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| 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.
