Researchers Create “Perfect EM Absorption” Rectenna for RF Energy Harvesting
New research out of the University of South Florida takes a new approach to radio frequency (RF) energy harvesting using metamaterials to create a new type of "rectenna."
With the proliferation of the Internet of Things (IoT) and other low-power, battery-based electronics, energy harvesting has received heavy investment.
Of all the forms of energy-harvesting being pursued, one that holds a significant amount of potential is RF harvesting.
An example overview of an energy harvesting system. Screenshot used courtesy of Texas Instruments and Frantz et al
Despite the investment and interest in this form of energy harvesting, there are a significant number of challenges facing preventing it from being the impactful technology it could possibly become.
Aiming to tackle some of these challenges, researchers from the University of South Florida have outlined a new technique in RF energy harvesting, one which they claim improves the technology significantly.
This article will explore the challenges facing RF harvesting and see how the new research addressed these issues.
Challenges with RF Harvesting
RF harvesting is the pursuit to capture stray electromagnetic (EM) waves from sources such as radio and cell phones and convert that energy into usable power for electronic devices. By nature, as is with a lot of RF technology, this can come with a lot of challenges.
An example RF harvesting node block diagram. Screenshot used courtesy of Mishra et al
The source of many of the significant challenges facing RF harvesting is the meager amount of power that the technology must harness.
In general, RF communications are plagues with various non-negligible forms of loss, which include path loss, energy dissipation, shadowing, and fading. This issue is compounded by the fact that RF emissions need to be kept underneath a certain energy threshold to prevent potential human health hazards.
Altogether, the reception sensitivity of RF energy harvesting technology is generally on the order of -10 dBm as compared to -60 dBms for RF communication.
This sensitivity can then pose several other challenges.
First, it becomes increasingly difficult to create antennas that can capture energy with high efficiency at extremely low power.
Further, the efficiency of the RF-DC conversion necessary to charge battery-powered devices has historically been extremely low at these low voltages. This challenge is primarily due to non-linear characteristics of rectifying diodes resulting in extremely low conversion efficiencies at low input RF power.
Though these might not be all of the challenges facing RF energy harvesting, together, these issues result in RF power-producing very limited energy, rendering the promising technology infeasible in many applications.
New Research and MPAs
Hoping to take a swing at RF energy harvesting challenges, in their recently published paper, researchers from the University of South Florida sought to improve the sources of inefficiency for RF energy harvesting.
To do this, the researchers developed a new high-efficiency RF harvesting device based on a metamaterial perfect absorber (MPA) antenna.
Electromagnetic metamaterials are described as “man-made materials consisting of metallic resonant structures that behave as homogeneous media.” The real value of metamaterials is that engineers can selectively choose the material’s effective electric permittivity (ϵ) and effective magnetic permeability (µ).
With that in mind, through carefully designed metamaterials, engineers could realize unprecedented EM properties that are not feasible in nature, one of which is "perfect" absorption.
Sample rectenna design. Image used courtesy of Fowler et al
By leveraging these metamaterials, the researchers created a metamaterial-based antenna that claims to achieve perfect EM absorption.
The device, dubbed a "rectenna," is stated to be tuned for perfect absorption at 0.9 GHz. It also contained embedded Schottky diodes to convert the captured RF waves to DC power.
All in all, this technique worked twofold: not only did it increase the energy absorbed by the antenna, but that increase in energy also led to more efficient RF-DC rectification.
Research Results and Future Work
With their new rectenna, the researchers claimed the ability to obtain 100 µW power for an incident intensity of 0.4 µW/cm2, a number large enough to power simple electronic devices. The rectenna was said to achieve a 16x improvement of RF-DC conversion efficiency at ambient intensity level vs. conventional technology.
Currently, the researchers state that their goals for the future are to take the device, which measures 16 cm x 16 cm and miniaturize it. Further, they have stated interest in pursuing an updated rectenna that can collect energy from multiple types of radio waves simultaneously.
Overall, this type of research could help be the step needed to push RF energy harvesting as an easier technology to incorporate into future devices.