Engineers from the University of Arkansas and the University of Michigan report creating a device that harvests energy from ambient electromagnetic radiation and powers an ultra-low-power temperature sensor. The work was published in the Journal of Vacuum Science & Technology B. This is already the second sensor developed by the team: it previously presented graphene solar cells for the same purpose.
The key element of the new device is a graphene inductor. An inductor is a coil that captures a magnetic field and converts it into electric current. In this case it harvests energy from magnetic fields produced by standard 60-hertz mains power (in the US). The signal then goes to a special circuit that converts alternating current into direct current and charges the sensor's capacitors (charge storage devices).
"The inductor adds charge to the capacitors slowly," explained the head of the work, physics professor Paul Thibado, "but faster than the sensor uses up that charge. So even when the electricity is cut off, the device can draw charge from the capacitors. In our configuration it will easily last 24 hours, and probably a week."
Thibado began studying graphene more than ten years ago. The first author of the paper was MD Rezaul Kabir, a graduate student in the materials science and engineering program. According to the researchers, they place three requirements on their sensors: power from locally harvested energy, a long service life without maintenance (potentially decades) and operation at nanowatt power (a nanowatt is a billionth of a watt). All three conditions are said to be met in the new sensor powered by ambient radiation.
So far the team has tested the concept on temperature sensors. In the future Thibado sees applications for long-lived low-power sensors in goods tracking, atmospheric monitoring, logistics fleet management, soil monitoring, wearable fitness electronics and medical monitoring. Because no batteries are needed, the device can be "set and forget," which reduces costs, labor and waste from replacing old power cells.
The ultimate goal is multimodal sensors that harvest energy from several sources at once (sunlight, heat, sound, motion, nonlinear processes and ambient radiation). For example, a sensor could draw energy from radio waves and from motion to have a backup when one source is cut off. This should help the development of the internet of things, where smart technologies are woven into everyday life. No timeline for commercial products was reported.