Hydrogen fuel cell electric vehicles could be coming to a showroom near you in just a few years. Many automotive manufacturers are turning to hydrogen as an alternate transportation fuel, with initial commercialization expected soon.
For these zero-emission vehicles, a fuel cell converts hydrogen and ambient air into electricity to run an electric motor. Unlike conventional battery electric cars, however, hydrogen fuel cell vehicles can be rapidly fueled (~3-5 minutes) at existing gas stations once appropriate infrastructure upgrades are in place. The state of California is leading in the national deployment of commercial hydrogen refueling stations with a plan to have 68 public fueling stations in place by 2015.
A principal challenge to the widespread adoption of hydrogen infrastructure is the lack of quantifiable data on its safety envelope and worries about additional risk from hydrogen. Using advanced laser-based diagnostics and
imaging capabilities in the Turbulent Combustion Lab (TCL), Isaac Ekoto (8367) and Adam Ruggles (8351) are working to provide that quantifiable data to accelerate the development of hydrogen fuel infrastructure.
“The use of hydrogen as a fuel presents some new challenges because of its unique storage requirements,” says Isaac. “To achieve sufficient energy density for relevant transportation uses, it needs to be stored at extremely low temperatures or under very high pressure, so an unintended release will behave differently than gasoline.”
Hydrogen does have certain added safety benefits. The high diffusion rate and buoyant nature means that leaks quickly dissipate into the atmosphere and move rapidly away from the source.
To assure regulatory officials, local fire marshals, fuel suppliers, and the public at large that hydrogen refueling is safe for consumer use, the risk to personnel and bystanders must be quantified and reduced to an acceptable level. Such a task requires validated methods to assess the potential harm from credible failure modes and a good understanding of effective mitigation measures to control any associated hazards.
Understanding ignition probability
Until recently, most methods used to analyze hydrogen infrastructure safety were adapted from those used for natural gas and industrial environments without much regard to the unique properties of hydrogen. This approach has often resulted in overly conservative rules and requirements that make infrastructure adoption prohibitive in densely populated areas.
To understand the thinking behind the specification of separation distances, one must consider the necessary sequence that results in a catastrophic event — essentially, a fire or explosion initiated by an unintended hydrogen release. First, the gas must be released from its containment system in sufficient quantities to create a hazard. A flammable mixture must then come into contact with an ignition source and ignite. Ignition, however, is not enough; the flame must be able to sustain itself long enough for a hazard to develop. Each process has its own specific probability that is dictated strongly by physical layout and system operation.
Isaac and Adam have examined well-characterized hydrogen jets in the TCL to address the need for suitable analysis tools for large-scale hydrogen storage safety and to better understand potential hazards from unintended releases.
They are able to recreate representative hydrogen leaks using custom burners and a laser spark apparatus to pinpoint ignition at various locations within the release plume. These capabilities enable statistical characterization of the release plume and insight into phenomenological processes during ignition and transition to sustained flame light-up.
To ensure the controlled laboratory experiments preserve relevant flow physics expected from releases from compressed storage, Adam recently designed a high source pressure hydrogen jet and integrated it into the lab. The ability to study realistic release scenarios using state-of-the-art measurement tools distinguishes the TCL from similar labs around the world.
“It’s very easy to study an atmospheric hydrogen jet to understand the fundamentals, but in any real-world release scenario, the stored hydrogen is going to be under extreme pressure or at a very low temperature,” says Isaac. “The next step in this lab is to add the capability to test hydrogen at those low temperatures.”
In a recent experiment, they attempted to spark ignite a hydrogen jet at numerous locations. “We can record whether the mixture ignites and whether ignition leads to a sustained flame or is simply extinguished,” says Adam.
