This could well be the first question on your lips when you hear that our Mirai fuel cell vehicle is powered by hydrogen. Here’s our answer.
Hydrogen? Is that safe?
The quick and the long answer
05/08/2015
This could well be the first question on your lips when you hear that our Mirai fuel cell vehicle is powered by hydrogen. Here’s our answer.
Toyota, of course, only releases a car into the market that is totally safe. The Mirai is no exception. Over the last decade, hundreds of Mirai test cars have been thoroughly road tested, crash tested and safety tested. They have racked up millions of kilometres over all sorts of demanding terrains. They have been put through their paces in the cold of northern Finland and the heat of southern Spain. Their hydrogen fuel tanks have even been shot at by high-velocity weapons.
The result? The Mirai has passed all the tests with flying colours. It’s as safe as any other Toyota vehicle. The fact that it is powered by hydrogen has absolutely no effect on its inherent safety.
Let’s unpack the short answer to look at three aspects of a hydrogen powered car like the Mirai that relate to safety: the car, the refuelling process, and the gas itself.
Their main source of strength originates from the carbon fibre shell. Over that is a glass fibre layer. Should the car be involved in an accident, any resulting damage to the hydrogen tank will be clearly visible on this layer. Tests can then be carried out to evaluate if the carbon shell itself is compromised. The glass fibre does not contribute to rigidity of the tank, but gives absolute confidence of its integrity. The whole tank is lined with plastic to seal in the hydrogen.
As mentioned above, the tanks have been subjected to extremely severe testing. They are designed to withstand up to 225% of their operating pressure, which is clearly a very comfortable safety margin.
In the improbable event of a leak, the Mirai contains highly sensitive sensors that detect minute amounts of hydrogen. These are placed in strategic locations for instantaneous detection of hydrogen. In the extremely unlikely event of a leak in the fuel system, the sensors immediately shut down the safety valves and the vehicle itself.
As a third layer of safety, the cabin is strictly separated from the hydrogen compartment to prevent penetration of any leaking hydrogen, which would instead gradually disperse into the atmosphere.
Refuelling is a critical process because it involves human action, which unfortunately can lead to unforeseen and unsafe scenarios, like trying to drive off while the fuel nozzle is still connected to the car. For this reason a number of safety precautions have been put in place.
First, the nozzle at the end of the hydrogen dispenser’s flexible hose contains a mechanical lock to ensure optimal connection with the car’s filling inlet. Unless this mechanical lock clicks into place securely, filling will not commence.
Secondly, a pressure impulse checks for any leakage in the system between the filling station and the car. If a leak is detected, refuelling is aborted.
Thirdly, the rate of filling is carefully regulated, to avoid overheating during transfer. Temperature sensors located in the car’s hydrogen tanks, the nozzle and the pump constantly communicate with each other by infrared to control the rate of flow of hydrogen into the car so that the temperature rise is not excessive.
The internationally applicable standards SAE J2601, SAE J2799 and ISO 17268 establish safety limits and performance requirements for gaseous hydrogen fuel dispensers. The criteria include maximum fuel temperature at the dispenser nozzle, the maximum fuel flow rate and the maximum rate of pressure increase.
By the way, if you try and drive off in your Mirai while the fuel nozzle is attached to the car, you won’t succeed! The car’s ignition is disconnected until you have replaced the nozzle in its holster and closed the car’s fuel cap. To be absolutely sure, a redundant safety system is embedded in the hose and locks the pump if a car would pull the hose too hard when driving off in the middle of refuelling.
Hydrogen is the lightest thing known to man and considerably (14x) lighter than air. The consequence is that should a leak occur, the hydrogen will rise into the atmosphere. And thanks to its status as ‘’smallest molecule ‘’ in the universe, it disperses quickly in air and any gas.
The advantage of this is clearly illustrated in gunfire tests conducted on a hydrogen tank. When the hydrogen ignites it appears as a localized jet flame, which is much safer than an accumulation of gas that could suddenly explode.
Finally, the Mirai’s tanks have a pressure relief device that releases the hydrogen gradually in case the temperature should rise abnormally (like in a fire). This prevents any overpressure or explosion; far from the stereotype of a hydrogen explosion. Moreover, the resulting fire leaves much of the car undamaged.
Hydrogen is as safe as any other fuel used in a car. It’s been used as an energy carrier for decades, and there is a vast amount of cumulative know-how and experience in Toyota and elsewhere to handle it safely. Furthermore, it is a carbon-free, non-hazardous energy source that can be produced from many renewable resources and emits no greenhouse gases when used as a fuel.
No wonder that “mirai” is a Japanese word meaning “future.”
