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|Fire Safety for Hydrogen Fuel Cell Vehicles|
Issue 49: Fire Safety for Hydrogen Fuel Cell Vehicles
By Carl H. Rivkin, P.E.
Hydrogen fuel cell vehicles have been deployed at the prototype level in the US and other countries such as Japan and Germany. However, planned commercial deployment will occur in the near future, and it is therefore important to have a basic understanding of the safety issues that these vehicles present.
Hydrogen fuel cell vehicles present different hazards than gasoline powered vehicles do. These hazards require taking different safety measures than those employed on gasoline-powered vehicles. As with gasoline powered vehicles, these hazards can be addressed with both engineering and operator controls.
There are several ways of classifying hazards associated with
hydrogen fuel cell vehicles. This article will group hazards into three
areas: dispensing and operating, onboard hydrogen storage and use, and
onboard electrical systems. This article does not address vehicles that
employ hydrogen internal combustion engines, although these vehicles
present many of the same hazards as hydrogen fuel cell vehicles.
A hydrogen fuel cell vehicle consists of the systems shown in Figure
1. A hydrogen fuel cell vehicle is an electric vehicle because it is
powered by an electric motor. The fuel cell stack converts hydrogen into
electricity that powers the motor. The term "stack" is used because the
conversion device is composed of many fuel cells that operate in series
to produce the electric current required to run the motor. A single
fuel cell would not produce sufficient power to meet the required
Vehicles typically use Polymer Electrolyte Membrane (PEM) fuel cells,
although there are other fuel cell technologies. Hydrogen is fed on the
anode side of the fuel cell and oxygen is fed on the cathode side of
the fuel cell. The PEM allows the hydrogen molecule that has separated
into OH - and H + ions to have the OH - ion flow through the membrane
and the H + ion to flow around the membrane in a circuit. PEM fuel cells
require less time to power-up and operate at lower temperatures than
other fuel cell technologies, which makes them suitable for vehicle
Dispensing gaseous hydrogen into fuel cell vehicles presents hazards
that are different than dispensing gasoline; hydrogen is transferred
into the vehicle at pressures that may exceed 10,000 psi (70 MPa). The
dispensing nozzle must connect to the vehicle so that, unlike gasoline
dispensing, it forms a closed, high-pressure system. Failures of the
nozzle to lock to the vehicle should terminate the dispensing operation.
However, there is still an opportunity to release hydrogen in the event
of a nozzle failure or other leak.
The failure of the nozzle to adhere properly to the vehicle also presents the hazard of an unrestrained high-pressure hose proximate to the individual performing the fueling. This hazard is addressed primarily by engineering the nozzle system for very high reliability.
Hydrogen storage and use on-board the vehicle
Almost all hydrogen fuel cell vehicles use high-pressure gaseous
storage systems to provide the hydrogen for the fuel cell stack. There
are cryogenic /compressed hydrogen storage systems that are being
developed, but the majority of vehicles that will be deployed relatively
soon will use high pressure storage systems. These systems are
typically either 5000 psi (35 MPa) or 10000 psi (70 MPa) systems.
Hydrogen storage systems are designed with sensor technologies that
will shut down the system if a leak is detected. The leaks are typically
detected by sensing a pressure drop in the system. If the system is
exposed to fire, the pressure relief device (PRD) will function and
relieve the contents of the tank. SAE J25781 states that the
PRD should be installed to direct flow away from people, assuming the
orientation of the vehicle is not changed from the routine orientation.
There is not standardized labeling system used on hydrogen fuel cell
vehicles, although most vehicles have some type of label that indicates
that hydrogen is on the vehicle.
The storage systems will also have to meet the Federal Motor Vehicle
Safety Standards (FMVSS) for crash safety so that the systems will
maintain their integrity after the specified crash conditions.
Electrical safety associated with electric motor and associated components
Hydrogen fuel cell vehicles use an electric motor to power the vehicle. This motor requires high voltage lines to run from the fuel cell, typically located in the center of the vehicle, to the electric motor located in the front of the vehicle. Hydrogen fuel cell vehicles would also typically employ a capacitor or battery to allow storage to start the vehicle and compensate for variation in fuel cell power output. There are a few basic safety rules that emergency responders should follow:
These lines typically run through the bottom of the vehicle so the
general safety rule is to avoid cutting through the floor line of the
Codes and Standards
There are fairly comprehensive codes and standards promulgated that address vehicle fueling and onboard vehicle safety. These codes and standards include:
Carl H. Rivkin is with the National Renewable Energy Laboratory
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