# Wireless transmission of energy

• physior
I'm not sure where that came from.In summary, wireless power transmission is becoming increasingly efficient, but there are still some ways to increase the power transfer.f

#### physior

hello!

as for wireless transmission of energy, what is the efficiency and how far the item can be?

thanks!

hello!

as for wireless transmission of energy, what is the efficiency and how far the item can be?
I hesitate to give numerical values. But I will say that the technology has advanced greatly over the last decade or so.

Two aspects are important to achieve better efficiency:
• Keep the effective transmission and reception coils as closely, spatially aligned as reasonably possible. This might ("might" is important here; not all systems do this) involve electronic circuitry that effectively changes the properties of the coils' position/alignment/radius, or switch between which coils are active and which are idle.
• Maintain a high resonance. This may involve active circuitry to maximize resonance between the transmitter and receiver coils.
Regarding the second point. Imagine a large pendulum across the room and you wish to transfer energy to this pendulum through a slinky. You can hold one side of the slinky and shake it; the other side of the slinky is attached to the pendulum.

If you just shake the slinky wildly, most of the energy you put into it is wasted, and very little gets to the pendulum.

If instead you carefully time you pulling and pushing to match the natural motion of the large pendulum, in time, after many pushes, you can transfer quite a lot of energy to the pendulum through the slinky (similar in a way to how you need to time your "pushes" correctly when pushing a child on a swing). It may take many, small pushes for the pendulum to gain appreciable movement, but in this way most of the energy of each push makes it to the pendulum (and is not wasted).

It is maintaining the resonance that is key to efficient, wireless energy transfer. [Edit: particularly, this resonance is the key if the coils are not so tightly coupled.]

I would start by googling terms like "wireless charging," "inductive charging," "magnetic resonance," "Qi inductive power standard"

Last edited:
hello!

as for wireless transmission of energy, what is the efficiency and how far the item can be?

thanks!

With coupled coils it works only at relatively small distances (distances up to 5x dimension of receiver/transmitter with efficiency up to 50%).
At much higher frequencies, and other methods, like with microwave beam transmission technologies, distance can be quite respectable.

Last edited:
hello!

as for wireless transmission of energy, what is the efficiency and how far the item can be?

thanks!

Most of the 'wireless' devices work in the 'Stored' reactive near-field energy region. One of the reasons wired energy transfers are usually very efficient is that energy is transferred by these reactive (non-radiative) fields that surround the wiring using the free charges in the conductors to guide (direct) energy from source to destination. The resistive and dissipative parts of the total transmission impedance is low so the real power losses are low. As distances are increased to the intermediate range the real parts of the impedance increase so real power losses increase.

http://research.microsoft.com/pubs/70504/tr-2007-143.pdf

People always talk of high Q circuitry to provide good coupling but, if the system is to transmit significant power (to be efficient - whatever that term means in this context), how can the necessary loading and Power throughput, give you high Q?
I do know that you can get ridiculous 'locking' and beating effects between two very high Q oscillators, separated by huge distances - but there is very little power transfer in that sort of situation.
I think it would be a good thing if there could be some solid definition of what is actually meant by 'wireless power transmission' and the sort of distances and powers involved. It is true that 'contactless' charging and powering of circuits is very achievable and also a very useful thing for us to look forward to (I love my toothbrush charger) but that is very much a half way house towards what many people seem to be envisaging. (That danged Nikola T raises his head again)

I can't help think that wireless transmission of power is going to be bad for the planet. Can we afford the efficiency of all our modern gadgets to drop from say 80-90% to 50%? Should governments set a minimum efficiency level?

billy_joule
(That danged Nikola T raises his head again)

AAAAARGH!

Drakkith
I can't help think that wireless transmission of power is going to be bad for the planet. Can we afford the efficiency of all our modern gadgets to drop from say 80-90% to 50%? Should governments set a minimum efficiency level?
The term 'Gadgets' presumably doesn't apply to washing machines, cookers and Immersion heaters so perhaps the wasted few Watts needn't bother us too much. It'll probably be made up for by less energy hungry equipment. I remember a massive hooha about equipment being on standby and the losses that could involve. A decent circuit for standby level should take less than a mW - how does that compare with opening the front door on a cold day?
Shale gas will take care of everything, in any case - and then we'll have Fusion power. The US will sort us all out.

can materials be interject ? what ?

what is this exactly?

I can't help think that wireless transmission of power is going to be bad for the planet. Can we afford the efficiency of all our modern gadgets to drop from say 80-90% to 50%? Should governments set a minimum efficiency level?
But the technology has improved greatly in recent years. Already, commercial electric automobile wireless charging is quite a bit better than that, and might get better still.

