Hydrogen Safety

Safety of Hydrogen as a Transportation Fuel

A major issue facing hydrogen as a fuel is public perception about its safety. While hydrogen has many safety issues that need to be addressed, images of the Hindenburg and the hydrogen bomb often cloud meaningful discussion of hydrogen’s safety as a fuel. The Hindenburg is perhaps the most spectacular disaster where hydrogen was erroneously reported as the culprit. While hydrogen did indeed burn in the disaster, a new coating used on the zeppelin cover was highly flammable and was the primary cause for the major fire engulfing the frame. The misidentification of hydrogen with nuclear power has caused similar consternation. While there are some who believe hydrogen energy is somehow inherently linked with the hydrogen bomb or the deuterium and tritium components of nuclear energy, this belief is simply rooted in a misconception that can be allayed through education and outreach.

Hydrogen Material Safety Data Sheet (from BOC Gases)

In addition to the safety concerns regarding hydrogen fueled vehicles, the design and building of hydrogen fueling stations pose safety concerns for consumers that must also be responsibly and thoroughly addressed. The main issue is the risk caused by hydrogen stored at high pressure in a tank and the subsequent risk of leakage, especially in non-vented area such as an enclosed garage. There are also concerns about hydrogen accidents at refueling stations, either while the vehicle is being refueled or as hydrogen is being stored and in some cases produced on-site. Tank design is crucial to reducing risks created by hydrogen to a level at or below current risks posed by gasoline. The U.S. Department of Transportation and the Society of Automotive Engineers have recommended standards that must be met for fuel tanks before they are approved for use on the road. There are recommended safety standards for the transfer of fuel from compressed tanks to the uncompressed gas in the fuel cell and for refueling connection devices to ensure that only proper fuels at correct pressures are pumped into the fuel tank. Manufacturers are running hydrogen tanks through a series of rigorous tests to ensure that under all extreme operating conditions the tanks will hold up and limit the risk for explosion and rupture to an acceptable level.

Some safety statistics for hydrogen and other fuels

Characteristic	Hydrogen	Natural Gas	Gasoline
Lower heating value kJ/g	120	50	44.5
self-ignition temperature (ºC)	585	540	228-501
Flame temperature (ºC)	2,045	1,875	2,200
Flammability limits in air (vol%)	4 – 75	5.3 – 15	1.0 – 7.6
Minimum ignition energy in air (uJ)	20	290	240
Detonability limits in air (vol%)	18 – 59	6.3 – 13.5	1.1 – 3.3
Theoretical explosive energy (kg TNT/m3 gas)	2.02	7.03	44.22
Diffusion coefficient in air (cm2/s)	.61	.16	.05

In addition to the safety concerns that consumers have regarding hydrogen fuel, there are important technical considerations to be addressed when working with hydrogen fuel. The amount of energy needed to ignite hydrogen is comparable to natural gas but is one-tenth the energy needed to ignite gasoline. In a number of areas, hydrogen has properties that are more beneficial than gasoline. Hydrogen is nontoxic and it is difficult to create a high enough concentration of hydrogen to combust due to its light and buoyant nature. Gasoline, when leaked, can puddle at the source and emit fumes that can build and linger.

Hydrogen car to the left, Gasoline car to the right

A number of studies have examined hydrogen and conclude that while hydrogen raises a different set of safety concerns, experience has shown that they can be addressed. The American Physical Society released a report prepared for their Panel on Public Affairs (POPA) describing many of these studies and their conclusions. These included tests by Lockheed Martin, Arthur D. Little, BMW and the University of Miami that all conclude hydrogen is no more dangerous than gasoline. BMW undertook a number of crash tests and found the safety of the fuel to be sufficient. The University of Miami, in its test, set fire to two cars, one with hydrogen and the other gasoline. While both created fires when ignited, the gasoline fire engulfed the entire car causing
total damage, whereas the hydrogen flame vented vertically and failed to spread to the rest of the vehicle. (see above picture) As early as 1994, the Sandia National Laboratories performed a vehicle safety study and concluded, “there is abundant evidence that hydrogen can be handled safely, if its unique properties—sometimes better, sometimes worse, and sometimes just different from other fuels are respected.” Similarly in 1997, a vehicle safety study by the automaker Ford concluded hydrogen is potentially a better fuel source than gasoline when proper controls are built into the vehicle.

Just as gasoline tanks in today’s cars are manufactured and tested under a number of codes and standards, hydrogen tanks too will need their own set of standards to meet safety concerns arising from the unique properties of hydrogen fuel. There has been a significant amount of work already performed to achieve this goal. The U.S. Department of Energy coordinates its codes and standards efforts through the National Renewable Energy Laboratory. The hydrogen industry trade group, the National Hydrogen Association, also has organized codes and standards working groups to address safety needs. International codes and standards work is coordinated through the International Energy Agency and the International Standards Organization. Other groups involved include the National Fire Protection Agency and the Society for Automotive Engineers. Fuel cell manufacturers, hydrogen tank manufacturers and automakers are also developing best practices for hydrogen use and safety.