“There’s a lot we can learn from EM waves that can help us in future for energy conservation, generation as well as transmission. This is an area of research that should be picked by universities.”
Where does the energy go when two EM waves interfere?
In radio engineering it’s well known that interference reduces the strength of the signal.
With respect to interference we talk about things like constructive and destructive interference. In constructive interference signals are in phase and reinforce each other.
In destructive interference signals are out of phase and cancel each other.
But when we talk in terms of energy, when we consider electromagnetic waves as propagating energy sources a different question arises
How does interference affect the energy of the wave,if it does at all?
Now it’s quite obvious that unlike signals that can lose information energy can’t be “lost”. However energy can be transformed as well as transferred. In that case does the transfer of energy happens within the wave or between the interfering waves.
Or to put it another way does the wave that carry energy lose it to another in case of destructive interference and gain it from another in case of constructive interference?
This understanding is of great importance in wireless energy transmission systems.
An electromagnetic wave consists of electric and magnetic components. The transformation of energy happens within these components
As shown in this paper by Kirk T MacDonald of Princeton University
http://www.physics.princeton.edu/~mcdonald/examples/destructive.pdf
-
A destructive interference in EM waves doubles the electric energy but destroys the magnetic energy of the wave.
-
A constructive interference doubles the magnetic energy but destroys the electric energy of the wave.
In both cases the total energy of the wave remains constant or conserved.
Effect of transformation on signal
We know that information is encoded in electric field by a modulating current at the transmitter. At the receivers end this electric field produces a current that is demodulated.
Now if we apply these observations to signal theory in radio engineering we find that since in destructive interference the electric component is modified so is the information.
But the loss of Information does not mean loss of energy. As noted, the wave energy is conserved in magnetic/electric fields.
The electrical nature of EM wave
In EM waves there is no electric charge. Just like you can’t get shocked by sunlight you also can’t get shocked by a radio wave.
But just as you can get burned by intense sunlight you can also get burned by radio waves,if the intensity is high enough.
https://physics.stackexchange.com/a/19633
A question arises then ,if there is no electricity in EM waves then what is the E component in EM.
Relationship between electric field and electric charge
That E in EM refers to an electric field. We know that a static Electric field a force. A moving electric field,such as the one in a propagating wave is energy.
More explanation…
It is common knowledge that electric charges create fields. Static charges create static field and moving charges create moving field. This is a well understood principle.
But electric charges are not the only things that create and electric field. A changing magnetic field can also create an electric field. This electric field will affect electric charge but they are not composed of it.
In other words electric field is not the cause of electric charge but the effect. An electric charge has an electric field. A moving magnetic field also has an electric field but without electric charge.
This is the fundamental principle of EM wave propagation where electric and magnetic fields feed off each other.
EM waves are pure energy that can be converted into other forms of energy like heat,or wind or electricity. We’re already familiar with the conversion process as in converting sunlight to electricity, or sunlight to heat etc.
Since energy cannot be destroyed all interference does is change one form of EM energy into another.
From the point of view of wireless transmission of energy the presence of other EM waves will not destroy the energy of the transmitting wave.
workrockin@gmail.com
Can we send electricity through wifi?
In one word, the answer is yes. If you want to understand how please keep reading. The concepts and principles that make Wireless Power Transfer (WPT) work are extremely simple.
Theory
As the name suggests WPT is the transfer of power without the use of wires. It works on the principle of electromagnetic induction. Oscillating electromagnetic fields generate current when received in a conductor. They can travel through free space without any need for a medium like a wave. Sunlight is the most common electromagnetic wave that we experience everyday.
WPT has two forms
-
Near field
-
Far field
Near field power transfer uses magnetic/electrical coupling to transfer power. It works at distances that are very small compared to the wavelength.
You have most probably encountered near field power transfer in induction cookers and more recently in phone chargers.
Near field devices use mostly use magnetic coupling. Power is transmitted via magnetic field over very small distances. If the conductor is not within this distance no power is transmitted.
[At small distances of near field, magnetic and electrical components of the wave can be considered separate. In other words, the electromagnetic wave is not yet radiating ]
Far field devices uses radiating electromagnetic waves to transmit power. Focussed beams tend to behave like light rays. And they can be collected just like we can collect rays of the sun using a mirror on a focal point. High gain point to point antennas can deliver power several KMs apart. Enough to charge batteries.
