Receiving Liquid Hydrogen

The methods used to receive liquid hydrogen into a container impact the pressure and temperature response during a filling process. Tests conducted with tanks that are not vented during filling as shown above provide insights into which methods are best used for various applications [1].

Bottom Filling

The left top and left bottom panels of the above graphic plots the pressure and fluid temperature responses from a nonvented liquid hydrogen filling from the bottom of a tank using a dip tube diffuser. The general trends would be similar for an open discharge pipe in the same configuration.

Tank pressure slowly rises as the liquid level increases and compresses the ullage gas. The ullage gas remains thermally stratified with gas temperatures slowly rising due to compression. As the liquid level rises, it submerges each temperature sensor causing a sudden drop to liquid hydrogen temperature at that location.

This method of filling works well for tanks that will be partially filled and/or stored for extended periods of time. Liquid hydrogen is conserved by not unnecessarily cooling the container walls above the liquid level.

Upward Discharge Filling

Injecting liquid hydrogen upward from the bottom of a container as shown in the middle panels above actively mixes the liquid in the tank. This mixing enables sufficient condensation at the liquid-vapor interface to keep the pressure near saturation condition until relatively high fill levels are reached.

If the discharge jet has sufficient initial momentum to reach the upper portion of the tank, then the fluid and wall temperatures will be rapidly cooled. For lower velocity injection, the temperatures in the gas and wall will behave similar to the bottom filling configurations, while the liquid remains relatively close to saturation due to constant mixing.

For applications where a well mixed liquid and controlled tank pressure during filling are needed, this filling method works well. Ground vehicles, aircraft, and marine vessels are examples where these characteristics may be desirable.

Top Spay Filling

A top spray liquid hydrogen injection configuration is shown in the  top and bottom right panels above. The resulting droplets drive the tank pressure and ullage gas toward saturation due to the mass transfer occurring between each droplet and the hydrogen vapor.

Design of the spray cone angle will dictate how much of the tank wall is cooled by the incoming droplets. The location and number of spray nozzles are also important considerations if this method is used.

Since top spray filling directly controls the ullage gas conditions, this configuration works well for applications where tank pressure control is a driver during filling. A spray nozzle can also be used in an internal mixing system to drop tank pressure during storage and mitigate the need to vent.

[1] "Hydrogen No-Vent Fill Testing..." Moran, Nyland, and Driscoll, NASA TM-105273, 1991.

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.

Cryogenic Systems Modeling and Analysis

Cryogenic Fluid Management (CryoFM™) Interactive Calculations Notebook

Last month I participated in NASA's annual Thermal and Fluids Analysis Workshop (TFAWS) as a short course instructor, panelist, and presenter on the topic of cryogenic fluid management. This is a critical topic for launch vehicles and spacecraft. It is also becoming a very important consideration for the rapid growth in production, energy storage, ground transportation, shipping, and aviation applications of liquid hydrogen systems.

What is Cryogenic Fluid Management?

Cryogenic fluid management deals with the systems, technologies, and operations associated with the liquefaction, storage, and transfer of cryogenic liquid propellants. Hydrogen, oxygen, and methane are the most commonly used fluids for this purpose.

Appropriate modeling and analysis is vital for development of high performing cryogenic systems. There are three broad categories of software tools typically used for this purpose:

1. Computational fluid dynamics (CFD): The highest fidelity option that also generally requires the highest level of resource commitment (i.e., computational, personnel, and licensing). CFD typically uses a very fine mesh of finite volumes to model the system. Setting up the model and the appropriate parameter adjustments requires experience with the particular CFD software being used and an understanding of how to best represent the actual system of interest.
2. Multi-nodal models: A moderate fidelity and resource option that divides a cryogenic system into discrete lumped nodes. The number of nodes can be few or many, and is a key determinant of the model resolution. Similar to CFD, the modeler's experience with the software and ability to accurately represent the actual system is critical.
3. System-level and first-order analysis: The lowest fidelity option with generally the lowest resource commitment. Reduced order system models and first order analyses can be used early in the development to narrow the trade space of feasible designs. Also useful as a check on the results obtained from higher fidelity tools.

