Wireless power transmission was the dream of Nikola Tesla over a hundred years ago. Yet, despite significant refinements of his work and the work of many others since then, true wireless power still seems to be something of a pipe dream today.
So, that begs the question of when, or if, a world without wires will ever be achieved? Let's take a look.
What is wireless power transmission?
WPT, or wireless power transmission, is the transmission of electrical power from one point to another through a vacuum or the air, without the need for wires or other physical means. WPT can be used, conceivably, to provide instantaneous power delivery or continuous delivery of energy on demand.
Modern-day applications of this kind of technology are proposed where conventional wiring is unaffordable, hazardous, or simply less convenient. Examples today include wireless charging pads for smart devices.
Broadly speaking, wireless power transmission can be achieved through a variety of techniques including:
- Inductive coupling
- Magnetic resonant induction
- Electrostatic induction
- Resonant inductive coupling
- Microwave power transmission
- Laser power transmission
The first four of these are generally only practicable for short-ranges, while the latter two are specifically designed for long-distance wireless power transfer.
What is wireless charging?
Wireless, or inductive, charging, is a type of power transfer that uses electromagnetic induction to provide electricity to portable devices like smartphones and tablets. Today, the most common form is the so-called Qi wireless charging standard for smart devices.
However, the technology can also be found in certain vehicles, power tools, other consumer electronics like toothbrushes, and some medical devices. To use it, compatible electronic devices are placed near a charging station and charge without needing to be precisely aligned or make electrical contact with it.
Generally speaking, there are three main types of wireless charging. These are:
- Charging pads - These use tightly-coupled electromagnetic inductive, or non-radiative charging to operate.
- Charging bowls or through-surface type chargers - These use loosely-coupled or radiative electromagnetic resonant charging to transmit charge over a few centimeters.
- Uncoupled radio frequency (RF) wireless charging - This type of system allows for "trickle" charging at distances of many meters.
All of these use the same principle to produce a time-varying magnetic field to induce a current in a closed loop of wire.
While relatively new to consumer products, you might be surprised to find out that wireless charging is actually quite an old concept — in fact it's just over 100-years-old. More on that later.
How does wireless charging work?
Most wireless charging works through a process known as inductive coupling. This involves the application of an alternating current through an induction coil in the charging station or pad (aka. the primary coil or transmission coil).
As any moving electrical charge creates a magnetic field, the transmission coil produces just such a field that regularly fluctuates in intensity as the AC current's amplitude is constantly altered.
This change in magnetic field strength generates something called an electromotive field, as was described by Faraday's law of induction.
This law states that an induced voltage in a circuit is proportional to the rate of change over time of the magnetic flux through that circuit. In plain English, this means that the faster a magnetic field changes, the greater the voltage of the circuit, and any change in the direction of the magnetic field also determines the direction of an induced current.
The voltage of a circuit, therefore, can be increased by adding more loops to a circuit. So, a coil with two loops has twice the voltage of just a single loop. This is the law that underpins the design and operating of electrical motors and generators and explains why these devices tend to have multiple coils.
It is for this reason that smartphone wireless charging pads have a relatively short range, because the copper coils within them are only a few cm in diameter.
By also increasing the size of the coil(s) used, the distance and efficacy of wireless charging can also be markedly increased. The bigger the coils, or the greater the number of them, the greater the area of effect.
In wireless charging, the magnetic field generated by the transmission coil induces another AC current in another induction coil within the portable device. Commonly known as a receiving, or secondary coil, the induced AC current is then converted to DC using a rectifier which, in turn, charges the device's battery or provides direct power to the device.
There can be one or more receiving coils (or antennae).
All well and good, but this kind of setup tends to have a relatively short range. To extend the range, resonant inductive coupling (or magnetic resonance) can be employed. This involves the addition of a capacitor to each induction coil to create, in effect, two LC circuits with a specific resonance frequency.
The amount of induced current in the receiving current can be increased by using an appropriate capacitance to ensure the loops resonate at the same frequency. This also enables the range of wireless charging to be greatly increased.
What are some of the major milestones on the road to wireless power?
To get an appreciation for the long history of wireless power transfer, let's take a quick look at some of the major milestones in the development of wireless charging to date.
1. Nikola Tesla kicks the whole wireless charging thing off
In the late-19th century, visionary inventor and engineer Nikola Tesla first demonstrated magnetic resonant coupling. This, in case you are not aware, is the transmission of electricity through the air by creating a magnetic field between two separate circuits (a transmitter and a receiver).
He was able to demonstrate this by wirelessly lighting up phosphorescent and incandescent lamps at his Colorado Springs laboratory, and later in a series of public lectures. Tesla would patent the technology under the name "resonant transformer" or "Tesla Coil."
