Instrumentation tips and tricks for fuel cell testing
For hydrogen to succeed as a scalable and affordable alternative energy source, technologies must be effectively optimized and tested. Here we discuss ways to improve the efficiency of fuel cell testing setups by optimizing the use of mass flow and pressure instrumentation.
Tip 1: Optimize flow control stability
Maintaining steady control of inlet gas flow prevents fluctuations in fuel cell power output, minimizing changing variables and simplifying testing. This is so useful that in some cases, the ability of a mass flow controller (MFC) to hold a setpoint is actually more important than accuracy or repeatability specs.
Tightly specifying control valves
One tactic to ensure steady mass flow control is to tightly specify valves. This requires striking a balance between control stability and pressure drop. A large valve, for example, tends to create a lower pressure drop than a smaller, more restrictive valve – but it also results in decreased control stability and resolution.
A properly specified valve must be large enough to reach full scale flow when fully opened, and small enough to maintain resolution at low flow rates. A handy rule of thumb is to use a valve at around 50-75% of capacity, just in case operating conditions end up changing.
Using the same MFC to flow both hydrogen and another gas
Tight valve tuning can be problematic for applications using one MFC to flow multiple gases, for example hydrogen and air or nitrogen. In this case, the higher molecular weight gas may be unable to supply the necessary pressure drop to reach the full scale flow rates. This issue must be resolved, as the outlet usually feeds directly into the fuel cell system – and system pressure must also be closely controlled.
When looking for regulation equipment that can be used across a wide range of gases and conditions, it is often worth compromising narrow valve tuning in favor of versatility.
Tip 2: Regulate fuel cell system pressure
It is critical to maintain both inlet gas flow and fuel cell system pressure. This can be difficult due to the dynamic relationship between flow and pressure.
Alicat mass flow units come with integrated pressure sensors, and can therefore control mass flow while simultaneously monitoring pressure. This is useful for setups with upstream valves, however certain systems require a downstream valve instead. In these cases, the pressure sensor measures upstream supply gas pressure instead of fuel cell stack pressure.
One solution for maintaining system pressure is to tightly control gas flow at the inlet of the fuel cell and to regulate back pressure at the outlet. During this process, it is important to keep the device electronics safe from high temperatures and humidity. A couple ways to do this are to use stainless steel instrumentation and sensors, or even to employ remote exhaust valves that can be positioned at some point in the line away from the electronics.
Dome loaded, mechanical back pressure regulators also work well in testing conditions, particularly for multi-phase fluids. Performance and control resolution can be increased by using an Alicat dual valve pressure controller as the pneumatic sub-system to provide pilot pressure.
Tip 3: Ensure compatibility with high humidity conditions
Maintaining gas humidity across a fuel cell stack ensures even power generation and keeps the system healthy. Unfortunately, high humidity gases can damage mass flow electronics and decrease measurement accuracy. For this reason, gas humidification systems are generally placed downstream of MFCs on the inlet of fuel cells. This, in addition to high system temperatures, makes it is particularly tricky to measure mass flow of the exhaust gas.
Coriolis mass flow technology is a viable solution. However, the ultimate goal is to make hydrogen technology scalable and affordable – and these devices tend to be more expensive than other mass flow technologies. Depending on system requirements, a more affordable, lower accuracy Coriolis device may be an ideal solution.
Tip 4: Simplify troubleshooting with easy to use instruments
Easy to read screens and local controls on flow and pressure instruments simplify troubleshooting and optimization. The backlit screens on standard Alicat units enable real time control of systems.
When actively managing parameters, the local controls allow changes such as setpoint control or gas composition selection to be done quickly and easily locally on each unit.
Tip 5: Easily test gas mixtures
It is common to experiment with different gas mixtures in early stages of hydrogen system testing. Beyond ensuring valves are properly specified to flow necessary gases, it is also beneficial to use highly versatile equipment. Alicat devices are calibrated to reliably flow 98+ gases (including custom-defined mixtures) and can therefore easily be used for fuel cell gas mixture testing. They additionally operate from 0.01% to 100% of full scale, meaning the same unit can oftentimes be used even as the processes are scaled up. This simplifies fuel cell systems and minimizes overall cost.
Tip 6: Prevent leaks
Hydrogen gas is expensive and poses numerous risks to safety, making leak prevention critical. The proportional control valves of controllers present a particular risk for leaks, especially when flowing small molecules like hydrogen. ASCO valves are a good solution as they have a supreme leak spec. It can also be beneficial to helium leak test all mass flow and pressure instrumentation as this can provide the utmost assurance that your devices are leak free.
Tip 7: Minimize test bench footprint
When scaling up test stands, space may become a concern. At higher flows, thermal units may require long straight runs of pipe. In contrast, laminar mass flow technology can be placed in any piping layout without the need for long, straight runs of pipe. Alicat devices are therefore valuable for their ability to minimize test bench footprint.
Have additional questions or concerns about fuel cell testing setups? Contact our applications engineers today!