System-Level and First-Order Analyses

Generally, the development of a new cryogenic system and assessment of key operations begins with system-level and first-order analyses. These activities can be performed faster and for lower resource expenditures compared to higher fidelity modeling. They enable assessment and modification of the early design and operational options.

Commercially available general purpose system simulation software options have limited cryogenic modeling capabilities. Conversely, while many cryogenic system specific software tools have been developed over the years, most are either proprietary or inconsistently maintained and documented. And validation of model results for all of the modeling options is an ongoing challenge for applying them to new cryogenic systems.

The short course I taught at the NASA TFAWS event was an attempt to address the documentation issue by presenting a publicly available report on passive cryogenic fluid management that can be accessed online by anyone at no cost. My subsequent technical presentation outlined the status and plans for a set of calculation software tools based on that report for quickly performing first-order analyses and building system-level models.

While both the training course and technical presentation were well received, several excellent questions from workshop participants have been on my mind:

• How can all of the planned cryogenic fluid management tools best be developed and maintained?
• What about users who don't have access to the tool platforms or aren't permitted by their organization to download them (e.g., Python)?
• If other platforms are of interest to specific users (e.g. Matlab), who will modify the tools for those users?

The Open Source Option

One potential approach to addressing these questions is to make the new cryogenic fluid management software tools open source. This approach would ensure that they are accessible; and would encourage  community development, maintenance, and expansion to other platforms and new capabilities.

The screenshot shown at the top of this post represents a small first step in that direction. It uses the Jupyter platform to integrate markdown outline, text, images, and equations with interactive calculations in the Python programming language. A fully functional instance of the notebook can be invoked in a web browser without downloading anything.

By hosting these tools in a public GitHub repository, full access is granted to anyone interested in using or modifying the tools subject to the open source license. Improvements and extension to other platforms can be likewise shared among the user community. If you have any feedback on this approach, or are interested in being part of a future beta test group for the software tools, send me a message at info@moraninnovation.com.

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.

Transferring Liquid Hydrogen

Schematic of Test Facility Used for LH2 Transfer Testing [1]

In the last post I talked about liquid hydrogen (LH2) storage. At some point there is a need to transfer that stored hydrogen to another container, or to feed it to an engine or fuel cell.

Pressurized transfer requires raising the pressure in a storage container in order to enable fluid flow to a lower pressure receiving tank or other destination. This can be accomplished with the injection of a pressurant gas, or by increasing the saturation conditions by warming the LH2.

Pressurant Gas Injection

There are only two practical pressurant choices in LH2 systems: gaseous helium (GHe) or gaseous hydrogen (GH2). GHe is the only gas with a lower boiling point than hydrogen and for that reason it  has been historically used in many aerospace LH2 systems. GHe pressurant is generally supplied via high pressure tanks. Note that the ullage gas portion of an LH2 container will always contain some partial pressure of GH2 vapor when GHe is used for pressurization.

The primary benefit of GHe is that it won't condense onto the LH2 surface (i.e. it's a non-condensable pressurant). This reduces the amount of pressurant mass needed to maintain tank pressure, and ensures a lower bound of container pressure for a given mass of injected GHe. Although GHe does dissolve into LH2, the rate is relatively slow and generally not an issue for most aerospace applications where it has been used to date.

However, GHe is an expensive pressurant option and many modern systems employ some type of helium recovery process as a result. Helium is extracted from natural gas as a byproduct of radioactive decay of uranium and thorium. Once helium is released into the air, it continues to rise into the atmosphere without reacting with other elements and is lost. It is therefore a nonrenewal commodity.

Alternatively, GH2 pressurant can be provided via high pressure tanks or by tapping off the stored LH2 (i.e. autogenous pressurization). While a GH2 pressurization system is generally cheaper and lighter to implement, the GH2 can condense onto the LH2 surface if the LH2 is subcooled relative to the container pressure. This can result in a rapid pressure drop (i.e. ullage collapse) if subcooled LH2 is circulated toward the interface due to sloshing or other means.

