# Can 5okW be transferred using resonant wireless power transfer?

I don't mind efficiency or loss, but simply can it be transferred? I'm wondering more about the size of the receiver than the transmitter. If the receiver is positioned within the power transfer field just how fast can the power transfer be initiated? I'm assuming the receiver needs to detect and initiate the resonance to initiate the power transfer and it can start very fast in the order of nano seconds.

• If you don't mind about efficiency ... sure it can. Basically what you are describing here is an extremely large microwave oven. – jippie May 27 '12 at 13:31
• – jippie May 27 '12 at 13:52
• By 5oKw, I suppose you mean 50kW? – starblue May 27 '12 at 17:50
• Yes, technically easy. Just [tm] a matter of engineering. What frequency? Add a resonant load to an EM field and transfer will initiate. Q of loaded circuit probably a major pickup time factor. One cycle is (of course) 1/f_MHz uS or 1/f_GHz ns. At 2.4 GHz 1 cycle ~= 0.4 nS. Size depends on frequency. Higher is smaller. – Russell McMahon May 27 '12 at 21:19
• Looks like it is possible - see my question electronics.stackexchange.com/q/25172/3552 with enough links to pages describing Bombardier PRIMOVE technology. – sharptooth May 28 '12 at 5:52

Your question contains several inconsistancies and things that don't make sense. You apparently want to transfer a sizeable amount of energy wirelessly into a car from some sort of transmitter. You say you want 50 kW energy transfer rate for 2 seconds, which is 100 kJ. Now for the inconsistancies and problems:

1. You say you don't mind "efficiency loss" (inefficiency?) in the question, and that you are "not concerned with energy loss" in a comment. Let me be really clear. At 50 kW efficiency is a big deal. Even if you don't care about wasting energy and its economic and environmental costs, dealing with the heat will be difficult and expensive. Let's say you have 50% efficiency. That means to get 50 kW to the load, you have to supply 100 kW and that 50 kW of that will make something somewhere get hot in a hurry. This is not something you get to ignore.

2. You mention storing this 100 kJ in supercapacitors. Do the math. Today you can get a supercap that is rated for 5 V and 1 F that is around 1 cubic inch in volume, or about 16 ml. The total energy it can store (ignoring that you'll never get all of it back out) is 12.5 J, which comes out to a volumetric energy density of 780 J/l. To store your 100 kJ would require 130 l, or a cube about 1/2 meter on a side. Note that this doesn't account for mountings, invevitable spaces between individual caps, interconnects, room for airflow or some other means of cooling, etc.

3. You basically want to create a transformer between a primary burried below the road surface and a secondary in car above the road surface. There will be some inevitable minimum separation between these, and the magnetic leakage field will be large. Anything in the vicinity that can conduct electricity, like the metal care frame and other mechanical structures, are going to have eddy currents created in them. A large metal object like a car frame can dissipate this power for a while, but it will rob power from the magnetic field and lower the Q of any resonance you hope to set up.

Put another way, the intended pickup coil will only be a small part of what the magnetic field produced by the transmitter "sees", so it's characteristics won't have a large effect on overall operation and won't intercept a large fraction of the total power.

4. The inevitable large transmitter field is going to effect everything in its vicinity, including any people in the car. I'm not going to sit my butt in a 100 kW (at least) magnetic field. Even if you can shield occupants and only .1% of the power gets to them, that's still 100 W. Have you done careful studies what a 100 W magnetic field at 10s to 1000s of MHz does to a human body? Even microamp currents in the wrong place in the brain or other parts of the nervous system could possibly cause trouble. And what about long term effects? Can you show that repeated exposure over 10s of years is safe even if a single exposure appears to cause no immediate harm? I didn't think so. There is still legitimate scientific debate over whether a cell phone routinely held to your ear has any long term effect, and that's a rather lower power level and is largely a electric field.

I'll make my comment an 'answer'. May get modified if questions get answered.

Yes, 50 kW is technically easy.
Just [tm] a matter of engineering.
Engineering may ir may not be easy :-).

What frequency?

Add a resonant load to an EM field and transfer will initiate.
Q of loaded circuit probably a major pickup time factor.

One cycle is (of course) 1/f_MHz uS or 1/f_GHz ns.
At 2.4 GHz 1 cycle ~= 0.4 nS.

Size depends on frequency.
Higher is smaller.

• Thanks! So the transfer needs to take place in 2 seconds, but the EM field needs to be large (assume 3ft circle) and the receiver about the size of an A4 piece of paper. Could that be constructed? – Anthony Jun 2 '12 at 12:18
• Whoa! What transfer? ie if you say "the transfer" and "2 seconds" then it is not certain you MEAN 50 kW. If something is rated at 50 kW transfer capability then it produces 50 kW for 0.01 seconds, or 1 second or 1 hour etc. Power is not energy. Energy = Power x time. If you have a specific amount of energy to transfer, say so. Tell us what you want and we'll tell you what you need. Veering towards "telling us how" can lead you (and us) astray. – Russell McMahon Jun 2 '12 at 15:10
• And :-) - You say "must be large" and "size of ...A4". This implies you have firm constraints in mind and that there may be issues that you are no mentioning. An assumption from this is that you want the A4 to be anywhere in the 3 foot circle. Next you'll say orientation is arbitrary, enclose it in mumetal, pot it in concrete and drop it in the ocean. Yes ? :-). Why not describe what you actually are trying to do well enough that we don't have to do the 1000 guesses game? – Russell McMahon Jun 2 '12 at 15:14
• Whoa Russell, yes eeek, reading my stuff back, yes obviously I'm carrying the picture in my head, sorry :) I'm trying to discover any practical reasons why we can't combine an Ultra Capacitor (UC's) charging a Lithium Battery for a Nissan leaf 25kW battery over a wireless power transfer mechanism. (I have a Nissan Leaf). I'm interested in UC's as a means of wirelessly capturing energy very rapidly and charging the battery over and above its current rate of consumption as it passes through the EM field. – Anthony Jun 3 '12 at 5:31
• The accuracy of the car being located over the transmitted is not guaranteed and such be an assumed factor hence suggesting a resonance approach. So what I am looking to establish is just how many miles of energy can I capture with a short pause of only two seconds. MIT are looking into continuous wireless charging, as am I, but I'm looking to establish the design factors that may cause misgivings in this approach. – Anthony Jun 3 '12 at 5:32