Wow! I am very grateful you put this much effort into my topic! You have given me a great understanding of what the "adventurers" are really up against.

Just one last question: would it be practical to carry a compressed emergency supply of hydrogen aboard the airship in a storage tank?

 

There are a few options to consider. First, you can store it as a compressed gas. At 10,000psi and room temp you are looking at about 30g/liter. You could also store it as a liquid at about 70g/liter. The 76,000 moles equates to about 152kg, so you need 5000 liters for gas and 2200 liters for liquid. But that is just for the hydrogen itself. You also have to carry the thick, heavy pressure vessels and/or the cryogenic tanks. Another option is to store it chemically, perhaps in ammonia, and carry a catalytic reformer. The idea here would be that the carrier and the reformer work out to a smaller mass than the storage containers for the other methods. But whatever means you use, your going to eat into your payload pretty quickly, especially since this whose discussion is for a 'small' airship. That 60,000 cubic feet is replacing air (at about 33.5g/mole) with hydrogen (at about 2g/mole). So the net difference of about 31.5g/mole is the lifting capacity, which means that our 76,000 mole gas bag is going to have a total lifting capacity somewhere in the 2400 kg range (and that has to include the mass of the airship itself!).

It's probably going to be hard to claim that you are carrying a significant reserve supply of gas.

Another thing to consider is having the upper surface of the airship consist of solar panels and have them provide the electricity. If you claim a 10% efficiency then you might be able to claim an effective conversion of perhaps 50W/m^2 of upper surface area. Crank the numbers on that and see if it sounds reasonable.

 

Thank you WBahn, you have been a great help to me!

 

There are a few options to consider. First, you can store it as a compressed gas. At 10,000psi and room temp you are looking at about 30g/liter. You could also store it as a liquid at about 70g/liter.
...

Are those calcs right? A litre of water is 1000g, surely a litre of liquid hydrogen which is a smaller denser particle would be a lot more than 70g?

 

It's very non-intuitive, but correct.

The average intermolecular spacing of the H2 molecules in the liquid state is considerably more than the spacing between H20 molecules.

Liquid hydrogen (at 20K) has a density of 71g/liter. Water, at 300K, has a density of 1000g/liter, a factor of 14 higher. But a water molecule is only about 8 times as massive as a hydrogen molecule, so they are nearly twice as many per unit volume.

Basically, hydrogen (and helium, for that matter) really, really want to be gases.

 

Thanks WBahn that info is much appreciated.

It also explains why, given the very high pressures and very poor energy density, that we don't have more pure-hydrogen powered vehicle running around.

I've been saying for years one of the "holy grails" in hydrogen fuel is to develop a system for pure hydrogen to be converted back into an easily handleable liquid fuel, so solar generated hydrogen etc could be turned into somehting like "fake gasoline". But to my knowledge there's nobody at all working on it!

 

Liquid hydrogen (at 20K) has a density of 71g/liter. Water, at 300K, has a density of 1000g/liter, a factor of 14 higher. But a water molecule is only about 8 times as massive as a hydrogen molecule, so they are nearly twice as many per unit volume.

Basically, hydrogen (and helium, for that matter) really, really want to be gases

A caution about regarding atomic or molecular weight as indicating the state (phase) of a substance at normal temperatures and pressures.

This works well with homologous series of the same kind of molecules such as alkanes (long straight chain hydrocarbons), the first four a gaseous, the next twelve are liquid and the remainder are solid here.

Whilst it is true that for elements, the first two are gaseous at STP, the next four (Lithium , Beryllium , Boron and Carbon) are solid under these conditions.

 

A caution about regarding atomic or molecular weight as indicating the state (phase) of a substance at normal temperatures and pressures.

This works well with homologous series of the same kind of molecules such as alkanes (long straight chain hydrocarbons), the first four a gaseous, the next twelve are liquid and the remainder are solid here.

Whilst it is true the for elements the first two are gaseous at STP, the next four (Lithium , Beryllium , Boron and Carbon) are solid under these conditions.

I can't for the life of me figure out what point you are making that is relevant to the discussion. Where did anyone use the atomic or molecular weight as indicating the state at normal temperatures and pressures? The observation was that liquid water, even at 15x the temperature, has significantly more molecules per unit volume than does liquid hydrogen and that the same is true for helium.

 

Isn't that the deduction you were implying with your statements I extracted

"But a water molecule is only about 8 times as massive as a hydrogen molecule "

Isn't that about molecular weight?

 

Are you saying that a water molecule isn't about 8 times (actually 9) as massive as a hydrogen molecule?

This part of the discussion was regarding this question:

Are those calcs right? A litre of water is 1000g, surely a litre of liquid hydrogen which is a smaller denser particle would be a lot more than 70g?

To which I replied that the numbers are correct and then explained that the particle density of liquid hydrogen is actually less than that of liquid water.

Fact #1: Liquid hydrogen at 20K has a density of ~71g/liter.

Fact #2: Liquid water at 300K has a density of ~1000g/liter.

Fact #3: Liquid water has a mass denity ~14x that of liquid water.

Fact #5: Liquid water has a molecular mass ~9x (I mistakenly said 8) that of hydrogen.

Conclusion: The molecular density of liquid hydrogen is considerably less than that of liquid water.

Where in any of that chain of reasoning is a claim that because hydrogen has a low molecular weight that it is a gas at STP?

 

Take it easy, of course water has a molecular weight of 18 (16 from one oxygen and 2 from two hydrogens), whereas hydrogen has a molecular weight of 2 (from two hydrogens). An 18 is actually 9 times 2 but 8 is close enough not to argue.

However you did append the statement that hydrogen (and helium) really want to be gasses - which is also true, but not due to molecular weight.

For that matter oxygen (molecular weight 32) and nitrogen (molecular weight 28) also want to be a gasses.

 

Take it easy, of course water has a molecular weight of 18 (16 from one oxygen and 2 from two hydrogens), whereas hydrogen has a molecular weight of 2 (from two hydrogens). An 18 is actually 9 times 2 but 8 is close enough not to argue.

However you did append the statement that hydrogen (and helium) really want to be gasses - which is also true, but not due to molecular weight.

But I never said anything about why they wanted to be gasses, just that they do!

I never said that they want to be gasses because of molecular weight. I didn't give a reason at all why they want to be gasses. The statement was nothing more than a handwavy observation and explanation of why, even at only 1/5 of the tempurature, the molecular density in the liquid state is roughly half that of water (and helium happens to have an almost identical molecular density, so I included it as well since the overall topic has been about lifting gasses for airships). The observation/explanation is simply that, for whatever unspecified reason, hydrogen and helium really want to be gasses, even at 20K. Even at 4K, helium has the roughly the same molecular density as hydrogen at 20K.

[/QUOTE]
For that matter oxygen (molecular weight 32) and nitrogen (molecular weight 28) also want to be a gasses.[/QUOTE]

Not at the temperature that we were talking about, namely 20K. We weren't talking about anything being a gas at 300K; the only thing at that temperature we were talking about was liquid water. At that temperature, not only is nitrogen not a gas, it is not even a liquid, since it freezes at 63K. Similarly, oxygen freezes at 54K. For comparison, the freezing point of hydrogen is 14K and, at atmospheric pressures, it won't freeze at all.

If I was going to make a claim about why they want to be a gas, it wouldn't be just that they are low molecular weight, but more importantly which Group of the periodic table they are in.