It's been about a year since my last hydrogen myth busting episode posts [1, 2]. This episode is about the hydrogen zombie myths that not only won't die but have actually made their way into reports from authoritative sources.
The focus will be on liquid hydrogen where many inaccuracies and outdated information are still being published. It's critically important to resolve this issue since public policy, regulations, funding priorities, and private sector investments are heavily influenced by these publications.
Two contemporary reports from the International Energy Agency (IEA) and the U.S. Department of Energy (DOE) will be used as examples [3, 4]. They are both excellent documents from an overall hydrogen survey standpoint, but unfortunately contain critical errors and omissions on the topic of liquid hydrogen.
Boil-Off Gas (BOG)
The potential for BOG generation is inherent in any cryogen storage vessel (liquid hydrogen, natural gas, oxygen, nitrogen, etc.). Environment heating - often termed "heat leak" in cryogenic engineering jargon - eventually leads to vaporization (aka "boil-off") of the liquid in a purely passive system if stored long enough.
There are several ways to address this, all of which have been operationally demonstrated over many years and have the highest technology readiness level possible (TRL = 9). Futhermore, all the necessary components are commercially available from long established supply chains:
- Vent the gas. To prevent a liquid hydrogen storage vessel from getting above its maximum design pressure, the BOG can be vented. Legacy systems have historically released this vent gas to ambient air or used a flare stack. The IEA and DOE reports (and most others) imply that this is the only method of handling the BOG issue. It is not. In fact, modern liquid hydrogen systems can and should be designed to completely avoid venting and associated BOG losses except in emergency off-nominal situations.
- Passive thermal design. Optimized insulation systems, support structures, penetrations, and other well established cryogenic design techniques are the first line of defense against BOG generation. In some cases, this is sufficient to keep the storage vessel within its specified pressure range without venting depending on the concept of operations (e.g., if the feed rates to a fuel cell, engine, or fuel transfer are sufficient to offset vaporization).
- Mix the liquid. Liquid hydrogen storage vessels become thermally stratified over time, with subcooled liquid settling toward the bottom in the absence of any external agitation. By periodically mixing the liquid, the tank pressure can be reduced as the fluid is brought toward equilibrium.
- Vapor cooled shielding. The environmental heat load can be reduced by adding a vapor cooled shield (e.g., tubing coil) that the hydrogen vapor is fed through prior to consumption in a fuel cell or engine. This lowers the vaporization rate within the storage vessel and can eliminate the need to vent depending on the concept of operation.
- Joule-Thomson cooling. When hydrogen near its liquid temperature is expanded in a valve, orifice, or similar device, there is a drop in temperature and a significant cooling effect. Using this effect with either internal or external heat exchangers can cool the stored hydrogen whenever vapor is fed to a fuel cell or engine and bring down the storage vessel pressure.
- Cryo-refrigeration or cryocoolers. In any application, cryo-refrigeration integrated with a heat exchanger can not only completely eliminate BOG (i.e., "zero boil-off"), it can also liquefy, re-liquefy, and condition liquid hydrogen to lower temperatures including densified states. This optional capability is built into the largest operational liquid hydrogen dewar tank in the world at NASA Kennedy [5, 6].
It's also worth noting that housekeeping and auxiliary power demands can be supplemented by BOG fed to a fuel cell. So although zero boil-off is possible with any liquid hydrogen storage system, it may actually be more desirable to design for some amount of BOG to provide this function. Trade studies early in the system development cycle are important to optimize all the design parameters (including BOG) to address needs, goals, objectives, requirements, etc.
Nevertheless, the DOE report states the following (p17): "Liquid hydrogen is not viable for long-term storage (>10 days)". This is a stunningly incorrect statement, particularly since liquid hydrogen is the only method of long-term (and large scale) hydrogen storage that has been in continuous use for more than 60 years and counting.
Key takeaway: Vent losses can be completely eliminated in any liquid hydrogen system with proper system design using proven methods (i.e., zero boil-off). Claims and assumptions based on "unavoidable boil-off" are wrong and result in erroneous conclusions.
The IEA report indicates that 20% of the initial energy in hydrogen is lost during liquefaction "conversion" (p140). The DOE report states: "Hydrogen liquefaction uses >30% of the hydrogen energy content" (p17). This is obviously a significant discrepancy.
The calculation is fairly simple based on the Carnot cycle temperatures, system pressures, liquefaction COP, hydrogen heating value, and thermophysical state point properties. Unfortunately, none of these assumptions are provided in either report, so it's difficult to pinpoint why the values are so divergent.
In any case, they are both misleading. Much of the energy "lost" during liquefaction is recovered in the eventual operation of a well designed liquid hydrogen system. This is due to the significant cold sink that the stored liquid hydrogen provides and can be applied to re-liquefaction, cooling of various streams and components, thermodynamic power or refrigeration cycles, and other system functions.
Key takeaway: Energy analyses should always consider the broadest system scope, operations, and lifecycle. Calculations of efficiency or energy losses that focus on an isolated process, subsystem, or component will result in misleading figures of merit.
Our legacy fossil fuel paradigm is based on separate locations and operations for exploration, extraction, refining, storage, distribution, and fueling. Even the incumbent liquid hydrogen providers have historically followed a similar paradigm of separate production, liquefaction, and delivery to their customers.
However, for many (if not most) emerging applications, the production, liquefaction, storage, and fueling of liquid hydrogen is best implemented in one location. Renewable energy and water (or other local feedstocks) inputs; liquid hydrogen output; all at the same site.
In this new paradigm, the distribution and transportation losses are nonexistent. Nonetheless, neither report - nor most others like them - even mentions this option. Instead, transportation losses are baked into every assumption and conclusion in both reports.
Key takeaway: Liquid hydrogen will be produced, stored, and delivered all in the same place at the point of use for many modern applications. This is a game changing paradigm shift that impacts every assumption and analysis.
Footnotes and References
- Hydrogen Myth Busting (Episode 1), 2022-Apr-22
- Hydrogen Myth Busting (Episode 2), 2022-May-01
- Global Hydrogen Review 2022, IEA, 2022-Sep-22
- Pathways to Commercial Liftoff: Clean Hydrogen, DOE, 2023-Mar-21
- Liquid Hydrogen Droplets to Millions of Gallons: Lessons Learned from Decades of Space Exploration Systems Operation, Testing and Technology Development, 2020-Sep-10
- Densified Liquid Hydrogen and No-Loss (Zero Boil-off) Systems, 2021-Dec-02