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| Source: "Liquid Hydrogen Systems Course", Moran, 2025. |
No Free Lunch (But Keeping it Chill Has Some Benefits)
It takes energy to store hydrogen whether it's liquefied, compressed, converted to/from another carrier fluid, or absorbed/adsorbed in a solid state material. So when does liquefaction make sense as the storage method of choice?
The answer lies in a properly executed systems engineering analysis and design trade study for the application being developed. There are a few key features and capabilities of liquid hydrogen (LH2) systems that come into play when comparing options:
- Higher volumetric density
- Lower pressure storage
- Higher storage mass fraction
- Cooling capacity
- Pressurization capability
The first three have been discussed in previous posts, and are the reason why so many mobility applications are using or contemplating LH2. These characteristics also increase in importance for large scale applications such as maritime, rail, long-distance trucking, offtake and fueling infrastructure, delivery and export, etc.
However, the last two capabilities are often underappreciated or ignored by those not experienced with cryogenic systems. The cooling capacity and self-pressurization benefits of LH2 are used in a variety of ways in a properly designed system.
Both of these capabilities can effectively recover some of the energy expended during liquefaction, a fact that many reports and studies tend to ignore. State-of-the-art liquefaction plants require about 10 KWh of input energy per kg of liquefied hydrogen, and conceptual plant designs are expected to bring this down by more than 50% (see below). But part of that energy is still accessible in the stored LH2 for those who know how to use it.
A Cool Thing to Have Around (And Who Needs a Compressor)
The cooling capacity of LH2 can be used in heat exchangers to reject heat from other subsystems; maintain structures below their maximum temperature limits when subjected to high heat loads; or keep superconducting components at the required cryogenic temperature. This can all be done as part of the warming process of a hydrogen stream being fed to a fuel cell or engine.
Cooling is also available through the Joule-Thomson effect which can be produced using a pressure drop device with LH2 or GH2 below 200 K (e.g., orifice, valve, etc.). Another cooling mechanism unique to hydrogen is the endothermic reaction of the para-to-ortho conversion that can be catalyzed in a vaporized hydrogen stream.
And when it's time to pressurize a hydrogen system, the LH2 will do the job with no input power needed. Metering a small amount of LH2 into an isolated volume and allowing it to vaporize and warm results in any pressure required if done properly. This "pressure-building" method has been widely and routinely used for decades in LH2 systems of all sizes and types. No compressor or compressed gas needed.
It all comes down to knowing how to make cryogenics work in your favor. And the first law of thermodynamics is where to start (see below). Applying good systems engineering can uncover a great deal of overall efficiency improvements keeping this fundamental equation in mind.
Author Bio
Matt Moran is the Managing Member at Moran Innovation LLC, and previous Managing Partner at Isotherm Energy. He's been developing power and propulsion systems for more than 40 years; and first-of-a-kind gas, slush, and liquid hydrogen systems since the mid-1980s. Matt was also the Sector Manager for Energy & Materials in his final position at NASA where he worked for 31 years. He's been a cofounder in seven technology-based startups; and provided R&D, engineering, and innovation consulting to several hundred organizations. Matt has three patents and more than 50 publications including his online Cryogenic Fluid Management guide and Decarbonizing Mobility with Liquid Hydrogen SAE report. He has created and taught liquid hydrogen courses, webinars, and workshops to global audiences.