Autogenous Pressurization

The simplest form of autogenous pressurization is caused by passive environmental heating resulting in GH2 boil-off gas generation and the subsequent increase in container pressure. This process occurs in every LH2 container and can be used to transfer hydrogen in either gaseous or liquid form. If the pressure approaches the maximum container design pressure when transfer is not needed, it must be vented or reliquefied via a cryo-refrigeration subsystem.

Existing vessels used to transport LH2 (over-the-road trailers, trains, barges, etc.) predominantly use a more controlled method of autogenous pressurization to transfer the LH2 at the delivery location. A small amount of LH2 is siphoned from the container and vaporized in a pressure building subsystem. The pressurized GH2 is then injected into to the vessel ullage to enable pressurized flow.

Another method of autogenous pressurization used in launch vehicles is to combust some of the hydrogen in a gasifier or gas generator. The heat of combustion is used to vaporize, heat, and pressurize GH2 flow for pressurization. Regeneratively cooled rocket nozzles are also used for generating high temperature and pressure GH2.

A final method that can be considered autogenous is raising the saturation pressure of the LH2 container. This can be accomplished with a heat exchanger in the LH2 that has warm gas circulating within it to raise the LH2 temperature. As the LH2 temperature rises, the tank pressure also increases. This method is most commonly used when GH2 is fed into an engine or fuel cell rather than the transfer of LH2.

Pumps and Bladders

LH2 pumps are sometimes used to increase the flow rate capacity during transfer or engine feed. These pumps range in size and complexity depending on the application (e.g. from small units for trailer off-loading, to very large and complex turbopumps like the ones used with the Space Shuttle main engines). Note that some pressurization of the supply container is still generally required to meet net positive suction head requirements for the pump and to prevent cavitation in the inlet line.

Metallic bladders separating the gas and liquid phases of an LH2 container have been historically used in a few applications to improve fluid management, minimize slosh, and reduce pressurant requirements. Fatigue failures and high residuals were two of the challenges encountered with this approach. However, recent development work with nonmetallic bladders that remain flexible at LH2 temperature is showing promise.

What About Delivery?

Once the liquid hydrogen is stored, transferred, and transported it needs to be delivered to it's point of use. As alluded to above, this has already been done for many decades with trailers, trains, and barges using LH2 dewars. More recently, shipping vessels have been developed for long distance transport. Other modes are in active development as well (e.g. airships).

But is delivery always necessary for LH2? For traditional fossil fuels the extraction, refining, and end use is generally widely distributed. This is not necessarily the case for LH2, however. Hydrogen can be generated, liquified, stored, and used in the same location.

This is an intriguing paradigm shift for the use of LH2 as a replacement for traditional fuels. Economics, geopolitics, renewable natural resources, and other parameters will dictate where and how LH2 is generated, stored, transferred, and used.

[1] “Liquid Transfer Cryogenic Test Facility: Initial Hydrogen and Nitrogen No-Vent Fill Data”, Moran, Nyland, and Papell, NASA TM-102572, Mar, 1990.

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.

Storing Liquid Hydrogen

Composite cryogenic demonstration tank (courtesy NASA)

Storage vessels for liquid hydrogen (LH2) can be broadly classified as single-wall tanks or double-wall dewars. In both cases, operating pressures are generally kept relatively low. As mentioned in my previous post, stainless steel and aluminum alloys are the most commonly used material for these vessels. Metal liners with a composite overwrap have also been used in some LH2 applications.

A very active area of ongoing development are vessels comprised only of composite material for applications where weight reduction is critical (see photo of an example above). Composite vessels can also potentially withstand higher pressures with reasonable wall thicknesses providing increased storage density and greater operational flexibility.

Single-Wall Tanks

Single-wall tanks are used in applications where storage times are relatively short and consumption rate is very high. The most common example are rocket stages that are loaded on the launch pad and consume most of the LH2 during the several minutes required to reach orbit.

Spray-on foam insulation (SOFI) is the most common option historically used for thermal protection of a single-wall LH2 tank. Key design considerations include: foam thickness; micro-cracking due to large temperature differentials; water uptake from the environment; repair and maintenance; and other factors.