This device was able to produce very high voltages and frequencies, and his improved later designs allowed for the technology to be used in a very safe and reliable manner. Although, as we have seen, inductive and capacitive coupling are "near-field" effects and cannot be used for long-distance transmission. However, Tesla was convinced he could develop a long range wireless power.
In 1902, Tesla began to experiment with a much larger apparatus, to see if his vision of a worldwide wireless power delivery system was possible. He foresaw a huge network of towers that could wirelessly light cities, send communications, and perhaps even power things like airplanes in the air.
His first prototype, the Wardenclyffe Tower, was promising, but ultimately the venture failed.
Nevertheless, this was a groundbreaking work well ahead of its time.
2. The invention of the radio helped push the concept further
While not, technically speaking, a form of wireless power transmission, radio works through a very similar concept. Identified and studied by the German physicists Heinrich Hertz in the later 1880s, it is so ubiquitous today we barely give it a second thought.
Radio works by transmitting electromagnetic waves at frequencies between tens to hundreds of Hertz through the air. These are generated by an electronic devices called transmitters which radiates out radio waves until they are received by another antenna — the receiver.
At the receiver, the radio waves induce a small alternating current, which is then translated into sound via a transducer. This entire process, in effect, is transmitting power over a distance without the need for wires.
With regards to power transfer only, the use of radio waves has not proved fruitful, yet. This is because of the relativity low-frequency of radio signals, and the fact that it is spread out in all directions. This means that very little energy can actually be transferred to a single receiver — hence the need for an amplifier in most situations.
However, using a device called a rectenna, or rectifying antenna. This is a type of receiving antenna that is used for converting electromagnetic energy into DC electricity. By using a rectenna, radio waves could, conceivably, also be used to transmit electricity over greater distances.
However, current work in this field is only able to provide small amounts of power, on the microwatt scale. While useful for small electronic devices like LEDs or silicon chips, it is a magnitude of scale lower than is needed for your smartwatch or TV. Though, it is important to note that radio-wave wireless power transfer is currently a rapidly developing field.
3. Microwaves have been used to send power wirelessly as early as the 1960s
For best results, an efficient power transfer would require transmitters that generate high-frequency waves, like microwaves. To achieve this, the microwaves need to be focused into narrow beams for transmission.
Early steps were made in this area during the Second World War, when devices like the klystron and magnetron tube were developed, as well as parabolic antennae.
One interesting example was made by William C. Brown in the 1960s. He was able to demonstrate long-distance wireless power transmission using a rectenna that could efficiently convert microwaves into DC power. In 1964, he even managed to demonstrate the technique by powering a model "helicopter" using microwaves beamed from the ground!
Brown would continue to refine the technique as the technical director of a JPL-Raytheon program until his retirement in the mid-1980s. Part of his work here enabled his team to beam 30KW of power over a distance of 1 mile (1.6 km) at over 80% efficiency.
4. Wireless power transmission was used in medical devices in the 1960s
One of the most important real-world applications of wireless power transmission was the use of inductive wireless energy transfer in implantable medical devices in the 1960s. Early iterations of these devices used a resonant receiver coil only, while later ones also came with resonant transmitters coils as well.
Such devices were designed for high efficiency, using lower power electronics without the need for wires. Today, the use of resonant inductive energy transfer is increasingly more common, with many commercially available implantable medical devices, like cochlear implants.
5. Early strides in wireless charging in vehicles were made in the 1970s
In the 1970s, various attempts were made to provide wireless charging in vehicles. For example, research in 1972, by Professor Don Otto at the University of Auckland.
Through his research, Professor Otto proposed that a vehicle could be charged inductively using transmitters embedded in a road's surface. Receivers on the vehicle, could conceivably then be used to power the vehicle as it traveled.
Later, in 1978, the first application of inductive charging was demonstrated by J.G. Bolger and his colleagues. They managed to produce an electric vehicle powered inductively using a system running at 180 Hz with 20kW.
At the end of the decade, in California, a wirelessly charged bus was also unveiled. Powered by inductive charging, similar ventures were also pioneered in France and Germany around the same time.
More recently, companies like Momentum Dynamics have been working in Norway on wireless charging systems for electric vehicles. Using a form of inductive charging technology, they hope to bring wireless charging to electrical vehicles, like buses or taxis, allowing them to charge without the need for charging stations.
This solution would see EVs topping up their batteries when idling, like waiting to pick up passengers, rather than needing to stop during their working day to recharge. The company is also working with others in China to develop a similar solution.
6. Long-distance charging was demonstrated in 2007
In 2006, MIT professor Marin Soljačićm first demonstrated that electricity could be transferred over distances greater than 6.6 ft (2 mt). This was achieved by using a highly resonant form of magnetic induction.
