Wireless Electricity has the Power to Change the World & It’s On Its Way
Imagine a world where electricity could be sent wherever it was needed in seconds. What if I told you this wasn’t all that far removed from reality.
Over the last twenty years, we’ve seen our phones go wireless. We’ve seen our computers go wireless. We’ve seen our internet go wireless. This begs the question, how long until we see electricity altogether go wireless?
To uncover the answer to this question, we need to rewind a couple of years.
In 1890, Nikola Tesla began experimenting with wireless electricity. Soon after he was performing public demonstrations sparking up lightbulbs from across a stage. No wires to be seen. What was considered magic at the time, has become the groundwork for scientists in the quest for wireless power.
Tesla showed that energy was not bound by physical links. He imagined a world where electricity could be transferred in seconds anywhere on earth. We have had the technology to transfer energy wirelessly available for over 130 years. How far are we from realizing Tesla’s dreams? Is it even possible?
By taking a look at how wireless energy transfer works, we can get a clearer picture of its limitations and how we can overcome them.
What is wireless power transfer?
In its most basic form, it’s exactly what it sounds like. The transfer of energy without the need for any wires through the use of electromagnetic fields.
Electric and magnetic fields are all around us. They can be formed from a single atom exchanging electrons or the molten core of our earth churning liquid metal.
All matter has a charge, be it positive, negative, or neutral. If an atom gains or loses a charge through the transfer of electrons, an electrostatic field will form around it.
Now electrically charged, these particles will begin to group and move. Once they start moving, they become a steady flowing electric current, and a static magnetic field will be formed around them.
These are considered static fields, meaning they don’t vary in intensity or direction with time. As these fields are static, they can contain energy, but can not carry harnessable power.
For these static fields to carry power, the particles need to accelerate so that their direction and intensity are no longer constant. This is typically done by inducing an alternating current that regularly reverses the direction and varies the voltage of the charges. These accelerating particles are known as time-varying currents and produce time-varying electromagnetic fields. Unlike static fields, these time-varying fields can carry harnessable power.
Or, in short:
- Stationary charges → Static electric fields that hold energy
- Steady currents → Static magnetic fields that hold energy
- Time-varying currents → Electromagnetic fields that carry power
How far can we transfer electricity wirelessly?
If we want to transfer electricity wirelessly, we need to create time-varying electric and magnetic fields from some kind of antenna. Now what?
Well, based on the transmission distance from the antenna, there are two different regions. These regions are defined by unique characteristics and require different technologies: those being near-field & far-field.
- The near-field or non-radiative region focuses on the range within 1 wavelength of the transmitter. One wavelength is the distance between a peak of a wave and its successive peak.
- In this range the time-varying electric and magnetic fields are separate
- Power is transferred via electric fields through capacitive coupling or via magnetic fields through inductive coupling
- Energy transfer in this space is non-radiative and energy stays within a short distance
- If there is no receiving device, no power leaves the transmitter
2. The far-field or radiative region focuses on the range greater than 1 wavelength from the transmitter
- Oscillating electric and magnetic fields merge and propagate as an electromagnetic wave
- Electromagnetic radiation can be focused by reflection or refraction into beams, typically in the form of microwaves or lasers
- Energy transfer in this space is radiative and energy propagates in all directions, so if the receiving antenna is far away, only a small amount of radiation will hit it
- Energy leaves the antenna regardless of if there is a receiver
What technologies can be used to transfer electricity wirelessly?
To better grasp the technology at hand, let’s use inductive coupling as an example for near-field and microwave technology as an example for far-field.
Inductive coupling is the oldest and most common method for wireless energy transfer. Nikola Tesla was conducting inductive coupling experiments using his “Tesla Coils” in 1891, and we are finally seeing widespread adoption in the market. You’ve probably seen a wireless charging pad or an electric toothbrush stand. Both of these are examples of inductive coupling.
Inductive Coupling is when energy is transferred between coils of wire through a magnetic field. It requires a transmitting and a receiving device.
The transmitting device consists of only three parts:
- An alternating current sending power
- An electronic oscillator generates a higher frequency alternating current. This is important as transmission efficiency improves with frequency.
- A copper transmission coil
The tightly wound transmission coil produces a magnetic field. This is known as magnetic flux. The magnetic flux density is determined by:
- the number of turns of the wire
- the diameter of the transmission coil
- displacement meaning the transmission distance between coils
- permeability of free space meaning the proportion between magnetic flux density and magnetic field strength
- the current meaning the flow of electrons measured in amps
The receiving device is basically the same, just in reverse order. The coil in the receiving device picks up the magnetic flux and a current is induced. The received alternating current can then be applied directly to power a device or rectified. To rectify the current means to convert it from alternating current to a direct current.
Unfortunately, regular inductive coupling only achieves high efficiency at very close distances. Power received decreases exponentially with distance. Every time you double the distance, the power received falls by a factor of 64. This is where resonant circuits and ferrite cores come into play.
