This is a pre-print of my October 2020 article for Cold Facts Magazine by the Cryogenic Society of America.
Although the minimum work to liquefy hydrogen is 3.92 kWh/kg, the current best performance for industrial scale hydrogen liquefiers is near 10 kWh/kg. In other words, the best hydrogen liquefiers are only achieving 40% of the theoretically possible efficiency. This energy cost to liquefy hydrogen is approximately 30% the energy content of the fuel at the lower heating value. Given the current trends towards renewable energy and low carbon content of cool electro-fuels like hydrogen, humanity is overdue for fundamental innovations in hydrogen liquefaction.
From what I can tell, three approaches to improving hydrogen liquefier efficiencies have dominated the community for several decades: 1) Bigger is better, 2) Mix the refrigerants, and 3) Build better bearings. Hydrogen liquefaction cycles approaching Gigawatts in size have been proposed using these concepts. The average size seems to be 30 tonne/day capacity, approaching $3M/tonne in CapEx and over 100 square meters in footprint. This liquefaction paradigm is not scaleable for our distributed renewable energy future. My goal is to present a new paradigm for hydrogen liquefaction that challenges the above approaches and spurs needed innovations before we build more inefficient large-scale systems.
Bigger is better dates back to Galileo who realized that length increases by x, area by x2, and volume by x3. This means that as an object gets bigger, the volume increases faster than the surface area. For those of us trying to keep something cool, minimizing the surface area to volume ratio is a good thing. These trends in liquefier efficiency versus size were verified 44 years ago by Strobridge and remain mostly valid. However, manufacturing economies of scale and distribution standards place inherent limitations on how big we can practically make a liquefier. Moreover, technology scaling rules are not universally applicable and new concepts are emerging that will change the way we design liquefiers.
Hydrogen liquefier technology has stagnated for several decades. Liquefiers using Claude and Reverse Turbo Brayton cycles are most common. For reference, Table 1 lists losses, by component, in a hypothetical 250 t/day Claude cycle liquefier operating at 10.8 kWh/kg.1 What is clear from the table is that the dominant loss is hydrogen compression.
|Hydrogen liquefier component||Percentage of system losses (%)|
In the last few years hydrogen electrolysis from water using Proton Exchange Membrane (PEM) systems have dropped dramatically in cost. Earlier in September, Cummins announced construction of the largest PEM electrolyzer system in the US (5 MW) to be built here in Washington State. Although most people understand how electricity can split water into hydrogen and oxygen, many do not understand how this very same process can be used to pressurize hydrogen via a process known as electrochemical compression. Once split from water, hydrogen ions can be pressurized via additional membranes and electricity to over 1 MPa at >95% efficiency, exceeding the very best mechanical compressors within the same solid-state device. This new compression paradigm could enable smaller hydrogen liquefiers to operate with higher efficiencies and more reliability than the larger traditional systems.
The second largest loss in the table is liquid nitrogen precooling. Smaller hydrogen liquefiers can utilize more efficient cooling cycles; potentially removing the need for refrigerants altogether. The theoretically ideal cycle is the Stirling. Philips Stirling coolers in the 1960’s performed better with hydrogen as the working fluid than helium above 50 K. An opportunity exists to create an open cycle Stirling cooler running with hydrogen that uses 3D printing for the regenerator matrix. 3D printing could allow for more thermodynamically optimal heat exchangers that follow branching ratios similar in geometry to a ‘river’. Whether acoustic, pulse, or you name it, this will be a complex optimization that will also likely require a better understanding of ortho-parahydrogen catalysis and complex Computational Fluid Dynamics (CFD).
The next largest loss mechanism is the cold expander. Cryogenic turbo-expanders running with hydrogen have been historically difficult to seal. Below 50 K the thermal deBroglie wavelength of hydrogen is larger than the mean distance of interaction. This means hydrogen is more quantum mechanical than classical in this regime. I keep trying to imagine a quantum mechanism of some kind that can stream work and/or entropy out of hydrogen. The entropy/energy of ortho-parahydrogen conversion is one opportunity as it’s by far the largest per mass of any cryogenic mechanism. Chemical mechanisms won’t help us at these temperatures. But once created, such a cold expander is the final piece that would allow a small, non-rotary, mostly solid-state, maximum efficiency hydrogen cooler.
Small, efficient hydrogen liquefiers would change plant operations substantially. Liquid hydrogen tanker trucks are limited to 5 tonne of capacity by highway standards, and a 5 tonne/day liquefier size corresponds well to the size needed by most renewable energy farms. Direct loading of tankers from a smaller liquefier would eliminate the need for the on-site storage that is so benefited by Galileo’s square-cube law. The 5 tonne/day size is much smaller and more portable, potentially small enough to fit in the column of a large off-shore wind turbine. This smaller form factor is also more amenable to manufacturing automation, and commodities of scale. All of these points could lead to more optimal, smaller hydrogen liquefiers.
I fully realize what I’ve mentioned here are extensions of what is currently practical. However, the future is close enough that a big project for an advanced small scale hydrogen liquefier could make a timely advance. Climate change and electrification are not going to wait. Systems change when more things become connected more simply. Why try to increase the efficiencies of compressors, expanders, and storage vessels when you can remove them all together?
Please reach out if you are interested in conducting a techno-economic analysis on this topic.
 Walter Peschka, “Liquid Hydrogen: Fuel of the Future,” Springer-Verlag/Wien (1992).
 Stang, Jacob & Nekså, Petter. (2010). Development of large-scale hydrogen liquefaction processes from 1898 to 2009. International Journal of Hydrogen Energy – INT J HYDROGEN ENERG. 35. 4524-4533. 10.1016/j.ijhydene.2010.02.109.