This post has been corrected.
A busy person’s iPhone can chew through plenty of energy in a day, but that doesn’t mean it needs to belly up to the wall trough every time its battery starts running low.*
Wireless power won’t be here tomorrow, but it’s coming. Energy is everywhere, in every action, most of it bleeding away to be recycled back into the endless machinations of the universe. Scientists and engineers are moving ever closer to figuring out how to harvest power from our environment, ourselves, and our devices themselves—from nanoscale pillars that could offset a device’s energy use by turning waste heat into electricity, to a spongey cell-phone case that works up a charge by sitting on a vibrating car dashboard.
This isn’t just wireless charging; this is harvesting energy from the world around us. Rather than use acres of solar panels or skyscraping wind turbines, energy harvesting engineers want to power your mobile devices from things like heat differentials, ambient vibrations, and your walk to work. That’s not just an engineering challenge, but also a design challenge. Tapping into the energy of everywhere shouldn’t add friction to the pace of modern life.
A few wireless power harvesters have already made their way to the shelves, but the biggest advances are still being worked out in the lab. Here is a review of some of the promising paths that engineers and designers are taking to power the mobile web.
Thermoelectricity
How it works: Electrons flee from hot to cold, creating a current. When this current crosses from one type of metal to another, it generates a charge. Tap that seam with a wire, and you’ve got electricity.
Where it’s working for mobile: Your body is a steady source of heat, so it’s an obvious source for electricity on-the-go. At 2010’s Glastonbury Festival, wireless carrier Orange introduced Power Wellies—mud boots with a thermoelectric panel in the sole for charging mobile phones. Vodafone upped the ante at 2013’s Isle of Wight festival with sleeping bags and Daisy Dukes with thermoelectric pockets. Orange says you’ll get an hour of battery life for every 12 hours of tromping through the mud, while Vodafone’s Power Pocket will give you 24 minutes of talk time for eight hours in the sack.
Last year a company called Epiphany debuted a coaster that harvests energy from your drink, hot or iced. The onE Puck charges your phone by using your beverage to heat or cool an inert gas inside, which expands or contracts to drive a piston and spin a generator. The principle isn’t the same as thermoelectricity, but both turn temperature change into power. With a piping hot mug of coffee, onE Puck puts out about as much wattage as a wall outlet—a big advantage over body-worn thermoelectrics.
Challenges: Harvesting thermoelectric heat for electricity is relatively easy, but currently it’s neither fast nor efficient. The best materials can harvest about 15 to 20% of the heat (no thermoelectric harvester can ever get more than 30%). But that’s not the biggest challenge. Thermoelectrics get more charge out of big temperature differences, so if the surrounding air’s temperature is too close to your body temperature, then your power stream will slow to a trickle. As for the coffee coaster, it may have good output, but needs to sit continuously under a hot mug for over two hours to offer a full charge—and topping off a hot coffee mug for that long is hardly the most convenient way to charge your phone.
Future potential: Because it has such a low ceiling for energy recovery, thermoelectricity isn’t the best source for power-hungry devices with high IQs. It could be perfect, however, for embedded medical monitors, and could keep fitness trackers ticking indefinitely. One promising thermoelectric material is Power Felt, a fabric being developed at Wake Forest University in North Carolina. Made from a tangle of carbon nanotubes and plastic fibers, it could be used to make full body heat-harvesting clothing lines. As long as the wires are unobtrusive, the large area of a heat-harvesting teeshirt could do a lot to make up for the general pokiness of thermoelectricity.
Piezoelectricity
How it works: Certain materials—quartz, most notably—generate an electric charge when they are compressed, shaken, or otherwise agitated in a way that makes them vibrate at a target frequency.
Where it’s working for mobile: Currently, the best uses are low-power-consuming, non-thinking instruments like this television remote from Philips and Arveni. There’s enormous optimism in the electrical engineering community about the potential of piezoelectric devices to dominate mobile energy, but current materials and devices still can’t gather enough energy for mobile computers. There’s been a patent on file for a piezoelectric keyboard since at least 1989, but until scientists figure out a way to amplify the amount of energy they can capture, we’re still years away from a laptop that charges itself by the keystroke.
