Batteries let us live life on the go. They power the laptops, smartphones, handheld games and music players that accompany us everywhere. Yet no matter where we take our favorite electronic gadgets, they eventually end up in the same place: plugged into an outlet to recharge.
That is changing. Batteries will always need recharging. But two new technologies will free us from the tangle of cords and plugs. They will make recharging a snap, even on the go.
Wireless electricity is the first of those technologies. Electric toothbrushes and some smartphones already rely on this technology. To recharge the devices, just place them on a special base or pad. Some electric car models can also recharge using wireless electricity while parked in their garages. Even more exciting are experimental vehicles that can recharge on the road, thanks to electronics buried under part of the roadway. That technological advance could eliminate two big downsides to electric vehicles: the worry about running out of power and the wait to recharge drained batteries.
Energy harvesting is the second technology. It could help us go truly cord-free. Energy harvesting taps into energy that’s already moving through our surroundings. The technology grabs that energy in the form of radio waves and microwaves and converts it to electricity. That power could run small gadgets and maybe even larger household appliances. Devices could charge themselves without our ever doing a thing.
Together, these technologies would free us from the bother of plugging in all our electronics. They also could cut waste and reduce greenhouse gas emissions.
Change is coming, but it won’t happen overnight. Taking full advantage of the promise of wireless electricity and energy harvesting will take much more research, as well as time and money.
Going the distance
Last August, two self-charging electric buses began running in Gumi. This industrial city sits in the center of South Korea. The batteries that power the buses never need plugging into a power source. Instead, buried coils lie under portions of the route that these buses follow. Those coils send energy to the buses wirelessly.
The innovative buses come from the Korea Advanced Institute of Science and Technology in Daejeon, South Korea. “The bus never needs to stop anywhere to recharge, because it occurs automatically,” explains engineer Nam P. Suh. He helped develop the electric buses while he was the former president of the Institute. Suh now works at the Massachusetts Institute of Technology in Cambridge.
The principle behind wireless electricity is pretty basic. It builds on the relationship between electric and magnetic fields.
Magnetism and electricity are “actually two faces of the same coin,” explains Elza Erkip. She teaches engineering at New York University’s Polytechnic Institute in Brooklyn. “In electrical engineering, we call it electromagnetic energy.”
“Whenever you have electrical signals, you also have magnetic signals,” says Erkip, explaining how they produce — engineers say induce — one another. “By their very own nature, they go together.”
You can show this relationship yourself. Wrap wire around a nail over and over. Now run electric current through that wire coil. This creates a magnetic field. The nail acts like an electromagnet and attracts small iron or metal objects within the field.
Even if you remove the nail, the magnetic field remains around the coil as long as the current flows. You also can curve the coil into an arc, ring or spiral, the way you might do with a Slinky toy.
The process works in reverse too. A magnetic field can induce an electric current. Most electric generating plants use this principle on a big scale. They spin coiled wire near huge magnets. The magnetic fields interact with the electrons in the wire and make current flow.
Wireless electricity uses both steps to transfer energy. First, an electric current causes a magnetic field. Then the magnetic field creates an electric current in another coil of wire.
Scientists and engineers call the process induction.
By the late 1800s, electrical engineer Nikola Tesla had already worked out the idea that induction can wirelessly transmit energy. Yet 120 years later, only a handful of wireless gadgets rely on induction for recharging. The technology’s limited range has held back its adoption. Most induction chargers won’t work over a distance of more than a centimeter (0.4 inch).Only recently have scientists figured out how to create a magnetic field with the consistent strength and reach to induce a current across gaps of a meter (3.3 feet) or more. That advance has made it possible to recharge the South Korean buses on the go.
To give those buses their wireless charge, an electric current runs through coils buried beneath the road. That current creates a magnetic field. The field extends above ground and interacts with coils underneath the bus. There it creates — again, induces — an electric current.
Note that this electric current doesn’t jump through the air, as a spark might. “Actually, power is being wirelessly transferred through the magnetic field,” explains Omer Onar. He is an engineer at Oak Ridge National Laboratory in Oak Ridge, Tenn.
For the Gumi buses, engineers shaped the magnetic fields of the buried coils so they would reach the buses. To understand how they did this, think about a bar magnet. A bar magnet’s magnetic field varies with the distance between its two poles. “We can control the distance between the north and south pole to reach a certain height,” explains Suh.
