Hydrogen – How Safe is It?

As we promote the adoption of hydrogen storage systems for renewable energy and highlight the many cross-sectoral benefits of this versatile energy carrier, the first reaction from many people is “Sure, but is it safe?” or “Isn’t hydrogen really explosive?”.  The answer to these two questions is respectively ‘Yes’ and ‘No’ but it’s important to understand why. The following is a Q&A on hydrogen safety based on the many questions that we and our associates in the hydrogen industry have been asked.

Q: Is Hydrogen Dangerous?

A: All fuels have some degree of danger associated with them, but hydrogen is arguably safer than gasoline, and it is inherently safer than the combustion of fossil fuels. The safe use of any fuel focuses on preventing situations where the three combustion factors—ignition source (spark or heat), oxidant (air), and fuel—are present. Understanding each fuel’s properties, we can design fuel systems with appropriate engineering controls and establish guidelines to ensure the safe handling and use of a fuel.

Some properties of hydrogen require additional engineering controls to ensure its safe use. Specifically, hydrogen has a wide range of flammable concentrations in air and lower ignition energy than gasoline or natural gas, which means it can ignite more easily. Consequently, adequate ventilation and leak detection are important elements in the design of safe hydrogen systems. And because hydrogen burns with a nearly invisible flame, special flame detectors are required

But the fact is that hydrogen can be used as safely as other common fuels we use today when guidelines are observed, and users understand its behavior.

Q: In what way is hydrogen different from natural gas or gasoline?

A: Hydrogen’s properties differentiate it clearly from other fuel sources. In many cases, hydrogen is safer than the fuel we currently use to power our cars. Here are a few key differences:

1)      Carbon-based fuels tend to spread as liquids. When conventional fuel combusts, it produces carbon particles that radiate high levels of heat. However pure hydrogen has no carbon to burn and therefore produces very little radiant heat. In practical terms, if hydrogen were ignited, it will burn quickly with a non-luminous flame that has only one tenth of the radiant heat of a hydrocarbon fire.

2)      Hydrogen is non-toxic. It is a gas under normal atmospheric conditions, and a release of hydrogen does not contribute to atmospheric pollution and will not contaminate groundwater.

3)      Hydrogen is fourteen times lighter than air and when released dissipates upwards at about 20 metres per second, allowing for rapid dispersal of the fuel in case of a leak. Propane and gasoline vapor by comparison are heavier than air and in the event of a leak will gather at ground level where they are much more dangerous.

4)      Hydrogen is four times more diffusive than natural gas and twelve times more than gasoline fumes, so leaking hydrogen rapidly disperses up and away from its source, typically before an explosive mixture can form.

Q: How is hydrogen stored?

A: Hydrogen storage tanks used in fuel cell vehicles such as cars, trucks and buses are extremely robust. Storage tanks are typically manufactured from carbon-fibre wrapped cylinders that are either lined with metal (Type III) or polymer (Type IV). These tanks are far stronger than conventional gasoline tanks. Toyota reached back to its early origins as a loom manufacturer to create triple-layer hydrogen tanks made of woven carbon fibre. In tests on their Mirai fuel cell vehicle tank, it withstood a crush test of 150 tonnes and even survived the impact of high velocity bullets fired point blank twice at the same location without rupture. Tanks are fitted with thermally-activated pressure safety relief devices designed to vent the tank’s contents if temperatures rise.

For renewable hydrogen storage systems, tank manufacturers such as Luxfer manufacture tanks that exceed global technical standards with design lives greater than twenty years.

Q: But what happens if a tank does leak?

A: Much as with other fuel systems, a hydrogen storage tank supplying a fuel cell could develop a leak, either through poor maintenance, a collision or exceeding system design life. However, hydrogen storage and fuel cell systems are designed to with a range of risk mitigation features such as pressure-relief devices, leak detection, heat detectors and venting systems that prevent build up of hydrogen by conducting it to the atmosphere. In the case of escape to atmosphere, the rapid upward release of hydrogen makes it almost impossible to experience the concentrations of hydrogen in which it becomes flammable. In fuel cell vehicles, should a collision take place, in-built collision sensors will activate a safe shutdown mode that is designed to isolate the high pressure hydrogen storage in the fuel tank and also the high-voltage elements that might cause ignition.

