Terrestrial applications of zero-boil-off cryogen storage

Storing cryogenic propellants with zero boil off (ZBO) using a combination of active (cryocoolers) and passive technologies has recently received a great deal of attention for long-term space missions. This paper will examine a variety of potential near-term terrestrial applications for ZBO and, where appropriate, provide a rough order of magnitude cost benefit of implementing ZBO technology. NASA's Space Shuttle power system uses supercritical propellant tanks, which are filled several days before launch. If the launch does not occur within 48-96 hours, the tanks must be drained and refilled, further delaying the launch. By implementing ZBO, boil off could be reduced and pad hold time extended by a factor of eight. At NASA's John F. Kennedy Space Center, vented liquid hydrogen (LH2) storage dewars lose 650 kg (500 gal)/day through boiloff. Implementing ZBO would eliminate this, saving $625,000 per year. Overland trucking of LH2 from the supplier to the launch site via roadable dewars results in a cryogen loss of 10% per tanker (1300 kg (1000 gal)/tanker). If this loss could be eliminated, the savings would be approximately $30,000 per year. Within the superconductivity community, there is skepticism about using coolers, based upon reliability concerns. One approach would be to design a hybrid system including a smaller dewar to hold the cryogen for a short time (approx 1 month) and a cooler sized for continuously re-liquefying the boil off. This approach would provide a system with both the high reliability of a stored cryogen combined with the low maintenance and small size of a commercial cryocooler, and could greatly benefit not only high temperature superconducting power applications, but cellular phone base stations or any commercial application that cannot afford a system failure.