How a lesson learned from lotus flowers could give us self-cleaning solar panels

Whatever engineering problem you’re facing, no matter how high-tech, it’s quite possible that evolution solved it millions of years ago. That’s the foundational idea of biomimicry, the design philosophy of solving human challenges by emulating the designs and concepts already found in nature. Biomimicry has previously given us everything from wind turbines modeled after humpback whales to gecko-inspired robots for cleaning up space junk.

Now an international team of researchers thinks they’ve found another problem that nature’s trial-and-error can be used to help solve: The tricky issue of how best to remove dust from solar panels in order to maintain their efficiency and light-absorbing abilities.

“Keeping windows, cars, and other surfaces clean is a very challenging in dusty environments, such as the desert,” Yair Kaufman, a project scientist at the University of California Santa Barbara, whose group studies the molecular interactions between interfaces, told Digital Trends. “However, there are plants and insects that do it effortlessly. [In our latest piece of work], we explain the so-called ‘self-cleaning’ mechanism which is employed by these plants and insects to remain dust-free.”

The power of self-cleaning

One of the examples of a self-cleaning plant the team looked at was the lotus flower. The lotus flower is perhaps nature’s best known self-cleaning plant, with a “lotus effect” that is the result of the ultrahydrophobicity of its leaves. The lotus flower has a thin waxy, hydrophobic coating that repels water, along with dust and pathogens.

“During the self-cleaning mechanism, a water droplet slides over the surface, and as it slides, the droplet detaches [and cleans] particles from the surface,” Kaufman explained. “Nevertheless, as we all know, when our windows are dirty, it is often not enough to wash them with water. One often needs to use soap and scratch the windows as well. [As we found], the nanotexture that covers these self-cleaning plants… reduces the forces that attach dust particles to the surface, which makes it easier to for droplets to remove dust from the surface.”

The same effect is found on certain insects that are not able to thoroughly clean all of their body parts, such as butterflies and dragonflies.

Self-cleaning technologies have been explored previously. However, most of the time these studied focused on the ability to expel water from surfaces. In parts of the world which use solar panels, but experience minimal rainfall, that is a far less significant problem than accumulated dust. This work (which was published in the American Chemical Society journal Langmuir) therefore focused more on the ability of these self-cleaning surfaces to remove dust particles. The forces that attach and detach particles from surfaces during self-cleaning, and the effect of nanotextures on these forces, are far less fully understood.

In an attempt to come up with an answer, the researchers prepared four silicon-based materials that could be used as a coating for solar panels. These included a smooth hydrophilic surface, a nanotextured hydrophilic surface, smooth hydrophobic, and nanotextured hydrophobic surfaces. This was carried out by wet-chemically etching the surface so as to create nanowires on the surface. The researchers then additionally applied a hydrophobic coating.

“The most important challenge was to determine the origin of the force that detaches particles from the surface,” Kaufman said. “Even more perplexing was the fact that, when we carefully looked at how water drops clean the surfaces, we noticed that the dust particles are actually removed by the back of the drop; that is when the water drops leave the surface, and not by the front of the drop. To address these challenges, we used an atomic force microscope to measure the molecular forces between dust particle and solar panels. Then we theoretically analyzed these forces and derived relatively simple equations that allow for designing self-cleaning solar panels in the future.”

Removing 98% of dust particles

The most efficient of the surfaces tested by the team — the superhydrophobic Si-based nanotextured surface — was able to remove a massive 98% of dust particles. As a result, the engineers involved in the project believe that modifying the surface of solar panels in this way could be used to significantly increase the potential of solar energy harvesting applications by removing dust.

“Dust is a big issue for solar panels,” Kaufman said. “For instance, look at what happened to the Mars rover Opportunity when it was exposed to dust on mars. The dust [on] Mars slowly covered its solar panel, which simply ‘killed’ the rover. Unfortunately, this example shows that dust can kill a project that costs billions of dollars. Here on Earth, dust in the desert blocks solar panel within hours to [a] few days. This dust often reduces the power that can be generated by solar panels by 30% to 80%. Therefore, to keep the efficiency of these solar panels, one needs to frequently clean them.”

While more work still needs to be done before possible commercialization can occur, this is nonetheless extremely promising work. And — alongside researchers Tabea Heckenthaler, Sumesh Sadhujan, Yakov Morgenstern, Prakash Natarajan, Muhammad Bashouti, and Yair Kaufman — we’ve got Mother Nature to thank.

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