Abstract
NASA has completed a series of tests at the Kennedy Space Center to demonstrate the capability of using integrated refrigeration and storage (IRAS) to remove energy from a liquid hydrogen (LH2) tank and control the state of the propellant. A primary test objective was the keeping and storing of the liquid in a zero boil-off state, so that the total heat leak entering the tank is removed by a cryogenic refrigerator with an internal heat exchanger. The LH2 is therefore stored and kept with zero losses for an indefinite period of time. The LH2 tank is a horizontal cylindrical geometry with a vacuum-jacketed, multilayer insulation system and a capacity of 125,000 liters. The closed-loop helium refrigeration system was a Linde LR1620 capable of 390W cooling at 20K (without any liquid nitrogen pre-cooling). Three different control methods were used to obtain zero boil-off: temperature control of the helium refrigerant, refrigerator control using the tank pressure sensor, and duty cycling (on/off) of the refrigerator as needed. Summarized are the IRAS design approach, zero boil-off control methods, and results of the series of zero boil-off tests.
Highlights
Numerous concepts have been explored to develop zero boil off (ZBO) systems for cryogenic operations, mainly liquid helium and liquid hydrogen systems
In the early 1990’s, Ergenics Inc. proposed a system that would capture boil off losses in a metal hydride compressor bed, used to run a Joule-Thompson refrigerator precooled with liquid nitrogen in batch processes [5]
Hydrogen Consultants Inc. later won a Phase II SBIR grant to prove the concept of a metal hydride compressor design that could run a continuous J-T refrigeration system sized to provide just enough refrigeration to overcome the steady heat leak into the tank [6]
Summary
Numerous concepts have been explored to develop zero boil off (ZBO) systems for cryogenic operations, mainly liquid helium and liquid hydrogen systems. Zero Loss Storage and Transfer – Use IRAS systems to remove system heat loads from steady heat leak (ZBO) and transient chill down operations With fixed helium flow rates and temperatures from the refrigerator, the cooling capacity is constant and the LH2 will eventually reach an equilibrium state in the tank.
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