“We’ve been consistently upping the system’s performance over the past year, and are now in possession of a prototype that is able to transmit three kilowatts (kW) at an overall efficiency of 95 percent. Today’s electric car models can be recharged overnight,”
http://www.fraunhofer.de/en/press/r...ging-electric-cars-efficiently-inductive.html

To me, overall 95% sounds a little optimistic. I don't know if I'd bet on that. But still, it's pretty clear that wireless power transfer technologies have come a long way.
---
Edit: Here's another link that might be more believable than the first:

"Partnering with the Energy Department, Oak Ridge National Laboratory has demonstrated a wireless charging unit with 85 percent system efficiency -- meaning only 15 percent of the energy is lost in the transfer from the grid service to the vehicle’s battery when charging wirelessly."

Last edited:
Already, commercial electric automobile wireless charging is quite a bit better than that, and might get better still.
It's more 'contactless' than 'wireless', surely. Electrical isolation is a good idea (like the toothbrush charger) but it ain't going to work from your house, across the pavement (=sidewalk) to your car, is it? At least without spraying kW of power out into the street.

It's more 'contactless' than 'wireless', surely. Electrical isolation is a good idea (like the toothbrush charger) but it ain't going to work from your house, across the pavement (=sidewalk) to your car, is it? At least without spraying kW of power out into the street.
The most common wireless automobile chargers on the market today are undercar. The transmitter is located in the driveway or parking space and the receiver is mounted under the car. Here is an example:
https://pluglesspower.com/chevy-vol...SOfo95ep-i97GjNGSn-_cwpTDDZVvgnM8IaAkcB8P8HAQ

For what it's worth, I still charge my car with an old fashioned cord. I use a cord for my cell phone too. Sure I may be lazy, but I'm not that lazy.

Then again my toothbrush is charged wirelessly and I wouldn't want to go back to a toothbrush charging cord. Perhaps in the future I'll change my mind about the car and cell phone.

Then again my toothbrush is charged wirelessly and I wouldn't want to go back to a toothbrush charging cord.

It's worth taking a moment to think about just how bizarre that sentence would have sounded just two generations ago...

collinsmark
It's worth taking a moment to think about just how bizarre that sentence would have sounded just two generations ago...
Most people didn't have a toothbrush in those days (gives a gummy smile
But seriously, those were the days before strong permanent magnet motors and small rechargeables. I think the actual toothbrush charging circuit could have been achieved long before the electric toothbrushes came on the market. I'd not be surprised to find a diagram in some sketch book of the blessed Nicola's. (ARRRRGH, again)

I'd not be surprised to find a diagram in some sketch book of the blessed Nicola's. (ARRRRGH, again)
The artistic drawing of Tesla's Long Island transmitter is simply funny.
Note just those people peacefully walking in close vicinity of the tower while the multimegavolt facility is in operation (Arrrggh!)
Tesla's patent which can be related to that thing: http://www.keelynet.com/tesla/01119732.pdf

People always talk of high Q circuitry to provide good coupling but, if the system is to transmit significant power (to be efficient - whatever that term means in this context), how can the necessary loading and Power throughput, give you high Q?
I do know that you can get ridiculous 'locking' and beating effects between two very high Q oscillators, separated by huge distances - but there is very little power transfer in that sort of situation.
I think it would be a good thing if there could be some solid definition of what is actually meant by 'wireless power transmission' and the sort of distances and powers involved. It is true that 'contactless' charging and powering of circuits is very achievable and also a very useful thing for us to look forward to (I love my toothbrush charger) but that is very much a half way house towards what many people seem to be envisaging. (That danged Nikola T raises his head again)

When there is a "transmitting" and a "receiving" circuit, we may regard the coupling between them as due to a mutual impedance - an impedance which is in both circuits. The mutual impedance defines the amount of coupling. If the energy is transferred by the magnetic field, like a transformer, the mutual impedance is an inductance. If the energy is transferred by radiation, the mutual impedance will have a resistive component. If energy is transferred by radiation, the maximum efficiency is 50%.

If energy is transferred by radiation, the maximum efficiency is 50%.
Hmm, I beg to differ. Will you elaborate?

He's talking about the Maximum Power Transfer theorem, I suspect. But transmitters do not usually operate that way.

Hmm, I beg to differ. Will you elaborate?
With a simple receiving antenna, half the power is absorbed by the load and half is re-radiated. This occurs because, when a current flows in the receiving antenna, it radiates.
For the case of two dipoles, the path loss will never fall below 3dB even when they are actually touching. I agree that a more complex receiving antenna can avoid this defect. It is the same reason that resistive paint does not provide complete absorption of radio waves.

The Wardencliff Tower was not wireless power distribution. It used the Earth as a wire.

As far as wireless power transmission goes, Bingnan Wang has achieved the highest efficiencies I know of using a metamaterial array.

https://merl.com/publications/docs/TR2014-096.pdf

Wow, 50% efficiency for energy transfer before the powered device even uses it. The utility companies would love this to happen past the wired power meter to your house.