6. Safety Considerations

As discussed above, there are many characteristics which introduce new challenges for the safe use of hydrogen as a fuel. Described in the literature are various ways of managing these challenges.

6.1. Main Considerations for FCEVs

In the event of an accident, the high pressure hydrogen should be vented in a safe manner to prevent a fire risk or an explosion. The vented hydrogen should be directed upwards and away from the vehicle. A. R. Carpenter and P. C. Hinze [12] suggest sounding an audible alarm when venting due to the fact that the hydrogen and/or hydrogen flames are difficult to see and can present a danger. It is noted that this could be designed into the relief valve or vent so that when venting the gas causes a loud whistle sound.

The FCEV should be designed, as is similar to a gasoline vehicle, so that the fuel tank is not in a position at which it would be compromised in the event of an accident.

In the event of an accident or leak, the fuel system should be designed to isolate the tank. This is done with excess flow valves and blow-off valves; in the event of a problem, solenoids are triggered to seal off the tank preventing the hydrogen from escaping. The author suggests using the same sensors that trigger airbags to trigger shut-off valves. Furthermore, leak detectors in the passenger area and at other positions should detect high concentrations of hydrogen.

6.2. Filling Stations

For consumers, a hydrogen filling station will work in a similar way to the familiar petrol station. The car will be parked next to a pump and a hose will be attached to the vehicle. Filling of the hydrogen will then commence. Note that the hose will be attached, not just inserted into the filler as on a petrol or diesel vehicle.

Break-away device

One issue with a hydrogen filling station is that if the vehicle was to accidentally move away during refilling, the dispenser hose would become detached and hydrogen would leak. It is noted that similar accidents have occurred in gasoline filling stations. In a report about hydrogen refilling, S. Kikukawa et al. [10] state that a break-away device should be used to prevent such accidents. Also considered is a built-in function for FCEVs that prevents movement during refilling.

Excess flow valve

An excess flow valve would close automatically if a pipe would rupture and a large amount of hydrogen would leak. Note that the probability of this event happening is very low.

Cooling the hydrogen

Another issue for hydrogen filling is that the compressed hydrogen gas at the station must be cooled to about -30 degrees to the hydrogen station. This is to prevent a rise in temperature in the tank beyond the operability limits. A pre-cooler can perform this task. In fact, due to the fact that the Riversimple vehicle requires only 1 kg of hydrogen, the fill rate can be lowered without extending the fill time beyond reasonable limits. This means that the gas does not need to be cooled and so energy is not expended in the cooling process.

Emergency shutdown

M. Casamirra, et al. [11] discuss the use of strategically placed “panic buttons” that would shutdown all hydrogen lines; essentially a manual emergency shutdown. These buttons would be placed inside and outside of the filling station. A backup generator is also to be used to ensure power for all emergency operations, including the emergency shutdown, emergency lighting, and fire pumps.

Layout

In terms of the layout of the filling station itself, NASA state that a weather shelter or canopy should not be enclosed by more than two walls set at right angles and should have vent space provided between the walls and the canopy [8].

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