Since liquid hydrogen has a high specific impulse as fuel for rockets, its technology has been developed in connection with space technology programs. On the other hand, with the recent advance of technologies such as fuel-cell vehicles and stationary fuel-cell systems, liquid hydrogen is also expected to be used as an efficient means of transporting and storing hydrogen used as a source for clean energy. Slush hydrogen is a two-phase solid-liquid cryogenic fluid consisting of solid hydrogen particles in liquid hydrogen. Compared with liquid hydrogen, the density and cryogenic heat capacity of slush hydrogen with a solid fraction of 50% increase by 16% and 18%, respectively. Various applications of slush hydrogen are anticipated such as fuel for reusable space shuttles and fuel cells, and as a more efficient means of transporting and storing liquid hydrogen. This paper describes the applications and technology developments of liquid and slush hydrogen.
We discuss the heat flow and work flow of an orifice pulse-tube refrigerator and a heat-driven thermoacoustic cooler consisting of a single looped tube. It is shown that, while the orifice of the orifice pulse-tube refrigerator is essential for increasing the cooling power, it decreases the efficiency because of the dissipation introduced by itself. It is also shown from the energy flow diagram that dissipation of the work flow inherently occurs in the heat-driven thermoacoustic cooler, resulting in a reduction in efficiency. We present that these unwanted dissipations would be avoided by the addition of a feedback path for the work flow. The modified refrigerator and the cooler would achieve efficiencies much higher than those of the original ones, especially at noncryogenic temperatures.