Safety with Liquid Hydrogen

Relative to conventional fuels, hydrogen has a wider flammability range in air (4–75%), higher permeability through some materials, and a lower ignition energy (0.02 mJ). These characteristics make it vital to provide adequate ventilation, prevent leaks, and eliminate ignition sources in any hydrogen system design and operation. System monitoring and detection is important to ensure that all safety precautions are active and operating as intended. 

The very rapid 20 m/s rise rate of gaseous hydrogen in air under ambient conditions greatly aids with ventilation and dilution. Hydrogen also has an auto-ignition temperature of 585°C, which is higher than most fuels. Hydrogen is colorless, odorless, and not toxic to breathe. However, low oxygen detection is needed anywhere hydrogen may accumulate near personnel since asphyxiation is possible if insufficient oxygen is available.

When combusted, hydrogen produces no smoke or soot, which eliminates associated inhalation risks common with fossil and other hydrocarbon fuels. The resulting flame also produces much less radiant energy compared to hydrocarbon fires, thereby reducing the zone of potential heat damage or burns. A hydrogen flame is nearly invisible under daylight conditions, requiring infrared sensors or cameras for detection. At night, the flame is pale blue in appearance. For all hydrogen systems, emergency and fire response planning and coordination is a critical consideration.

The use of LH2 introduces additional safety concerns beyond gaseous hydrogen due to the temperature extremes and phase change characteristics inherent in cryogenic fluid systems. Personnel training, appropriate protective clothing, human interface designs, and safe operations are key to mitigating frostbite and other physiological risks. Appropriate equipment design that eliminates the possibility of human contact with cryogenic surfaces is preferable whenever possible. Exclusion zones, caution and warning systems, safety sensors, and approved operational procedures further mitigate risks to personnel.

The large temperature ranges in cryogenic systems require careful materials selection and design to accommodate differences in thermal expansion and contraction. Phase change from liquid to vapor (and sometime the reverse) occurs throughout a LH2 system, resulting in potential rapid pressure changes in isolated volumes. This must be addressed with appropriate design, operations, and pressure relief devices. If maximum vent relief flow rates are high enough, flaring may be required.

Mitigation of ice buildup is necessary where it may cause key components to not operate properly or create other hazards. Likewise, prevention of oxygen condensing out of the air is addressed with proper insulation on any surfaces that may reach low enough cryogenic temperatures. Any LH2 spills will begin to immediately vaporize and rise as the vapor warms. However, the initially cold hydrogen vapor will be denser than air and can result in temporary regions of high concentration near the ground.

Material selection for hydrogen service must address the design and operational requirements for strength, ductility, fatigue, permeability, and other material properties. Approved cleaning processes must be followed to ensure that unacceptable contaminants are not introduced to the system from materials. Purity levels of the hydrogen are generally dictated by the fuel cell specifications or other feed requirements. Purging and inerting of the assembled system is required for various operations to prevent the introduction of air or other contaminant fluids.

Retrograde US Energy Policy

"The biggest story in the data is the dramatic growth of [US] solar energy, with a 30 percent increase in generation in a single year, which will allow solar and wind combined to overtake coal in 2024." [1]

This pie chart and quote may be one of the last bits of promising US energy news we'll get for the next few years. Many colleagues have asked my opinion about the prospects for hydrogen in the US under the new administration. Here's a breakdown of what we already know, and my guess about what's to come.

Federal Energy Policy

The new federal energy policy can be summarized as a huge step backwards that prioritizes oil and gas while demonizing intermittent renewables [2]. This ignores the fact that solar and wind are the lowest cost power generation sources to bring online and operate, which is the primary reason they have grown so rapidly in recent years.

Mitigation of greenhouse gases and pollution are existentially crucial additional benefits of renewables, making them the logical focus for growth from both an economics and environmental perspective. But propaganda trumps cost of electricity, public health, casualty losses, and the future quality of life for coming generations in the current administration.

The new secretary of energy has parroted this policy, with additional emphasis on liquefied natural gas (LNG) exports from the US. LNG is 85-95% methane, which is a 25 times more potent greenhouse gas than carbon dioxide over 100 years (85 times more over 20 years). Gas leaks and intentional venting are prevalent sources of methane emissions from production, transport, and end use of LNG.

What About Hydrogen?

There is no mention of hydrogen whatsoever in any US energy policy documents released by the new administration. So what does that mean for federal policy regarding hydrogen? Let's connect some dots by enumerating a few key benefits of hydrogen in the energy sector:

1. Hydrogen produces no greenhouse gases and no pollution of any kind when used to produce electricity with fuel cells. If burned in a turbine or other combustion engine, it produces some NOx (as all combustion processes do) that can be minimized with various design and operational parameters.
2. Hydrogen can store energy at nearly unlimited scale from intermittent renewables when excess generation capacity is available, and be used to generate electricity when demand exceeds generation capacity.
3. Hydrogen's unparalleled specific energy relative to any other conventional fuel enables high performance sustainable solutions across multiple mobile and transportation sectors (e.g., aviation, rail, maritime, trucking, etc.)
4. Hydrogen provides unique energy resiliency and eliminates fuel logistics dependencies for remote or isolated regions.

Note that none of the above benefits are aligned with the new federal energy policies. Nor were they eight years ago when we saw this energy policy disaster unfold the first time around. Looking back at that timeframe may help make a clear-eyed assessment of what's to come.

