Our eyes are tuned only to a narrow band of possible wavelengths of electromagnetic radiation, on the order of 390-700 nanometers. If you could see the world at different wavelengths, you would know that in an urban area, you are illuminated even in the dark - infrared radiation, microwaves and radio waves are everywhere. Some of this electromagnetic radiation from the environment is emitted by objects that scatter their electrons all over the place, and some carry the radio and Wi-Fi signals that underlie our communication systems. All of this radiation also carries energy.
What if we could harness the energy of electromagnetic waves?
Researchers at the Massachusetts Institute of Technology presented a study that appeared in the journal Nature detailing how they got down to practical implementation of this goal. They developed the first fully bendable device that can convert energy from Wi-Fi signals into usable DC electricity.
Any device that can convert AC signals to direct current (DC) is called a rectifying antenna. The antenna picks up electromagnetic radiation, converting it into alternating current. It then passes through a diode, which converts it to direct current for use in electrical circuits.
Rectennas were first proposed in the 1960s and were even used to demonstrate a microwave-powered helicopter model in 1964 by inventor William Brown. At this stage, futurists already dreamed of wireless transmission of energy over long distances and even the use of rectennas to collect space solar energy from satellites and transmit it to Earth.
Optical rectenna
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Today, new technologies for working at nanoscale allow a lot of new things. In 2015, researchers at the Georgia Institute of Technology assembled the first optical rectenna capable of handling high frequencies in the visible spectrum from carbon nanotubes.
So far, these new optical rectennas have low efficiencies, around 0.1 percent, and therefore cannot compete with the increasing efficiency of photovoltaic solar panels. But the theoretical limit for rectenna-based solar cells is probably higher than the Shockley-Kuisser limit for solar cells, and can reach 100% when illuminated with radiation of a certain frequency. This enables efficient wireless power transmission.
The new part of the device, manufactured by MIT, takes advantage of a flexible RF antenna that can capture wavelengths associated with Wi-Fi signals and convert them to alternating current. Then, instead of a traditional diode to convert that current to DC, the new device uses a "two-dimensional" semiconductor just a few atoms thick, creating a voltage that can be used to power wearable devices, sensors, medical devices, or large-area electronics.
The new rectennas are made of two-dimensional (2D) materials - molybdenum disulfide (MoS2), which is only three atoms thick. One of its remarkable properties is the reduction of parasitic capacitance - the tendency of materials in electrical circuits to act as capacitors, holding a certain amount of charge. In DC electronics, this can limit the speed of signal converters and the ability of devices to respond to high frequencies. The new molybdenum disulfide rectennas have an order of magnitude lower parasitic capacitance than those developed so far, which allows the device to capture signals up to 10 GHz, including in the range of typical Wi-Fi devices.
Such a system would have fewer problems with batteries: its life cycle would be much longer, electrical devices would be charged by ambient radiation and there would be no need to dispose of components, as is the case with batteries.
“What if we could develop electronic systems that would wrap around a bridge or that would cover an entire highway, the walls of our office, and give electronic intelligence to everything that surrounds us? How are you going to power all this electronics?”Asks co-author Thomas Palacios, professor in the Department of Electrical Engineering and Computer Science at Massachusetts Institute of Technology. "We've come up with a new way to power the electronic systems of the future."
The use of 2D materials allows flexible electronics to be produced cheaply, potentially allowing us to place them over large areas to collect radiation. Flexible devices could be used to equip a museum or road surface, and it would be much cheaper than using rectennas from traditional silicon or gallium arsenide semiconductors.
Can i charge my phone from Wi-Fi signals?
Unfortunately, this option seems highly unlikely, although over the years the topic of "free energy" has fooled people over and over again. The problem lies in the energy density of the signals. The maximum power that a Wi-Fi hotspot can use without a dedicated broadcast license is typically 100 milliwatts (mW). This 100 mW radiates in all directions, spreading over the surface area of a sphere centered on the AP.
Even if your mobile phone collected all this power with 100 percent efficiency, it would still take days to charge the iPhone battery, and the phone's small footprint and distance to the hotspot would severely limit the amount of energy it could collect from these signals. MIT's new device will be able to capture about 40 microwatts of power when exposed to a typical Wi-Fi density of 150 microwatts: not enough to power an iPhone, but enough for a simple display or remote wireless sensor.
For this reason, it is much more likely that wireless charging for larger gadgets will rely on induction charging, which is already able to power devices up to a meter away if there is nothing between the wireless charger and the charging object.
However, the surrounding RF energy can be used to power certain types of devices - how do you think Soviet radios worked? And the upcoming "Internet of Things" will definitely use these food models. All that remains is to create sensors with low power consumption.
Co-author Jesús Grajal of the Technical University of Madrid sees potential use in implantable medical devices: a pill that a patient can swallow will transfer health data back to a computer for diagnosis. “Ideally, we would not want to use batteries to power such systems, because if they let through lithium, the patient could die,” Grajal says. "It is much better to harvest energy from the environment to power these small laboratories inside the body and transmit data to external computers."
Current device efficiency is around 30-40% compared to 50-60% for traditional rectennas. Along with concepts such as piezoelectricity (materials that generate electricity when physically squeezed or stretched), electricity generated by bacteria and environmental heat, "wireless" electricity may well become one of the power sources for microelectronics of the future.
Ilya Khel