Hydrogen Safety

Hydrogen safety: one perception. Mention “hydrogen” and some people think of the Hindenburg or the Hydrogen bomb.

The Hindenburg: the Hindenburg dirigible burned as it tried to dock at Lakehurst New Jersey in May 1937.  The fact that 60 of the 97 passengers and crew survived the fire proves that hydrogen did not explode, but in fact burned relatively slowly.

But the Hindenburg fire has virtually no bearing on the safety of a fuel cell electric vehicle:

  • Hydrogen in the Hindenburg was carried in flimsy cloth bags, each coated with a flammable mixture of cellulose acetate or cellulose nitrate impregnated with aluminum chips....a mixture similar to rocket fuel! By contrast, the hydrogen on a FCEV is stored in extraordinarily strong carbon fiber composite tanks; these tanks have been shown to survive a 50 mph rear-end collision without leaking.
  • The Hindenburg carried 2,300 times more hydrogen than is stored on a FCEV [1]. 

The Hydrogen Bomb: a hydrogen bomb requires three ingredients that would never be present on a FCEV: deuterium, tritium and an atomic bomb! Hence any association of hydrogen fuel with the so-called “hydrogen bomb” is completely unwarranted.

The term “hydrogen bomb” is a misnomer,  shorthand for a thermonuclear fusion device as opposed to a fission or “atomic bomb” such as those dropped on Hiroshima and Nagasaki.  A fusion bomb requires an atomic bomb to trigger the fusion of deuterium and tritium gas. Deuterium and tritium are two isotopes of hydrogen, which is the only connection with the word “hydrogen”: deuterium has one neutron, and tritium has two neutrons, while hydrogen atoms such as those in ordinary hydrogen gas have no neutrons, and could not be used to sustain a fusion reaction..

Hydrogen safety: the reality. Hydrogen, like any fuel, contains significant energy, and can therefore pose a serious risk in some circumstances.

Hydrogen Vehicle Safety. The Ford Motor Company, in their report to the US Department of Energy included this statement [2]:

    “Overall, we judge the safety of a hydrogen FCV system to be potentially better than the demonstrated safety record of gasoline or propane, and equal to or better than that of natural gas.”

In a collision, a trapped survivor in a hydrogen-powered fuel cell electric vehicle would have considerably lower risk of death or injury from fire than a survivor in a gasoline car due to four factors:

  1. Compressed gas tanks are far stronger than gasoline tanks and can survive much greater impacts (see below), minimizing the chances of a hydrogen release after an accident.
  2. In the unlikely event that any hydrogen was released, it would rise and disperse faster than any other fuel. Gasoline vapors, on the other hand, are heavier than air and linger in the vicinity of a wrecked car, posing a large risk to any trapped survivors [See sidebar at right -->].
  3. The hydrogen stored on a FCEV will have less than half the energy of the gasoline on a regular car due to the higher efficiency of a FCEV.
  4. FCEVs can be designed with an inertial switch to cut off any hydrogen flow from the high pressure tank via a solenoid valve located inside the neck of the tank; this valve would close within milliseconds of any collision, thereby keeping all hydrogen safely inside the composite tank that has been shown to survive collisions up to 52 mph.

Residential Hydrogen.  Hydrogen was routinely used in home heating systems in the form of “town gas” prior to World War II. Town gas was a made by gasifying coal in each city, resulting in a mixture with roughly 50% hydrogen and the rest methane (the main ingredient of natural gas), carbon dioxide and a few percent carbon monoxide. Millions of Americans heated their homes, meals, and lit their lights with hydrogen before natural gas became widespread. Unfortunately a few did commit suicide with town gas, but that was due to carbon monoxide poisoning, and had nothing to do with hydrogen.

Town gas or “coal gas” was also used in England for lighting in the 19th century, with 48 km of cast-iron hydrogen pipelines laid in London by 1815 [3].  Wooden gas pipes were also used in the US to transport town gas as late as 1870.

