Professor Hongrui Jiang
Cribbing on nature's designs can be one of the best routes to energy efficiency, as recent work from University of Wisconsin-Madison demonstrates. Electrical and computer engineering Professor Hongrui Jiang has adopted a passive method of re-orienting solar panels in the direction of sunlight, imitating how sunflowers rotate east to west to follow the sun through heliotropism adaptation. Unlike "active" solar systems that track the sun's position through GPS repositioning panels with motors, Professor Hongrui Jiang creates a passive method to re-orient solar panels in the direction of the most direct sunlight. His design, published in Advanced Functional Materials uses a combination of a smart material liquid crystalline elastomer (LCE), which phase changes and contracts in the presence of heat, together with carbon nanotubes, that absorb a wide range of light wavelengths. "Carbon nanotubes have a very wide range of absorption, visible light 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." 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, the heat differential between environment and inside the actuator, causes the LCE to shrink. This makes the entire assembly bow in the direction of the strongest sunlight. The actuators will cool and re-expand as the sun moves across the sky, 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. In Jiang's tests, the system improved the efficiency of solar panels by 10%, an excellent increase considering material improvements in the solar panels themselves tend to only net a few percent on average. A passive system also means no energy grazing motors or circuits. "The whole point of solar tracking is to increase the electricity output of the system," says Jiang who is now researching to refine the materials for use in driving larger solar panels, where net energy gain from his system will be maximised. University of Wisconsin-Madison
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