Q: What would happen in a car crash?

A: Simulated fires in vehicles compared a hydrogen fire with a gasoline fire. In one frequently cited example, the hydrogen fire resulted in a vertical flame plume which increased the vehicle’s interior temperature by a maximum of 1 – 2 degrees Fahrenheit, while the outside temperature closest to the flame was no hotter than what a car experiences from sitting directly in the sun. The passenger compartment was unharmed. By comparison, the gasoline fire gutted the car and would have killed anyone trapped inside. In fact in these tests, the same car was used. The hydrogen fire produced so little damage that the vehicle could be used for the gasoline test in which the car was not so fortunate.

Q: What are some of the downsides?

A: Here are a few of the challenges associated with hydrogen safety..

Hard to Detect:  Hydrogen is odorless, colorless, and tasteless making it undetectable by human senses. For these reasons, hydrogen systems are designed with ventilation and leak detection. Note that natural gas is also odorless, colorless, and tasteless, but a sulfur-containing odorant is added so people can detect it. There is no known odorant light enough to “travel with” hydrogen at an equal dispersion rate, so odorants are not used to provide a detection method. Many odorants can also contaminate fuel cells. A flame can be detected using special thermal imaging cameras and/or UV measurement.

Lower Ignition Point:  Hydrogen gas requires very little energy to ignite – a mixture of hydrogen gas and air can be ignited along a very wide band of between 4 % to 75 % ‘volume fraction’. To prevent static charges accumulating in hydrogen fuel cell and storage systems, design calls for materials that are good conductors of electricity and builds in ‘potential equalisation’ to ensure no build up of charge. 

Gas Diffusion & Embrittlement: Due to the small size of its molecules, hydrogen can diffuse in or even through materials. Some metals can become brittle when exposed to hydrogen, so selecting appropriate materials is important to the design of safe hydrogen systems.

Q: Okay, But what about the Hindenburg?

A: It appears that any conversation on hydrogen inevitably gravitates toward the Hindenburg Zeppelin disaster of 1937.  Amory Lovins, founder of the Rocky Mountain Institute, commented on the accident in his 2005 paper ‘Twenty myths about hydrogen’. Lovins noted that the explosive power of hydrogen per unit volume is 22 times less than that of gasoline vapour and leaking hydrogen is far more likely to burn than to explode because it burns at concentrations far below its explosive limit. He went on to suggest that contrary to a popular misunderstanding, the attributes of hydrogen might actually have helped prevent further loss of life in the 1937 Hindenburg disaster. He cites an investigation by NASA scientist Dr. Addison Bain who found that the disaster would have been essentially unchanged even had the dirigible been lifted not by hydrogen but by non-flammable helium, noting that there was no explosion. He notes that the 35 passengers and crew who died largely lost their lives by jumping out, or by the burning diesel oil, canopy, and debris. Bain’s investigation found that the cloth canopy was coated with ‘aluminium impregnated cellulose acetate butyrate’ – akin to compounds that have been used as rocket propellant. Zeppelin designers had expressed concerns that electrical charge build-up on the skin could not readily dissipate through the non-conductive butyl fabric coating of the airship, leading to the hypothesis that the combustible fabric was ignited by electrical discharge and only once the outer covering started to burn did he consider that hydrogen played a role in the fire.

Whatever the triggers and contributory factors of the Hindenburg disaster, scientific advances in the intervening 83 years have enabled a far deeper understanding hydrogen’s characteristics and inherent safety attributes, underpinned by greatly advanced technology. As integration of hydrogen systems into our mainstream energy mix grows, we are confident that this flexible energy vector will become the accepted standard for safe use across a wide range of applications.