Commercial Hydrogen. Hydrogen has been used safely in several key industries for many decades. In 2006, at least 7.2 billion kilograms of hydrogen was produced, primarily from oil and natural gas [4]. This amount of hydrogen could supply 36 million FCEVs. Most of this hydrogen is already used for the transportation sector to make gasoline at oil refineries, which consumed 65% of all hydrogen made in 2006. Another 23% is used to make ammonia for fertilizer. About 10% is used for making various chemicals, float glass, semiconductors, for metal annealing, and for cooling electric

The following picture shows the wreckage of a natural gas vehicle (NGV) after a rear-end collision that demolished the car.  Natural gas on this NGV is stored in composite fiber-wrapped tanks, the same type of tanks used to store hydrogen on FCEVs These tanks are so strong that they survived the crash and protected the driver.

al generators. 

 

 

 

(Source: Edward B. Cohen, American Honda Motor Co. Inc. June 12, 2009)

 

Hydrogen in the Space Program. Hydrogen is the optimum fuel for rockets, since it has the highest specific impulse (essentially energy per unit weight) of any fuel.  Weight is all important in rockets, since every extra pound of weight requires more energy to carry it into space...the same “weight compounding” problem that burdens battery electric vehicles.  That is why the Shuttle main engines were fueled by liquid hydrogen and liquid oxygen.  Hydrogen also powered the fuel cells on the Shuttle providing electricity and water for the astronauts to drink. Hydrogen is the ideal environmental fuel in space, producing no pollution that would otherwise accumulate in the closed environment. The NASA space program was the motivating driver behind the development of liquid hydrogen fueling and delivery technology in the 1960’s, and also the development of fuel cells. 

Footnotes: [1] The Hindenburg carried approximately 6.7 million standard cubic feet of hydrogen with an energy content of 1,900 gigajoules; a FCEV carries about 0.8 gigajoules of hydrogen or 2,300 times less energy.  Source: Ref 2 below:

[2] “Direct hydrogen fueled proton exchange membrane fuel cell system for transportation applications: Hydrogen vehicle safety report,” prepared by the Ford Motor Company for the U.S. Department of Energy Office of Transportation Technologies under contract # DE-AC02-04CE50389, Dearborn, Michigan May 1997.

[3] T. I. Williams, “A history of the British gas industry,” Oxford Press, 1981, p.15.

[4] B. Suresh, S.Schlag, M. Yoneyama, CEH Marketing Research Report: Hydrogen Chemical Economics Handbook, SRI Consulting, October 2007

Hydrogen vs. Gasoline Fires

The following pictures compare hydrogen and gasoline fires from an experiment conducted by Mike and Matt Swain at the University of Miami.

The car on the left contained high pressure hydrogen tanks with 175,000 btu of energy and the car on the right had a conventional gasoline tank with just five pints of gasoline or about 70,000 btus of energy.  Spark plugs were installed outside both vehicles to ignite a leaking hydrogen tank and a leaking gasoline line.

Hydrogen Vehicle:          Gasoline Vehicle:

The pictures below were taken 3 seconds after ignition

 Hydrogen:                     Gasoline:

One minute after ignition (below); hydrogen fire nearly extinguished;

 Hydrogen:                       Gasoline:

One minute 30 seconds after ignition (gasoline image enlarged to show detail; all hydrogen fuel burned so picture of hydrogen car is minmized and removed in subsequent frames) 

Two minutes 20 seconds after ignition, just before interior deflagration (explosion of flammable gases inside car):

Two minutes 20 seconds, one frame later showing interior deflagration in the gasoline vehicle as flammable gases from plastic and upholstery ignites:

(Source: M.R. Swain, “Fuel Leak Simulation,” Proceedings of the 2001 DOE Hydrogen Program Review, NREL/CP-570-30535, pg 679)

Fire Risk “Radar Chart” As another measure of fire risk, the “Radar Chart” below compares three attributes of fuel vapors:

  • LFL--lower flammability limit, the minimum percentage of fuel vapor mixed in air that will ignite
  • Diffusion -- the speed that gases disperse in air
  • Buoyancy -- the speed that gases rise in air

All three of these attributes should be large to reduce the risk of a fire.  In the chart above, the fuels closest to the center of the 3-D “radar” chart have the greatest risk.

By this 3-element risk metric, gasoline is the most dangerous, propane the next most dangerous, then methane (natural gas) and hydrogen is the safest fuel on all measures except lower flammability limit, where methane has slightly higher LFL than hydrogen.

(Source: Ford hydrogen safety report)

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