In June 2021, a Department of Energy (DOE) Small Business Innovation Research (SBIR) grant was awarded to The Protium Company to further facilitate the development of their cryogenic tank technology. Commonly used for the storage of cryogenic liquids, or liquified gases held at deep-freezing temperatures of -150 ⁰C or lower, Protium’s tank technology is bound to play an instrumental role in the development of a renewable liquid hydrogen economy.
Storing hydrogen as an energy source is tricky and done in two ways: in room-temperature gaseous form at very high pressure, or in liquid form at very cold temperatures. Both extremes, high pressure or cold temperatures, are necessary in order to maximize the energy density of the hydrogen.
Most hydrogen fuel-cell vehicles today run on hydrogen gas held in tanks at pressures of up to 10,000 psi [pounds-per-square-inch]. But since liquids are more dense than gas, it’s more advantageous for industrial gases like hydrogen to be stored at cryogenic temperatures to better maximize their energy density.
Yet, when these cryogenic liquids are dispensed into an environment that is above those deep-freezing temperatures, the fueling process can become complicated. So Protium will be directing their SBIR funds, totaling $178,039, specifically toward reducing what’s known as “boil-off” losses that represent this prevalent challenge in liquid hydrogen fueling.
“When you fill one of these cryogenic liquid hydrogen tanks at room temperature, that’s like putting water on a hot stove where it immediately flash-boils and you get all this vapor coming off,” said Eli Shoemake, chief commercial officer of Protium. “Normally, you’d have to fill up your tank for a long time before the tank gets cold enough that it can actually hold and store the liquid. But while you’re doing that, basically all of that hydrogen that is boiling off is wasted and you can’t hold it in your vehicle because it just vents to the atmosphere.”
Not allowing a tank holding liquid hydrogen to vent gaseous hydrogen can present a hazard. Protium aims to better control the bleed off and put the vented hydrogen gas to work in a fuel cell.
“So essentially, what happens is [the tank] is full of liquid, as heat goes into the liquid, the liquid will boil, which means you generate gas, and essentially, you have to let that gas leave the tank, otherwise, you’re going to over-pressurize your tank and it’ll blow up. So essentially, what you have with these liquid tanks is if you don’t want them to blow up, then you’re going to be pulling the hydrogen out of the tank at whatever rate it’s boiling off,” Shoemake said. He added that the gas that boils off is then used in a fuel cell instead of wasted, ideally.
Therefore, Protium has developed a heat exchanger to be built into their tanks to mitigate the amount of boil-off losses during fueling. By transferring the liquid hydrogen within the tank through this heat exchanger, where it warms to a gas before being dispensed into the fuel cell at a steady rate, Protium hopes to maximize the efficiency of their cryogenic tanks by isolating the warming process from the rest of the storage tank.
“We 3D-printed a heat exchanger into the wall of that tank so that while the hydrogen goes through that heat exchanger pathway, the hydrogen is warming up before it exits the tank to the fuel cell,” Shoemake said. “That’s heat that normally would be warming your liquid hydrogen in the middle of the tank and making your boil-off rate worse. So we’ve kind of been using this boil-off gas that’s coming out of the tank to intercept heat that otherwise would go into the tank, which kind of acts like an additional layer of insulation. That allows us to use less insulation in the tank, making it higher performance and lighter, so that’s really the benefit.”
Installing this heat-exchanger into the tank is a novel approach to hydrogen fuel technology, which has been around since the Apollo-era “Space Race.”
“All of these cryogenic storage tanks came out of the Apollo-era and the Space Race when people would ask the question, ‘How do I hold liquid hydrogen forever?’ in these vacuum-insulated containers,” Shoemake said. “[Traditional models] have very good insulation that keeps the hydrogen cold indefinitely, with very little boil-off loss out of the tank and that’s great if you want to hold your hydrogen for days or weeks or months.
“But for most vehicles, it really doesn’t make sense to try and keep that hydrogen forever when you’re just going to use it and burn it up in the fuel cell. So we said, ‘Let’s not just design a tank where that hydrogen boils off and leaves the tank, but leaves the tank at the rate your fuel cell wants to use it.’”
According to Shoemake, the general consensus is that there will always be a 0.5 percent – 1 percent boil-off loss in cryogenic tank technology. So by implementing a measured, but steady, boil-off rate of liquid hydrogen into hydrogen gas through the heat exchanger, Protium’s tanks are already poised to increase the efficiency of traditional cryogenic tank technology. But to take their innovative technology one step further, they’re also exploring how to improve the cooling aspect of their cryogenic tank system in an attempt to further reduce their boil-off rate to zero.
“We’re going to look at creating a liquid hydrogen tank that actually has zero boil-off, because instead of all this heat getting through into the liquid and boiling off your hydrogen, we have basically the entire tank wrapped in this heat exchanger that’s attached to a giant refrigerator that will keep the whole tank cold,” Shoemake explained. “Then you’ll be able to have storage tanks hooked up to a refrigeration system that could hold liquid hydrogen indefinitely with no losses, which right now is a big issue when you’re talking about transporting liquid hydrogen or storing liquid hydrogen for a long time. So if we can get rid of that 1 percent loss by building this refrigerator into the tank, that potentially could make the whole liquid hydrogen chain a lot more efficient.”
With these technological advancements in mind, Protium primarily envisions their tanks being used in unmanned aerial vehicles [UAVs], or drones.
“The size and scale we’re focused on right now is in aerospace, specifically, because it makes a lot of sense,” Shoemake said. “With our technology, one of its key advantages is that it’s really lightweight compared to conventional technologies and it also has the benefit that with liquid you can get a lot more [hydrogen] in a smaller space. Those two things combined are very important for aircrafts.”
But while applications in UAVs and drones are the focus for now, Shoemake doesn’t rule out their technology being scaled for other vehicles in the future.
“Marine vehicles are another one we’re looking at a lot for our cryogenic tanks, where it can make a lot of sense to try and keep your tank size small and you might want to go liquid,” Shoemake said. “Potentially on-road trucks and vehicles as well, but probably not passenger cars, because our tank is designed to fuel it up and then your fuel is going to be dispensed at some rate no matter what. If you have a passenger car, you probably don’t want to park your car for a week and then come back and have no fuel. But something like a long haul semi-truck that’s constantly on the road driving from warehouse to warehouse, maybe this makes a lot of sense.”
Top photo caption and credit – Dr. Ian Richardson and team perform a test on a 3D printed liquid hydrogen tank that is then used to power a stationary version of an Insitu EagleWorX UAV fuel cell. The test is conducted with Jeff Knapp (blue coat, wide-brim brown hat), the Chief Engineer of Insitu’s EagleWorX division at the Outdoor Research Facility (ORF) on the campus of Washington State University, Monday, July 13, 2020 – photo: Bob Hubner, WSU News.
Scott King writes about the environment, science, and technology for the Ally. Support his work.