently highlighted in Nature.