Aerogel blankets are another potential option for single-wall LH2 tanks. While this option mitigates some of the issues with foam mentioned above, other design considerations come into play (e.g. cost, installation, total insulation mass, etc.).

Double-Wall Dewars

Dewars are comprised of an inner wall that contains the LH2 and an outer wall exposed to the environment. The space between the walls is evacuated and generally contains insulating materials. These vessels are sometimes referred to as vacuum jacketed and are based on the same principle as "vacuum flasks" used to store hot or cold beverages.

Dewars are heavier than a single-wall design with comparable storage capacity because of the additional containment wall. However, their thermal performance is far superior due to the minimization of conduction and elimination of convection heat transfer within the vacuum jacket. For this reason, virtually all current stationary and transportation LH2 vessels are dewars.

Insulation options inside the vacuum jacket to further improve thermal performance include: reflective surfaces, perlite, glass bubbles, aerogel beads, multi-layer insulation (MLI), and others. MLI is the highest performing practical insulation option within a vacuum jacket. Double aluminized mylar with dacron netting spacers is a common MLI configuration.

Design parameters that effect MLI performance include: materials used, number of layers, layer density, boundary temperatures, compression load, and degradation factors. Degradation due to seams and penetrations must be minimized with proper design and installation techniques to ensure acceptable storage performance.

Penetrations

Penetrations refer to any solid conduction path that is in thermal connection with the inner tank wall and its contents. In well-insulated tanks, penetrations and their associated thermal conduction often impose the largest heat load into the LH2. Some key penetrations for LH2 vessels of any type include:

• Supports, flanges, ports and similar structural components
• Fill line for loading LH2 into the vessel
• Drain line for removing LH2 (a single fill/drain line is sometimes used)
• Feed line for high LH2 consumption rate applications
• Pressurization or other pressure building subsystem
• Vent line for pressure relief and fluid conditioning (sometimes tied into the pressurization line with appropriate isolation valving)
• Sensors for temperature, pressure, and mass gauging or fill level monitoring

In the next post I'll talk about at how LH2 is transferred in and out of a storage vessel.

Reference

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.

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.

Energy Shapeshifting with Hydrogen

Day operation HyERA™ example system model: renewable power is greater than demand yielding hydrogen production and storage

A recurring question when it comes to hydrogen is whether it is an energy carrier, an energy storage method, or a fuel. The answer is yes. This versatility is why hydrogen represents a key solution to mitigating climate change across virtually all industry sectors.

No carbon emissions. No harmful environmental impacts. No strategic materials supply chain issues. No disposal or recycling challenges. And with a system lifetime measured in many decades with routine maintenance and no significant replacement costs.

There is another truly unique capability of hydrogen systems that is often overlooked: the capability to produce large quantities of potable water. But first, let's start with its role as an energy carrier.

Energy Carrier

An energy carrier is a fuel or system that enables the conversion of potential energy to other forms of energy (e.g. mechanical, thermal, chemical, or electrical). The conversion process can be shifted in time, location, or both relative to when and where the potential energy was originally created.

A common and currently ubiquitous example is natural gas. Produced by decaying organic material, it has intrinsic potential energy in the form of its heat value (a measure of energy density). It can be delivered as a gas in pipelines, or liquefied for transport. Unfortunately, natural gas is a significant source of greenhouse gas emissions via leaks and combustion emissions.

Hydrogen has all the energy carrier characteristics of natural gas without the greenhouse gas or other environmental impacts. It can be similarly delivered as gas, or liquefied for transport. Its heat value is more than double that of natural gas or any other fossil fuel. Further, the energy conversion efficiency when hydrogen is consumed in a fuel cell is much higher than any combustion process.

Energy Storage

Energy storage methods retain energy from a power source and store it for later use. This capability is particularly critical for balancing electrical demand in grids or microgrids that rely wholly or in part on intermittent renewable power sources (e.g. solar or wind).

Commonly used forms of energy storage include hydroelectric (i.e. "hydro") for large scale storage, and lithium-ion batteries for small to moderate energy storage. For grid-scale energy storage where the local terrain doesn't support hydro - which is most locations - there are few feasible options and even fewer proven systems. Lifecycle battery costs at this scale become untenable.

