When it comes to squeezing maximum amounts of energy out of the daylight hours, plants have a head start thanks to evolution.
Now, engineers have designed solar panels that mimic the sunflower’s sun-chasing talent, through clever use of nanotechnology.
By moulding temperature-sensitive materials into thin, supportive structures, scientists have come up with tiny ‘stems’ that bend towards a bright light source, providing a moving platform that could dramatically improve the efficiency of a range of solar technologies.
Researchers from the University of California Los Angeles and Arizona State University refer to their system as a sunflower-like biomimetic omnidirectional tracker. Or ‘SunBOT’, if you like your acronyms.
In biological terms, any general movement in response to specific changes in the environment is described as a nastic behaviour. Flowers that open at dawn and close at dusk are a good example of this.
Chemists have had little trouble making synthetic nastic materials and structures that open and close, or bend and twist in response to changes in light intensity or fluctuating temperatures.
But nature has another, slightly more complicated behaviour that directs the movements of organisms towards good things and away from threats.
These tropic behaviours are what we see when sunflowers tilt their flowers to face the Sun, warming their reproductive bits in order to attract pollinators.
Sun-chasing actions, or heliotropism, would be mighty handy for things like photovoltaics, which are most efficient when bathed in a dense glow of radiation hitting their surface straight-on, rather than from a more shallow angle.
In practical terms, compared to rays from an overhead illumination source, light coming in at an angle of around 75 degrees carries as much as 75 percent less energy.
To solve this problem of oblique-incidence energy-density loss, the research team looked to gels and polymers that respond predictably to light or heat.
A handful of different materials were selected as candidates worth closer investigation, including a hydrogel containing gold nanoparticles, a tangle of light-sensitive polymers, and a type of liquid crystalline elastomer embedded with a light-absorbing dye.
Each arrangement was shaped into a millimetre-wide thread several centimetres in length. When targeted by a laser, the tiny artificial stalks responded rapidly to the light’s warmth, shrinking on one side and expanding on the other to cause the thread to kink and lean towards the laser.
To put their synthetic sunflowers to the test, the researchers assembled an array of SunBOTs and submerged them in water, letting them sit right at the water-air boundary.
To detect the harvesting capabilities of their invention, the team then determined how much light was converted to heat by measuring the water vapour their setup generated.
Changes in the amount of vapour indicated that the SunBOTs were up to four times better at harvesting energy at steep angles than a boring old flat, static surface.
By demonstrating that a variety of materials could serve as a synthetic tropic material, the researchers argue their devices could potentially be a solution for just about any system that experiences a loss of efficiency due to a moving energy source.
For example, lawns of these miniature sun-worshippers could theoretically be used to tilt just about any solar process towards the light, from itty-bitty solar cells to evaporation devices that can purify water.
According to the SunBOTs’ designers, the sky (if not beyond!) seems to be the limit for this kind of technology.
“This work may be useful for enhanced solar harvesters, adaptive signal receivers, smart windows, self-contained robotics, solar sails for spaceships, guided surgery, self-regulating optical devices, and intelligent energy generation (for example, solar cells and biofuels), as well as energetic emission detection and tracking with telescopes, radars and hydrophones,” they write in their report.
Even if just a handful of those predictions eventuates into real-world use, the future of synthetic tropic materials is certainly looking brighter.
This research was published in Nature Nanotechnology.