Over the past two decades, many wireless systems have been developed and widely used around the world. The most important examples are cellular mobile radio and Wi-Fi systems. Just like radio and television broadcasting systems, they radiate electromagnetic waves/energy into the air but a large amount of the energy is actually wasted, thus how to harvest and recycle the ambient wireless electromagnetic energy has become an increasingly interesting topic.
One of the most promising methods to harvest the wireless energy is to use a rectenna, which is a rectifying antenna — a special type of receiving antenna that is used for converting electromagnetic energy into direct current (DC) electricity. There are at least two advantages for rectennas: (1) the life time of the rectenna is almost unlimited and it does not need replacement (unlike batteries). (2) It is “green” for the environment (unlike batteries, no deposition to pollute the environment)
They are used in wireless power transmission systems that transmit power by radio waves. In future they may power our smartphones, laptops, wearables, and other electronics instead of batteries. Since the 1970s, one of the major motivations for rectenna research has been to develop a receiving antenna for proposed solar power satellites, which would harvest energy from sunlight in space with solar cells and beam it down to Earth as microwaves to huge rectenna arrays.
A proposed military application is to power drone reconnaissance aircraft with microwaves beamed from the ground, allowing them to stay aloft for long periods. In recent years, interest has turned to using rectennas as power sources for small wireless microelectronic devices.
The largest current use of rectennas is in RFID tags, proximity cards and contactless smart cards, which contain an integrated circuit (IC) which is powered by a small rectenna element. When the device is brought near an electronic reader unit, radio waves from the reader are received by the rectenna, powering up the IC, which transmits its data back to the reader.
Researchers from MIT and elsewhere have taken a step in that direction, with the first fully flexible device that can convert energy from Wi-Fi signals into electricity that could power electronics.
Rectenna is a combination of a rectifier and an antenna. The wireless energy can be collected by the antenna attached to rectifying diodes through filters and matching circuit. The rectifying diodes convert the received wireless energy into DC power. The low-pass filter will match the load with the rectifier and block the high order harmonics generated by the diode in order to achieve high energy conversion efficiency which is the most important parameter of such a device.
A rectifying antenna or rectenna was developed by Raytheon. The structure consisted of a half-wave dipole antenna with a balanced bridge or single semiconductor diode placed above a reflecting plane. The output of the rectenna element is then connected to a resistive load. 2.45 GHz is widely used as the transmitting frequency because of its advanced and efficient technology base, location at the center of an industrial, scientific, and medical (ISM) band and its minimal attenuation through the atmosphere even in heavy rainstorms.
There are basically two approaches to increase the efficiency at the low microwave power density. The one is to increase the antenna aperture as shown in. There are two problems for this approach. It produces a high directivity and this is only applied for exclusive applications as SPS satellite experiment and not for low power applications like RFID or microwave energy recycling. The other approach is to develop a new rectifying circuit to increase the efficiency at a weak microwave input.
The rectenna can be divided into narrow-band rectennas and wide-band rectennas. Many narrow-band rectennas have been developed. For example, a dual-polarized patch rectenna at 2.4 GHz and a high conversion efficiency rectenna at 5.8 GHz were designed by McSpadden in 1994 and 1998 respectively. However, very few broad-band rectennas have been developed which are the most desirable rectennas for wireless energy harvesting. They may collect energy from systems operating at different frequencies to maximize the output power at a given location
Schottky diodes are usually used because they have the lowest voltage drop and highest speed and therefore have the lowest power losses due to conduction and switching. Large rectennas consist of an array of many such dipole elements.
Converting Wi-Fi signals to electricity with new 2-D materials
The researchers demonstrate a new kind of rectenna, described in a study appearing in Nature in Jan 2019, that uses a flexible radio-frequency (RF) antenna that captures electromagnetic waves — including those carrying Wi-Fi — as AC waveforms.
The antenna is then connected to a novel device made out of a two-dimensional semiconductor just a few atoms thick. The AC signal travels into the semiconductor, which converts it into a DC voltage that could be used to power electronic circuits or recharge batteries.
In this way, the battery-free device passively captures and transforms ubiquitous Wi-Fi signals into useful DC power. Moreover, the device is flexible and can be fabricated in a roll-to-roll process to cover very large areas.
