BMW iX5 Hydrogen Enters Production

Depends on how big the tank is and how well insulated from heat.

As a gas, hydrogen is routinely supplied compressed to about 2500 psi in high-pressure heavy-duty tanks similar to oxygen for welders. When you shut the valve tight on a non-leaky compressed gas tank, the full amount of gas stays in there as long as you want until you open the valve again.

Propane and butane are LPG products that are petroleum hydrocarbons having higher energy density per liter of gas but as a gas that is still not enough to get any serious range per refuel compared to a liquid.

Now propane and butane gas can not be compressed anywhere near 2500 psi at ambient conditions before they condense into the much higher-energy-density liquid state, not that much lower in miles per liquid gallon compared to gasoline. The LPG tank does not need to be that heavy either, the typical barbecue tank is welded sheet metal, not a heavy-walled cylinder, and only needs to hold a few hundred psi. You fill up the tank, shut the valve and store a few liters of liquefied petroleum gas until whenever you need it later.

A heavy high-pressure tank of hydrogen gas would work just fine but won't get you far on the road since it really doesn't have that many grams of hydrogen even at 2500 psi.

So unless you're going for only short hops between refuels, the hydrogen needs to be in liquid form itself as an on-board vehicular fuel.

That means cryogenics.

Hydrogen is so light that it must be refigerated into liquid form rather than compressed. And that requires hundreds of degrees below zero, at some energy expense elsewhere. Then the super-cooled liquid hydrogen needs to be stored in the most highly insulated tank for best results, the better the insulation, the slower it evaporates.

So the liquid hydrogen tank still needs thick walls but filled with insulation (or evacuated) rather than heavier solid metal. Not such a high pressure tank either, the cryogenic fluid is so cold it refigerates everything it comes into contact with, and once you've got it in the fuel tank it's made everything so cold that it can refigerate itself by slowly evaporating.

You can never fully shut a liquid hydrogen (or oxygen, nitrogen, helium, argon) tank. As the bulk of the cryogenic liquid slowly evaporates the gas in the upper part of the tank is only allowed to rise to about 200 psi, and then excess gas is vented to the atmosphere through a fixed regulator at the rate which the liquid is evaporating. There are additional pressure-relief devices to prevent catastrophe in case the main regulator fails.

If you are constantly consuming gas at a faster rate than the gas would be venting otherwse, there is nothing wasted out the vent.

But when it's just sitting there in the tank, the fuel's disappearing at a steady rate and directly goes away completely, without driving a mile.