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CAFE Electric Aircraft Symposium Set For May 1



 
 
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  #31  
Old June 3rd 15, 11:29 PM posted to rec.aviation.piloting
[email protected]
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Posts: 2,877
Default CAFE Electric Aircraft Symposium Set For May 1

Skywise wrote:
Larry Dighera wrote in
:

Hello Brian,

You seem to have firm grasp of the physics involved. Are you able to
suggest how efficient an electric power system powered by LH2 would have
to be to offset the power density difference from gasoline/kerosene?


A quick disclaimer.... I don't have any degrees in this stuff or
work with it. I just happen to be very interested in the sciences
in general and have taught myself a few things over the years.
Having said that, I try very hard to check my facts and figures
before I say anything. I have an allergy to foot-in-mouth disease.

On to your question....

Per numbers in http://www.tinaja.com/glib/energfun.pdf

Liquid H2 has an energy density of 2600 Watt-hours/liter.
Gasoline is 9000 Watt-hours/liter.

[I used energy density per volume rather than per mass
because that's the limiting factor in any vehicle, the
volume of the 'gas tank']

If we make an assumption for discussion's sake that an LH2
powered system were 100% efficient, then the gasoline system
would only need to be 2600/9000 = 29% efficient to reach
parity with LH2. But note that nothing is ever 100% efficient.
There are _always_ conversion losses. It's a matter of how
much.

Per:
http://en.wikipedia.org/wiki/Interna...rgy_efficiency

Engine efficiency is limited by thermodynamic laws. "Most steel
engines have a thermodynamic limit of 37%." Further, "most engines
retain an average efficiency of about 18%-20%."

Right away we see it's at least potentially possible for gasoline
to still beat out 100% efficient LH2. But let's go on the low
side and assume a gasoline engine is 18% efficient. Then we need
to figure out the efficiency required of an LH2 system to beat
gasoline:

9000 * 18% = 1620
1620 / 2600 = 62%

Therefore an LH2 system would have to be 62% efficient overall to
beat a typical gasoline engine.

Per the same Wikipedia article, "Electric motors are better still,
at around 85%-90% efficiency or more, but they rely on an external
power source (often another heat engine at a power plant subject to
similar thermodynamic efficiency limits)."

OK. So an electric motor _by itself_ is more than efficient, but
as stated it has to get it's electricity from somewhere else. We
are assuming an LH2 powered source.

Let's go with the high side of 90% on the electric motor. So we
have to now figure out what efficiency is required in converting
LH2 to electrity so a 90% efficient electric motor produces
1620 Wh/l of LH2...

1620/.90/2600 = 69%

Now that leaves us with finding out how efficiently LH2 can be
converted to electricity.

Per: http://energy.gov/eere/fuelcells/fuel-cells

"Fuel cells can operate at higher efficiencies than combustion
engines, and can convert the chemical energy in the fuel to
electrical energy with efficiencies of up to 60%."

So we may be coming up a bit short.

However, all my pondering here is surely a gross oversimplification.
And it's possible I goofed on my math or went astray with my logic.
And I imagine different sources will give different numbers. But I
hope it gives you some idea. There are surely other factors that
need to be taken into account. Some may make things work out better,
others may make things worse.

Brian


The efficiency numbers look to me to be back of an envelope correct.

The real world, however, has very little interest in the energy efficiency
of things like airplanes.

Some numbers that people care about are endurance, operating cost. initial
cost, and life time maintenance cost.

Given some reference platform, such as a C-172, what would be the enduraance
of a LH2 system for that platform versus gas?

How much does 1 hour of LH2 cost versus gas?

In what column do we put the typical 1%/day evaporation loss of LH2 and
the venting equipment you would have to have in a hanger to get rid of
it?

We can swag what a LH2 system would cost from commercial stuff, but how
much of an adder will aircraft certification cost?

LH2 tanks have limited life; inspection and replacement costs?

Energy efficiency is nice to talk about, but it is dollars that make
things happen.


