Calculating Pressure & Mass for Hydrogen Tank Filling

[H2Fill]

TL;DR

I want to fill a secondary tank with 10kg of H2 at 700bar, how do I calculate the necessary pressure/mass of the primary tank.

As I stated in the summary I'd like to calculate the required pressure and mass of a tank of hydrogen that would allow me to fill a second tank with 10kg of H2 at 700bar.

I am trying to get an idea of the feasibility of doing this, putting aside, for now, how the primary tank would itself be filled and any safety concerns.

I appreciate any help.

 

[jrmichler]

Talk to your supervisor and discuss what assumptions are necessary to start solving this problem.

 

[H2Fill]

Looks like assumptions are already being made right here.

This isn't something I am responsible for and is only tangentially related to what I do for a living. I would like to know how to figure it out though if you can help.

If I start with the ideal gas law assume a volume and add mass until I get the pressure I want keeping all else constant. This would give me a mass for the secondary tank.

Then I could keep adding mass until the delta pressure is relatively small that would give me a pressure and mass for the primary tank that could give up what the secondary tank needs without losing pressure.

If the pressure increases too much without adequate mass I can increase the volume of the tank and start over.

Thoughts?

 

[berkeman]

As I stated in the summary I'd like to calculate the required pressure and mass of a tank of hydrogen that would allow me to fill a second tank with 10kg of H2 at 700bar.

What volume of $H_2$ has a mass of 10kg? Do you have power at the site where you want to make this transfer? If so, can you use a compressor to aid in the transfer, so that the lowering pressure of the main tank is less important? It looks like 700 bar is near the upper end of the typical $H_2$ tank pressures, so it would be useful to be able to use a (hydrogen-safe) compressor stage between the primary and secondary tanks to aid in the filling.

https://energycentral.com/sites/def...c/ece/nodes/404391/h2-test2.png?itok=VFKyPhm8

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Hydrogen can be stored physically as either a gas or a liquid. Storage of hydrogen as a gas typically requires high-pressure tanks (350–700 bar [5,000–10,000 psi] tank pressure).

https://www.energy.gov/eere/fuelcells/hydrogen-storage

 

[berkeman]

Looks like assumptions are already being made right here.

I think we are all a bit concerned about the safety aspects of handling $H_2$ if you don't have a lot of experience in it yet. I think that's one reason @jrmichler alluded to your supervisor in this project.

putting aside, for now, how the primary tank would itself be filled and any safety concerns.

 

[jrmichler]

The IP address for the OP was from Ford Motor Company. Hence my recommendation to talk to their supervisor because this sounded like the type of project handed to a young engineer to see if they know how to start working on a challenging problem.

An engineer who thinks this is merely a pressure and mass calculation, and does not think to look further, fails. Which is why my answer was so short.

 

[H2Fill]

Thanks for the input. I wasn't really expecting the hostility I seem to be getting. Just to make it clear, I am in no way responsible for designing or building a system to do this. A conversation I had made me curious what a tank that could do this would like like. Since I don't do this for a living and was unclear where to start, I thought I'd post something here. It's starting to look like that was mistake, though is has lead me to a few other interesting posts.

Do you have power at the site where you want to make this transfer?

For the sake of argument there is no power. This is hypothetical, I am just curious how you would go about doing this calculation.

I think we are all a bit concerned about the safety aspects of handling H2 if you don't have a lot of experience in it yet. I think that's one reason @jrmichler alluded to your supervisor in this project.

Lets pretend it's nitrogen then. I put in the note about ignoring safety to try and avoid going down this rabbit hole of how hydrogen is, surprise, explosive.

An engineer who thinks this is merely a pressure and mass calculation, and does not think to look further, fails. Which is why my answer was so short.

Wouldn't posting here count as looking further, really not quite sure what issue you have with the question? You made a bad assumption and now are doubling down on it. If you don't want to help that is fine, stop wasting your time investigating my family history and guessing my ulterior motives. Just ignore the topic, you will feel better.

 

[jrmichler]

At first glance, this appears to be a simple optimization problem. You partly recognize this in Post #3. Start by searching compressibility factor hydrogen. You will quickly find that the effect is significant at 700 atm.

Then dig out a thermodynamics book and study the chapter on filling a tank from a high pressure tank. You can find some good hand waving discussion by searching fill scuba tank temperature.

Lets pretend it's nitrogen then. I put in the note about ignoring safety to try and avoid going down this rabbit hole of how hydrogen is, surprise, explosive.

SURPRISE! Even inert gases are explosive at much lower pressures. Search scuba tank explosion and water heater explosion to learn about this. Keep in mind that scuba tanks run at less than half the pressure you are talking about, and water heaters less than 1/50 the pressure.

I am trying to get an idea of the feasibility of doing this

Part of the feasibility, and only hinted at in your Post #3, is the cost of compressing the hydrogen to pressures higher than the fill pressure. And that cost is a function of supply tank size.

All of which is why I said:

An engineer who thinks this is merely a pressure and mass calculation, and does not think to look further, fails.

We are here to help you. We tailor our answers to the ability of the OP based on what they post, and on their profile information. If, for example, you had said that you were a middle school student, you would have had entirely different answers. And those answers would have been less direct.