This will primarily deal with pressure vessels as used as fuel tanks in GURPS Vehicles. This is a result of my looking into the rules for hydrogen storage, specifically, but I'll probably generalize some.
Chapter 7 of GURPS Vehicles, 2nd Edition, has tables giving stats for fuel tanks and typical fuels. The table gives the density of Hydrogen as .58 lbs/gallon. According to the Chemistry Web Book, this is the density at 20 K and 1 atmosphere pressure. The assumption is that liquid hydrogen is stored in a cryogenic fuel tank at 20 K. This works out to about 115 cubic feet of hydrogen at 300 K (room temperature) and 1 atmosphere pressure.
Following is a table of the properties of hydrogen at other temperatures and pressures:
| Temp (K) | Press. (psia) | Density (lbm/cf) | cf at STP | gal of LH |
|---|---|---|---|---|
| 300 K | 14.7 psia | .0051093 | .13 cf | .001156 |
| 20 K | 14.7 psia | 4.4197 | 115.6 cf | 1 |
| 300 K | 3000 psia | .92606 | 24 cf | .21 |
| 300 K | 6000 psia | 1.6571 | 43 cf | .375 |
| 300 K | 10000 psia | 2.4155 | 63 cf | .55 |
The cf at STP is the cubic feet of gas at STP for one gallon at the conditions stated.
The gal of LH column has how many gallons of LH can be stored in 1 gallon at this temperature and pressure.
For liquid hydrogen, fuel tanks should always be constructed as cryogenic tanks per MA LLoyd's Vehicles Additions. These have a x5 cost multiplier with the same volume and weight and include relief valves and insulation. Realistically, they should have an increased volume to account for insulation.
The pressure vessels listed in MA Lloyd's Vehicles Additions can't hold the same amount of hydrogen, so should probably use 3000 psia or 6000 psia. I'll work up some weight numbers for pressure vessels to see how they work out.
Using some accumulator volumes and weights, I come up with about 6 lbs/gallon/ksi for the mass of a pressure vessel. In reality, this varies quite a bit. Based on this, here's a chart of pressure vessels:
| TL | Pressure | Cost | Mass | Volume |
|---|---|---|---|---|
| 7 | 3000 psi | 18 | .15 | |
| 7 | 6000 psi | 36 | .15 | |
| 7 | 10000 psi | 60 | .15 | |
| 8 | 3000 psi | 12 | .15 | |
| 8 | 6000 psi | 24 | .15 | |
| 8 | 10000 psi | 40 | .15 | |
| 9 | 3000 psi | 9 | .15 | |
| 9 | 6000 psi | 18 | .15 | |
| 9 | 10000 psi | 30 | .15 | |
| 10 | 3000 psi | 6 | .15 | |
| 10 | 6000 psi | 12 | .15 | |
| 10 | 10000 psi | 20 | .15 | |
| 11 | 3000 psi | 3 | .15 | |
| 11 | 6000 psi | 6 | .15 | |
| 11 | 10000 psi | 10 | .15 |
Cost, Mass, and Volume are per gallon. Pressure rating is nominal. Typically, a pressure vessel will fail at 3-4x the rated pressure. Failure if a gas failed pressure vessel can be catastrophic. Failure of a completely liquid filled pressure vessel is much less dangerous.
At TL 7 and 8, pressure vessels are primarily made of metals. At higher TL, composite materials are more common.
Hydrogen can be stored in solid form in a metal hydride matrix. This form of storage is very dense and still relatively safe. Looking at some of the technologies below, hydrogen can be stored at a 5-7% mass ratio, where the hydrogen stored is 5-7% of the weight of the metal hydride. Carbon nanotubes also have potential for storing hydrogen at even greater mass ratios. Both metal hydride and carbon nanotube storage allow hydrogen to be stored at relatively low pressures, though some require higher temperatures to release the hydrogen effectively.