Triac/Josh Sisk
It’s Thanksgiving weekend here in the United States, and if you’re an American, that probably means being somewhere really stupid for some period of time, like an airport or hometown bar or some hideous department store. If that’s the case, just try this for a minute: stop, close your eyes, and just listen. Even in the small town daylit bar I’m currently writing this in, that ambient sound is, in an objective sense, pretty loud: football on TV, the supernaturally loud voice of a bartender, some quieter conversations, and dirty glasses crashing under the bar. The jagged ambient sound is all just air particles moving around in the relative atmospheric viscosity of Earth’s more habitable elevations. Some of those assembled particles might deliver some meaningful noise or message to be decoded by your ears, but most of them just dissipate as wasted energy.
Maybe environmental noise doesn’t have to be totally wasted. Nanomaterials expert Steve Dunn and photochemist James Durrant, at London’s Queen Mary University and Imperial College respectively, are working on a method to harvest environmental noise for the cause of more efficient solar cells. Which is to say, they’re imagining using noise pollution to, ultimately, stem air pollution.
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We need more efficient cells because, however cleaner solar energy is over conventional energy sources like coal and oil, solar panel technology is still way less efficient than coal and oil. For a current state-of-the-consumer-art monocrystalline silicon panel, we’re only at about a best case of 20 percent efficiency. Granted the wasted part of the solar equation (heat, mostly) doesn’t involve irreplaceable materials and climate change doom, but as solar efficiency goes up, the more and more absurd burning old dinosaur fossils looks.
We should probably do a quick refresher on how solar cells actually work to start. You have a sheet of material and it’s loaded with electrons because, well, that’s just what matter is, protons and neutrons being orbited by a bunch of electrons that move around a lot and jump back and forth to other atoms. In some atoms, electrons are more into moving between atoms, and that’s when you have a conductor, a material more apt to pass electrons along it. The movement of electrons along a material is current, and using current you can make an industrialized society pretty easily.
Our local G-type main-sequence star can induce current through the photovoltaic effect. Photons racing at us from the sun crash into a material—perhaps silicon—and it’s possible for one of those photons to knock an electron out of position. This creates what’s known as an exciton, the suddenly at-large electron and the hole where it used to live. An exciton has new energy, captured from the incoming photon, and usually it’s just lost as heat. This loss and resulting heat is in a sense everything we hold dear. But it could also be electricity if, instead of that electron falling back into its hole, it’s captured in an electrical circuit. Do this a whole lot of times and that circuit is now carrying current. There you have it: solar power.
Tied into the making solar panels more efficient is the materials problem. Monocrystalline and polycrystalline silicon cells are expensive and fragile. Monocrystalline cells, at the higher end of the efficiency spectrum, are made of silicon crystals that have to be grown and then cut into thin wafers. That’s pretty cool, but there are other materials out there.
The compound zinc oxide (ZnO)—see also: calamine lotion, cigarette filters, food additives, and rust-proofing—is one such material and it’s very plentiful. ZnO is used as a coating or additive to flexible, transparent polymer sheets as a way to capture the photovoltaic effect in materials that may not be otherwise conduct electricity very well. As a solar power harvesting technique, it’s still only delivering about 1 percent efficiency.
Turns out that process has a bit to do with all of the insane noise that constantly surrounds us. Sound, it seems, can “loosen” electrons, boosting efficiency by up to 50 percent—not huge, but a crucial step.
It seems reasonable that sound and efficiency be connected. Dunn and Durrant’s scheme goes like this: in addition to being alright at conducting electrical current in solar power schemes, zinc oxide is useful for its piezoelectric properties.
A Contact Mic for Solar Panels
If the reader’s ever made sounds louder via electricity, they may already understand a bit about piezoelectricity (or the device just known as a “piezo,” or contact mic). Put simply, if you apply a strain to certain types of materials, they develop a polarization charge. So, for example, if you place some certain types of material against a moving source, it’s possible to convert that meeting into electricity. I could, for example, slap a very cheap device against a railroad track or the bridge of a musical instrument and amplify it.
A cross-section of the zinc-oxide/polymer (P3HT) material/Advanced Materials
The researchers’ discovery, published in the journal Advanced Materials, is that it’s possible to blast sound at solar cells embedded with tiny zinc-oxide nanorods and as a result generate tiny piezoelectric currents, which boosts the overall photovoltaic effect of the cell, depending a bit on what variety of sound is used.
“In our solar cells, we think the vibrations from the sound waves cause the zinc-oxide rods to bend, which generates small voltages across each rod,” Durrant said in an interview. “[This] in turn make the solar cell work more efficiently. They help separate the charges in the solar cells, the electrons and holes.”
Acoustic vibrations are shown to enhance the photovoltaic efficiency of a P3HT/ZnO nanorod solar cell by up to 45%
In addition to more scientific sound sources like simple frequencies and ambient sounds, the pair tested out some different varieties of pop music on the cells: “Adele, Beethoven, and Persian music. The cell efficiency increased most with Adele—but this was definitely not [within] a properly controlled scientific experiment,” Durrant noted.
Consider that more of a bonus to the duo’s larger work on marrying piezoelectrics and photovoltaics. The largest frequency range delivered in the most brutal way possible would have the most actual effect on efficiency, so one might assume an ear-bleeding dose of Merzbow would have the most impact.
Of course, blasting anything sonic at solar cells in the interests of squeezing more power from them is pretty much the opposite of efficiency. Powering the speakers takes more electricity than you’re getting in return. But the overall idea is that such a system could capture the sort of noise we were talking about earlier: the preexisting sounds of reality. “The effect would most likely be useful in consistently noisy environments, such as on a machine or road side,” Durrant said. Also: the tops of buses, around heavy machinery, at at Skrillex concerts—anywhere that loud and preferably electrically-generated sounds can be found on a regular basis. In other words, in an industrialized society.