The diagnostics enable the researchers to freeze a moment in time and visualize the distribution of the flammable range. From their results, Adam and Isaac have been able to identify the ignition probability for all locations of a given unintended hydrogen release. Their methods represent a tremendous technical advance over the current method used to establish separation distances for hydrogen, the lower flammable limit (LFL) determined from mean concentration boundaries. For hydrogen, the LFL is about 4 percent.
Grossly excessive safety distances
“Basing hydrogen separation on the LFL leads to safety distances that are grossly excessive. Yes, the flammable limit of hydrogen is much wider, but at what point is there a true hazard? Right now, we can predict the probability of ignition with much more specificity over the old methods based on LFL,” says Adam.
In establishing safety distances for a hydrogen fueling station, the probability of a sustained flame developing is the key metric, not ignition probability. “The hazards of a leak only occur when ignition has transitioned to a sustained flame,” he adds. “If you only consider ignition probability, you end up with distances that are quite large and can inhibit the development of stations in crowded areas. But if you consider the lower probability of sustained flame development, the distances shrink even further. This provides a technical case for building stations with a smaller footprint without compromising safety.”
Isaac and Adam are working to develop validated methods for predicting flame light-up transition.
Hydrogen fueling stations for light-duty vehicles are just one application for this research. Another growing area of hydrogen utilization is in the materials handling sector through the use of hydrogen fuel cell-powered forklifts. In a separate project, Isaac and recently retired
Sandian Bill Houf (8365) examined the risks and potential impact of an accidental release inside a warehouse.
“The codes and standards for how much refueling you could do indoors were based on floor layout, overall volume, and the ventilation system. Essentially, if you had a certain level of active ventilation, you could refuel as much as you wanted,” Isaac says.
Some surprising results
After testing different scenarios they came up with some surprising results. “We found a critical period after a release of about five seconds up to a minute in which the hydrogen was above the LFL before it diffused out,” says Isaac. “If the released gas came into contact with an ignition source within that window, things went bad quickly no matter what kind of ventilation system was in place.” This work has already had an impact on the codes governing hydrogen fuel cell use in warehouses.
The researchers are working with several different codes and standards communities, including the International Organization for Standardization (ISO), International Fire Code (IFC), and National Fire Protection Association (NFPA). Adam and Isaac have performed targeted experiments to answer specific questions, from which they ultimately plan to develop a toolkit that couples release/ignition behavior and hazard modeling with quantitative risk analysis tools that determine failure frequencies.
“The idea is to develop a toolkit that the operator can use to minimize risk as they design a system or facility like a hydrogen fueling station or a pipeline,” says Isaac. “Each setup is unique, so rather than give specific parameters, the toolkit will enable operators to optimize their design using risk reduction strategies.” With sufficient funding, they hope to have a beta version ready within two years.
They also have begun conducting experiments on other fuels like liquid natural gas to refine the understanding of the flammable envelope. “When we started this research 10 years ago, the understanding of hydrogen was way behind other gases. But now, despite its limited use, our understanding of hydrogen has advanced far beyond what we know about other gases because of this targeted effort,” says Isaac. “Everything we do here in the Turbulent Combustion Lab with hydrogen can be done with other gases.”
He and Adam feel as if they’ve just scratched the surface in studying hydrogen releases. Right now the TCL has a simple setup, with an unimpeded vertical jet. “We’d like to do different orientations and with barriers. It’s not likely a release will come from a perfectly concentric circle — most cracks are elongated and we don’t know how that changes the release,” says Adam. “We want to keep taking this one step closer to what could happen in the real world, outside of a lab.”
As Adam and Isaac push toward a better understanding of the true hazards of hydrogen leaks, they are adding to Sandia’s vast body of knowledge and experience when it comes to high-pressure hydrogen systems.
“This effort demonstrates how Sandia’s expertise in combustion science, laser diagnostics, risk assessments, and high-pressure hydrogen science and engineering, is uniquely leveraged to remove barriers to a clean and secure transportation energy future” says Daniel Dedrick (8367), Sandia’s hydrogen and fuel cells program manager.