Overgeneration from renewables that is currently curtailed when insufficient demand exists can be used to electrolyze water into hydrogen and oxygen. The hydrogen is then stored until demand exceeds renewables capacity, at which point it is fed to fuel cells to generate the additional electricity needed.

Hydrogen energy storage is rapidly being recognized as a frontrunner solution for large scale applications where it has already been deployed in many locations. And more capacity is coming online at an accelerated rate. 

Universal Fuel

As previously mentioned, hydrogen has more than double the energy density of any fossil fuel option. It has been successfully demonstrated in nearly every combustion process imaginable, from internal combustion engines to turbines. Adjustment of the oxygen-fuel ratio is the primary modification required.

Ground, sea, and air demonstration vehicles using hydrogen have already been fielded with many more under development. These applications particularly benefit from the increased energy density of hydrogen relative to legacy fuels. And of course, carbon emissions are eliminated.

Over the last few years, introduction of a low percentage of hydrogen into natural gas pipeline networks has been tested in several global regions with promising results. Modification to residential and industrial burners will be required to increase the hydrogen percentage to more than 10%.

In short, there are no current fossil fuel applications that cannot be transitioned to hydrogen. Additionally, the higher flame temperatures that can be achieved with hydrogen will enable new applications and potentially enhance the efficiency of existing combustion-based systems.

Potable Water

The fact that hydrogen can be produced from water using renewable energy sources, and then returned to water form during usage, is generally well known. But many other feedstocks can be used to produce hydrogen. And when the hydrogen is used, potable water can be extracted in significant quantities.

Two of the more intriguing feedstock options are seawater and biomass waste. A hydrogen electrolysis plant located near a coast or on an island could desalinate seawater and produce hydrogen for energy storage, fueling, and/or export.

Likewise, dual production of potable water and hydrogen can be realized with agricultural waste and other recycled biomass as feedstock. In this case, potable water and hydrogen can be generated where little or no water is available.

The implications for drought stricken regions is immense, particularly where renewable resources such as solar are available in abundance. Secure and reliable energy, water, and food becomes possible in regions that are vulnerable to weather or other threats to these vital necessities.

Hydrogen has the potential to address all of the above. And in the process, restore the health and stability of our global environment for future generations.

Night operation HyERA™ example system model: renewable power is less than demand requiring hydrogen usage and producing potable water

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 Nuclear Option for Hydrogen

Illustration of a conceptual spacecraft enabled by nuclear thermal propulsion (Credits: NASA)

This week I attended the Nuclear and Emerging Technologies for Space (NETS) conference. It was a bit of nostalgia for me since I worked briefly on the NASA-led SP-100 program in the mid-1980s. The SP-100 was meant to demonstrate a nuclear fission reactor that could provide 100 kW of electrical power to a spacecraft. It was cancelled before flight hardware was built, in part due to public resistance about launching a nuclear reactor.

Later on at NASA I also worked on the solar dynamic Brayton power system originally planned for the space station; and Stirling microsystems development for multiple applications. Both technologies convert heat to electricity and can use radioisotope or fusion reactor heat sources. Later still, I briefly supported the Jupiter Icy Moon Orbiter program that was slated to use a small nuclear fission reactor for propulsion and power. 

None of these systems have flown on any spacecraft. But there is reinvigorated interest in nuclear fission based power and propulsion to expand the capabilities and performance of scientific and human exploration space missions. 

So why write about all this in a blog devoted to hydrogen? Because nuclear fission reactors have significant potential implications for hydrogen systems in space as well as on the ground in the energy sector. First, a very quick summary of the primary types of space-based nuclear power and propulsion technologies. 

RTGs, NTPs, NEPs, and Fission Surface Power for Space Missions

Radioisotope thermoelectric generators (RTGs) convert the thermal energy from a decaying radioisotope source to electrical energy using an array of thermocouple junctions. There is no nuclear fission involved. RTGs have been used on more than two dozen spacecraft over the past 60 years and enable very long mission durations far from the sun.

