Wireless power transfer

Wireless power transfer is a way to send electricity without using wires. Imagine charging your phone or a toy without plugging it into a wall socket. That’s what wireless power transfer does! It works by using special machines that create invisible energy fields. These fields can send power through the air to another machine that catches the energy and uses it.

There are two main ways to do this. One way works over short distances, like charging a phone on a special pad. This uses magnetic fields created by coils of wire. Many everyday things use this, such as electric toothbrushes and cooking pans. The other way uses beams of energy, like laser light, to send power over longer distances. This could be used for things like satellites in space.

One big thing to think about with wireless power is making sure it’s safe for people and animals. We need to be careful that the energy fields don’t hurt living things.

Elementary overview

Wireless power transfer is a way to send electricity without using wires. It uses special fields to move energy from one place to another. These fields can be electric or magnetic, or they can be types of waves like radio waves, microwaves, infrared, or even visible light.

A wireless power system has two main parts: a transmitter and a receiver. The transmitter is connected to a power source, like a wall outlet. It changes this power into a changing electromagnetic field. The receiver picks up this field and changes it back into electricity that can power devices. The transmitter might use different kinds of antennas—like a coil of wire, a metal plate, or a device that sends out radio waves or light—to create the field. The receiver uses a similar device to catch the field and turn it into usable electricity. The frequency, or how fast the field changes, helps decide what kind of waves are used.

Wireless power transfer is similar to how radios and cellphones work, which also use electromagnetic fields to send information without wires. But with wireless power, the goal is to send enough energy to power devices, not just to send small amounts of power for information. This means how well the system works over distance is very important. Wireless power can also help power devices that send or receive information, making networks that share both power and data at the same time.

History

19th century developments and dead ends

The 1800s had many ideas about sending electricity without wires. In 1826, André-Marie Ampère found a link between electric currents and magnets. Michael Faraday showed in 1831 that a changing magnetic field could create an electric current. Many inventors tried wireless energy transfer, but they didn’t fully understand it until James Clerk Maxwell created his theories in the 1860s. These theories explained electricity and magnetism together as electromagnetism and predicted waves that could carry energy.

Around 1884, John Henry Poynting described how energy moves in electromagnetic fields, and Heinrich Rudolf Hertz proved these ideas in 1888 with experiments showing radio waves.

Nikola Tesla

After 1890, inventor Nikola Tesla tested ways to send power using special coils that create high voltages. He showed he could light bulbs from far away using resonant inductive coupling. Though he couldn’t make a successful product, this method is now used in many electronics and short-range wireless power systems.

Tesla dreamed of creating a system to send power long distances directly to homes and factories. He thought of using balloons to hold wires high in the air. He built a test site in Colorado Springs in 1899 and did many experiments. He believed he could send power around the world using the Earth itself, but his big plan, called the World Wireless System, was never finished. In 1901, he started building a large tower in New York called Wardenclyffe Tower, but ran out of money before it was done.

Post-war developments

Before World War II, there wasn’t much progress in wireless power. Radio was used for messages, but not for power because the waves spread out too much. After the war, new technologies like klystron and magnetron tubes helped create focused beams of microwaves. In the 1960s, William C. Brown built the first system to send power over long distances. In 1964, he created a device called a rectenna that could turn microwaves into electricity and used it to power a small helicopter from the ground.

Field regions

Electric and magnetic fields are made by tiny particles called charged particles, like electrons. When these particles are still, they create static fields that hold energy but can't send power. But when charges move, like in alternating current (AC) electricity, they create changing electric and magnetic fields around them. These changing fields can push electrons in a receiving device, making them move back and forth as alternating current, which can power something.

These changing fields around an antenna can be split into two areas based on distance. Close to the antenna (within about one wavelength), the electric and magnetic fields act separately. Here, power can be sent using either electric fields between metal parts or magnetic fields between coils of wire. These fields don't spread out far, so they work best over short distances. Farther away (beyond about one wavelength), the fields change into electromagnetic waves, like radio or microwaves. These waves can travel long distances, especially if focused into a narrow beam using special antennas.

Near-field (nonradiative) techniques

Near-field techniques use electric and magnetic fields that stay close to the transmitter. These fields get weaker quickly with distance. This means these techniques work best over short distances, usually just a few times the size of the antenna.

Inductive coupling

Main article: Inductive charging

Inductive coupling uses magnetic fields to transfer power between coils of wire. One coil, called the transmitter, creates a magnetic field when electricity flows through it. This field passes through a second coil, called the receiver, which uses the changing magnetic field to create electricity. This is similar to how a transformer works.

Inductive coupling is the most common way to wirelessly charge devices. It’s used in things like electric toothbrushes, shavers, and charging pads for phones, tablets, and computer mice. It’s also used to power medical devices inside the body, like heart pacemakers, without needing wires through the skin.

Resonant inductive coupling

Main article: Resonant inductive coupling

Further information: Tesla coil § Resonant transformer

Resonant inductive coupling is an improved version of inductive coupling. It uses special circuits called resonators to increase the distance over which power can be transferred efficiently. This method can work over longer distances — up to about ten times the size of the coils.

Capacitive coupling

Main article: Capacitive coupling

Capacitive coupling uses electric fields instead of magnetic fields to transfer power. It works by creating an electric field between two plates, which can induce electricity in a second set of plates. While this method has some advantages, it usually needs very high voltages.