The Path Ahead

Within this new reality, what is the future for hydrogen in the US? Regrettably, here are my predictions:

• Federal funding for hydrogen programs, including the hydrogen hubs, will be largely gutted. One potential exception is military applications where hydrogen addresses strategic defense and national security challenges that no other approach can match.
• States and local policies and funding will help in a few US regions. California will remain the hydrogen hotbed it has been for many years. Hawaii, New York, Pennsylvania, and parts of New England also have or may provide supportive policies for hydrogen. Texas will be a wildcard since there is much in place for hydrogen production, but may have fractured policies depending on the area (e.g., Gulf coast vs rural areas). However, many other states and locales already have policies that are hostile toward renewables and hydrogen, and will be emboldened to double down on derailing permitting and similar tactics with the new federal policies.
• Private sector funding for hydrogen systems and products has been extensive in some industry sectors and regions. Many of these hydrogen applications have demonstrated performance and economic viability at various commercial readiness levels. It's unlikely that private investors will walk away from sunk cost investments if there is an opportunity to get a reasonable return. The challenge is which global markets are the best targets if most of the US is off the table, which leads to my final prediction.
• Global regions will likely stay the course, or even accelerate hydrogen plans, as the US backs away. China will build on its lead as the largest producer and user of hydrogen and associated systems. The European Union, United Kingdom, India, South Korea, Japan, and Australia may find increased interest in new hydrogen projects in their regions with the drying up of US funds and incentives. The same for other countries and regions with established and emerging hydrogen programs such as Canada, South America, Middle East, Africa, and other countries in the Asia and Indo-Pacific regions.

[1] John Timmer, Ars Technica, Jan 27, 2025.

[2] Executive Order, Jan 20, 2025.

[3] Secretarial Order, Feb 5, 2025.

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 break-through liquid, slush, and gaseous hydrogen systems since the mid-1980s. Matt was also the Sector Manager for Energy & Materials in his last position at NASA where he worked for 31 years. He's been a co-founder in seven technology startups; and provided R&D and engineering support to many organizations. Matt has three patents and more than 50 publications including the Cryogenic Fluid Management series. He also teaches courses, workshops, and webinars on liquid hydrogen systems.

Cryogenic Hydrogen Thermal Design Options

A key storage consideration for liquid hydrogen is the vaporization rate caused by environmental heat loads, often referred to as boil-off. The above graphic shows some of the established methods for mitigating or eliminating boil-off categorized by the input power required [1].

Passive techniques require no input power and include design and material selection for insulation, structural supports, piping, and other tank interfaces that minimize heat transfer to the inner tank wall. Hybrid methods require some input power for valve actuation, mixers, pumps, or other components to reduce the boil-off rate. 

Finally, active techniques require power input for cryo-refrigeration or densification processes. Depending on the concept of operation for the system, application of the appropriate combination of these methods can minimize or eliminate boil-off losses [2].

Hydrogen Storage Options

The most common commercially available storage options along with some of their key characteristics are shown above [1]. Each of these methods has advantages and disadvantages that are critical considerations for selecting the best storage method for a given system and use case.

Low-to-moderate pressure storage in salt caverns or other compatible underground locations enables large quantities of gaseous hydrogen to be stored for long periods of time. Electrical energy input is required for a blower or compressor to inject the hydrogen and buffer gas underground at the desired storage pressure. This approach relies on the necessary local geology as well as connection to a gaseous distribution system similar to natural gas infrastructures. A hydrogen liquefaction plant can also be sited nearby using the underground hydrogen storage as feedstock after removal of the buffer gas.

Material-based or solid-state storage is a broad category that encompasses methods to store hydrogen in a matrix material via microscale adsorption or chemical absorption. Many materials and methods have been developed, with metal hydrides currently the most common type in commercial use. Thermal energy is generally required for the solid-state storage reaction to occur as adding hydrogen is exothermic requiring cooling, and removal is endothermic with heat addition. Metal hydride systems are generally better suited to stationary applications due to their low mass fraction and should be installed where low-cost process heat and cooling are available or low pressure is especially important. However, newer technologies using lightweight matrix materials such as aerogels may hold promise for some mobility applications.

Compressed gaseous hydrogen storage requires electrical energy to create the high pressures required. Cooling is also needed to bring the gas stream back down to ambient temperature due to the heat of compression. Composite overwrap pressure vessels (COPV) designed to withstand the high pressures are commercially available at 350 or 700 bar. Initial mobility demonstrations with hydrogen often use COPV storage, and for some applications this option is sufficient to meet the system goals. For many mobile applications, however, the volumetric energy density and mass fraction of compressed hydrogen storage is too low to meet performance requirements.

Cryogenic LH2 has nearly double the volumetric storage density of 700 bar compressed hydrogen at ambient temperature, and along with low-pressure storage conditions enables a much higher mass fraction. For this reason, many transportation applications transitioning to hydrogen are storing or planning to store in cryogenic liquid form. The primary electrical energy input required for LH2 storage is the liquefaction process. As previously mentioned, LH2 has been the primary storage and distribution method in the space industry for many decades.

In aviation, H2Fly demonstrated LH2 onboard storage during successful flight testing of their small demonstrator aircraft. ZeroAvia and Airbus have publicly shared their LH2 design plans for new aircraft. Nikola and Hyzon have demonstrated long-range truck routes with LH2, and First Mode has demonstrated hydrogen in a large mining truck. Operational mobility systems using LH2 include Hyundai Rotem trams and marine vessels from multiple companies.

Cryo-compressed hydrogen is another option that has intriguing advantages for some mobile applications. In this supercritical storage state, the hydrogen is compressed at cryogenic temperature resulting in higher potential volumetric density compared to LH2 storage. However, the mass fraction is generally less than LH2 storage systems due to the need for thicker walled vessels to withstand the higher pressures. The U.S. Department of Energy (DOE) has funded development of this technology over many years, and it is now being commercialized by Daimler Truck, Verne, and others. Daimler has also developed a ‘subcooled’ transfer process that can fuel a truck with 80 kilogram of hydrogen in 15 minutes or less without a return vent line resulting in onboard cryo-compressed storage. [2]