Soljačićm demonstrated that it was possible to transfer 60W of power to a similar dual resonance receiver over a distance of 6.6 ft (2 mt). Not only that, this was achieved with an amazing 40% efficiency.
7. The Wireless Power Consortium was founded in 2008
In 2008, in response to the widescale proliferation of mobile phones, tablets, and other devices, strides were made in the research of mid-range wireless power and charging technology in order to remove the need for tethering and the use of wall plugs for charging. As part of this effort, the Wireless Power Consortium was born, to develop interoperable standards within the industry.
This ultimately led to the Qi inductive power standard that was first published in 2009, for high-energy charging and powering of portable devices up to 5 watts over distances of 1.6 inches (4 cm).
8. Focused EM beams might be the future of wireless power
One interesting avenue of research into wireless power transfer is the use of EM beams as the main vehicle of the transfer. Microwaves, for example, have been experimented with to provide point-to-point energy transfer without the need for wires.
NASA conducted research in the 1960s to investigate the possibility of harvesting energy from space using solar-paneled satellites and "beam" the energy back to Earth. The work was conducted at NASA's Jet Propulsion Lab where, after some trial and error, researchers demonstrated the transmission of 30kW over 0.93 miles (1.5 km) using 2.38 GH microwaves with an 80% efficiency.
Further work on a similar concept, called SPS-ALPHA, was later developed by NASA in the early-2010s.
More recently, work in this area has focused on the long-distance powering of drones. In the late-1980s, for example, Canada's Communications Research Center was able to develop a small prototype airplane called the Stationary High Altitude Relay Platform (SHARP).
This plane was powered using microwaves and a rectenna and was able to fly 13 miles (21 km) in the air and remain airborne for months without the need to recharge. A similar, more advanced, craft was also developed at Kyoto University in the early-1990s called Microwave Lifted Airplane eXperiment (MILAX).
In the early-2000s, NASA managed to develop the world's first laser-powered aircraft too. A small prototype was developed that was powered by electricity generated by photocells that generated power from a ground-based IR laser.
9. Various companies are now working on wireless power for your home
In more recent years, the private sector has been increasingly getting in on the act to help bring wireless power transfer to the mainstream. Various companies, like Wi-Charge, Energous, and Ossia, are currently developing methods to powering devices wirelessly using infrared and RF technology in a safe and reliable fashion.
Wi-Charge's solution uses focused beams of IR light directed at a receiver on an enabled device which converts the beam into useful electricity. Energous, on the other hand, is developing radiowaves to enable the charging of many enabled devices within a 49-foot (15-meter) radius.
Ossia is developing a means of wireless power transfer aimed specifically at the automotive market. They hope to provide a means of in-car wireless charging of compatible devices in the future.
These solutions could conceivably render charging cables a thing of the past — something that would very handy in places where electrical cables are potentially hazardous or inconvenient, like bathrooms.
10. Long-range wireless power transfer could literally be just over the horizon
For wireless power transfer to rival conventional wired power, a means to transfer it over long-distances is needed. This is where companies like the NZ-based Emrod, could soon revolutionize the way power is transmitted around the world.
They are developing a means of safely, and wirelessly, distributing power in collaboration with Powerco (New Zealand's second-largest power distributor). Emrod has recently reported promising results with their current prototypes, with large amounts of power being transmitted between two points efficiently.
Their solution uses a series of antennae, relays, and receiving rectenna to convert microwave energy into electricity. These microwaves are within the non-ionizing industrial, scientific, and medical band of the radio spectrum that includes frequencies commonly used in Wi-Fi and Bluetooth communications.
11. The future needs to be faster and over a greater distance
The recent developments in wireless power transfer are impressive, but they are only just the beginning. However, it is important to point out that most experts stress that current solutions are not entirely wireless, as the transmitters themselves need to be plugged into the mains in some fashion.
Not only that, but consumer takeup is also somewhat limited at present. When users begin to trust and buy into it en masse, the demand for flexibility and robustness will likely greatly improve.
This market pressure will force manufacturers to develop more robust, reliable, and longer-ranged wireless charging solutions. At present, for domestic applications, consumers have the choice between short-range but fast charging (akin to a wire) or longer-range by trickle charging.
Work on long-range power wireless distribution is potentially very promising, but it is far from a viable alternative to traditional copper wires — at least for now.
Over the coming years and decades, however, some of the most common uses for cables in your home may become a thing of the past, and the same may hold true for your EV car as well. However, larger-scale distribution of electricity from power stations or space is likely not going to be possible for quite a while yet.
Once reliable and safe solutions for both the large-scale, long-range distribution for utilities and businesses along with short-to-medium range solutions for consumers can be solved, and the benefit of both combined, only then would wireless charging truly come of age.