Safety in the underbody
Hydrogen is an environmentally friendly energy carrier used to power fuel cell vehicles. DaimlerChrysler Research has developed an array of safety measures that make the risks associated with handling this promising energy carrier manageable. You cannot see it. You cannot smell it. It is ethereally light in weight, rises swiftly and dissipates rapidly in air. It is neither toxic not corrosive nor hazardous to water. Nor does it ignite spontaneously — and yet most people treat it with the utmost respect.
That’s hydrogen, the lightest of the elements. But these favorable attributes appear to be somewhat misleading whenever the term 'oxy-hydrogen gas' is mentioned. “Well, it’s true,” says Erwin Wüchner of DaimlerChrysler Research. “Hydrogen does ignite very easily. What’s more, if it’s also confined within a solid container during the process, it reacts very violently.”
Dr Wüchner and his colleagues at the DaimlerChrysler Research Center in Nabern, Germany, are working on the development of hydrogen storage systems, for instance for the Citaro fuel cell bus or the F-Cell passenger car, which is based on the A-Class. At the same time, the scientists in RBP (Research Body and Powertrain) are intensively addressing hydrogen-related safety issues. “One has to clearly understand the special characteristics of hydrogen and act accordingly. Then it’s just as safe an energy carrier as is petrol or natural gas,” says Wüchner summing up the basic tenets of DaimlerChrysler’s approach to H2 safety technology.
The position that the German-American automaker has taken on this issue is shared by others. Scientists at institutions of the European Union, at universities and other research facilities as well as utility companies likewise believe that the risks involved in handling the easily ignitable gas are manageable.
In their daily work, DaimlerChrysler researchers apply the tried-and-proven methods of fault analysis in order to eliminate all conceivable risks. “We focus early and systematically on the question of what faults are possible, even in theory,” explains Wüchner. “And for every conceivable fault we develop a safety concept that is subsequently subjected to further testing and improvements. If we encounter a safety risk, we change the concept for the component in question, or we develop a new one until a potential fault becomes manageable.”
Take, for instance, the development of the exhaust system for the 60 fuel cell vehicles that are currently being tested around the globe. When the fuel cells are turned off, small amounts of hydrogen can migrate into the exhaust system, where they normally dissipate swiftly. The issue was: What happens if H2 accumulates there? In a test, the exhaust system was filled with hydrogen, which was subsequently ignited. The system remained intact, but there was a loud bang that approached the pain threshold.
To spare future customers such an experience, the engineers installed a flame arrester. A fine-meshed grid is now in place to ensure that a flame front cannot spread from the tailpipe into the exhaust system. This solution demonstrated that the potential fault 'H2 accumulation in the exhaust system' is manageable. During the development work on fuel cell vehicles, it has always been a top priority to eliminate faults whenever possible and to manage those faults that cannot be eliminated.
The A-Class-based F-Cell vehicles are therefore equipped with special safety features. All hydrogen-related components, such as the pressure tanks, lines, valves and seals, were subjected to many specific endurance tests. These included shake and shock testing as well as vibration and temperature tests. In addition, the passenger cabin of the F-Cell vehicles was separated from the propulsion system by an airtight partition. Escaping hydrogen could also accumulate in certain cavities within the underbody, where the fuel cell stack and the ancillary components are located.
“At sensitive locations in the F-Cell vehicle we have therefore installed hydrogen sensors that switch off the fuel cell system if a hazard is detected,” explains Wüchner. Developed by DaimlerChrysler, these sensors react to low concentrations of hydrogen. As soon as a concentration of two percent by volume is reached —in other words, considerably below the ignition limit of four percent — the gas valves are closed.
Another safety device in the F-Cell vehicles monitors the two hydrogen tanks, which have already undergone extensive tests and certification procedures. These tanks are robust aluminium cylinders coated with a carbon fibre composite. Their normal operating pressure is 350 bar, but they can withstand pressures exceeding 800 bar — more than twice as high. When the vehicle is parked, the valves of both gas cylinders are closed and automatically monitored in order to ensure that they are tightly shut.
