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Old 07-13-2017, 04:37 AM   #1 (permalink)
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H2 equivalent to 1 liter of gasoline

A question I had was how much hydrogen gas (and at what pressure !) equals 1 liter of gasoline. Wikipedia mentions gasoline has 33,41 kWh per gallon (or hence 125 kWh per liter) and (idealhy.eu mentions that) hydrogen gas contains 0,003 kWh per liter (at a pressure of 1 bar). I would thus assume that to attain a same amount of energy (125 kWh per liter), I'd need to use a 41666 bar compression on that 1 liter tank.

Is this actually correct ? I don't see where I made a mistake, yet such kind of pressures are not used in everyday life (the biggest compression rate used is some 700 bar).

Can anyone verify whether my calculation is correct or not ?

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Old 07-13-2017, 08:16 AM   #2 (permalink)
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I think you will find that the hydrogen is stored at 350 or 700 bar, therefore to hold 1 litre petrol equivalent you require a far greater volume than 1 litre.
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Old 07-13-2017, 08:37 AM   #3 (permalink)
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I gallon of gas equivalent is about 2kg.
So a liter would be about half a Kg.
Forget volume, do calculations in mass any time possible.

I think that is correct.

What are you trying to do with hydrogen exactly?
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Old 07-13-2017, 10:23 AM   #4 (permalink)
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Quote:
Wikipedia mentions gasoline has 33,41 kWh per gallon (or hence 125 kWh per liter)
Your conversion is backward. 33.41kWh/gal / 3.78541 L/gal = 8.83 kWh/L

So your calculations following that point are off by a factor of about 14x.
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Old 07-13-2017, 11:29 AM   #5 (permalink)
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I knew something looked wrong in there but I just couldn't figure it out. Thanks for pointing it out for me and we'll the OP I guess. 3 1/2 hours of sleep got me a little fuzzy.
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Old 07-13-2017, 12:14 PM   #6 (permalink)
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Re: thanks for noticing this samwichse; updated the calculation:

1 liter of gasoline= 33,41 kWh per gallon (or hence 8,83 kWh per liter)
hydrogen gas contains 0,003 kWh per liter (at a pressure of 1 bar). I would thus assume that to attain a same amount of energy (8,83 kWh per liter), I'd need to use a 2943 bar compression on that 1 liter tank.

This still seems huge (the biggest compression rate used is some 700 bar). I guess it's doable if I am to use a 10 liter tank pressured at 294,3 bar but still that's a relatively large tank for just 1 liter of gasoline equivalent.

The other thing I've been wondering about: does this take into account the size the gas still takes in after compression (I would assume that 1 liter of hydrogen, after compression to say 294 bar only takes in about 1/294th of the space (so 0,003397 liter). Or well, about that amount of space ...
----------------------

The other calculation I did actually used weight rather than volume (as oil pan 4 suggested). It went as follows:

1 liter of gasoline = 0,26 gallon of gasoline
14 liter of hydrogen = 1 kg of hydrogen @ 1 bar (see uigi.com/h2_conv.html )
1 kg of hydrogen = energy in 1 gallon of gasoline (see heshydrogen.com/hydrogen-fuel-cost-vs-gasoline )
0,26 kg hydrogen = 3,64 liter of hydrogen (compressed at 1 bar) -0,26 X 14-
So in this latter calculation, it looks as if a mere 3,64 liter tank with hydrogen, without any compression at all would do

This obviously doesn't seem correct, but I don't see the mistake here either.
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Old 07-13-2017, 02:33 PM   #7 (permalink)
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I think 1kg hydrogen is equivalent to a gallon of gasoline. And 1 gallon of gas = 33.7kWh.
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Old 07-14-2017, 12:50 PM   #8 (permalink)
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The real problem with compressed H2 is the tank. From a quick search, the air in a typical scuba tank weighs about 6.5 lbs. Firgure H2 is about 1/14 as dense as air, and you get about 1/2 lb of H2 in a tank that weighs 30 libs or so.

Then there's the energy needed to compress it to 3000 psi...
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Old 07-14-2017, 02:03 PM   #9 (permalink)
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3,000? How about 10,000PSI?
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Old 07-15-2017, 03:12 AM   #10 (permalink)
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Quote:
Originally Posted by jamesqf View Post
The real problem with compressed H2 is the tank. From a quick search, the air in a typical scuba tank weighs about 6.5 lbs.
H2 is about 1/14 as dense as air, and you get about 1/2 lb of H2 in a tank that weighs 30 libs or so.
Have you read my other post ?

I calculated out you could get about 6,95 kWh in a SCUBA.
Haven't tried it in practice though, so if you have, let me know.

I got the SCUBA tank idea from USH2. If it won't work, I guess I could always change it for a CNG tank, but I don't see any structural difference between the 2, so I assume both should work.

As for the energy needed and the compression used (3000 instead of 10 000 psi): using less compression means less energy needed (and less energy loss as well). Obviously it still remains inefficient from this standpoint, but on the other hand it has no (polluting) exhaust gases and you could use renewable electricity to make the hydrogen.

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