“But is Hydrogen safe?”

The question I get asked most about hydrogen, especially for transportation, is whether it is safe. The short version is yes – it’s very safe in a car. The slightly longer answer is that it far safer than gasoline and battery powered vehicles. Diesel vehicle safety is probably comparable in terms of immediate risk, but hydrogen has fewer environmental implications.

Most of the reason hydrogen is safe is that it’s packaged in a car in a way that makes it unlikely to have serious issues. The other mitigating factors are how little energy is released in the event of an accident compared to other fuel types.

While there are many other end-uses of hydrogen, I am focusing on vehicles because they are what most people will be interacting with, whether it is buses, heavy duty trucks, or even passenger cars.

Safety – a relative viewpoint

Fun fact: there are ~4000 gas station fires in the US every year[1]. Only New Jersey is exempt, because in New Jersey people can’t pump their own gas. Gasoline is not a safe fuel, but we have normalized the risks. Hydrogen in practice is more safe because we have many more safety restrictions in place.

Fun fact 2: it takes about ~500-600 gallons of water to put out a fire for a standard gasoline combustion type car. It takes well over 3000 gallons to put out a BEV fire, and the batteries can re-ignite, often requiring submersion of the BEV to ensure it stays extinguished. As an example, a Tesla fire in Alabama used 36,000 gallons of water. Were a hydrogen car to catch on fire, it is unlikely that the hydrogen itself would be part of the fire, and it would be relatively easy to put out. In the event that the hydrogen did catch on fire, the amount of energy in the hydrogen is about 40% that of an equivalent range of gasoline, and a very small fraction of the energy produced by burning batteries.

A BEV has significantly worse fire safety than a hydrogen vehicle owing to the high flammability of the batteries. A long-range BEV battery provides nearly 1000 pounds of material for combustion.

In short, the biggest risk of hydrogen – catching fire — is a much worse problem in other vehicle types.

By the numbers- why H2 is a fire risk

Compared to gasoline, hydrogen requires 1/10th the energy to start a fire, has a significantly wider burning range, and a much wider explosive range. That being said, the behavior of hydrogen when it gets out into the wild is to rapidly disperse because it is 7% the density of atmosphere. If it instead ignites, it will create a jet-flame rather than explode. There are some exceptions – see below for production and other risks.

Gasoline may require more energy to start a fire, but given its behavior of pooling and spreading combined with the much lower mixing ratios required for an explosion, it’s just as dangerous if it gets out,  and it is far more likely to get out in an accident than hydrogen. Moreover, the amount of energy in a standard gasoline vehicle is more than double that of a standard hydrogen vehicle owing to the H2 car’s higher efficiency, so the danger can be greater.

Why H2 flammability is less of a concern

Compressed hydrogen fuel tanks are made of carbon fiber. The tanks can take a 21 caliber bullet without taking damage. They are meant to hold up in event of a crash, and if your hydrogen tank is hit hard enough to be crushed in an accident, you have likely been hit hard enough that you won’t be alive. Other components in the fuel line can rupture from lighter accidents, but at that point the hydrogen would come out in more controlled release instead of a massive depressurization event.

Compare this to a standard gas tank – they are relatively easily punctured or cracked in the event of an accident. And compared to BEV – whereby damage to a lithium ion battery can cause thermal runaway and a very hot fire – H2 has much better fire safety.

Liquid hydrogen – all bets are off

Some vehicle designers are thinking about doing liquid hydrogen onboard rather than gaseous hydrogen. The safety features described above go out the window here. Liquid hydrogen is extremely dangerous – it’s rocket fuel. Worse, it has all the bad flame characteristics of gasoline – it spreads out in a pool and offgases – combined with the higher flammability of hydrogen. In a final major issue, liquid hydrogen is cold enough (-253C) that liquid oxygen (-183C) will distill out of the air, creating mixtures of liquid oxygen and liquid hydrogen. This can easily lead to a vapor-gas explosion.

Liquid hydrogen tanks are robust, but not nearly as robust as compressed gaseous hydrogen tanks. For these reasons, you should only expect to see liquid hydrogen on much larger vehicles with professional drivers. Nonetheless, liquid hydrogen is very dangerous.

At production and other risks

One of the biggest risks in hydrogen is trapped volumes of hydrogen mixing with air and reaching ratios where explosions are possible. This is more likely at a production site, but it could happen in other locations. The good news is that hydrogen is extremely slippery – it tries to get out wherever it is. The bad news is that if it doesn’t get out, it will collect and then explode. This happened in 2019 at a production site in Norway and set off airbags in nearby cars. Worse was an explosion at a production site in South Korea where two people died. Expect these issues to decrease drastically as we better identify and control these issues.

What about the Hindenburg and Hydrogen Bombs?

The Hindenburg used an incendiary paint to seal the blimp[3]. This paint caught fire then the hydrogen caught fire. This would be the equivalent of using fireworks to make a gas tank. Generally speaking, it’s bad practice to paint your airship with incendiary chemicals.

Hydrogen bombs are fusion bombs. For those of you that watched Oppenheimer, this would be the big bomb they were referring to – it starts with a nuclear fission reaction to trigger a fusion reaction, both of which are atomic reactions. Any hydrogen that a normal person deals with doesn’t undergo an atomic reaction, it undergoes a chemical reaction (H2 + O2 à H2O), which is many orders of magnitude less energetic than atomic reactions. So the association of hydrogen with bombs is talking about a very different kind of hydrogen.

Conclusion:

In most cases, hydrogen will be very safe for end users. In vehicles in particular, H2 is considerably safer than gas-powered or battery-powered vehicles.

[1] https://www.nfpa.org/education-and-research/research/nfpa-research/fire-statistical-reports/service-or-gas-station-fires

[2] https://h2tools.org/bestpractices/hydrogen-compared-other-fuels

[3] https://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/h2_safety_fsheet.pdf