The thermocouple junctions in an RTG can be replaced by a Stirling engine for much higher efficiency and higher associated electrical power output. Heat drives the Stirling cycle via a working gas (e.g. helium) resulting in pressure-volume mechanical power that is converted to electricity. Stirling technology is at a high readiness level and will likely start replacing RTGs on some future missions.

Nuclear thermal propulsion (NTP) uses the fission process to heat up a propellant - generally hydrogen - to a very high temperature and then accelerate it through a nozzle to create thrust. Nuclear fission replaces the heat of combustion in traditional chemical propulsion such as rocket stages using hydrogen and oxygen. For missions beyond earth orbit, NTP enables much faster transit times. 

Nuclear electric propulsion (NEP) utilizes the fission process to generate electrical energy which then drives an electric propulsion system (e.g. arcjet or ion thruster). Depending on the propulsion system used, the thrust is produced by accelerating hydrogen, xenon, argon, or another fluid. Some of the electrical energy generated also provides primary power to the spacecraft. NEP provides much greater power capacity and higher propulsion efficiency for spacecraft.

Finally, small modular nuclear fission reactors on the moon, Mars, or other celestial bodies can enable extended power intensive surface operations. In addition to mobility and life support functions, these reactors would enable full scale in situ resource utilization such as the production of hydrogen and oxygen from harvested ice.

NIMBY Everywhere

A nontechnical barrier that all nuclear fission systems face, whether in-space or on the ground, is sometimes termed "not in my backyard" (NIMBY). NIMBY comes into play on many topics that impact the public, from landfills to prisons. In the case of nuclear reactors, the relevant "backyard" can be a very large area indeed based on air or sea circulation patterns.

The general public wariness of anything related to nuclear power is not unwarranted. For every well known nuclear related disaster - Three Mile Island, Chernobyl, Fukushima - there are many more "near misses". These almost accidents are caused by operator error, various system failures, fraudulent inspection or maintenance records, and many other factors.

In addition to the public risk perception related to operating nuclear systems, there is also the issue of nuclear waste disposal. NIMBY comes into play even more strongly on this issue. As a result, public acceptance of nuclear fission for any application - space, power plants, or otherwise - is a challenging consideration.

Nuclear Power for Hydrogen Production

Existing nuclear power plants, and the utilities that own them, have a rather intractable economic problem. These plants are no longer cost competitive compared to other power generation options. And in the U.S., for example, many state and regional public utility commissions are balking at passing these higher costs on to the consumer.

But nuclear reactor cores cannot simply be shut down like a fossil fuel power plant. The half-life of the radioactive fuel used in them is unaffected by the whims of political policy or utility economics. Decommissioning a nuclear power plant is a complex and expensive undertaking that requires ongoing site control and monitoring for a very long time.

One potential option is to repurpose some of the existing global nuclear power plant capacity to produce hydrogen via water electrolysis. This approach would contribute to the reduction of greenhouse gases as the generated hydrogen displaces legacy fossil fuels. It also redeploys nuclear power assets that are no longer economically competitive for grid power generation.

Whether these advantages offset the previously mentioned public safety and nuclear waste disposal concerns is a matter of considerable debate. It will require circumspect policy formulation that ideally engages the public at large. And the path forward will undoubtedly vary among nations based on regional energy policy, resources, existing infrastructure, and many other parameters.

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.

Hydrogen is Best Served Cold

Storage and Delivery Options

One of the key decisions in developing a hydrogen system is deciding how - and in what form - to store and deliver it. The most common approaches are ambient temperature compressed gas up to 700 bar, or low pressure cryogenic liquid near 20 K.

Other methods have been tried and are the subject of ongoing research and development. These include: materials that trap hydrogen by molecular or chemical means; cryo-compressed (transcritical); and hydrogen "carriers" that are converted prior to use (e.g. ammonia).

Any application should be evaluated to determine the best approach given the system objectives, requirements, constraints, and concept of operations. For large scale applications, and vehicles with onboard hydrogen storage (land, sea, air, or space), liquid hydrogen is often the best solution.