Fine-tuning energy transfer
Shaping a magnetic field isn’t the only thing that’s important for wireless electricity. “The farther you get away from the coil that’s making the magnetic field, the weaker the field gets,” explains engineer Katie Hall at WiTricity Corp. in Watertown, Mass. The company is developing wireless electricity for use in homes, businesses and transportation.
If the magnetic field is too weak when it reaches a device, it won’t generate enough electric current to power it. So in order for wireless electricity to be useful, a system must deliver enough energy over the necessary distance. And it must work efficiently.
To do these things, engineers fine-tune the resonant frequencies of the sending and receiving ends of the system. Frequency is the rate at which something vibrates or makes waves. A resonant frequency is the natural rate at which something wants to vibrate.Think about a violin. “When I pluck a string on the violin, it vibrates at a certain frequency,” explains Erkip. “That’s the resonant frequency of that string. That’s the natural frequency it tends to operate at.” Other strings will have their own resonant frequencies.
Electrical materials work the same way. “The resonant frequency of an electrical circuit or a device is the natural frequency it operates at,” explains Erkip. Engineers can design a circuit so that it will have a particular, desired resonant frequency.
That’s important because wireless electricity works best when the resonant frequencies of both the energy transmitter and the energy receiver match. Just as a leaky bucket won’t hold water, mismatched frequencies waste lots of energy.
On the flip side, matching the resonant frequencies maximizes the energy transfer. The same concept explains how an opera singer can shatter a glass by holding a high note. Sound waves transfer energy from the singer to the glass. If the resonant frequency of the glass matches the singer’s high note, enough energy can build up to shatter the glass.
The wider adoption of wireless electricity could let it recharge more and more things. For example, a wireless electricity source in a kitchen counter could charge multiple devices at once. Just plop laptops, music players, game consoles or other gadgets onto the counter, suggests Hall of WiTricity. Everything would charge without cord clutter.
Engineers also are looking into how wireless electricity might power medical devices inside the body. One example would be the pumps fitted in the chests of patients with heart failure. Unfortunately, the electric cables that go through the skin to the pump can also let infections enter the body. Wireless electricity could cut that risk by getting rid of the wires.
Energy from the air
Energy harvesting is a totally plug-free idea. It uses some of the electromagnetic energy that already is “all around you,” explains Manos Tentzeris. He teaches engineering at the Georgia Institute of Technology in Atlanta.
Light is one form of electromagnetic energy. Photovoltaic solar cells convert it into usable electricity. The radio waves and microwaves that carry radio, television, Wi-Fi and other signals are another form of electromagnetic energy. They also can be converted into electricity. And the environment around us already is flooded with those waves, Tentzeris points out.
“A tiny fraction of [this energy] is sufficient to power most of your portable devices,” he says. In 2011, his group made the first device to grab that energy. It produced electricity using the signals from a television station as its source of energy. The television station was about 500 meters (1,640 feet) away.
Inkjet printers already can print the devices that would do the energy harvesting. The printers use special inks. The inks form the antennas and wires of the device. The base can be paper, plastic or other flexible materials. None of the items is very expensive, notes Tentzeris.
Here’s how it works: The harvesting equipment uses its antennas to pick up microwaves, radio waves or other electromagnetic energy. This works much like how a smartphone picks up a signal. The waves arrive at the harvesting equipment in an up-and-down pattern.
Part of the harvesting device, called a diode, is like a valve or gate. It forms a “one-way street” for electricity, says Allen Hawkes. “Current can go one direction, but not in the other direction.” Hawkes is an engineering student at Duke University in Durham, N.C. Hawkes, together with Duke engineers Alex Katko and Steven Cummer, recently designed a new form of energy-harvesting device. They outlined how it works in the December 2013 issue of Applied Physics Letters.
The diode locks onto the energy waves and holds them at their peak. Waves exiting this electronic gate would now look like a straight line. (Think of the flat water as a wave finishes running up the beach.) “That straight line is the electrical signal,” says Katko.
The scale of an energy harvesting system could be “as big or as small as you need,” Katko says. “You could make it so it covers the roof of your house, or the size of the back panel of your cell phone.”
Energy harvesting’s first uses might include the slow charging of low-power devices such as smoke detectors. Later, the technology could help charge batteries on phones or other gadgets. Some devices, such as light or sound sensors, might no longer even need batteries, says Tentzeris.
Eventually, energy harvesting might power “80 to 90 percent of household equipment,” says Tentzeris. It could power lamps, radios, computers, TVs and maybe even refrigerators.
