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Wireless Charging
- Thread starter Doug_G
- Start date
Wow!!
his calculations showed that a set of resonant coils and discs could transfer about 10 kilowatts with a 97 per cent efficiency within 7 microseconds
his calculations showed that a set of resonant coils and discs could transfer about 10 kilowatts with a 97 per cent efficiency within 7 microseconds
This is either meaningless or inscrutable:
Does this mean that there is a 10kW charging current applied for 7 microseconds? If so, that's a insignificant 0.0194 watts-hours of power added to the battery, which can't be right.
Does this mean the coils can transfer 10 kilowatt-hours in 7 microseconds? If so, that's a jaw-dropping 5.14 MW rate, which can't be right.
So what does this statement in the article mean?
They were talking about an amount of transfer from my interpretation of the article which would point to your second calculation as I did. Something is misreported I believe.
I took it to mean the power transfer starts within 7 microseconds (when you drive over a coil), not that it completes within that time. But I agree that it is badly worded.
Hi,
I'm not so sure it applies to the time to start an energy transfer.
Below is a link to an undated white paper by Prof. Fan and others.
In it they describe two types of simulations. The first is with dielectric discs which they claim to have an energy transfer efficiency of nearly 100%. The second simulation uses conducting wire loops in which they state "Even without the optimization of the device design, the energy transfer efficiency is shown to be around 85%. The energy exchange time is about 10 microseconds."
Wireless Power Transfer to Moving Vehicles
Larry
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I'm not terribly impressed by simulations. In this sort of device second and third order effects can be very important. If you leave anything out of your model the result can be quite unrealistic.
Hi Doug,
Let's suppose for the discussion purposes that second and third order effects are negligible. If Robert's calculation is correct wouldn't it take an impractically large number of coil to coil energy transfers to result in a useful amount of charging?
Thanks.
Larry
It's hard to comment without knowing how their scheme is intended to work, but the large number of coils is probably just the beginning.
- A car moves one meter (39") in 30-odd milliseconds at highway speeds. That's not a lot of time for coils to be aligned (maybe that's where the 7 microseconds comes from?)
- If the coils aren't physically large, then lateral alignment will also matter
- Large coils don't like to run at high frequencies
- Coil resonant frequency is sensitive to temperature, both in the car and the pavement, which can be at different temperatures and have different response curves
- Steel is a lossy material and can alter the magnetic circuit. Aluminum produces a scattering field at a different phase. Both metals can detune the coils. Since every car is different there will be a different impact on the roadway coils.
- There would be a heck of a lot of electronics and wiring under the pavement
I just can't see how laying sensitive, high-voltage electronics underneath this is going to be a practical model -- sounds like "great in theory, horrid in practice" ideas. Here in New England, they can't keep a normal asphalt interstate highway in decent condition for more than about three years without resurfacing (scrape off about 2cm of asphalt, rough it up, relay new). Salt-laden water everywhere for four months of the year. This does not sound like a happy environment for delicate high-voltage equipment.
Hi Doug,
Perhaps I have made a mistake in my math or reasoning, but even without the complications that you mention I figure that at 70 mph a Roadster expends 300 Wh/mile. If Robert's .0194 watt-hour per energy transfer is correct then it would take about 15,500 transfers per mile to equal the discharge rate. So that would require coils being buried at least every four inches.
Larry
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Yeah. That's what I meant by "a heck of a lot"!
Robert.Boston also has a great point. Every spring we're "Potholes 'R Us" around here. One good January thaw can wreak havoc with the roads. A few years back the weather kept alternating every two weeks between deep-freeze and thaw, and they were still fixing roads the following year.
Hi Robert,
Why? It's not the first time an EV manufacturer decided to go with a proprietary charging system independently of the SAE. :wink:
Larry
