PULLMAN, Wash. – Washington State University researchers have developed a mathematical model and a set of recommendations to improve liquid hydrogen storage tank operations that could someday make hydrogen a more viable alternative for powering vehicles and other industrial processes.
The researchers used real-world tank data to identify operational regimes in which hydrogen boils off and is lost, which can be as much as 25% of the hydrogen delivered to storage tanks. The work is published in the journal Cryogenics .
"If we want to reduce reliance on fossil fuels and come up with fuel that is clean and produced from renewable energy sources, then liquid hydrogen is a most suitable candidate for that purpose," said Konstantin Matveev, professor in the School of Mechanical and Materials Engineering and a co-author on the paper. "Now we have a tool that can model important parts of the liquid hydrogen supply chain and using that tool, we can make this technology for the green economy more feasible."
Hydrogen-powered vehicles are an alternative to gasoline or diesel-powered combustion engines because they don't emit harmful greenhouse gases. They are particularly appealing for heavy machinery, such as forklifts or trucking, where electric vehicles require too many batteries. One company, Plug Power, currently operates about 250 liquid hydrogen tanks that power 70,000 hydrogen-powered forklifts around the world, moving approximately 30% of groceries in the U.S.
But storing and transporting hydrogen is a major challenge for the industry. Liquid hydrogen is the most convenient form of hydrogen for most industrial uses, but keeping it liquid means it has to be stored at extremely low temperatures. Any time that the hydrogen encounters normal air temperatures, it boils off very quickly. To keep the hydrogen liquid and move it in and out of tanks, a large number of structural elements and mechanisms are employed, such as insulating shells, pressure valves, fluid circuits, and pumps to minimize boil-off losses.
"There are several complex processes happening at the same time, which makes developing a theoretical model really important not only to understand the current operations, but also to invest in technology to improve those operations," said Jake Leachman, corresponding author on the paper and a professor in the School of Mechanical and Materials Engineering.
One area where a lot of loss happens is when hydrogen is being transferred.
"The transfer line has to be cooled down, and during that process, around 13% of hydrogen molecules stored in the liquid form are lost due to evaporation and can't be utilized as a liquid hydrogen fuel," said Kyle Appel, first author on the paper and a recent master's degree graduate from the School of Mechanical and Materials Engineering.
In their work, the WSU research team developed a theoretical model for real-world tank performance and verified it using data from a fleet of Plug Power's in-service tanks. The researchers showed that changes in liquid hydrogen tank operations can yield significant boil-off loss reductions, and that it is possible to get to zero boil-off with additional system modifications. For instance, they showed that changing the pressure limits when the relief valves are activated can decrease hydrogen loss by about 26%.
"That's just changing the set parameters of a valve, which is pretty simple," said Appel.
The mathematical model they developed is computationally efficient to run, too, said Matveev. Previous, more involved models have taken days to run, required a supercomputer, and could only simulate the tank's operations for a few hours. WSU's new simplified model calibrated against real-world test data can simulate hundreds of hours of operation in minutes.
"Using this tool, you can effectively explore a variety of operational changes, so our contribution here is also in developing an efficient mathematical model that can be used in industry, by customers, designers, and government entities," said Matveev.
The researchers are continuing to work with Plug Power as they look at ways to implement their recommendations for liquid hydrogen tanks. They also want to refine their model to better understand transfer operations, pumps, and other devices in the hydrogen systems. The researchers are doing additional studies for the Federal Aviation Administration, evaluating and modeling the storage of liquid hydrogen at airports.