Resonant inductive circuits utilize the natural phenomenon of resonance. If you strike a tuning fork, another tuning fork across the room will vibrate in tune. This is because all objects have a natural frequency. When an object receives a vibration that matches this resonant frequency, its response is far stronger. This is applied to inductive coupling by tuning the transmitter and the receiver to the same resonant frequency. Resonant inductive coupling is effective at ranges 4–10 times further than non-resonant inductive coupling.
In addition to resonant circuits, ferrite cores can be used to improve transmission. Ferrite “flux confinement” cores confine the magnetic field, improving efficiency and reducing interference. These cores are, however, bulky and heavy, so small wireless devices opt to avoid them.
Even with the use of resonance and ferrite cores, induction coupling is still only efficient at close ranges. But what about far-field? How effective is our technology at transferring power long-range?
To see this, let’s take a look at microwave technologies.
Microwaves have commonly been used in point-to-point communications systems, satellite communications, and radars. This same technology is capable of sending power long distances, however, that comes with one main challenge. Far-field systems are radiative, meaning that energy gets transmitted in all directions while trying to reach its target. Imagine shooting a powerful hose straight up to try and fill a kiddie pool. Probably not the easiest task. This is okay for communications technology, as only a small percentage of the signal needs to be received. Energy transfer, on the other hand, is only useful if it can be received efficiently.
Power transmission via microwaves requires the following:
- An alternating current supplying power
- An electronic oscillator or magnetron to generate short radio waves
- A transmitting antenna
- A receiving rectenna that can convert the energy from microwave into direct current
To minimize losses, antennas can focus microwaves into tight beams that form to match the shape of the receiving area. Even with these highly directed beams, directivity is heavily limited by diffraction in the atmosphere. To account for the diffraction, massive receiving areas are required to recoup the energy transmitted. Just how massive are we talking? NASA conducted a study regarding the possibility of space-based solar power. To send 2.45GHz of energy via microwaves, the demo required a transmitting antenna with a 1-kilometer diameter and a receiving rectenna with a 10-kilometer diameter.
Microwave power transfer is also limited by safety. A human-safe power density for microwaves is around 1 milliwatt per centimeter squared. Using a 10-kilometer rectenna at this power, we could receive about 750 megawatts of power. This is the power level found in a typical power plant. Which sounds great! Until you contrast it to the 10,000 megawatts that could be produced, had we just used a 10-kilometer solar farm in the first place. This brings us back to the ever-present issue of efficiency.
What companies are leading the way?
Companies around the world have jumped at the opportunity to get involved in the rapidly growing space of wireless electricity. These companies have been mostly focused on near-field technologies, where we have seen major strides in commercializing products. The Wireless Power Consortium has brought many of these companies together to collaborate with one goal in mind: worldwide compatibility of all wireless chargers and wireless power sources. They’ve established a Qi standard, delivering up to 15 watts wirelessly to smartphones. Companies such as Qualcomm are producing wireless charging for electric vehicles. The applications and efficiencies of near-field technologies are growing day by day.
In the far-field range, we have seen fewer steps towards public adoption. Understandably so. Companies like NASA, WiTricity, Ossia, Energous, and Reasonance have all put on demos showing the possibilities of far-field energy transfer, yet none have led to a commercial product.
The progress has been slow. Or at least it was until the past couple of years. New Zealand-based company, Emrod, has made waves in the industry. Their current prototype can transport energy over 40 meters using beam-formed microwaves and rectennas at 70% efficiency. They were able to reach this efficiency using relay devices to maintain the strength of the beam. This is the closest to a commercially viable product in the far-field space and plenty to get excited over.
How can this technology be applied in the future?
Since Nikola Tesla’s days, the world-changing potential of wireless electricity has been evident. We have the technology, it’s just a matter of efficiency. Wireless electricity still has a ways to go before reaching comparable levels of efficiency as a wired connection, however, there are signs that we’re well on our way.
Although having wireless charging topping up our Teslas and iPhones would be nice, the real value of wireless electricity is in far-field technologies.
Wireless electricity is at the forefront of solving some of the world’s most pressing issues. From global equity and damage relief to climate change and transportation, wireless power transfer can change our relationship with energy altogether.
With space travel becoming more accessible, it’s likely we’ll see solar satellite farms collecting energy from earth’s orbit. Solar farms in space can harvest twice as much energy as they do on earth. With wireless power transfer, this energy could be beamed to the places that need it most. Whether that be remote villages, regions ravaged by a natural disaster, or the 13% of the world still lacking access to electricity. These beams can be used to power energy-guzzling planes and ships and would open doors for long-range space travel. Solar satellites and wireless power transfer can make universal, on-demand accessibility to power a reality. But that’s not all.
Possessing the ability to transfer electricity globally opens up entirely new options for renewable energy production. Renewable sources, be it solar farms in space or wind farms in the middle of the ocean, can be consolidated and mass-produce energy. That energy can then be instantaneously transferred to where it is needed. This eases access to renewable sources for regions where it was previously unattainable.
We may not be transferring electricity wirelessly around the world just yet, but we’re well on our way. As each day goes by, our technologies get more efficient and within due time, Tesla’s dreams will be realized. Universal access to power will no longer be an issue of how or a question of when. It will be among us and changing the lives of millions.
