On Wed, 16 Apr 2008 11:07:53 +0000 (UTC), Dylan Smith
wrote in
:

On 2008-04-15, Larry Dighera wrote:
Another limitation is that for something the size of a C-172, your
battery has to deliver around 120 kW to get off the ground and
climb to altitude.

I don't see that fact as being too limiting. Why do you feel that's
an issue?

120kW, or about 160 horsepower, at any sane voltage is going to be a
tremendous amount of current. If your supply voltage to the motor was
600 volts, you'd need to deliver 200 amps. This requires a serious piece
of cable to do efficiently (i.e. without getting insanely hot).

I would use bus bar instead of cable. Here's a chart showing the
ampacity for copper bus bar:
http://www.stormcopper.com/design/Am...uick-Chart.htm
It indicates that 1/4" X 1" copper would conduct 400 amps with a 30 °C
temperature rise.

It also needs batteries or a power source with a very low resistance to not also
get very hot.

Internal resistance is a serious issue, and deserves serious
consideration. The battery internal resistance is considerably better
than that of the fuel cell. From the battery specification sheet here
http://a123systems.textdriven.com/product/pdf/1/ANR26650M1_Datasheet_MARCH_2008.pdf
it would appear that it's not impossible (10 m0hms typical). The fuel
cell internal resistance is considerably higher, but I took that into
consideration in my previous rough calculations.

With typical high current motive applications like trains
or cars you can just add more metal to the conductors to the motors. You
have a weight issue with aircraft, though, with both the control
circuitry and the high voltage, high current wiring.

That is true. To decrease conductor weight silver might be
substituted for copper, but it would only provide about a 10%
improvement.

It seems fuel-cells have a rather high internal resistance at high
currents. That's where most of the losses will be.

Of course gasoline also requires tanks, but they are often just sealed
parts of the wing structure, so their weight isn't really significant.
I don't know the strength of carbon-fiber or Kevlar composite, but
pressure cylinders constructed of them are about 60% lighter than
comparable Al cylinders

It's not just the tanks - you also have to make an idiot proof fuelling
system that can be operated by the typical 17 year old line boy, but is
capable of handling *five tonnes* per square inch of pressure.


It would appear that has already been done:
http://www.fuelcells.org/info/charts...ngstations.pdf
http://www.sciencedirect.com/science...1eb7fca01a8c1d
http://biz.yahoo.com/prnews/080331/lam082.html?.v=101
http://www.ieahia.org/pdfs/honda.pdf

To put that into perspective, that's like two SUVs sitting on each
square inch of pipe, connector and tank. Without even considering the
energy content of the actual fuel, the potential energy of even an
inert gas at those sorts of pressure would result in very bad stuff
happening if someone got careless with the fuelling equipment.


It's currently being done, so evidently the technology exists.

While the engineering challenges can be solved, it's never going to be
anything remotely resembling low cost due to the enormous pressures
involved, and the safety issues with handling anything at those enormous
pressures.

It appears that it's currently cost effective enough to be viable in
the marketplace. Perhaps you are able to provide some information
that supports your assertion.

Thanks for your input, Dylan.