Liquefied Hydrogen

Liquid hydrogen has several critical advantages compared to other forms of hydrogen storage:

• Relatively high volumetric density at low operating pressures
• Long history of production, storage, transport, and usage
• Off-the-shelf subsystems and components available
• Many legacy stationary, over-the-road, train, barge, and launch systems
• Zero boil-off possible with proven cryo-refrigeration technology
• Low temperature provides ancillary system capabilities

The above advantages are partially offset by the unique design and operational considerations associated with liquid hydrogen. Some key considerations include: liquefaction (and re-liquefaction), material properties, fluid thermodynamics, phase change, and thermal management.

Although there are well established and validated methods to fully address these considerations, the requisite knowledge base is not widely disseminated yet. The primary motivation for this blog is to share that knowledge base with a wider audience to accelerate the safe and effective adoption of liquid hydrogen in new areas of application.

Below is a presentation I gave a few months ago at the Center for Hydrogen Safety Asia-Pacific Conference that addresses zero boil-off and densified liquid hydrogen systems. Future posts will provide more details on these and other related topics.

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.

Hydrogen Myth Busting (Episode 2)

Global temperature anomalies (Source: NASA GISS)

In my previous post, I started to address some common and recurring myths about hydrogen. This post will continue along the same theme, but with myths that have more nuances to unpack.

Myth #5: Making Hydrogen From Renewables Isn't Practical

There are multiple variations on this myth. Some of the most persistent of them include:

• There aren't enough renewable resources to support all the carbon-free hydrogen production needed
• Renewables should only be used to directly meet the electrical demand; producing hydrogen with them is wasteful
• Conclusions regarding hydrogen production from renewables specific to a particular region are globally true everywhere

    Not enough renewables

Let's start with the first variation. A typical set up for this argument is that if all fossil fuel applications were immediately switched to hydrogen there wouldn't be enough renewable energy capacity to support its production. 

The fundamental flaw with this argument is that every energy transition occurs over decades; there has never been and will never be an immediate transition to any new energy paradigm. If this argument had carried the day prior to the industrial revolution, our energy and transportation systems would still be based on horses.

Hydrogen production using renewables is growing via the deployment of new solar, wind and other generating capacity. These renewable resources can be used during periods of over-generation (more on this below), or supplied from dedicated new microgrids.

 

    Renewables should only be used for electrical demands 

In a fantasy computer simulation, it may be possible to make the sun shine and the wind blow whenever needed to perfectly match the electrical demand at all times. Unfortunately, electrical demand never matches available renewable energy in the real world.

The historical solution to this problem has been to use base load generating sources such as coal-fired or nuclear power plants plus peaking capacity (e.g. natural gas or diesel) to balance power vs load. However, as more renewables come on line, this balancing act becomes untenable when renewables reach about 30% or more of the generating mix.

As fossil fuel generating capacity continues being replaced by cheaper renewables, energy storage will become increasingly necessary to maintain the balancing act. If the local geographical features support pumped hydro (water) storage, that may be the cheapest and simplest solution for that locale. If the scale is not too large, batteries may also provide a reasonable solution.

But for large grids, or microgrids that also incorporate fueling functions, hydrogen is the solution that can be implemented anywhere at any scale. And the lifecycle costs and supply chain risks are lower than batteries under these conditions for an equivalent storage capacity.

    The same hydrogen solution applies everywhere

Consider a densely populated small region in the upper northern hemisphere with limited solar irradiation and no real estate for wind power. Large scale production of hydrogen from renewables in this region may not make much sense.

Now consider a sparsely populated larger region near the equator or in the southern hemisphere where there is an abundance of solar and wind resources and vast real estate available at a low price. Hydrogen production from renewables in this region is not only feasible but may represent a lucrative export opportunity. It can be transported to that densely populated region similar to oil or liquified natural gas.

Between these two extremes are a plethora of regions with varying natural, economic, and geopolitical conditions that dictate what type of hydrogen infrastructure makes sense for that location. And yet it is not uncommon to see articles and policymakers who declare a recent study for a specific region to be "the answer" on how hydrogen should be implemented globally.

