by Dr. William Notardonato, James Fesmire and Adam Swanger all from Cryogenics Test Laboratory, NASA Kennedy Space Center

Low molecular weight and high specific impulse make liquid hydrogen (LH2) a superior propellant for rocket applications, but its low normal boiling point (NBP) and density make utilization difficult and costly compared to other cryofuels. The US Atomic Energy Commission, and subsequently NASA and the US Air Force, pioneered the development of large-scale LH2 systems in the 1950s, but over time the technology in the industrial gas industry has seen tremendous gains.

The space industry uses its cryogens in a different manner than most industries, storing large quantities for periodic batch use rather than continuous feed. It also requires the product to be supplied to the end process in liquid form, as opposed to liquid storage but gaseous supply. NASA’s cryogenic ground systems and processes are still based on proven but inefficient technologies from the 1960s. As a result, over the duration of the space shuttle program approximately 50 percent of the hydrogen purchased was lost, vented to the atmosphere due to system heat leak and cooldown of hardware [1].

Integrated Refrigeration and Storage (IRAS) is a technology to help maximize efficiency of spaceport cryogenic processes by integrating modern cryogenic refrigeration units with liquid storage vessels. Brayton cycle helium refrigerators are available in a range of capacities and temperatures, with demonstrated high efficiency and low maintenance. In an IRAS system, a suitable refrigerator supplies a direct flow of gaseous helium refrigerant to a cold heat exchanger (HX) integrated within the tank, and distributed throughout the bulk volume of liquid.

Distribution of cold power is the key to obtaining an effective overall heat lift without the large conduction heat leak penalty associated with “point-cold” cryocooler arrangements. IRAS technology provides an approach of directly coupling the cold HX with the cryogenic liquid to minimize thermal resistance and expedite heat transfer. This approach offers full control of the state of the cryofuel using addition and removal of thermal energy, instead of being limited to management via addition and removal of mass (i.e. pressurization and venting). Such control allows for greater operational efficiency, greater control of ground operations and enhanced performance benefits.

Researchers at NASA Kennedy Space Center (KSC) developed the IRAS technology to demonstrate several novel cryogenic operations. It features zero loss storage and transfer, removing system heat loads during both steady-state heat leak and transient cooldown operations; propellant densification to control the storage state of LH2 below the NBP; in-situ liquefaction to provide liquefaction of gaseous hydrogen inside the storage tank, and zero loss cooldown to provide cooldown of storage tanks with no product loss.

The ability to control thermal energy in a tank is a new operational capability that enables users to examine cryogenic storage systems from a different perspective. Consider a map that shows the net rate of heat flow crossing the system boundary on the X-axis—normal heat leak plus the vaporizer heating minus the refrigeration power; and the net rate of mass crossing the boundary on the Y-axis—mass flow rate of pressurant gas minus the rate of venting. The net heat flow can be non-dimensionalized by dividing by the normal heat leak and is hereby defined as the refrigeration ratio (eqn. 1), while the net mass flow can be non-dimensionalized by dividing by the normal evaporation rate (NER) and is defined as the mass ratio (eqn. 2).

In Figure 1, the positive X coordinates are when the tank is receiving net heat from the environment, negative X coordinates are when the tank is dumping heat to the environment, and the Y-axis denotes an adiabatic system. Similarly, negative Y values signify net mass flow out of the tank, positive Y values signify mass flow into the tank, and along the X-axis the system is closed. At the origin, the system is closed and adiabatic, and the pressure will remain constant. An isobar can be drawn through the origin at some negative slope—all operations above that isobar cause an increase in tank pressure, and operations below the line result in a pressure decrease.

Cryogenic operations are particularly limited in capability without IRAS technology. Daily operations typically consist of venting at the NER (point A on the isobar) in order to accommodate heat leaking into the tank. If the vent valve is closed, vent flow drops to zero and heat leak causes self-pressurization, shown as point B. Prior to liquid transfer the tank is pressurized by the vaporizer, adding heat and moving the operation to point C. Occasionally liquid tanks are also pressurized by gas trailers, as indicated by point D. Finally, when the tank is vented the operation would lay somewhere along the line between E-A, depending on the vent flow rate, and will eventually settle back at point A for daily operations. Without IRAS, all operations will occur on or to the right of the line EABD, the “passive line.”

IRAS opens up the wide range of cryogenic storage and transfer operations located to the left of the passive line—most notably, the origin (point F), where the system is adiabatic, closed, isobaric, and defined as zero boiloff (ZBO). When the refrigeration ratio increases beyond ZBO the tank pressure will decrease and liquid will densify (points G and H). Steady liquefaction occurs along the isobar when the mass flow into the tank is provided (points I and J).

The GODU-LH2 project recently completed at NASA KSC demonstrated operations located at all points of Figure 1, and showed that full control of the state of the fluid is possible as well as practical [2,3]. Red points represent legacy operations while blue points represent new capabilities proven by the GODU-LH2 project using IRAS. For this work, NASA engineers took a 125,000 liter LH2 tank, depicted in Figure 2, retrofitted it with a novel internal HX, and coupled it to an 860 W Brayton helium refrigerator [4,5]. Testing also included simplified, large-scale production of slush hydrogen. The technology of IRAS is extensible to other cryofuels including liquid methane (or LNG) and liquid oxygen. In addition, other fluid control issues related to boil-off, such as aging of LNG or enrichment of liquid air, could be addressed.

While ZBO keeps cryogenic liquids indefinitely by matching the tank heat leak, IRAS goes further to provide full control of the state of the fluid: gas, liquid, densified liquid, or slush. Cryogenic storage design approaches are generally built around passive systems (i.e. without active refrigeration), making the term “non-storable” synonymous with cryogenic propellants. However, new design approaches that take advantage of IRAS technology will render this description obsolete, and provide several benefits, including simplified liquid densification and keeping; operational reliability and safety; and logistical flexibility. The technology of “storable” cryofuels makes possible new levels of efficiency as well as new approaches to the supply of hydrogen for transportation applications.

References:
[1] J.K. Partridge, “Fractional consumption of liquid hydrogen and liquid oxygen during the space shuttle program,” in Advances in Cryogenic Engineering, AIP Conference, Volume 1434, 2012.
[2] W.U. Notardonato et al., “Ground Operations Demonstration Unit for Liquid Hydrogen Initial Test Results,” in Advances in Cryogenic Engineering, IOP Conf. Series: Materials Science and Engineering 101, 2015.
[3] A.M. Swanger et al., “Integrated Refrigeration and Storage for Advanced Liquid Hydrogen Operations,” in Cryocooler 19: Proceedings of 2016 International Cryocooler Conference, 2016.
[4] J.E. Fesmire et al., “Integrated heat exchanger design for a cryogenic storage tank,” in Advances in Cryogenic Engineering, AIP Conference Proceedings, Volume 1573, 2014.
[5] A.M. Swanger et al., “Modification of a Liquid Hydrogen Tank for Integrated Refrigeration and Storage,” in Advances in Cryogenic Engineering, IOP Conf. Series: Materials Science and Engineering Volume 101, 2015. ■