Introduction

Conclusions

Safety

Hydrogen gas has a long, and possibly undeserved, reputation for danger, partially due to the Hindenberg disaster of 1937 (contemporary research revealed that the fire was caused more by the flammable coating of the zeppelin than by the hydrogen itself).

However, as a gas that is significantly more dispersive and energy-intensive than other forms of fuel, hydrogen does pose certain risks.

One recent study of the safety of parking hydrogen fuel cell vehicles (FCVs) in standard parking garages and home garages found that no significant safety modifications would be necessary to prevent hydrogen from leaks from accumulating past its flammability concentration of 4.1%.[1] Because hydrogen is extremely dispersive, it may become diffuse fairly rapidly, especially under conditions of normal ventilation.

However, the study required that FCVs have extensive safety monitoring systems that would prevent an unstable leak from causing tank rupture, and it assumed that vehicles would be equipped with sensors to monitor hydrogen concentration in and around the vehicle.

One unique concern is that due to hydrogen's extremely light molecular wave, it may collect along the ceilings of garages and tunnels in the case of vehicle leaks. Traditional ventilation systems focus on clearing volatile materials from the floors of such structures, which may lead to increase explosion and flame propagation risk. [2]

Hydrogen embrittlement

Of particular concern is a phenomenon unique to hydrogen gas. Because of its small size, H2 can be absorbed into the metallic lattice of steel, where certain impurities such as titanium cause it to dissociate into atomic hydrogen. Atomic hydrogen, in turn, will form anionic compounds around the edges of microcracks and voids, which facilitates their propagation. [3]The results of hydrogen infiltration into steel are shown in the graph below.

The yield and fracture stress for a steel sample are shown to decrease with increasing hydrogen concentrations.

Source: Schvachko, V.I.

Hydrogen embrittlement prevents the use of current natural gas pipelines and other vessels from serving as hydrogen conduits, because a special coating may often be needed to prevent structural failure in the long-term from embrittlement.

 

Sources:

[1] "Support Facilities for Hydrogen Fueled Vehicles Conceptual Design and Cost Analysis Study." California Fuel Cell Partnership.

[2] Professor Frederick Dryer, Princeton University, personal conversation. 15 Nov 2004.

[3] Shvachko, V.I. "Micromechanical Aspects of Reversible Hydrodgen Embrittlement" Materials Science, Vol. 36, No. 4, 2000.