Slush hydrogen is a mixture of liquid and solid hydrogen at the triple point considered as a possible vehicle fuel. What is the need of having it at the triple point? Couldn't any other set of thermodynamic conditions consistent with liquid–solid equilibrium work as well?
I'll have a stab at this one, despite the fact I've forgotten almost the totality of engineering thermodynamics, as I do recall seeing these papers before.
One of the references1 in the Wikipedia article describes the production of slush hydrogen through the auger technique, contrasting it with the freeze-thaw method. Specifically, the auger method can be performed above the triple point pressure of hydrogen, whereas the freeze-thaw method uses the latent heat of vaporisation to freeze some of the hydrogen. In this technique the vaporisation of some of the hydrogen (prompted by the application of a vacuum) at the triple point causes some of the liquid hydrogen to freeze, generating a crust of solid hydrogen which is then broken up to make slush.
As far as I can tell, the triple point conditions are only required to make the freeze-thaw process go, and are not required for storage.
The auger technique, also described in the report1, seems to work on the basis of shaving solid hydrogen which is refrigerated with liquid helium. Voth compares the technique to a conventional ice shaver (albeit much, much colder!). In this instance there's no requirement for triple point conditions, which is given as one of the advantages of the technique.
Finally, Haberbusch and McNelis2 give us this sad quote, in reference to a 1991 paper:
"According to Hans, projected annual demand for slush hydrogen in the year 2010 is 584 million pounds to support a fleet of 20 spaceplanes"
One day, maybe...
(1) Voth, R.O.; Producing Liquid-Solid Mixtures of Hydrogen in an Auger; National Bureau of Standards Department of Commerce; 1978, pp. 1-16
(2) Haberbusch, M.S. and McNelis, N.B.; Comparison of the Continuous Freeze Slush Hydrogen Production Technique to the Freeze-Thaw Technique; NASA Technical Memorandum 107324, 1996, p. 1
I have studied triple-point systems, though not principally in hydrogen. I have worked with phase equilibrium in dynamic (moving) water systems, looking at changes in density and surface tension. Because of this, I have studied the phenomenon more broadly.
While a system of mass and energy to remains (is maintained) in phase-equilibrium (i.e. the triple point), the introduction of new forces – be they pressure, e-field, magnetic polarization, thermo, etc – can lead to an ongoing reaction cycle.
I won't speculate too deeply about what that does in this H system, but will leave this paragraph for another specialist to answer.
The phase-equilibrium point can make it easier to push a reaction toward a specific kind of change / characterization of the media. Particularly, the closeness to the gas phase means the volatility of [part of] the media is being utilized to
- bond in external media (doping)
- perform redox or some other electronic reaction
- liberate some media in a thermo exchange that crystalizes others
- change the crystallization morphology
Pragmatically, I've read this is applied in reactor\feedstock scenarios, where the 'reaction' portion would in this case be held at the triple-point, whilst the feed/flow of media causes the outlet portion to take on a certain character, as the inlet media is prepped. The principle to understand here is that the reaction portion act more homogeneously, rather than local regions of differentiated materials.