Entering the Cryo Zone with Hydrogen

Saturation temperature vs pressure for hydrogen, oxygen, and methane [1]

At ambient temperature and pressure (i.e., 298 K and 1.01 bar), hydrogen is a gas with approximately 7% the density of air. This results in an advantageous rise rate of nearly 20 m/s (or six times faster than natural gas) enabling rapid dispersion of any leaks. However, this low density is a challenge for storage.

The two most commonly used methods of increasing hydrogen density in order to store significant quantities are compression and liquefaction. Compressed storage up to 700 bar is commercially available and increases the density of hydrogen by a factor of 477 times greater than ambient pressure; whereas, liquefaction increases it by 866 times compared to ambient conditions (i.e., nearly double the density of compressed hydrogen at 700 bar).

Liquefying Hydrogen

In order to liquefy hydrogen it must be cooled to a very low temperature (e.g., 20 K at 1.01 bar). This is accomplished with a cryogenic liquefier or cryocooler. Various thermodynamic cycles and equipment are available for this purpose.

All liquefaction processes are limited by the ideal Carnot efficiency which is calculated as the ratio of the cold refrigeration temperature divided by the difference in cold refrigeration and warm rejection temperatures. The actual performance of hydrogen liquefiers are a fraction of the ideal efficiency, ranging from about 30-40% of Carnot for state-of-the-art systems.

Two-stage hydrogen liquefiers generally bring the hydrogen gas down to the 80-100 K range in the first stage (i.e., sensible cooling); and then cool and liquefy it in the 20 K range in the second stage (i.e. sensible and latent cooling). Efficiencies are much higher for the first stage due to the higher refrigeration temperature.

Hydrogen liquefaction must also address the conversion of ortho-to-para hydrogen that occurs at cryogenic temperature. This change in equilibrium electron spin state is an exothermic process that is generally accelerated with a catalyst during liquefaction.

Liquid Hydrogen Storage Behavior

Insight into the behavior of hydrogen and other fluids at cryogenic temperatures can be gleaned by examining their saturation temperature at the vapor pressure of interest (see above plot). In a container of liquid hydrogen, the interface between the liquid and vapor is always at the saturation temperature corresponding to the container vapor pressure.

However, the temperatures in the hydrogen vapor space of the container - also known as the ullage - are at or above the saturation temperature (i.e., superheated). For a stationary tank, the ullage thermally stratifies with the coolest temperature near the interface and warmest temperatures near the top of the container.

Conversely, the liquid hydrogen in such a container is at or below the saturation temperature (subcooled). A stationary container with subcooled liquid will also thermally stratify with the coldest temperatures near the bottom of the tank. If the subcooled liquid is circulated toward the interface by a mixer, or from momentum forces in a mobile application, the tank pressure will drop to a new saturation condition.

Over time, the liquid hydrogen in a container will warm toward the saturation temperature (but not above it) due to heat transfer from the environment. When all of the liquid reaches saturation temperature, it will begin to boil off and raise the tank pressure. This additional vapor must either be vented when the tank pressure reaches the maximum design limit; or reliquefied to maintain "zero boil-off" storage.

Cryogenic Material Properties

Most materials behave very differently at cryogenic temperatures compared to ambient conditions. These differences must be well understood by engineers, designers, and operators of cryogenic systems.

Thermal properties of materials such as conductivity and specific heat are highly nonlinear functions of temperature in the cryogenic range. As a result, heat transfer and energy balance calculations often require integrating the property of interest over the temperature range. Simply using an average value between the upper and lower temperature can result in significant calculation errors.

Mechanical properties that can vary significantly at low temperature include:

• Yield and ultimate strength: generally increases at lower temperatures for most solids
• Ductility: some materials remain ductile (e.g. aluminum alloys, austenitic stainless steel with > 7% nickel, most face-centered cubic metals); while some materials become brittle (carbon steel, most plastics, most body-centered cubic metals)
• Elastic modulus: varies
• Fatigue strength: varies

All of the above has implications for the selection of materials in liquid hydrogen system design. Storage tanks of 300 series stainless steel are common. Aluminum alloys are also used in some applications, and titanium alloys are suitable but rarely used outside of the aerospace industry.

Seals for fittings, gaskets, and valves must be comprised of compatible elastomers for cryogenic hydrogen service. Likewise, instrumentation and sensors designed for cryogenic temperatures are required for liquid hydrogen system monitoring and process control.

In the next post I'll touch on tank design options, insulation systems, and filling/draining operations.

References

[1] Cryogenic fluid management of liquid hydrogen, oxygen, and methane: Part 1 - passive technologies, systems, and operations. Moran Innovation LLC, 2022.

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 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 cofounder in seven technology based start-ups; and provided R&D and engineering support to hundreds of organizations. Matt has three patents and more than 50 publications including the Cryogenic Fluid Management report series. More about him can be found here.

The Coming of Age for Liquid Hydrogen Systems: 1960 - present

Apollo fuel cell (left); shuttle external tank (middle); new LH2 dewar tank at KSC under construction

In my last post I wrote about the use of hydrogen in the development of jet engines in the 1930s, and successful test flights with a liquid hydrogen jet aircraft in the 1950s. However, the applications where liquid hydrogen (LH2) systems were fully deployed at very large scale were in the space program and rocketry [1].

Large Scale Liquid Hydrogen Systems Deployment

The initial use of LH2 in rockets was with the Centaur which was first flown in 1962. Integrated with several first stages over the years, it evolved into a workhorse upper stage with over 245 launches and counting.

