Electric Motorglider Flies

Larry Dighera wrote:
On Tue, 15 Apr 2008 01:05:04 GMT, wrote in
:


Despite the fact that electric motors must use iron/steel in their
construction, they are significantly lighter (50%) than their
internal combustion counterparts. But when the wiring, controls,
batteries and perhaps fuel-cells are considered, I would guess the
weight of an electrically powered aircraft would be roughly comparable
to one powered by an internal combustion engine. So, with
significantly less power/energy density than gasoline, batteries will
not provide the same range/duration until they are improved further.
But it is encouraging to see progress being made at last.

Not going to happen.


I hesitate to attempt to infer your meaning in that phrase, but if you
mean Li-ion batteries, perhaps. If you're referring to electrically
powered aircraft, they have already happened, and development is
progressing.

Energy densities

fuel MJ/kg MJ/L

JET-A 43 33
ethenol 30 24
Li-ion battery (projected) 1 2
NiMH battery .2 .4
ultracapacitor .02 .05

Regenerative fuel cell come in a bit under 2 MJ/kg.

http://en.wikipedia.org/wiki/Energy_density


Thank you for the factual data. It's interesting that gasoline is
omitted:

As were many other things.


http://hypertextbook.com/facts/2003/ArthurGolnik.shtml
Liquid Fuel MJ/litre litre/Tonne GJ/Tonne MJ/kg
Gasoline, aviation 33.0 1412 49.6 36.4


Here's a little more data on Li-ion cells:

http://en.wikipedia.org/wiki/Lithium_ion_battery
Specific energy density: 150 to 200 Wh/kg (540 to 720 kJ/kg)
Volumetric energy density: 250 to 530 Wh/l (900 to 1900 J/cm?)
Specific power density: 300 to 1500 W/kg (@ 20 seconds and 285
Wh/l)



There's a great comparison chart of energy densities he

http://en.wikipedia.org/wiki/Energy_density

Which is where the above came from.


Here are a few of the entries:

Storage Type Energy Density By Mass (MJ/kg)
================================================== ================
lead acid battery 0.090.09?0.11[36]sm=n
lithium ion battery-present capability 0.230.23?0.28
lithium ion battery-predicted future capability 0.540.54?0.9sm=n
Regenerative Fuel Cell (fuel cell with internal Hydrogen reservoir
used much as a battery) 1.62
Lithium ion battery with nanowires 2.54-2.72
TNT 4.184
dry cowdung and cameldung 15.5
calcium (burned in air) 15.9
PET pop bottle plastic 23.5?23.5
ethanol 30
aluminum (burned in air) 31.0
Jet A aviation fuel 42.8
gasoline 46.9
compressed natural gas at 200 bar (2,900.8 psi) 53.6
compressed hydrogen gas at 700 bar (10,423.5054 psi) 143
Enriched uranium (3.5% U235) in light water reactor 3,456,000
nuclear fission (of U-235) (Used in nuclear power plants)
88,250,000


From the data in the chart it would appear that a best-case Lithium
ion battery with nanowires (2.54-2.72 MJ/kg) that would provide the
equivalent energy of a given amount of gasoline (46.9 MJ/kg) would
weigh 17.24 times as much as the gasoline it replaces. That doesn't
look too terribly feasible for aviation use. Oh well....

Or any other vehicle.

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.

However, hydrogen gas compressed to a pressure of ~10,500 psi (143
MJ/kg) would only weigh ~1/3 as much as the equivalent gasoline energy
it replaces. If that hydrogen were used along with atmospheric oxygen
to produce electricity by a fuel-cell with a typical efficiency of
~36% http://en.wikipedia.org/wiki/Fuel_cell#Efficiency, and the
efficiency of the electrical motor, wiring, and controller were 90%,
and the weights of the total systems were roughly equivalent, it would
appear that there would be a close approximation of performance of
today's aircraft including waste heat, but not noxious emissions nor
noise. I'm not sure exactly how the overall efficiency would be
affected by the use of pressurized oxygen, or if both the hydrogen and
oxygen were produced by the electrolysis of water by photovoltaics.
(Now, if the compressed hydrogen were carried in a tubular wing spar,
imagine it's rigidity... /dream mode)

You are forgetting about the enormous weight of a tank capable of
containing hydrogen at 10,500 psi as well as the problem of hydrogen
embittlement at those pressures.

The very last thing you would want to do is put it in a wing spar.

Of course these rough theoretical calculations are predicated on
existing technologies, and don't consider the inevitable future
technical advancements.

Which are no better than a wish and a hope in the real world.


Thank you for providing the catalyst that led to this insight into the
issue.


Electricity is great stuff, but damn awkward to carry around.

So it appears.

--
Jim Pennino

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