What you’ll learn:
- What is a metasurface and how can it be used to capture ambient RF energy?
- How an energy-harvesting metamaterial was constructed using this technique.
- The test setup used and results of the RF-harvesting scheme across various parameters.
Energy harvesting, with its many manifestations, is often an attractive and viable solution to providing long-term low levels of operating dc power. RF-energy harvesting is especially attractive since that energy is pervasive, while capturing it incurs no discernable negative impact on intended users—it’s truly “going to waste.” Further, unlike many (but not all) fixed-in-place solar, vibration, or thermal-harvesting installations, most RF-harvesting arrangements can be mobile to go with system they’re powering.
Key to use of ambient RF energy as a harvestable source is the energy collector. It acts as a transducer to capture electromagnetic energy and transform it into useful electrical energy and power in the form of voltage and current. While a regular antenna can be used for this function—after all, that’s what an antenna does—the capture efficiency is quite low and generally insufficient unless a fairly large and resonant antenna is used.
Addressing this challenge, a team at the University of South Florida has developed a metasurface-based antenna that meets a self-declared usefulness “threshold” of providing 100 µW of power from its 16- × 16-cm surface. With its delivered energy level of 0.4 µW per square centimeter—approximately the intensity of the radio waves 100 meters from a cell-phone tower—they were able to capture and use this power in real-time to power a small LED.
“With the huge explosion in radio-wave-based technologies, there will be a lot of waste electromagnetic emissions that could be collected,” said Associate Physics Professor and research team leader Jiangfeng Zhou. “This, combined with advancements in metamaterials, has created a ripe environment for new devices and applications that could benefit from collecting this waste energy and putting it to use.”
The team used a harvesting device based on a metamaterial perfect absorber (MPA) with embedded Schottky diodes as a rectenna (rectifying antenna) to convert captured RF waves to dc power. The Fabry-Perot (FP) cavity resonance of the MPA greatly improves the amount of energy captured. Furthermore, the FP resonance exhibits a high Q-factor and significantly increases the voltage across the Schottky diodes, which improves the rectification efficiency, particularly at low intensity.
“We also placed a cell phone very close to the antenna during a phone call, and it captured enough energy to power an LED during the call,” said Zhou. “Although it would be more practical to harvest energy from cell-phone towers, this demonstrated the power-capturing abilities of the antenna.”
Electromagnetic metamaterials (EM) are man-made, typically periodically arranged, metallic-resonant structures that behave as homogeneous media with effective electric permittivity (ε) and effective magnetic permeability (μ). These two values can be engineered to provide unique EM properties that don’t exist in nature, such as a negative refractive index, diffraction-unlimited optical imaging, EM invisibility cloaking, and perfect absorption.
Metamaterials also provide the flexibility needed to design electrically small and low-profile antennas. In Moreover, the ability to manipulate their electric permittivity and magnetic permeability further provides the ability to match input/output impedance of antennas to that of the surrounding environment for optimal energy transfer.
Leveraging the Metamaterial Perfect Absorber
In this project, the engineered MPA helps to convert RF waves more efficiently to dc power by perfectly capturing and storing the RF wave energy inside the Fabry-Perot meta-cavity. The MPA-based rectenna consists of a 4 × 4 array of double-gap split-ring resonators (SRRs) and a copper ground plane (not shown) with the same size, separated by a distance (s) (Fig. 1).
The sample was fabricated on a copper-coated FR4 circuit board (dielectric constant ε = 4.34) by using lithography followed by chemical etching. A Schottky diode (Skyworks SMS-7630-079LF) was soldered across one gap of each SRR to create a dc voltage by rectifying resonant current excited by the incident RF wave. The rows of SRRs are connected via copper strips along the x-direction, thus forming a series connection of four effective “batteries.”
Four rows of SRRs are connected through two thicker strips at the left and right ends, forming a parallel connection along the y-direction. The polarities of the diodes alternate in adjacent rows and columns to create the correct polarities of effective “batteries” in series and parallel connections. The alternating arrangement of diodes also helps to harvest both the forward and backward currents induced by the positive and negative half cycles of the incident RF wave, respectively.
The team carried out 3D full-wave simulations to solve Maxwell’s equations and obtain numerical solutions via Computer Simulation Technology (CST) Microwave Studio using a finite integration technology.
Tests and Results
Tests were performed in an anechoic chamber located at The MITRE Corp. (Bedford, Mass.), with the rectenna sample is connected to a 1-kΩ load as a proxy for a functioning load (Fig. 2).
A calibrated, fully controllable signal was transmitted by a horn antenna at normal incidence to the rectenna sample, which was placed about 380 cm away from the transmitting antenna. A metal ground plane was placed behind the sample with the purpose of creating a Fabry-Perot cavity to increase the amount of captured RF radiation. All instruments were controlled by a LabVIEW program and the measurement was automated by sweeping both the power and frequency of an incident RF wave.
The team performed performance and efficiency tests across a range of parameter values. They first measured the sample for relatively high intensities of incident RF waves (Fig. 3).
The intensity range of 2.6 μm/cm2 to 65 μm/cm2 is well above what would typically be expected from ambient RF signals. It would likely only be found within close proximity (<25 m) to a strong RF source such as a cell-phone tower or immediately next to a weak RF source.
With or without the ground plane, the maximum energy-harvesting efficiency occurs at 0.90 GHz, which is comparable to the frequency predicted by the absorption cross-section. Without the ground plane, the SRR array reaches the highest efficiency of ∼60% at 0.90 GHz when the intensity of the incident RF wave reaches 65 μm/cm2.
However, when the ground plane was placed at the optimum distance (s = 4 cm), the energy-harvesting efficiency improves considerably and reaches the highest efficiency of ∼140%. (Yes, it’s possible: efficiency above 100% indicates that the ground plane has caused the effective area of the rectenna to become significantly larger than its physical area.)
They also evaluated efficiency at intensities that are comparable to what can be found from ambient sources (up to 1.0 μW/cm2) (Fig. 4). These intensity levels were chosen to correspond to those encountered on the streets of urban environments, such as a GSM cellphone (< 1.930 μW/cm2 at 900 MHz and < 6.40 μW/cm2 at 1800 MHz) and 3G (< 2.4 μW/cm2 at 2110 MHz). The efficiency here is clearly much lower than it was for the high-power measurements. However, the presence of the ground plane increases the efficiency by a large factor (up to 16) in this case.
The team concluded that designing RF-harvesting rectennas based on metamaterial perfect absorbers is a promising solution for collecting ambient RF energy in low-power-density environments because they are tunable, highly efficient, and electrically small.
The perfect-absorber design of the metamaterial rectenna dramatically improves the RF-dc conversion efficiency (especially for lower available power densities) by increasing the effective area of the antenna. This eliminates the reflection due to impedance mismatching and helps overcome the high resistance below the turn-on threshold.
The current version of the antenna is much larger than most of the devices it would potentially power, so the researchers are working to make it smaller. They also would like to make a version that could collect energy from multiple types of radio waves simultaneously so that more energy could be gathered.
Their research, which was funded by the Asian Office of Aerospace Research and Development and the Alfred P. Sloan Foundation, is detailed in a crisp, nine-page, highly readable paper “High efficiency ambient RF energy harvesting by a metamaterial perfect absorber” published in Optical Materials Express. Somewhat surprisingly, there’s no supplemental material or video accompanying the paper.