Challenges: Unlike thermoelectricity, piezoelectricity doesn’t have a theoretical ceiling to the percent of available energy it can capture. The biggest challenge is how fickle the piezoelectric materials are about the frequencies to which they attune. Besides the problem of the “Goldilocks zone,” ambient vibrations from the surrounding environment can also disrupt a piezoelectric material’s efficiency. Some ceramic materials can harvest a wider bandwidth of vibrations, but they tend to break down and aren’t flexible enough for mobile devices.
Future potential: One promising solution to the problem of piezoelectric materials’ fickleness is to alter their structure, making them flexible. Dr. Xudong Wang of the University of Wisconsin led a team that created a sponge-like piezoelectric material, which he believes could easily scale up to power a phone. According to Wang, a phone wrapped in his piezoelectric material could be charged on a car dashboard. “The weight of the cell pushes the phone into the vibration,” he explains, “so we get away from the resonant frequency issue.”
Biomechanical energy
How it works: Biomechanical devices use our bodies’ motions to move tiny power generators. They often use degenerative force—like the breaking force your knee generates to straighten your leg just before each step.
How it works for mobile: No legit biomechanical devices have hit the shelves yet, but Bionic Power’s PowerWalk military-grade knee brace could soon be converted into a consumer model. According to the company’s website, an hour of walking with a brace on each knee could charge up to four cellphones. It doesn’t take too much imagination to see how popular these would be with outdoorsy types.
Bicycle hub dynamos aren’t technically biomechanics, but they’ve existed for years to power safety lights and touring odometers. Recently several companies have come out with USB adapters (here’s an exhaustive review of them). Add phone charging during your commute to the things a bicycle does more efficiently than a car.
Challenges: There are few ways to get around the fact that most biomechanical devices will be braces, which means that early adopters of this technology will look like geeks. Biomechanical harvesters can generate a ton of power, but they’re just clunky. And while knee braces could get smaller and smaller, they’re still one more thing to put on in the morning. Ideally, a personal energy-creating device wouldn’t require an accessory beyond what we’d normally wear. The inconvenience factor could limit biomechanical power adoption outside niche activities (hiking, biking, soldiering).
Future potential: One possible alternative could be biomechanical accessories, like this spring-loaded backpack. Slimmed down, biomechanical bags could become a staple for the commuters of the future.
Mobile solar
How it works: Solar power works because certain materials create an electrical current when they are exposed to sunlight.
How it works for mobile: At press time, there were at least a dozen different solar options for charging your phone. All of them take a long time. So charging your stuff with solar power is possible now, but it’s more of a hassle than finding a wall plug. The only halfway-serious option is the panel from Chargers, a German mom-and-pop startup. It’s pricier than the others ($190), but uses an external battery that stores enough solar energy to charge your phone twice. They’ve also designed their solar panel to attach directly to your backpack or purse.
Challenges: At the root of all solar energy challenges is something called the Shockley-Queisser limit: the physics rule that says cells can’t harvest more than 29% of the solar energy they intercept. On the mobile scale, the problems have more to do with time and space. Personal-sized panels are only big enough to harvest sun power at a trickle, even in the sunniest conditions (with the sun at a good angle in the sky, and your panel at a good angle relative to the sun). Mobile power is about being on the go, not babysitting your panel for ten hours while it charges your iPad.
Potentials: There have been the inevitable rumors that Apple is going to make the (still mythical) iWatch solar-powered, with Samsung to follow. This is Apple’s year to make something big happen, and it would make sense that the world’s most innovative company would be first to unlock mobile computing’s next, biggest prize.
*An earlier version of this post incorrectly stated that a busy person’s iPhone could us as much energy per year as a refrigerator. That claim was based on comparing 2G and 3G electricity usage with 4G data loads. 4G cellular is much more efficient at transferring data, and so actual average iPhone energy usage is about 11 percent (Table 1) that of a refrigerator.