“What if we could develop electronic systems that we wrap around a bridge or cover an entire highway, or the walls of our office and bring electronic intelligence to everything around us? How do you provide energy for those electronics?” says paper co-author Tomás Palacios, an EECS professor and director of the MIT/MTL Center for Graphene Devices and 2D Systems in the Microsystems Technology Laboratories. “We have come up with a new way to power the electronics systems of the future — by harvesting Wi-Fi energy in a way that’s easily integrated in large areas — to bring intelligence to every object around us.”
Promising early applications for the proposed rectenna include powering flexible and wearable electronics, medical devices, and sensors for the “internet of things.” Flexible smartphones, for instance, are a hot new market for major tech firms. In experiments, the researchers’ device can produce about 40 microwatts of power when exposed to the typical power levels of Wi-Fi signals (around 150 microwatts). That’s more than enough power to light up an LED or drive silicon chips.
Another possible application is powering the data communications of implantable medical devices, says co-author Jesús Grajal, a researcher at the Technical University of Madrid. For example, researchers are beginning to develop pills that can be swallowed by patients and stream health data back to a computer for diagnostics.
“Ideally you don’t want to use batteries to power these systems, because if they leak lithium, the patient could die,” Grajal says. “It is much better to harvest energy from the environment to power up these small labs inside the body and communicate data to external computers.”
All rectennas rely on a component known as a “rectifier,” which converts the AC input signal into DC power. Traditional rectennas use either silicon or gallium arsenide for the rectifier. These materials can cover the Wi-Fi band, but they are rigid. And, although using these materials to fabricate small devices is relatively inexpensive, using them to cover vast areas, such as the surfaces of buildings and walls, would be cost-prohibitive. Researchers have been trying to fix these problems for a long time. But the few flexible rectennas reported so far operate at low frequencies and can’t capture and convert signals in gigahertz frequencies, where most of the relevant cell phone and Wi-Fi signals are.
To build their rectifier, the researchers used a novel 2-D material called molybdenum disulfide (MoS2), which at three atoms thick is one of the thinnest semiconductors in the world. In doing so, the team leveraged a singular behavior of MoS2: When exposed to certain chemicals, the material’s atoms rearrange in a way that acts like a switch, forcing a phase transition from a semiconductor to a metallic material. The resulting structure is known as a Schottky diode, which is the junction of a semiconductor with a metal.
“By engineering MoS2 into a 2-D semiconducting-metallic phase junction, we built an atomically thin, ultrafast Schottky diode that simultaneously minimizes the series resistance and parasitic capacitance,” says first author and EECS postdoc Xu Zhang, who will soon join Carnegie Mellon University as an assistant professor.
Parasitic capacitance is an unavoidable situation in electronics where certain materials store a little electrical charge, which slows down the circuit. Lower capacitance, therefore, means increased rectifier speeds and higher operating frequencies. The parasitic capacitance of the researchers’ Schottky diode is an order of magnitude smaller than today’s state-of-the-art flexible rectifiers, so it is much faster at signal conversion and allows it to capture and convert up to 10 gigahertz of wireless signals.
“Such a design has allowed a fully flexible device that is fast enough to cover most of the radio-frequency bands used by our daily electronics, including Wi-Fi, Bluetooth, cellular LTE, and many others,” Zhang says.
The reported work provides blueprints for other flexible Wi-Fi-to-electricity devices with substantial output and efficiency. The maximum output efficiency for the current device stands at 40 percent, depending on the input power of the Wi-Fi input. At the typical Wi-Fi power level, the power efficiency of the MoS2 rectifier is about 30 percent. For reference, today’s rectennas made from rigid, more expensive silicon or gallium arsenide achieve around 50 to 60 percent.
There are 15 other paper co-authors from MIT, Technical University of Madrid, the Army Research Laboratory, Charles III University of Madrid, Boston University, and the University of Southern California.
The team is now planning to build more complex systems and improve efficiency. The work was made possible, in part, by collaboration with the Technical University of Madrid through the MIT International Science and Technology Initiatives (MISTI). It was also partially supported by the Institute for Soldier Nanotechnologies, the Army Research Laboratory, the National Science Foundation’s Center for Integrated Quantum Materials, and the Air Force Office of Scientific Research.