Electric Motorglider Flies

On Tue, 15 Apr 2008 17:45:04 GMT, wrote in
:

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?


However, hydrogen gas compressed to a pressure of ~10,500 psi [700 bar] (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

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
http://www.mhoxygen.com/images/Cylinder-dimensions.pdf. It would
appear that carbon fiber or Kevlar composite pressure cylinders may be
strong enough to contain the high pressure.

There's a tensile strength chart he


http://en.wikipedia.org/wiki/Tensile...sile_strengths

Material Ultimate strength (MPa) Density (g/cm³)
================================================== =========
Steel,
high strength alloy
(ASTM A514) 760 7.8
Carbon Fiber 5650 1.75


as well as the problem of hydrogen emb[r]ittlement at those pressures.


From the information at the link below it's not clear if carbon
composite materials are subject to hydrogen embrittlement.

http://en.wikipedia.org/wiki/Hydrogen_embrittlement
Process
The mechanism begins with lone hydrogen atoms diffusing through
the metal. When these hydrogen atoms re-combine in minuscule voids
of the metal matrix to form hydrogen molecules, they create
pressure from inside the cavity they are in. This pressure can
increase to levels where the metal has reduced ductility and
tensile strength, up to the point where it cracks open ("Hydrogen
Induced Cracking", or HIC). High-strength and low-alloy steels,
aluminum, and titanium alloys are most susceptible. Steel with a
ultimate tensile strength of less than 1000 MPa or hardness of
less than 30 HRC are not generally considered susceptible to
hydrogen embrittlement.



However according to the articles below, hydrogen embrittlement
doesn't seem to be an issue with carbon fiber composite cylinders:

http://en.wikipedia.org/wiki/Hydrogen_tank
A Hydrogen tank (other names- cartridge or canister) is used for
hydrogen storage, most tanks are made of composite material
because of hydrogen embrittlement. Some tanks are used for fixed
storage others are exchangeable for refueling at a hydrogen
station[1].


http://www1.eere.energy.gov/hydrogen...s/32405b27.pdf
The 5,000 and 10,000 psi tanks developed by QUANTUM Technologies
have been validated to meet the requirements of DOT FMVSS304,
NGV2-2000 (both modified for 10,000 psi hydrogen) and draft
E.I.H.P standard. Typical safety tests completed, in order to
ensure safety and reliability in an automotive service environment
included: Burst Tests (2.35 safety margin), Fatigue, Extreme
Temperature, Hydrogen Cycling, Bonfire, Severe Drop Impact Test,
Flaw Tolerance, Acid Environment, Gunfire Penetration, Accelerated
Stress, Permeation and Material Tests.


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

Why do you believe that is true?


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.


You've got to start somewhere, right?