When you have a device that needs power or has a battery that needs recharging, your first thought is usually to plug it into a wall socket. Most consumer devices have an adapter to connect to a compatible alternating-current power source. Inside your device, the AC power is used as-is or converted to a lower voltage, either AC or direct current.
A wireless power transfer is a means of getting that AC power from the wall to your device without connecting wires between them. It can also be the transmission of power on a much larger scale over a greater distance, such as beaming solar power from an orbiting satellite to a power station on the ground.
The same waves and electromagnetic fields are used for power transmission as are for radio, except the goal in radio is to get accurate data. This is accomplished using a relatively weak signal. If a device were built to transfer more power to the receiving end, this could also be a means of getting power to wireless receivers and transmitters themselves.
This article explores the seven main methods used today to transfer power without wires. Two — lasers and microwaves — are used to cover long distances, while the remaining five work only over very short distances.
Seven Methods To Wireless Power Transfer
A laser can be used to beam power over a relatively long distance in the form of visible light, which is a type of electromagnetic radiation. This is known as a radiative or far-field method because the beam is focused in a particular direction and radiates outward whether there's a receiver in its line of sight or not. Electricity is converted into a laser beam that is focused on a photovoltaic cell optimized for laser light. The laser beam must be aligned with the receiving device so that as much of the energy as possible is focused on the target instead of dissipating. Like a radio wave, a laser beam will become weaker and spread out as the distance traveled increases. A blue laser is better than a red one for this application; the higher frequency from the shorter wavelength of this color of light helps reduce the scattering effect.
Because lasers can be very destructive, blinding people and burning holes through things, extreme caution must be taken to ensure that nothing is exposed to the beam. Rigid safety standards must be met for laser-based devices. Water vapor, dust and other reflective objects in the light path can absorb or scatter the light, drastically reducing the already low efficiency of light-based wireless power transfer. Lasers do have advantages though, among them are the small size of the optics and supporting electronics, lack of interference with cell phones and Wi-Fi, and the ability to maintain a narrow beam over a long distance.
Lasers are being used to transmit power in several aerospace applications: recharging unmanned drones in flight, receiving solar power from satellites, powering a space elevator to lift objects into orbit, and for military weapons. The first approved commercial application to deliver power to devices on the opposite side of a room was shown in 2018.
3. Capacitive Coupling
Capacitive coupling is a non-radiative, near-field technique for wireless power transmission. This means that the energy stays very close to the transmitter: within one antenna diameter. This is just a few inches in many cases. It doesn't radiate off into the distance. If there is no antenna present to receive energy — in other words, nothing for the transmitter to couple to — the transmitter doesn't send it.
In this method, electricity is converted into an electric field with time-varying oscillations via a metal plate electrode. A similar plate at the receiving end converts the oscillating electric field back into an electric current. The two plates form a capacitor and the energy is transferred by electrostatic induction. The wave frequency is much lower than lasers or microwaves.
Capacitive coupling is often used for charging pacemakers, portable consumer devices and smart cards. It also has the potential for use in semiconductors to power different layers of integrated circuits. It is generally used only in low-power applications because very high voltages are required to transmit significant power levels. The electric fields in capacitive coupling are absorbed by materials near the electrodes, including human tissue. The fields do remain fairly confined between the plates though, which reduces interference and alignment issues.
4. Resonant Capacitive Coupling
A variation on capacitive coupling, resonant capacitive coupling adds circuitry to the transmitter and receiver to tune them to resonate at the same frequency. This significantly increases the coupling, which in turn increases the power-transfer capability. It also increases the maximum allowable distance between the electrodes to up to 10 times the antenna diameter. Resonant capacitive coupling is generally used for similar applications as capacitive coupling, especially where slightly greater distance or power transfer is required. Both capacitive methods are still limited when compared to lasers or microwaves.
5. Inductive Coupling
Inductive coupling is another near-field, non-radiative method of wireless power transfer. Like capacitive coupling, the energy stays very close to the transmitter, within one antenna diameter. If there is no antenna to couple to, the transmitter doesn't send energy. Unlike capacitive coupling, the incoming electricity is converted into a time-varying, oscillating magnetic field by coils of wire. Similar coils at the receiving end convert the oscillating electric field back into an AC electric current, which can be rectified to DC. The two sets of coils form a transformer. Like capacitive coupling, the wave frequency is much lower than lasers or microwaves.
Inductive coupling is currently the most popular wireless power-transmission technology. It's included in many commercial products, from wet appliances, such as electric shavers and toothbrushes, to laptops, cell phones, game controllers, media players, industrial heaters and induction stovetops.
6. Resonant Inductive Coupling
Resonant inductive coupling adds circuitry to the inductive coupling transmission and reception points to tune them to the same frequency. As with resonant capacitive coupling, this significantly increases the coupling efficiency, which in turn increases the power transfer capability. It also increases the maximum allowable distance between the sets of wire coils to up to 10 times the antenna diameter.
Resonant inductive coupling is generally used for close-range applications similar to simple inductive coupling, especially where there's a need for greater power transfer or distance. It's used for charging biomedical devices, such as insulin pumps and pacemakers, and for powering RFID and smart cards. Higher-current applications include charging electric automobiles and powering electric trains, buses and magnetic levitation systems.
7. Magnetodynamic Coupling
This method of coupling magnetic fields uses two synchronously rotating armatures to transmit power. Each armature has a permanent magnet on it, one in the receiver and one in the transmitter. When the transmitter armature rotates, its magnet, which is coupled to the receiver's magnet by a magnetic field, causes the receiver's armature to rotate at the same rate, as if they were mechanically coupled. An electric motor or some other mechanism may rotate the transmitter armature. On the receiving side, electric power is generated when the receiver's armature rotates an electric generator shaft or similar device.
The wave frequency of magnetodynamic coupling is very low, generally lower than any of the other methods described here. It's used for charging electric buses and other vehicles. It also has applications powering biomedical implants, such as pacemakers and insulin pumps. The electric vehicle charging system would have the transmitter armature embedded in a garage floor, while the receiver armature would be in the underside of the vehicle being charged. Such as system may be highly efficient and generate less electromagnetic interference than inductive coupling systems' high-frequency magnetic fields.
Wireless power transfer is a developing technology with great potential to change how we connect things to their power sources. For some types of consumer devices, such as cell phone chargers, it's a convenience. For others, such as electric toothbrushes, it's a safety feature. For still others, such as pacemakers, it's a necessity to prevent wires through the skin. Transferring power over large distances — via microwave or laser, for example — can be quite hazardous to humans, other living creatures and even inanimate objects. With the appropriate safeguards, however, these methods and others yet to be discovered can help guarantee our planet's access to life-sustaining energy in the long term