Before that can happen, though, systems would have to harvest much more power. And each appliance would need a way to tap into that power. They can’t do that yet.
Getting to market
Energy harvesting and wireless electricity have come a long way. But lots of work remains.
The South Korean buses are up and running. Yet more advances must take place before everyone can afford and drive wireless electric cars.
“We are trying to come up with better coil designs. We are trying to improve the efficiency. And we are trying to achieve a very high power level,” says Onar, of Oak Ridge National Laboratory. Higher power would allow faster charging of car batteries.
The real world is also very different from a lab. Engineers control lab conditions. However, homes have clutter and activity. Both can interfere with electromagnetic signals. That’s why someone standing in front of the TV can interfere with the remote control. It’s also why the Wi-Fi signal or phone reception might be bad in parts of your home.
Price matters, too. “If something costs too much money, people aren’t going to buy it,” says Hall of WiTricity.
“We’re always trying to make things smaller,” she adds. People prefer to lug less weight around. Smaller gadgets generally use less energy too.
Still, engineers are looking ahead to new uses for plug-free power. Suppose you want to put sensors in a remote location to record climate data. Energy harvesting could power those sensors. However, like sunlight, the strength of an energy source can vary from hour to hour, and day to day.
At New York University, Erkip is working on that problem. Sometimes a sensor might send its data right away. Other times, it might just save up any collected data until there’s enough power to transmit them.
Ideally, broadcast signals could do double duty. A wireless router might transfer both data and power to your laptop computer. A cell tower might do the same for all the smartphones in a neighborhood.
In either case, says Erkip, “The interesting problem becomes how much of which job the signal does.” The answer will depend on the device and its use at any moment.
Right now, wireless energy and energy harvesting work best under different conditions. Wireless electricity is best where someone needs to provide a lot of power at a constant rate. Thus, wireless electricity would be a good match for charging cars’ or buses’ batteries. However, wireless electricity has a limited range. So far its longest reach is several meters.
Energy harvesting can’t produce as much current as wireless electricity yet. But energy harvesting works over greater distances. The energy waves it taps can come from hundreds of meters or several kilometers away.
In time, the two technologies should be able to work together. A rooftop system might harvest lots of energy. Wireless electricity transfer then might deliver that energy throughout the house.
Both technologies are also eco-friendly. Each can cut down on disposable batteries. They also can cut demand for gasoline and other fossil fuels. Emissions from those fuels feed global warming.
So far, both energy harvesting and wireless electricity remain in their infancy. That means there is plenty of time and space for young scientists to come up with new ideas for a cord-free future. “Join us in thinking of all the places we could use this,” invites Hall.
coil Concentric rings or spirals formed by winding wire or other fiber around and around a core.
constant Continuous or uninterrupted.
diode An electronic part that works like a one-way valve for electric current.
electric current A flow of charge, called electricity, along a given path. That flow usually reflects the movement of negatively charged particles, called electrons.
electromagnetic radiation Forms of light. Electromagnetic radiation is typically classified by its wavelength. The spectrum of electromagnetic radiation ranges from radio waves to gamma rays. It also includes microwaves and visible light.
electromagnetic force (also known as electromagnetism) One of the four fundamental forces of nature. It’s the force that causes electrically charged particles to interact. The regions over which these interactions occur are known as electromagnetic fields.
engineer A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.
fiberglass A material made of glass strands.
fossil fuels Any fuel (such as coal, oil or natural gas) that has developed in the Earth over millions of years from the decayed remains of bacteria, plant or animals.
frequency The number of times a specified periodic phenomenon occurs within a specified time interval. (In physics) The number of wavelengths that occurs over a particular interval of time.
greenhouse gas A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.
induce To produce or cause something to happen. In physics, electromagnetic induction is the production of electricity using varying magnetic fields.
magnet A material that usually contains iron and whose atoms are arranged so they attract certain metals.
magnetic field An area of influence created by certain materials, called magnets, or by the movement of electric charges.
microwaves A form of electromagnetic energy whose wavelength is longer than visible light but shorter than radio waves.
poles (in physics and electrical engineering) The ends of a magnet.
resonant frequency The natural rate at which something vibrates or makes waves.
router In computer science, a device that handles the exchange of digital information between different points in a network.
sensor A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly.
smartphone A cellular (or mobile) phone that can perform a host of functions, including search for information on the Internet.
transmit To send or pass along.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.