The hydrogen lines of the fuelling system are also routinely monitored. They are equipped with sensitive pressure monitors that check the integrity of all gas lines and connections. A control system records the pressure when the ignition is turned off and compares this value with the pressure during the next start. Other potential sources of faults may occur in the stack and in the membranes that separate hydrogen from oxygen in the fuel cells. Since such a defect could result in an oxy-hydrogen-gas reaction, engineers have taken appropriate precautions. The housing containing the fuel cell stack is constantly ventilated during operation, so that even minimal amounts of hydrogen are dispelled.
“In the event that an accumulation of H2 in the stack box should occur anyway, an H2 sensor will respond,” says Wüchner describing this seamless safety design. In addition, a pressure test is performed before every start to indicate whether hydrogen could have escaped into the exhaust system due to membrane leaks. Engineers are planning yet another improvement for the next generation of F-Cell vehicles: any escaped hydrogen will no longer be conducted into the exhaust system to be dissipated there. Instead, free hydrogen resulting from the operation will be piped into the intake air, so that it can react with atmospheric oxygen in the catalyst layer on the cathode in the stack to form harmless water.
This is another example of the basic tenets behind the safety engineering of the hydrogen experts at DaimlerChrysler. “We’re not content with relying on sensors. Instead, we are committed to the use of robust solutions to proactively prevent malfunctions,” Wüchner emphasises. DaimlerChrysler’s safety design anticipates any conceivable event: a minor fault will illuminate a warning light, which advises that a trip to the repair shop is needed. Higher H2 concentrations trigger onboard safety measures, up to and including a system shutdown. As Wüchner sums it up: “Safety is assured in and by the vehicle itself.”
Hydrogen is much lighter than air and consequently tends to rise quickly. What’s more, its nearly invisible flame doesn’t emit much heat. Experts therefore don’t consider a hydrogen-fuelled vehicle more hazardous than a petrol-powered one. If a crash should result in a leak, the hydrogen flame would shoot upward and burn up in a very short time.
Nevertheless, hydrogen, which is colourless and odourless, is highly explosive due to its high combustion energy and its minimum ignition energy of only 0.02 millijoules. Even a small electrostatic discharge can provide that amount of energy. The burning rate ranges from 102 to 346 cm per second. The ignition or explosion limits define a range between 4 and 77 volume percent, which means that hydrogen is ignitable over a wide range of concentrations.
On the other hand, the detonation limits are between 18 and 59 volume percent. Thus the detonation limits of hydrogen are substantially narrower than its ignition limits. What that means is this: in the event of early ignition, hydrogen burns up before it can reach its detonation limits and trigger the pressure increase and its explosive effect.
“Filling ’er up” with a fuel cell car is similar to refuelling a natural gas-powered vehicle in that the gas hose and the filler neck are connected by an airtight coupling. The electrostatic charge on the driver or the vehicle represents a theoretical hazard at the hydrogen filling station. When the refuelling coupling is brought into contact with the filler neck, a spark might occur that, in the presence of a leak, could ignite the highly flammable hydrogen.
Such accidents have occurred in the past when fuel vapours were ignited. For that reason, all car tyres as well as the tarmac of filling stations now possess a certain level of conductivity ‘to ground’, so that a vehicle becomes electrostatically discharged in less than a second. By the time the driver has left the vehicle and walked to the pump there is no further danger of an electrostatic spark-over.
As an additional safety precaution, the refuelling coupling at hydrogen filling stations is automatically tested for an airtight seal. What’s more, the refuelling line is briefly tested for pressure. If the pressure drops, the pump dispenses no hydrogen. DaimlerChrysler Research is also developing an innovative filler neck with an integral data interface. An infrared diode on the filler neck transmits data to a receptor on the hose coupling. This enables the vehicle for instance to let the refuelling pump know the operating pressure and temperature of the onboard fuel tanks.
A resulting advantage is that the filling station can fill up the vehicle’s hydrogen tank more precisely. This simply hasn’t been possible in existing systems, because hydrogen heats up during the refuelling process, so that only approximate adjustments are presently possible for the temperature-related increase in pressure. The new data interface will also go some way toward supporting additional safety improvements.