Power from thin air
iPhones, iPads, and now Google Glass. Each of these gadgets brings a revolution in how we receive and send information. But they have another thing in common: they gobble energy. The first users of Google Glass – those spectacles with a camera and tiny screen – complain that the headset stops working after about four hours, which is hardly impressive for a device billed as the ultimate in being wired 24/7. The same is true for smart watches and other “wearable computing” devices that monitor heart rate and keep track of position via GPS: you really don’t want to recharge them every day (or even every 6 hours for the Gear, Samsung’s smart watch). “Energy is the main constraint for portability,” says EPFL professor Adrian Ionescu. “You need to be able to consume less battery power while operating over longer timescales.”
Aware of the problem, manufacturers have been looking for computer chips that consume less power, while also trying to develop more efficient batteries. Yet so far, the improvements in battery performance have been incremental. The alternative might be to do away with onboard power altogether and instead extract ambient energy that is otherwise going to waste. “Energy harvesting” is not in itself new – automatic watches have been powered by arm movements for decades, and many calculators are solar-powered. What remains to be developed is technology that can run today’s more energy-hungry portable devices, as well as autonomous sensors to monitor everything from fridges to bridges in the so-called Internet of Things. Here are four of the more promising ideas.
The portable generator
One relatively early solution was a backpack that converts some of the kinetic energy produced by walking into electricity. Developed by Larry Rome and his colleagues at the University of Pennsylvania in 2005, the device was designed mainly for soldiers, scientists, explorers and others who need autonomous power in the field. The pack is attached to springs and slides up and down on the frame as the person walks; the pack’s motion then turns a small gear that drives a generator and produces electricity. The backpack can produce up to about 30 watts, enough to power the most energy-demanding smart phones, but there is a significant drag: the device adds 2-3 kg to the pack’s weight.
A lighter but less powerful alternative, developed by scientists at the Georgia Institute of Technology, exploits what is known as the triboelectric effect: the separation of electric charge caused by friction between two different surfaces (seen, for example, when balloons stick to walls after being rubbed on a person’s hair). A backpack contains a compressible diamond-shaped plastic frame with nanoscale aluminium holes and copper teeth on opposite faces that generate surface charge when they mesh. The pack’s up-and-down movement opens and closes the diamond, creating up to 1.25 watts of electricity that can power devices directly or charge a battery. Lead researcher Zhong Lin Wang has set up a company to commercialise the device, but he admits there are still technological hurdles to overcome, particularly regarding the durability of the plastic-based product. “We’ve done tests in our lab and after 10 million compressions the device still works,” he says. “We need to extend that to billions.”
Another solution is smart clothes that generate energy through bodily movement. Liwei Lin’s group at the University of California at Berkeley has produced tiny fibres from a polymer material known as PVDF that accumulates charge when it is mechanically deformed – a phenomenon known as the piezoelectric effect. The fibres, only 500 nanometres in diameter, can convert as much as 20% of applied kinetic energy into electricity, which means, says Lin, that 1 million of them packed into just 1mm2 of fabric covering a moving part of the body such as the elbow “would be able to power an iPod”. But producing fibres in large quantities with a consistent performance remains a technological challenge.
Steve Beeby of the University of Southampton is leading a team that aims by 2015 to make films of piezoelectric and thermoelectric materials for printing onto fabrics. These films would provide electricity for portable devices through people’s movement as well as the thermal gradients between them and their surroundings.
Energy from the air
At the University of Washington in Seattle, a team is working to pluck energy from thin air. Shyam Gollakota and colleagues have built credit card-sized devices that use wireless signals from television and mobile-phone transmissions to communicate among themselves and to generate power. They say that their devices could be used inside autonomous smart sensors and potentially incorporated into battery-using devices such as smart phones, providing enough power after the battery has fully discharged to allow the user to send text messages.
EPFL’s Ionescu predicts that energy harvesting will appear on the market gradually, supplementing rather than replacing rechargeable batteries over the next five to 10 years; better energy-management software will also be needed to prioritise the limited energy available to devices. Ionescu says that harvesting is limited because it is “opportunistic”, scavenging whatever energy happens to be available locally. He notes that solar power is less efficient indoors and that technologies exploiting thermal gradients work better during the winter, when there is a higher temperature difference between people and their surroundings. Electromagnetic scavenging would work only when antennas are relatively close by and not at all in remote areas such as mountains. The technique might also potentially disrupt mobile communications, since it would consume some of the finite energy beamed by antennas.
Even so, Ionescu is convinced that such technology will eventually become commonplace. Until then, maybe, your smart glasses might just be the most expensive pair of eyewear you’ve kept in a drawer because recharging it proved too much
of a hassle.