While far field power transfer is used heavily in communication it’s use in energy transfer is limited.
[Some folks want to send a satellite into space and beam solar energy to earth from space. Due to the distance involved I’m not a big fan of the approach. But often crazy ideas seem to work so who knows?]
A huge business opportunity exists to couple radio wave’s power transmission with alternative sources of energy generation for WPT on ground.
About WiFI in particular
Wifi is an electromagnetic wave. And like all EM waves it carries electrical and magnetic energy. A receiver like a phone connected to WiFi router converts this power into current that is then filtered by the chip to get useful data. This process is called demodulation.
To transmit power over wifi all we need to do is skip the demodulation and store the current from the antenna directly into the battery.
Although any radio wave can be used as a medium for WPT, wifi offers advantages that are hard to beat or even match. They are:-
-
Open spectrum. Wifi does not need any additional licenses from regulatory bodies. Being a part of the ISM band it’s free to use by any one. This makes it friendly towards entrepreneurs and small business owners who would want to start a business in the field of energy. Or maybe researchers/college students who want to experiment in WPT systems to improve efficiency.
-
Ease of learning and operation. Since it’s so easy to get into more people can understand and work with the technology. Making skill acquisition as well as the professional support needed to get the infrastructure working easy. If someone starts a business it won’t be too difficult to find or train staff.
-
Availability. Billions of devices are available to create wifi signals. No investment is needed to create new equipment. Standard off the shelf products are more than competent to do the job. Which is to take an input power source and create a wifi signal to be beamed into free space by a directional antenna.
-
Frequency spectrum. Among all the other alternatives wifi is the best way to transmit long distance power. The frequency is not high enough to suffer from atmospheric attenuation (scientific jargon for power loss), but also not small enough that it needs huge antennas (antenna length is related to frequency/wavelength of the signal) to be of any use.
Difference between home wifi devices and ones used for power transmission
It all comes down to antennas. Home devices use omnidirectional dipoles that scatter the waves. While this is good for communication it’s not efficient for energy transmission.
High gain parabolic antennas improve efficiency significantly for the same devices.
[And before you ask yes these are readily available in the market]
Inside every big problem there’s a small problem trying to get out.
While the objective of WPT seems to be reducing the dependence on wires and batteries, a much more lucrative opportunity exists in integration as well as augmentation of existing systems.
Rather than thinking of WPT as a technology that replaces /disrupts old things it’s much more beneficial to think of it as something that creates new things.
Things like efficient energy transport for alternative sources of energy. Today it does not exist. Energy is everywhere. Wind, sun, tides and hot springs. But it’s not always easy to transport this energy. WPT can provide a solution.
Local energy generated can be transmitted using highly focussed antennas. On the receiver these beams can be collected to generate current that can be stored in batteries.
By the way, this does not mean that wifi can’t transfer traditional energy. It certainly can especially when there’s a surplus that might go to waste. But for traditional sources it can be a bit more expensive.
The thing about renewable energy sources is that they’re always creating power. Sun never stops. Nor does the wind or the waves. They keep on giving energy. More than we can ever use. We just need a way to transmit the power to where it’s needed. As opposed to keeping it where it’s generated.
Microwaves like wifi travel at the speed of light. They may not deliver the same amount of power as wires but they drive energy very fast. Consistently. It adds up.
workrockin@gmail.com
Why can’t devices be powered in the air without plugging in or using wireless charging statio
Actually we’re transmitting power wirelessly all the time. Cellular towers, wifi devices all work on the principle of transmitting electrical power over the air.
When these signals are demodulated in the device the current stream is converted into a digital signal which is then processed by the client.
Therefore we have both transmitters and receivers capable of manipulating energy wirelessly.
However the energy is very small. Comparatively speaking our electrical devices need much more power to operate. Signals over the air decay rapidly as the distance from the source increases.
In effect wireless electrical transfer would only be good at very small distances or the power of radiation would have to be drastically increased.
