Hydrogen is a wasteful fuel

When you start looking in earnest at hydrogen as a fuel, it becomes apparent that it is a fuel which is difficult to move and store, and its energy density per unit volume under all conditions is extremely poor, even though the energy density of hydrogen per unit mass is excellent.

The most obvious options are high pressure gas storage and making liquid hydrogen (LH2). For transportation fuel uses, compressed gas storage is the least bad of a terrible set of choices. However, even at 700 bar (10,000 psig or 5 tons per square inch of pressure), hydrogen is still only 41 kg/m3.

Hydrogen becomes a liquid at atmospheric pressure at a temperature of around -253C, or 20 Kelvin, i.e. 20 degrees above absolute zero. At that mind-bogglingly low temperature, it is still not very dense: only 71 kg/m3. And whereas to compress hydrogen from the 30-70 bar pressure at the output of an electrolyzer to 700 bar(g) can be accomplished for about 10% of the energy in the hydrogen, liquefying hydrogen takes 25-35% of the lower or net heating value energy in the hydrogen you’ve compressed. For methane, to make LNG, that amount is about 8-10%.

The problems of hydrogen liquefaction are considerable. First, hydrogen heats up when you expand it any time you start at a temperature above about -73C (200 Kelvin). This behaviour arises from hydrogen’s unusual negative Joule-Thomson coefficient above 200 Kelvin. That means, if you want to liquefy hydrogen, you first have to cool it down considerably as a gas before you begin. Generally liquid nitrogen precooling is used for this purpose, necessitating an air liquefaction plant as part of the works.

After precooling, the hydrogen can be liquefied by either a helium or hydrogen Claude refrigeration cycle. The energy input required is considerable. And hydrogen has another wrinkle: spin isomerization. The electron spins of the two hydrogen atoms in a hydrogen molecule can be either aligned (ortho) or opposite (para). When you condense gaseous hydrogen, you get a mixture of about 75% ortho and 25% para-hydrogen. As the liquid sits in storage, ortho gradually converts to para, releasing heat. And that released heat escapes the only way it can – by boiling hydrogen you’ve spent so much energy to cool and condense.

A catalyst is required to carry out the conversion more quickly so the heat can be recovered prior to storage, rather than causing excessive boil-off while the LH2 is being stored, but you’re still recovering that heat at 20 Kelvin and rejecting it to the environment at 293 Kelvin. That takes a lot of work (electricity), regardless of how you slice it.

With some liquefied gases, such as anhydrous ammonia, you can keep them liquid at room temperature merely by keeping the pressure high enough. But forget about doing that with hydrogen. The critical temperature is -240C. Above that, the liquid phase no longer exists.

Keeping heat out of LH2 at 20 Kelvin is easier said than done. Vacuum insulated “dewar” type tanks can be constructed, and for applications like this, spherical containers are the optimal shape with the lowest surface area per unit volume. A land-based LH2 dewar tank about as big as you can make it, referred to as a Horton sphere, reportedly can have excellent performance, where only 0.2% of the hydrogen in the tank boils off each day. Any tank smaller than that, or of a less optimal cylindrical shape, allows even more to boil off per day. And in transit on a ship recapture and re-condensation of the boil-off gas is not possible due to the complexity of the refrigeration system required.

The best you can do is to burn it, hopefully as a fuel, or if in port, just burn it to prevent it from becoming a greenhouse gas – hydrogen’s global warming potential is at least 11x as great as CO2 on the 100 year time horizon.

As this piece has shown hydrogen is extremely ineffective as a fuel. For those applications which require liquid fuels (limited to transoceanic aircraft and ships), it would be much more sensible to use biofuels, perhaps adding some green hydrogen to make up the hydrogen deficiency in biomass relative to the fuels we’ve become used to from petroleum. We should simply abandon the notion of using hydrogen or molecules whose sole energy content comes from hydrogen, as replacement fuels. That approach is just simple-minded fuel substitution thinking.