Electrodynamic wireless power transfer

Electrodynamic wireless power transfer uses a receiver with a moving magnet. When a changing magnetic field is applied, the magnet moves, and this motion is turned into electricity.

Magnetodynamic coupling

Magnetodynamic coupling uses rotating parts with magnets to transfer power. One rotating part in the transmitter creates a magnetic field that turns a similar part in the receiver, which then generates electricity. This method is being tested for charging electric vehicles.

Zenneck wave transmission

A newer method uses special waves called Zenneck waves to transfer power across metal surfaces. This could allow power to be sent over metal obstacles.

Far-field (radiative) techniques

Far-field methods can send power over long distances, sometimes many kilometers, because the distance is much larger than the size of the devices. These methods use special antennas or focused laser light to create a beam of energy that matches the shape of the receiving area. The effectiveness of these beams is limited by how they spread out over distance.

Visible light from lasers and microwaves from special antennas are the best types of radiation for transferring energy. The size of the parts used can depend on how far the power needs to travel, the wavelength of the energy, and how the beam spreads out. Shorter wavelengths, like those from blue lasers, spread out less than longer wavelengths, like red lasers.

Microwave power transmission can be more efficient than lasers and is less affected by things like dust or fog in the air. To calculate how much power can be sent, scientists combine different factors like how strong the signal is and how much is lost along the way. This process is called calculating a link budget.

Microwaves

Power can be sent using radio waves, making it possible to send energy over long distances with focused beams. A special device called a rectenna can change microwave energy back into electricity. Some experiments have shown that rectennas can be very efficient, changing most of the microwave energy into electricity. There have been ideas to use microwaves to send power from solar power satellites in space to Earth, or to send power to spacecraft leaving Earth.

However, for most space uses, the antennas needed to send microwaves are very large because of how the beams spread out. On Earth, large receiving areas can allow for lots of power while keeping the amount of energy safe for people. After World War II, scientists began researching using microwaves to send power. By 1964, they even made a small helicopter that was powered by microwaves.

Japanese scientist Hidetsugu Yagi and his colleague Shintaro Uda created a special type of antenna in 1926. Experiments with microwave power transmission have been done in places like California and Reunion Island, showing that these methods can work over distances of about a kilometer.

Under test conditions, microwave systems have been shown to change some of the energy into usable power over a distance of one meter. Some new ideas suggest using higher frequencies for shorter distances. In 2013, an inventor showed how to send power wirelessly over distances up to 30 feet using radio frequencies like those used by WiFi. In 2015, scientists at the University of Washington showed that WiFi signals could power small sensors and even charge batteries over similar distances. In 2017, officials approved the first system for sending power through the air using radio waves, and in 2021, they allowed a system that could send power to small devices like sensors over the air.

Lasers

Power can also be sent using lasers by changing electricity into a laser beam that is received by special solar cells. This is known as power beaming because the power is sent like a beam to a receiver that can change it into electricity. Benefits of using lasers include:

• Distance: Lasers can send power over very long distances without much loss.
• Size: Laser devices can be very small.
• Interference: Lasers do not interfere with radio signals like WiFi or cell phones.
• Access: Only the devices in the path of the laser beam receive power.

However, there are some challenges with using lasers:

• Efficiency: Changing laser light into electricity is not always very efficient.
• Weather: Clouds, fog, rain, and other weather can block laser beams.
• Access: Lasers need a clear path to reach their target, either directly or through a special glass fiber.

Laser power beaming has been used in military and space applications, as well as to power sensors in factories and for consumer electronics. The first laser power system for consumer use was shown in 2018 and could power devices across a room while following safety rules. It was approved by the US Food and Drug Administration.

Other challenges include how the laser beam travels and keeping the beam strong over long distances. Scientists have shown that model planes can be powered by laser beams, and researchers in China have tested using lasers to charge small devices or drones. In 2025, a project successfully sent a lot of power over a long distance using lasers, which was a big improvement over earlier tests.

Atmospheric plasma channel coupling

This method uses electricity to create a path through the air by ionizing it, similar to how lightning works. When there is a big enough difference in electric charge between two points, the air between them can break down and allow electricity to flow. Scientists are studying ways to control this process using lasers to guide the electric flow. This research could be used for things like controlling lightning or creating new types of antennas, but these applications are still being explored.

Energy harvesting

Main article: Energy harvesting

In wireless power, energy harvesting means changing energy from around us into electricity. This helps power small wireless devices without using batteries. The energy can come from many places, like electric or magnetic fields, light, heat, or movement. Even though the power collected is small, it is enough to run tiny devices, such as remote sensors. This technology helps these devices work for a long time without needing new batteries.

Uses

Wireless power transfer has been around since the 1800s. Inventors used transformers to move electricity without wires. One early use was for induction heating, which is still used in induction cooking.

As cordless devices became popular, scientists made special charging pads. These pads are useful for wet areas, like for electric toothbrushes and electric razors, to keep users safe. In the 1960s, doctors started using wireless power for tiny medical devices inside the body, such as pacemakers.

Today, many gadgets like mobile phones, tablets, and laptops can be charged without plugging them in. Groups like the Wireless Power Consortium work to make sure devices can use the same charging pads. The Qi standard lets you place a phone on a flat pad and it charges automatically.

Scientists have also worked on charging devices from far away. In 2007, a team at MIT showed they could send power over two meters. Others have tried using microwaves or lasers to power devices like drones from even farther distances.