--
Jim Pennino
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  #32  
Old June 4th 15, 06:55 AM posted to rec.aviation.piloting
Skywise
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Posts: 140
Default CAFE Electric Aircraft Symposium Set For May 1

wrote in :

The efficiency numbers look to me to be back of an envelope correct.


And that's all I guarantee them for!

I was about to do some more calculations to try to answer your
questions, and in the process of searching for numbers I found
the following Wikipedia article.

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

At a glance this article seems to cover a lot of the issues
being discussed. I for one would like to read it before trying
to answer. It might be worth other's to read as well.

Brian
--
http://www.earthwaves.org/forum/index.php - Earth Sciences discussion
http://www.skywise711.com - Lasers, Seismology, Astronomy, Skepticism
Sed quis custodiet ipsos Custodes?
  #33  
Old June 4th 15, 06:11 PM posted to rec.aviation.piloting
[email protected]
external usenet poster
 
Posts: 2,877
Default CAFE Electric Aircraft Symposium Set For May 1

Skywise wrote:
wrote in :

The efficiency numbers look to me to be back of an envelope correct.


And that's all I guarantee them for!

I was about to do some more calculations to try to answer your
questions, and in the process of searching for numbers I found
the following Wikipedia article.

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

At a glance this article seems to cover a lot of the issues
being discussed. I for one would like to read it before trying
to answer. It might be worth other's to read as well.

Brian


After reading that article, I have come to the conclusion that hydrogen
as fuel will become a general economic reality only if at least one of two
things happen:

The world runs out of petroleum, natural gas, and coal and there is no
other choice.

There is huge advancement in electricity production, such as cheap
fusion.

http://en.wikipedia.org/wiki/Hydrogen_economy#Costs


--
Jim Pennino
  #34  
Old June 4th 15, 09:37 PM posted to rec.aviation.piloting
Skywise
external usenet poster
 
Posts: 140
Default CAFE Electric Aircraft Symposium Set For May 1

wrote in :

After reading that article, I have come to the conclusion that hydrogen
as fuel will become a general economic reality only if at least one of two
things happen:

The world runs out of petroleum, natural gas, and coal and there is no
other choice.

There is huge advancement in electricity production, such as cheap
fusion.

http://en.wikipedia.org/wiki/Hydrogen_economy#Costs

I haven't finished the article, but that's also been my general
opinion.

Your item number one has a second ramification. Fossil fuels are
currently the primary mineral source for hydrogen production. So
running out of fossil fuel means switching to hydrogen requires
switching hydrogen production methods. Of course, there's plenty
of sea water, but that leads to your number two.

I think item two is required no matter what we do. Either we use
the electricity directly, or use it to power the production of
things like hydrogen as an energy storage medium. Remember, hydrogen
is not an energy SOURCE, because it has to be manufactured. Fossil
fuel is an energy source because all we have to do is extract it
and do a little refining. It already exists in tangible form.

Brian
--
http://www.earthwaves.org/forum/index.php - Earth Sciences discussion
http://www.skywise711.com - Lasers, Seismology, Astronomy, Skepticism
Sed quis custodiet ipsos Custodes?
  #35  
Old June 5th 15, 07:32 PM posted to rec.aviation.piloting
Larry Dighera
external usenet poster
 
Posts: 3,828
Default CAFE Electric Aircraft Symposium Set For May 1

On Wed, 3 Jun 2015 21:57:44 +0000 (UTC), Skywise
wrote:

Larry Dighera wrote in
:

Hello Brian,

You seem to have firm grasp of the physics involved. Are you able to
suggest how efficient an electric power system powered by LH2 would have
to be to offset the power density difference from gasoline/kerosene?


A quick disclaimer.... I don't have any degrees in this stuff or
work with it. I just happen to be very interested in the sciences
in general and have taught myself a few things over the years.
Having said that, I try very hard to check my facts and figures
before I say anything. I have an allergy to foot-in-mouth disease.

On to your question....

Per numbers in http://www.tinaja.com/glib/energfun.pdf

Liquid H2 has an energy density of 2600 Watt-hours/liter.
Gasoline is 9000 Watt-hours/liter.

