July 07, 2019 by Emily Gray-Fow
Rectifying antennas (‘rectennas’) aren’t exactly a new technology.
Based on concepts originally developed by Nikolai Tesla and further developed by Raytheon's William H. Brown in the 1960s, rectennas enable the harvesting of wireless electrical energy from ambient AC electromagnetic waves by using variations on the theme of a dipole antenna with a radiofrequency (RF) diode connected across the dipole elements.
While back in the 1960s there was not much in the way of reliable ambient wireless signals, we are now living in an era where Wi-Fi and other RF signals are commonplace both indoors and out. Naturally, this has led researchers to look more deeply into how to create the most practical rectenna-based power supply that can make use of these signals.
One issue that slowed the development of rectennas as a practical power source was the need for technology, in general, to catch up with what rectennas can offer us. In the 1960s and '70s, and even into the '80s, there was little in the way of ubiquitous low-power gadgetry that was part and parcel of daily life.
Once we reached the 21st century, we started to see the onset of relatively low-power gadgets like smartphones and low-power LED screens, plus a rapid increase in the level of ambient electromagnetic (EM) waves thanks to Wi-Fi and other data signals.
Until recently, solutions were still rigid, making it harder to fit rectennas into all of the places where a battery-free power source would be useful. Outdoors, EM signals are still too unreliable to be useful as a sole power source without battery back-up, but in many indoor environments, Wi-Fi is a given. This makes it an attractive focus for rectenna design.
Until recently, there has been some difficulty in designing a rectenna solution that can fit almost anywhere, especially in this age of flexible, foldable electronics. Ultimately, this has led to the development of the first flexible battery-free rectenna that can harvest power from ambient Wi-Fi signals.
There has also been plenty research into rectennas that can harvest power from other parts of the EM spectrum, most notably in the development of optical rectennas that harvest energy from solar radiation, microwaves, and even thermal radiation.
First optical rectenna capable of converting light to DC current, developed by Georgia Institute of Technology researchers. Image courtesy of Georgia Tech.
All rectennas share a few common features: they harvest energy from the EM spectrum in the form of waves and produce a DC current. Beyond that, they can vary quite a lot in size, with some thermal rectenna designs being 200 µm or less. Larger scale rectennas and arrays of rectennas offer the chance of transmitting relatively large amounts of power, where smaller iterations could offer battery-free power to numerous portable and mobile devices.
Like other antennas aimed at the portable/mobile electronics market, many of today’s rectenna designs are flat; the move towards transparent and flexible electronics hasn’t passed rectennas by.
In 2014, researchers from Université Paris-Est demonstrated a transparent array that output DC voltages of 70 and 190 mV at 1 and 5 μW/cm2 respectively. The rectenna array showed experimental voltages of 0.55 V and 1.4 V in the previous conditions of RF illumination.
Research into inkjet-printed circuits has included work on manufacturing rectenna arrays.
Recent work with molybdenum disulfide (MoS2), a 2D semiconductor that is just 3 atoms thick, has enabled the creation of flexible rectennas that offer around 40 microwatts of power when exposed to 150 microwatts of Wi-Fi signal.
EECS postdoctoral researcher Xu Zhang says: “Such a design has allowed a fully flexible device that is fast enough to cover most of the radiofrequency bands used by our daily electronics, including Wi-Fi, Bluetooth, cellular LTE, and many others”—all whilst minimising parasitic capacitance and series resistance.
Rectennas are being considered as a supplementary power source for smartphones and other electrical gadgets. Image courtesy of Pixabay.
The possibility of battery-free, wire-free power transmission opens up a number of avenues for the development of different types of devices. Improvements in thickness, flexibility, and improved manufacturing methods mean possible uses include:
Flexible rectennas as a power source for ingestible sensors and other medical devices where battery leakage could potentially cause harm to the patient.
Transparent, flexible rectennas could be used to coat windows, walls, and other surfaces to power IoT devices and sensors in the home and workplace.
Rectennas as a supplementary power source for smartphones, IoT devices, and other gadgets.
Optical rectennas could replace PV solar cells, offering as much as 90 per cent efficiency, instead of the current levels which remain below 30 per cent.
As part of a space-based solar power (SBSP) system, rectennas could be used to convert energy beamed from satellite solar power collectors, as well as potentially being used to collect the power from the sun in the first place.
Power for low-energy M2M (machine to machine) communications in an industrial setting, at home, or in the office.
Power for satellites and space-based habitats, whether beamed from earth or harvested directly from sunlight or other satellites (with satellite to satellite power transmission a real possibility).
Overall, the potential for harvesting energy that currently goes to waste on a massive scale using low-cost inkjet-printed rectennas could go a long way towards offsetting the additional power drain the IoT will place on the world as technology advances. We are also always happy to discover ways to extend battery life, given the rate of development in current smart devices.
Some of the more spectacular applications for rectennas like space-based solar power are likely to see use in the next 20-or-so years as we also see increased investment in Mars exploration. We will have to wait and see whether the end-users will be robots or people, though!
As engineers, rectennas are certainly something we need to keep an eye on. There are many use cases that will make them increasingly attractive over the next few years, both on land and in orbit.
