The scientists harvesting energy from humans to power our wearables

Inside a lab at the Massachusetts Institute of Technology (MIT), Sangtae Kim has been tinkering with a paper-thin device the size of a stamp. Kim is interested in harvesting energy from humans (though not the kind that turns people into batteries in the movie, The Matrix). He wants to harness motions, such as walking and running, to power sensors and wearable gadgets.

“It provides a new way of harvesting human energy,” Kim says of his prototype device, which he described recently in an article co-authored with his adviser, Prof Ju Li and other researchers.

“Any motion is possible to harvest, but you wouldn’t want clothes full of harvesters. I would target the soles of shoes – that’s where the most energy is located,” says Kim.

The idea of using movement to generate electricity isn’t new, though it’s far from commonplace. There are stationary exercise bikes with motors to turn sweaty workouts at the gym into energy. But portable energy harvesters that use human motion haven’t hit the market, partly because they have yet to generate enough energy, says Harry Zervos, an analyst with market research firm IDTechEx.

The human energy concept is promising. Using energy that would otherwise be wasted is appealing at a time when plans to fight climate change include using less energy or energy with lower carbon emissions.

Kim is targeting a growing consumer electronics market. Shipments of wearable electronics worldwide are estimated to increase from a predicted 111m devices in 2016 to 214.6m in 2019, according to IDC, a market research firm. IDTechEx expects the annual wearables sales to jump from $20bn in 2015 to nearly $70bn in 2025.

Quest for smaller more powerful batteries

Wearables collect and communicate data wirelessly, like mobile phones, and prolonging battery life is one of the big technical challenges for designers. And just like mobile phones, they are on their way to becoming thinner and more sophisticated. Designers of wearables – such as Apple Watch, Google Glass and fitness and health wristbands – are hunting for technology that could keep those gadgets running for longer in between charging. This means batteries that can pack more energy into smaller space or devices that could otherwise provide an energy boost without needing to plug into the wall.

Lithium-ion batteries, which also power the majority of consumer electronics such as laptops, are the go-to power source for wearables. But their performance declines when they have to shrink to fit tighter spaces, according to Christine Ho, CEO of Imprint Energy, a battery developer in California.

“It’s a conundrum for product designers, who are starting to realise they need to think more creatively,” Ho says. “New batteries have the opportunity to meet the demand.”

Gadget makers, such as Samsung, are experimenting with batteries that are thinner and flexible so that they can more easily fit into wearables that wrap around the finger, wrist and ankle. Imprint Energy is developing ultra-thin and flexible zinc batteries, which can store more energy in a given volume than lithium and are safe and non-toxic, says Ho.

But those emerging battery technologies tend to be expensive and hard to mass produce. Sales of ultra-thin and flexible batteries for wearable devices are expected to reach $300m by 2020, says Tony Sun, an analyst with market research firm Lux Research. The sales could jump to a whopping $4bn by then, he predicts, but only if those batteries can come down in price to the levels of lithium-ion batteries. This would require significant investments for technical breakthroughs, Sun adds.

Energy harvesting

The idea of harnessing human movements came to Kim one night in December 2013, when he got an email from Li, who had just attended a meeting of materials science researchers where talks about lithium ion batteries included a discussion of stress on the battery. Applying stress on a lithium battery alters the voltage and reduces the battery’s capacity. But what if you could turn this stress to an advantage?

Two metal electrodes make a sandwich around a layer of electrolyte. When it’s bent the lithium ions move across the electrolyte producing an electron current that can be harnessed. Photograph: MIT

“It was a two-sentence email that completely woke me up,” Kim recalls. “Then I started to design this device. It took me a year to build it and another year to fully understand what it was doing. We wanted to make sure it wasn’t a side effect.”

What Kim came up with is a device that has a similar structure as a battery: two conducting electrodes separated by a liquid electrolyte. Unlike a battery, the energy harvester uses the same compound, a mix of lithium and silicon, for both electrodes. This creates a volleying effect when physical stress is applied.

The pressure forces one electrode to spit out lithium ions and in the process upsets an equilibrium that causes the other electrode to open up and accept the rejected lithium. The electrolyte forces them to separate into lithium ions and electrons. The electrons travel through a circuit and get captured as electricity. The electrons then meet up with lithium ions on the other end and move into the electrode.

Unbending the device takes away the stress and causes the electrons and lithium ions to travel the other direction. That reversal creates another flow of electrical current before the two return home to the original electrode.

Kim’s prototype doesn’t yet generate enough electricity for wearables. He says he needs to boost its efficiency – the percentage of mechanical energy that gets converted into electricity – from 0.6% to 6%, to make it powerful enough for devices such as wristbands.

Increasing the energy output and lifespan of an energy harvester will be crucial to popularise its use, says Kevin Lloyd, co-founder and head of technology at California-based Whistle Labs, which has developed a GPS-based pet collar for tracking the location, health and other activities of your furry friend.

“When we look at building wearable devices, we are balancing science, battery life and cost,” Lloyd says. “You don’t want to have to take off your pet’s collar to charge every day. When you are on vacation for one or two weeks, you want it to last for that entire time.”

Another obstacle to make the energy harvester work: our body movements aren’t predicable or consistent, making it difficult to generate a reliable amount of energy at any given time, Ho says.

Tackling the challenge of designing his research into a wearable gadget will be important, Kim acknowledges, adding that he’s already heard from a few companies, including makers of smart watches and medical devices, that are interested in incorporating the energy harvesting process.

“But we need to make this technology more mature first, before thinking about commercialising it,” he says.

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