If you’ve ever taken a camera snorkeling, you’ve probably noticed that everything a few feet down comes out looking blue. Your camera’s fine. Water absorbs red light fastest, orange isn’t far behind, and by 33 feet or so you’re mostly working with blues and greens.<\/p>\n
That’s great for moody vacation photos and not so great if you want to run a solar panel down there. The panels on your roof are built to use a wide slice of the spectrum, reaching well into red and infrared light, and that’s the light the water strips out before it gets 33 feet down.<\/p>\n
So a team led by Wen-Hua Zhang at Yunnan University in Kunming, China, built a solar cell that pretty much ignores the light that doesn’t make it that deep. Then the researchers put panels made the same way on a small underwater drone, sent it down near Weizhou Island, in the South China Sea, and charged lithium-ion batteries at 33 feet. The study came out in the journal Joule<\/a> on September 11.<\/p>\n Every solar cell material has what’s called a bandgap. It’s basically the minimum amount of energy a bit of light has to carry before the cell can turn it into electricity. Light with less energy than that doesn’t get used at all, and light with a lot more wastes the extra as heat.<\/p>\n Zhang’s team went with a perovskite, a type of crystal you can print from a liquid ink, and set its bandgap at about 1.96 electron volts. Which means it effectively can’t see anything redder than orange. At 33 feet, that’s no loss, since those colors are already gone.<\/p>\n Perovskites are a pretty big deal in solar research because you can tune which colors they soak up. The catch is that they tend to break down when moisture gets to them. That makes them a slightly odd pick for something you plan to drop in the ocean.<\/p>\n The team’s fix was a polymer called PHMG (polyhexamethylene guanidine hydrochloride, if you’re keeping score at home), which it mixed into the ink ahead of drying. The researchers say it cuts down on defects in the crystal, keeps its ions from wandering around, and helps the finished film shed water. For the trip into the sea, the modules also got wrapped in several protective layers, including an aluminum oxide coating and cover glass, and potted in epoxy.<\/p>\n Here’s where it gets a little weird. In the lab, the team passed light through filters cut to match what’s left at 33 feet, and a small cell turned 34.71% of that light into power. Put the same cell through a standard full-sun test and it drops to 17.08% (an outside lab certified it at 16.79%).<\/p>\n So does that mean it’s making twice as much power underwater?<\/p>\n Nope. There’s far less light 33 feet down to begin with, so the cell’s total output is still a lot lower than it’d be on a roof. It just wastes less of the little light it gets, which is what Zhang’s team tuned it to do.<\/p>\n The drone took the modules down in two groups of four, stopping at three different depths for two hours apiece. Each group added up to roughly 18 square inches of active area, a little smaller than a 4-by-6 photo print.<\/p>\n If milliwatt-hours don’t mean much to you, that’s fair. A milliwatt-hour is a thousandth of a watt-hour, so 324 of them would run a small 5-watt LED bulb for about four minutes. No one’s running a fridge off this.<\/p>\n The team did use the charged batteries to light an LED sign with the university’s name on it, though, which I’ll admit is a nice touch.<\/p>\n And the result beat what the researchers had in mind. Zhang told Earth.com his team expected the cells to stop charging somewhere around 16 to 20 feet down, and figured on something like 50 to 100 milliwatt-hours at 33 feet. Getting more than 300 did “really surprise us,” he said.<\/p>\n Because just about everything we leave in the ocean runs on a battery or a cable. Seabed sensors, underwater cameras and the gear that relays their data all need power, and that usually means someone’s sailing out to swap batteries or running a line down from shore or from a ship. The Irish underwater drone we covered this week<\/a> spent 18 days hunting vents in the Greenland Sea on a 3,000-volt cable from a research ship, for example.<\/p>\n Zhang told the same site the cells could work at coral reefs and fish farms, or on underwater vehicles and lights. A patch of panels this size probably isn’t going to keep a big drone moving. For a sensor that mostly sits still and sends the occasional bit of data, though, topping up its battery from sunlight sounds like a pretty good deal to me.<\/p>\n Zhang calls the work “the first functional validation of submerged solar cells”<\/a> at around 33 feet, and he says earlier underwater solar studies all stayed at about 6.5 feet or less. I’d put a small asterisk on that.<\/p>\n More than a decade ago, the US Naval Research Laboratory<\/a> put gallium indium phosphide cells, a wide-bandgap type more often used in space, in a glass sphere and lowered them about 30 feet into the water. The Navy lab reported roughly 0.65 watts per square foot (7 watts per square meter) at that depth, and it called those results preliminary.<\/p>\n I can’t line those figures up cleanly against the Chinese ones. The Navy reported power per unit of area, while Zhang’s team reported how much energy went into batteries over two hours. What the Yunnan group does have is an open-sea trial that actually charged batteries, plus a pile of durability testing, and you’d want that durability work before anyone leaves one of these down there for months.<\/p>\n In the lab, cells ran for 1,160 hours (about 48 days) under simulated 33-foot light and barely lost anything, according to the researchers. Cells kept in a nitrogen-filled glovebox for 300 days still had about 96% of the efficiency they started with.<\/p>\n Zhang’s team also put a lifetime on them: about 5.5 years. That’s how long the researchers estimate a cell would run at 77 degrees Fahrenheit and 33 feet before it drops to 80% of its original output. The estimate comes from cooking cells at higher temperatures to wear them out faster, then extrapolating down to normal water temperature. It’s an educated guess, and I’d treat it as one rather than a warranty.<\/p>\n Then there’s the ocean itself. Zhang told ZME Science that stuff growing on the cells, junk floating in the water and salt eating at the protective layers all limit how well underwater solar can work. Anyone who’s pulled a boat out after a season knows things grow on everything down there. (Barnacles are enough of a pain that a solar raft off Maine<\/a> hangs its oyster baskets in the sun just to kill them.) Zhang’s group says it wants to keep the modules in real water for longer stretches, which should answer some of that.<\/p>\n And yes, there’s lead in these. They’re lead halide perovskites, so lead is part of the crystal itself. Earth.com reports that a 200-day tank test kept lead levels in the water around the cells below 1.15 parts per billion. But that’s a tank result, and the cells’ actual time in the ocean so far comes down to those two-hour dips.<\/p>\n We probably won’t have to wait long to find out how deep this can go, though. In an interview Earth.com published on September 14, Zhang said his group is already running experiments at 66 to 98 feet, and he thinks the cells can work well at those depths.<\/p>\n Image credit: Joule<\/strong><\/em><\/p>\n","protected":false},"excerpt":{"rendered":" If you’ve ever taken a camera snorkeling, you’ve probably noticed that everything a few feet down comes out looking blue. … <\/p>\nSo how do you build a solar cell for the ocean?<\/h2>\n
The batteries charged at every depth<\/h2>\n
Why would anyone want solar power 33 feet underwater?<\/h2>\n
Is this really the first time anyone’s done it?<\/h2>\n
How long would one last down there?<\/h2>\n