Myth #6: Hydrogen Isn't Green

As with the previous myth, there are variations on this one:

• Since most hydrogen has historically been produced with steam methane reforming (SMR), it isn't a solution to climate change
• Hydrogen may contribute to global warming

    Hydrogen production

Hydrogen and lithium ion batteries can be used to store energy. When that energy is used, no carbon byproducts are emitted. These are simple irrefutable facts based on the associated chemical processes.

If the feedstock for producing hydrogen is water that is electrolyzed using solar or wind, then it is a "green" method of production. If batteries are charged using the same renewables, then the charging process is green.

Neither hydrogen nor batteries are inherently green or not... it is the lifecycle ("cradle to grave") processes associated with them that determine the impact on the environment. In the case of hydrogen, the number and capacity of electrolyzer installations powered by renewable energy are growing rapidly and represent a truly green source of energy storage and fuel.

    Hydrogen leaks and global warming

This relatively new myth is based on a recent study that has been warped beyond recognition to produce tabloid worthy - and grossly inaccurate - headlines. The study in question poses the following (paraphrased) scenario:

If hydrogen were to be produced in the quantities required to replace fossil fuels; and large aggregate leaks of hydrogen were permitted to occur throughout this new global hydrogen infrastructure; and furthermore, these leaked quantities of hydrogen managed to reach the upper atmosphere without already combining with oxygen in the lower atmosphere or water; it may combine with hydroxides in the upper atmosphere to form water vapor.

 So what's the issue? Keep reading...

This process may inhibit the amount of upper atmosphere hydroxides available to react with the large amounts of leaked methane; thereby inhibiting the ability to mitigate the impact of methane sources and leaks.

While methane is a very potent and prevalent greenhouse gas, it's rather difficult to overlook the circular argument of this scenario. Namely, that transitioning to hydrogen might inhibit the upper atmosphere mechanism that helps to mitigate the greenhouse gas effects of one of the fossil fuels that hydrogen will replace.

The study goes on to recommend that implementation of global hydrogen infrastructures should address leakage to ensure very little reaches the upper atmosphere. The methods and technologies for minimizing hydrogen leaks are well known within the hydrogen community and are already used in any appropriately designed system.

This is a key takeaway and valid consideration as we transition to hydrogen. Unfortunately, that valuable nugget rarely seems to make its way into the subsequent articles that misrepresent the findings. Instead, a more sensationalized tale is spun about the effects of large scale hydrogen usage and greenhouse gases. While that may draw a lot of readers, click-throughs, and online traffic, it is clearly misleading.

Myth #7: Any Particular Technology is the Only Solution

Lithium-ion batteries have evolved to become a truly amazing energy storage technology. High round trip efficiencies during charge and discharge cycles. Portability ideal for very small scales up to automobile applications. Continuing improvements that extend useful life before replacement is necessary.

Li-ion batteries also have their weaknesses. Charging time and heating; temperature effects on performance; thermal runaway if punctured or crushed; strategic materials issues; recyclability; limited practical scalability; weighs the same fully charged as fully discharged.

Hydrogen has its own strengths and weaknesses. Its much higher energy density relative to batteries extends the range and capacity of land, air, and marine electric vehicles. And the vehicle becomes lighter as the hydrogen is consumed, unlike batteries. There are no strategic material nor recycling issues. The operational lifetime of hydrogen systems are measured in decades. Hydrogen systems are scalable to meet very large energy storage requirements, and any application that currently uses fossil fuels.

On the downside, hydrogen has lower round trip efficiency when used in a fuel cell compared to Li-ion batteries (although much higher than any combustion process). High compression as a gas or liquefaction at cryogenic temperatures is required to overcome its otherwise low volumetric density. And workforce training is needed to ensure the required skillsets are sufficiently available for properly designing, commissioning, and operating new hydrogen infrastructures.

Other solutions that reduce or eliminate greenhouse gas emissions have their own unique pros and cons. No single technology or solution is universally optimal for every application. The path to a more secure energy and environmental future requires careful consideration of all feasible options that get us closer to that goal. An important step forward is dispelling myths promulgated by vested interests or the misinformed.

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.