With fast refuelling, longer range, and only water as a by-product, fuel cell electric vehicles (FCEVs) powered by hydrogen may be in the early stages of implementation but are already showing a lot of promise. To safely implement this exciting technology, the risks of hydrogen flammability must be managed.
In this regard, leak testing of fuel cell components has become critical. Manufacturers may need to test the tightness of each individual bipolar plate – of which there can be around 400 in each fuel cell stack. Carelessness is not an option.
“As soon as hydrogen reaches concentration of 4.0% in air, it can be ignited,” explains Dr Rudolf Konwitschny, application specialist at Pfeiffer Vacuum. “There are numerous examples in history where hydrogen explosions have occurred. We must avoid the low explosion limit of hydrogen in the air.”
There are many safety regulations guiding and governing the fuel cells field. Component suppliers and OEMs around the world will be familiar with those of the International Organization for Standardization (ISO). For ISO/IEC member states there is no obligation to convert regulations into national legislation. However, standards become legally binding when laws or legal regulations, such as EU directives, refer to them. For example, European standards known as ‘EN’ have been mandated by the European Commission as a way for companies to meet the requirements of the EU directives.
Fuel cell-related standards
ISO 22734 and IEC 62282 are both important documents for the hydrogen world. ISO 22734 focuses on hydrogen generators using water electrolysis, while IEC 62282 refers to fuel cell technologies and modules, with a focus on safety.
ISO 22734 says that “the cell stacks shall be subjected to the common pressure test,” where “the oxygen and hydrogen sides of each cell stack shall be connected to a common pressure source and tested simultaneously.” Test conditions are also provided: pressure should be no less than maximum operating pressure and the test duration should be 2 minutes.
In addition, precise temperature conditions are listed in ISO 22734, but there is no explanation of the influence of these parameters on the test result. According to Konwitschny, this is something that manufacturers should be aware of.
“Temperature is one of the environmental parameters that influences leak testing by pressure decay or with air in general,” he explains. “Pressure must be maintained at a very precise level because, depending on the size and volume of the part you want to test, this can be a major contributor to the reading of a pressure gauge. For very small parts such as the bipolar plate of a hydrogen fuel cell, we are talking about temperature constancy in the range of less than 0.1°C.”
Leak testing methods and regulation conformity
In the hydrogen-specific IEC 62282, just two air-based methods are mentioned: pressure decay and flow measurement. Additional leakage tests for hydrogen gas component connections and piping joints are described in ISO 22734, including the bubble test and tracer gas leak detection. However, what the regulations lack is any clear and meaningful guidance with respect to pass/fail criteria.
“There are other regulations which are specific to leak testing, and they mention more methods on a more general basis,” adds Konwitschny. “ISO 20485 summarises some methods and there is also EN 1779.”
EN 1779 lists a total of seven quantitative integral measurement methods based an air and specific tracer gases for pressurised components. These methods include leak detection with ammonia, pressure decay, bubble testing, tracer gas, and more. Published a few years later, ISO 20485 describes tracer gas leak detection methods only, including one additional tracer gas method not mentioned in the older regulation.
There are further differences between the hydrogen-specific and general regulations. For instance, the pressure decay test described in ISO 22734 specifies that no pressure drop within 2 minutes is allowable. On the other hand, EN 1779 understands tightness as a measurable quantity and explicitly prohibits leakage rate “=0”.
Greater harmonisation between leak detection regulatory standards would no doubt be welcomed by the FCEV market. In principle though, the safety-driven philosophy of most current regulations is aligned. To summarise, all fuel cell manufacturers are required to measure the gas escape of a part, feed the data, document the escape of hydrogen, and apply a safety margin. They must then publish the measurement to the company that integrates the cell stack into a complete system. That company can then ventilate the system to avoid an explosive mixture.
Beyond the regulations
There are many other considerations for selecting a leak detection method beyond compliance, including cost. “Cost is a question of both investment and running expense,” says Konwitschny. “If you have a small-scale production, the initial cost of the test system is important because you need to subdivide this on the part that you want to produce. If you are in high-volume production, the cost of a test device is negligible. Instead, you are absolutely focused on speed and running costs; you need ultra-short cycle times.”
Cycle times can be slow for pressure decay and flow measurement methods due to the long stabilisation times needed for the compression heat to dissipate prior to the test phase. Cycles are significantly quicker with alternative methods, such as vacuum testing with tracer gas. Still, when applied properly, the performance of air-based methods is sufficient for many applications in the fuel cell industry.
Although tracer gas testing is not discussed much in the fuel cell-related regulations, this mature method is used widely in many industrial applications, where it is often considered the most sensitive of leak testing methods. Tracer gas leak detection has been well implemented in the FCEV industry, too. Pfeiffer Vacuum is one of the world’s leading leak detection companies and has supported the development of numerous FCEV prototypes with its well-recognised tracer gas leak detection systems and air flow-based devices.
There are many more considerations when selecting a leak testing system. Ultimately, when choosing a method for any application, the golden rule is to ensure the test simulates the real-life pressure gradient.
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