There are many problems associated with hydrogen--one of which is energy density--which for liquid hydrogen is a depressingly low 30,000 BTU/liquid gallon, which puts it even below the heating value of methyl alcohol.

The system you are talking about Bruce, I think is a supercritical storage system. Supercritical gasses are so compressed that they approximate a liquid in their density. They can flow like a liquid, and behave as if they are a liquid in most respects, but it is possess a lower density than fully liquidified gas--as such they can easily transform back to 100% vapor by simply passing through an expansion nozzle.

As far as liquid hydrogen systems go, a supercritical storage system may have the advantage in that quite a bit of energy of compression can be saved. In a fully compressed and liquidified hydrogen system, as much as 30% of the heating value of the hydrogen must be expended to compress and liquify the hydrogen. This is a very significant amount of energy. This would be like expending 40,000 BTU of heat energy just to liquify and store a gallon of gasoline.

Nevertheless, if the energy constraints may be side stepped by clean, plentiful and cheap, nonfossil energy sources, hydrogen becomes attractive because given a plentiful supply of electricity and water, hydrogen can be generated anywhere it is needed. Again, hydrogen as an energy source is a misnomer, because it is more correct to think of hydrogen as an energy storage medium and not as a fuel.

Supercritical systems may infact be something that might be an attractive intermediate between compressed gas storage (which has even more terrible energy density issues) or technically far more challenging full cryogenic systems.

I have often thought of liquid hydrogen powered energy systems in terms of a large truck service plaza: using electrical energy to electrolyze water and then compress and liquify the hydrogen, a service plaza sized to offer the chemical energy equivalent of 100,000 gallons per day of No. 2 diesel fuel will require a 500,000 gallon cryogenic storage tank (as big as the two spherical tanks at Cape Canaveral Kennedy Space Center to support space shuttle launches!) and will consume more than 600 Megawatts of electricity! This gives some indication of just what it takes to use hydrogen as an energy storage medium. Going to a municipal hydrogen gas supply (remote hydrogen generation) and liquifying it on the spot (seems reasonable,) the same Truck Service Plaza will consume 150 Megawatts of electrical power just to run the hydrgogen liquification plant. Clearly we have some substantial hurdles to clear before liquid hydrogen can be used as a motor fuel...