CAFE Electric Aircraft Symposium Set For May 1

On Fri, 8 May 2015 01:39:59 +0000 (UTC), Skywise
wrote:

Larry Dighera wrote in
:

I hadn't considered the explosive environment created by venting liquid
hydrogen. How is that dealt with by suppliers, laboratories and users
today?

Perhaps the venting H2 could be captured and run through the fuel-cell
and the resulting electric power stored in batteries for future use to
preclude the explosive atmosphere forming.

Just more equipment that can fail and adds weight and cost. I'm
sure the airlines would like that. Or, would you like that on
your car?


I wasn't considering electric airliners yet; I was thinking more along the
lines of GA-sized aircraft. I'm still curious how venting H2 is kept from
producing an explosive atmosphere in laboratories and at the gas plants where
it is produced. Surely the technology exists...


I'm wondering if the heat produced by a fuel-cell could be used to
change the liquid H2 into the gaseous phase, and if the resulting
cooling of the fuel-cell will contribute to its efficiency.

Why would you want to heat liquid H2?

I was thinking it might be necessary to heat the LH2 so that it could keep up
with the fuel demand of the fuel-cell producing the power to produce the motive
thrust.

The reason tanks vent is because it's boiling off. It's very difficult to
insulate a tank to LH2 temperatures, so some of it boils off.

It would be interesting to know just how difficult it is to insulate a LH2
vessel, so that the boil-off rate is reasonably slow.

If you don't vent it... KABOOOM!

Yeah. I recall the resounding POP that occurs when a burning splint is
inserted into the inverted test tube containing the evolved H2 from
electrolysis. It's a definite issue, as is the explosive atmosphere created by
venting gasoline vapors. But, obviously methods have been successfully
developed to deal with it.


Remember Challenger? That's what happens when the tank breaches.


My recollection was that the seals on the Solid Rocket Boosters on the sides of
the big central O2-H2 tank had failed, and the hot SRB gases had breached the
big tank. I wasn't aware of an H2 venting issue.

Speaking of rockets, ever notice how they are constantly venting
while on the pad? They close the valves just before lift-off. If
the launch is delayed the valves are reopened to prevent too much
pressure from building. After launch it's not a problem because
the fuel is being consumed fast enough.


Agreed.

Ever notice the main rocket nozzles suddenly ice-up shortly after ignition? I
believe that's a result of the cold liquid combustion gases being routed
through tubing coiled around the rocket motors to keep them from melting and
assist in atomizing the gases, so that they will react more readily. Just a
guess.

Personally, I find the whole argument on hydrogen as a replacement
for gasoline a joke.

The limited research I conducted years ago seemed to suggest that there wasn't
much else that approached the energy density of gasoline/kerosene. So, while
perhaps not ideal, hydrogen is a somewhat viable alternative to petroleum, that
has the potential to provide efficiencies several times better than the ~30%
efficiency obtained with internal combustion power plants. If you consider
that only one third the fuel will be required to achieve the current
performance, the numbers begin to make more sense.

With 70% of the energy blowing out the exhaust as heat, internal combustion
engine efficiency is comparable to an incandescent lamp that consumes ~90% of
its energy usage to produce heat, and only ~10% to produce light. LEDs, on the
other hand, can be 90% more efficient than tungsten filament lamps, and they
last many times longer too.

It's basic physics. So unless the laws of physics go out the window....

I understand what you are saying, and I agree; the solution isn't obvious, but
it may be possible. Apparently a lot of large commercial entities seem to
think so...

Well, there are those who think science and basic physics are a conspiracy to
keep the truth from being revealed... But I'm not assuming anyone here is in
that camp. Yet.

Brian


Hey. Let's leave T. Cruz and Santorum out of this discussion. :-)


--
Irrational acts are ultimately founded on irrational beliefs.
-- Larry Dighera