THE IDEA behind the Internet of Things (IoT) is that the world would be a better place if all sorts of objects that are not currently computerised were to become so. Microchipped bridges could report when they needed maintenance. Billions of tiny computers affixed to buildings could monitor air quality and traffic patterns. Scattered across fields, such computers could analyse nutrients and water in the soil. Arm, a Britain-based designer of the sort of low-power chips the IoT will need, reckons there could be a trillion connected gizmos in the world by 2035—more than 100 for every person on the planet.
That raises the question of how to power them. Regularly replacing a trillion batteries would be inconvenient, to say the least. Researchers have built chips that scavenge energy from light, heat or vibration. But such sources produce only a trickle of power, and are usually used to supplement a battery rather than to replace it.
In a paper published in Scientific Reports, Aline Eid and her colleagues at the Georgia Institute of Technology, in Atlanta, propose a better idea. Using a clever piece of kit called a Rotman lens, they have designed a small, flexible antenna intended to harvest electrical power from signals emitted by so-called 5th-generation (5G) mobile-phone masts. The hope is that mobile-phone networks could pull double duty as a ubiquitous wireless power grid.
Like the more-familiar glass lenses used in cameras and spectacles, Rotman lenses, which were invented in 1963, focus and redirect electromagnetic radiation. They are commonly used to produce “steerable” radar beams that can be sent off in different directions without requiring the emitter to be rotated physically. “Most people use Rotman lenses in transmission mode,” says Dr Eid. “We decided to flip that concept, and use them in receiving mode.”
Her lens allows the antenna to collect electromagnetic radiation from all directions, removing the need to align it with a distant transmitter and letting it work in cluttered urban environments in which waves scatter unpredictably. That improves the amount of harvestable power dramatically. And 5G also offers more power in the first place. The standard on which it is based covers communications on a wide range of frequencies. The range between around 24Ghz and 100Ghz, which is known as the millimetre-wave band, is sparsely used. With little risk of interference with other devices, many regulators are happy to allow masts designed to use those frequencies to transmit at higher power than older generations of kit.
The upshot, say the team, is that their antenna should be able to harvest useful quantities of power even at long range. Feeding their work into computer models used by the mobile-phone industry suggests it should be able to harvest around six microwatts of power at a distance of 180 metres from a mast. That is enough to let the sort of simple, low-power chips that will make up the IoT do useful work. Jimmy Hester, another of the paper’s authors, points out that 180 metres is roughly the size of a 5G-network cell, meaning that—in a city, at least—no gadget is ever likely to be much farther than that from a transmitter.
The fact that the antenna is flexible helps. The team’s laboratory prototype is about the size of a playing card. But if space is at a premium it can be folded in half, says Dr Eid, with little impact on performance. If more power is needed, a bigger antenna can be used. An ultra-frugal chip might be able to get away with a smaller one. The antennas are manufactured by printing the electronics directly onto a flexible substrate, meaning they are cheap and quick to produce.
If it can be made to work in the real world, such a system might offer more than mere convenience. Arm points out that fitting every one of the trillion devices it forecasts with a button battery (the sort found in watches) would require three times more lithium than the world currently produces in a year. Much better simply to pull the power from the air. ■