Different Failure Modes

Randy Rager does not like an idea for a seriously fuel-efficient car:

Exactly what happens in the case an of accident overloading those not-quite-clever energy reclamation bumpers and cracking the main pressure reservoir? Or worse, the vehicle gets t-boned?

Granted, the bumpers are a bit silly idea, but I think Randy doesn't have a clear idea of the failure modes of a pressure vessel full of gas in one case and liquid in the other.

When most people think about explosions - even those who know the real thing and not just movie special effects - they are visualizing a rapid expansion of gas. Gases are highly compressible, and this means that increasing pressure a lot of gas (and energy) can be stored in a rather small volume.

A mental calculation based on the ideal gas equation of state yelds that 10 liters (a small cylinder) of nitrogen at 200 bar correspond to 2000 liters in normal conditions* - the numbers will vary, but the ratios are constant in all units. This tendency to expand is what makes makes gas explosions nasty, and causes gas cylinders to go off like rockets if they are ruptured or punctured.

But liquids are a different matter: they are orders of magnitude less compressible than gases. So, if 10 liters of water are subjected to a 200 bar pressure, the final volume will be only slightly less (back-of-the-envelope calculation: 10 mL) than 10 liters. Release the pressure, and there will be no dramatic expansion, no rocket effect. Pressure testing of various components is often done with liquids (water or hydraulic oil) rather than gases for this precise reason.

High-pressure hydraulic systems are widely used in industrial machinery, aircrafts, ships, earth-moving machines, trucks and battle tanks. Granted, many of those aren't really designed to resist collisions and other impacts, but explosions of pressure reservoir usually isn't a matter of great concern. On the other hand, battle tanks are designed to resist enemy fire, so one would think that hydraulic systems wouldn't be so popular if they posed a severe explosion risk - in fact, the main risk is flammability of the hydraulic fluid.

Moreover, the most dangerous aspect of a car accident is the impact itself, so unless the pressure tank you have onboard is extremely fragile, it adds little risk.

Fianlly, hydraulic accumulators are a technology which has already been introduced on some vehicles - albeit in a rather experimental fashion; they comprehend pressurized nitrogen tanks, and safety does not seem to be the main obstacle to development.

So, to answer Randy's questions: pressure relief valves are available to relieve overpressure in hydraulic systems - and in case of accident, what happens is probably a loud bang and a puddle of hydro fluid.

*Normal conditions are 0 °C and 1.00 bar.