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 (not the kind that turns people into batteries, as in ‘The Matrix’ movie). He wants to harness energy from body 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’s stationary exercise bikes with motors which turn sweaty workouts at the gym into energy. But portable energy harvesters that use human notion haven’t hit the market yet, partly because they have yet to generate enough energy, says Harry Zervos, an analyst with market research firm IDTechEx.
Kim is targeting a growing consumer electronics market. Shipments of wearable electronics worldwide fare estimated to increase from a predicted 111 million devices in 2016 to 214.6 million in 2016, according to IDC, a market research firm. IDTechEx expects the annual wearable sales to jump from $20 billion in 2015 to nearly $70 billion 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, fitness and health wristbands – are hunting for technology that could keep those gadgets running longer between charges. They need batteries that can pack more energy into a smaller space or devices that can otherwise provide an energy boost without having to plugin & re-charge.
Lithium-ion batteries are the standard for most electronics like laptops, and are the go-to power source for wearables. But their performance declines when shrunk to fit into 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 realize they need to think more creatively,” said Ho. “New batteries have the opportunity to meet that demand.” And other emerging battery technologies tend to be expensive and hard to mass produce.
Energy harvesting – The idea of harvesting energy from human movement 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 about stress on the battery. The application of stress on a lithium battery alters the voltage and reduces the battery’s capacity. But what if you could turn this stress into an advantage?
“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.”
The design has a similar battery structure: two conducting electrodes separated by a liquid electrolyte. But unlike a battery, the energy harvester uses the same compound, a mixture of lithium and silicon, for both electrodes. This creates a volleying effect when physical stress is applied.
Stress-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 (stress relief) 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 generate enough electricity for wearable, yet. He needs to boost the percentage of mechanical energy conversion from 0.6% to 6%, to power devices such as wristbands.
Read Article (Ucilia Wang | theguardian.com | 02/04/2016)
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