[I used energy density per volume rather than per mass
because that's the limiting factor in any vehicle, the
volume of the 'gas tank']

If we make an assumption for discussion's sake that an LH2
powered system were 100% efficient, then the gasoline system
would only need to be 2600/9000 = 29% efficient to reach
parity with LH2. But note that nothing is ever 100% efficient.
There are _always_ conversion losses. It's a matter of how
much.

Per:
http://en.wikipedia.org/wiki/Interna...rgy_efficiency

Engine efficiency is limited by thermodynamic laws. "Most steel
engines have a thermodynamic limit of 37%." Further, "most engines
retain an average efficiency of about 18%-20%."

Right away we see it's at least potentially possible for gasoline
to still beat out 100% efficient LH2. But let's go on the low
side and assume a gasoline engine is 18% efficient. Then we need
to figure out the efficiency required of an LH2 system to beat
gasoline:

9000 * 18% = 1620
1620 / 2600 = 62%

Therefore an LH2 system would have to be 62% efficient overall to
beat a typical gasoline engine.

Per the same Wikipedia article, "Electric motors are better still,
at around 85%-90% efficiency or more, but they rely on an external
power source (often another heat engine at a power plant subject to
similar thermodynamic efficiency limits)."

OK. So an electric motor _by itself_ is more than efficient, but
as stated it has to get it's electricity from somewhere else. We
are assuming an LH2 powered source.

Let's go with the high side of 90% on the electric motor. So we
have to now figure out what efficiency is required in converting
LH2 to electrity so a 90% efficient electric motor produces
1620 Wh/l of LH2...

1620/.90/2600 = 69%

Now that leaves us with finding out how efficiently LH2 can be
converted to electricity.

Per: http://energy.gov/eere/fuelcells/fuel-cells

"Fuel cells can operate at higher efficiencies than combustion
engines, and can convert the chemical energy in the fuel to
electrical energy with efficiencies of up to 60%."

So we may be coming up a bit short.

However, all my pondering here is surely a gross oversimplification.
And it's possible I goofed on my math or went astray with my logic.
And I imagine different sources will give different numbers. But I
hope it gives you some idea. There are surely other factors that
need to be taken into account. Some may make things work out better,
others may make things worse.

Brian



Hello Brian,

Thank you for your fair and conservative analysis. Very much appreciated.

So generally on a theoretical basis, it is within the realm of possibility that
using LH2 to generate electric power with a fuel-cell to power an electric
motor might be completive in terms of performance with today's General Aviation
internal combustion powered aircraft, because the efficiency of the
electrically powered system is potentially so much greater than the IC
technology, that it compensates for the reduced power density of the LH2 fuel
compared to petroleum. The laws of physics don't prohibit it.

Of course, for this to be realized, significant engineering remains to be
accomplished, but the path to electrically powered aircraft isn't a dead end
due to the laws of physics.

Larry
  #36  
Old June 5th 15, 10:13 PM posted to rec.aviation.piloting
Skywise
external usenet poster
 
Posts: 140
Default CAFE Electric Aircraft Symposium Set For May 1

Larry Dighera wrote in
:

Of course, for this to be realized, significant engineering remains to
be accomplished, but the path to electrically powered aircraft isn't a
dead end due to the laws of physics.


VERY SIGNIFICANT engineering.

None of my rough calculations take into consideration the means
by which to store LH2 safely in a vehicle. For one thing, there
will certainly be a weight penalty. If enough, it may offset any
efficiency gains. Steel pressure tanks aren't the lightest things
in the world. I recall work being done in the past in the space
program to make composite LH2 tanks, but they just couldn't ever
withstand the pressures. http://en.wikipedia.org/wiki/Lockheed_Martin_X-33

But here's another safety thought. Would you want planes flying
around that are basically hydrogen 'bombs'? I mean, it's bad
enough when any aircraft crashes, but one with a tank of pressurized
LH2 on board?

hmmm... BLEVE anyone?

Brian
--
http://www.earthwaves.org/forum/index.php - Earth Sciences discussion
http://www.skywise711.com - Lasers, Seismology, Astronomy, Skepticism
Sed quis custodiet ipsos Custodes?
 




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