We have all been there. In a rush to leave the house we grab our phones and head out the door, realising all too late that the battery is dead because we forgot to plug it into the tablecloth.
Or perhaps we have not. But this could be the future that scientists hope to usher in with electronic sheets that charge our mobile phones, laptops and other gadgets by harvesting energy from the world around us.
In a step in that direction, scientists at Massachusetts Institute of Technology (MIT) have created super-thin, bendy materials that absorb wireless internet and other electromagnetic waves in the air and turn them into electricity.
The lead researcher, Tomás Palacios, said the breakthrough paved the way for energy-harvesting covers ranging from tablecloths to giant wrappers for buildings that extract energy from the environment to power sensors and other electronics. Details have been published in the journal Nature.
“When you have one of these energy-harvesting devices you are collecting energy 24/7 and you could be storing that in a battery to use later,” Palacios said. “You could cover your desk with an electronic tablecloth and even though you’re only at the desk for so many hours a day, it would be harvesting energy the whole time.”
Palacios and his colleagues connected a bendy antenna to a flexible semiconductor layer only three atoms thick. The antenna picks up wifi and other radio-frequency signals and turns them into an alternating current. This flows into the molybdenum disulphide semiconductor, where it is converted into a direct electrical current.
Ambient wifi signals can fill an office with more than 100 microwatts of power that is ripe to be scavenged by energy-harvesting devices. The MIT system has an efficiency of between 30% and 40%, producing about 40 microwatts when exposed to signals bearing 150 microwatts of power in laboratory tests.
“It doesn’t sound like much compared with the 60 watts that a computer needs, but you can still do a lot with it,” Palacios said. “You can design a wide range of sensors, for environmental monitoring or chemical and biological sensing, which operate at the single microwatt level. Or you could store the electricity in a battery to use later.”
Medical devices are another potential application. Because wifi and similar radio-frequency signals pass through people, energy-harvesting covers could be applied to implants to provide them with enough power to beam health data to an outside receiver.
Researchers have made energy-harvesting “rectennas” before, but existing devices are made from conventional semiconductors which are rigid, fragile and practically impossible to make in large sheets. By contrast, molybdenum disulphide film can be produced in sheets on industrial roll-to-roll machines, meaning they can be made large enough to capture useful amounts of energy.
“In the future, everything is going to be covered with electronic systems and sensors. The question is going to be how do we power them?” said Palacios. “This is the missing building block that we need.”