During the Apollo program, the massive Saturn V rocket that sent astronauts and payloads to the moon used LH2 in its second and third stages. The second stage held 260,000 US gallons (984 000 liters) of LH2; the third stage had 66,770 US gallons (252 750 liters) of LH2 onboard.

The Space Shuttle stored its hydrogen in the enormous brownish-orange External Tank (ET) recognized by anyone who watched a launch live or on video. Loaded into the ET for every shuttle launch was 390,000 US gallons (1 476 000 liters) of LH2.

Contemporary rockets that use LH2 include the European Space Agency's Ariane 5 with over 110 launches. NASA's new Space Launch System (SLS) also uses LH2 and is expected to perform its first launch in the second half of 2022.

All of the above use cases of LH2 over the past six decades has required extensive ground support systems and associated logistics. Large scale production, distribution, storage, fueling, and other operations are well established for LH2 as a result [2].

Hydrogen Fuel Cells

Propulsion wasn't the only use of hydrogen in the space program. The Apollo program used hydrogen fuel cells for power, heat, and potable water for the astronauts [3]. Likewise, the Space Shuttles relied on hydrogen fuel cells to provide power during every one of the 135 missions they flew.

NASA also developed regenerative (i.e., reversible) fuel cells that can operated "in reverse" as an electrolyzer. This technology enabled the use of a single unit to generate hydrogen from water when a power source is available (e.g., solar panels during the day), and then generate electrical power in fuel cell mode when needed (e.g., at night).

Hydrogen fuel cells have more than double the efficiency compared to combustion processes for generating electricity. Yet they retain a key advantage of traditional fossil fuels - the capability to store and distribute large quantities of fuel to be used when needed for energy production.

References

[1] "Hydrogen Systems Development: Past, Present and Future", Oct 9, 2021.

[2] "Transitioning to the Liquid Hydrogen Era", Mar 26, 2022.

[3] "Power and Water the NASA Way", Apr 26, 2016.

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 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 cofounder in seven technology based start-ups; and provided R&D and engineering support to hundreds of organizations. Matt has three patents and more than 50 publications including the Cryogenic Fluid Management report series. More about him can be found here.

The Evolution of Hydrogen Systems: 1930 - 1960

Technological evolution often requires decades of incubation and advancement before large scale commercial adoption is achieved. First discovered as a discrete substance by Henry Cavendish in the late 1700s, hydrogen has followed a circuitous path of discovery and application in a variety of fields.

Its primary large scale commercial use was in the petroleum and chemical industry where it's still a critical element of fossil fuel upgrading processes. Various other industrial processes - including applications as wide ranging as food preparation and semiconductors - use hydrogen.

Although it's been demonstrated in nearly every type of internal combustion engine as a replacement for fossil fuels, it's primary use for power and propulsion (until recently) has been in the aerospace industry.

Liquid Hydrogen

Liquid hydrogen (LH2) has been in routine and continuous use in the space program since the early 1960’s. However, many are not aware that its roots in aerospace trace much further back in aviation to the initial jet engine research and development in the late 1930’s; and later with successful flight demonstrations of a liquid hydrogen fueled jet engine in the mid-1950’s [1].

Initial jet engine testing done by the German's in 1937 used hydrogen in part due to its ease of ignition and high flame speed. First used on a 250 pound thrust (lbf) jet engine operating at 10,000 rpm (and later on a 989 lbf jet engine), hydrogen proved to be an ideal fuel for this new propulsion technology.

About twenty years later, Pratt & Whitney Aircraft developed a jet engine with an afterburner that operated on liquid hydrogen. The project was started in 1956 and resulted in a 4700 lbf jet engine intended for a supersonic reconnaissance aircraft under development by Lockheed. The engine was a success, but the aircraft concept was cancelled in favor of the Blackbird SR-7 development.

Aircraft Flight Testing with LH2

Liquid hydrogen was eventually tested successfully in a series of B-57 flights at the NASA Lewis Research Center from 1956 through 1959. The aircraft was modified with an LH2 tank under one wing; a helium pressurant tank under the other wing; and a heat exchanger to vaporize and warm the hydrogen prior to engine injection.

No modifications were made to the Curtiss Wright J-65 turbojet engines that typically operated on JP-4 (kerosene) fuel. Multiple in-flight tests involved taking off with JP-4 and then switching to hydrogen in one of the two engines during flight to demonstrate various operational conditions.

Three successful flight test campaigns were completed with 38 transitions from JP-4 to hydrogen that thoroughly demonstrated the feasiblity of using LH2 for jet aircraft. In parallel with the flight tests, wind tunnel and fixed engine tests were also performed. The hydrogen jet engines were found to significantly outperform their JP-4 counterparts in terms of engine mass, thrust, stable operation, and fuel consumption.

Beyond Aircraft

Paradoxically, aviation did not become the primary use case for LH2 despite these early successes. However, it did set the stage for LH2 use in rockets and future space vehicles. More on that in the next post.

As a final thought, it is interesting to note that electric cars have followed a similar path. First introduced by Thomas Edison circa 1913, they were initially unable to compete with internal combustion engines. Now both technologies are aggressively competing to overtake fossil-fueled aircraft and vehicles in the marketplace.

References

[1] "Hydrogen Systems Development: Past, Present and Future", Moran, Oct 9, 2021.

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 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 cofounder in seven technology based start-ups; and provided R&D and engineering support to hundreds of organizations. Matt has three patents and more than 50 publications including the Cryogenic Fluid Management report series. More about him can be found here.