From this FCC document
“Although the FCC permits an effective radiated power (ERP) of up to 500 watts per channel (depending on the tower height), the majority of cellular or PCS cell sites in urban and suburban areas operate at an ERP of 100 watts per channel or less.
An ERP of 100 watts corresponds to an actual radiated power of 5-10 watts, depending on the type of antenna used. In urban areas, cell sites commonly emit an ERP of 10 watts per channel or less. For PCS cell sites, even lower ERPs are typical. As with all forms of electromagnetic energy, the power density from a cellular or PCS transmitter rapidly decreases as distance from the antenna increases.”
From the point of view of physics you can capture this energy and use to store in a battery, no problem. But it would be a net loss as compared to the power delivered to you with wired lines.
So yes we can charge devices wirelessly.
Useful Trivia
Sun radiates 3.83 x 10^26 watts of energy wirelessly, a small fraction of which we look capture in fixed solar panels.
https://www.quora.com/What-is-the-estimated-power-radiated-by-the-Sun
I think it might not be such a bad idea to create devices that radiate sun’s power as electromagnetic waves that can be stored in remote batteries. Losses here don’t matter because everything is free. We can try to optimize for convenience and create a wireless power distribution system that augments (not replaces) the wired power distribution.
How can we build sustainable communication infrastructure?
Our communication infrastructure today consists in large part of undersea and underground cables. These form what is known as the backbone.
This part of the network I believe is mostly insulated from the shocks and disturbances of nature. Infact nature itself provides that insulation. Tsunami’s, Hurricanes,Cyclones affect only the surface of the earth. Underwater it’s peaceful.
Earthquakes have the potential of damaging the fibre but they too are not always earth shattering. And even when they are they affect a tiny region for a small time. Fibres are strong and have very tough shielding. It can be cut but not smashed so easily.
Fiber optic cables have in fact successfully been used as sensors to detect earthquakes!
https://www.photonics.com/Articles/Fiber_Optic_Cables_Find_Use_as_Seismic_Sensors/a63059
But here’s the kicker, climate change won’t be just one thing. It will be unpredictable. We can’t exactly say which part will be affected by what natural force.
Cell towers and other access points that are “out in the open” will probably take most of the battering. Although they are designed to be weather proof they are still susceptible to extreme weather like heavy rainfall or snow. They need regular maintenance. If nothing else rust will eat them away Any disruption in electricity will also render them useless.
On the other hand there are plans to create an internet backbone in space that will run on the sun’s energy. This architecture won’t be susceptible to natural disasters on Earth. When it turns into a reality, it will make our communication system even more robust.
All of this is speculation of worst case scenario that has not yet happened. A negative scenario is on the opposite end of the positive one. So what’s the positive then?
I believe that the communication industry can lead the change in sustainable technology. How?
All of outdoor mobile access points run on electricity powered by non renewable energy sources. The first step could be to provide alternative sources of power. Solar energy coupled with batteries should be a good start.
For places where solar energy is infeasible portable source of power like bio- fuel or hydrogen cells can be used to generate energy required for operation. The advantage that outdoor cells have is for the amount of service they provide they use very little energy. Almost negligible if you count the per user consumption.
But since the demand for communication technology is huge we need a lot of them. Because we need a lot of them they become great large scale labs for alternative sources of energy not only helping it refine for home usage but also kick-starting an industry by providing a ready made market. For example solar cells may not be adequate for home usage but they are ready for small cell towers.
But that is not going to be enough. The design should evolve to make access points smaller, compact and better adapted to the environment. Rather than setting up big towers we should use what is available to us in plenty. Like trees.
At the moment the trees are being viewed as a hindrance to build the infrastructure. But they themselves can provide the foundation for a new smarter telecommunication infrastructure at a cheaper rate.
[Some radio engineers will argue that trees absorb electromagnetic spectrum because of their high water content. But then water is everywhere. We have 70 percent water in our bodies. We simply can’t do way with it. The problem can be solved with directional antennas and multiple access points serving smaller areas. Or by moving the radios to an indoor structure for wider omnidirectional pattern. The possibilities are many. ]
Climate change is not something that we need to be afraid of. We can instead use it as an opportunity to upgrade our infrastructure to be more smarter and sustainable far into the future. Which at the moment it isn’t.
workrockin@gmail.com
workrock 