Hydrogen? Yes, but...
Hydrogen is a clean fuel – it doesn’t produce any carbon dioxide or other pollutants when it burns, just water vapour. It’s also a very energy dense fuel – you get a lot of energy per kilogram of hydrogen, compared to any other fuel.
All true, and all very well – but it’s not the solution to our transport fuel problem. It might have a niche role, and that’s about it. So what’s wrong with it?
Firstly, you can’t go and mine or drill for it – there isn’t any down in the ground – and you can’t grow it in fields. You have to make it somehow. That’s easy enough, but you need an energy source to do it. So it’s only as clean as the energy source you use to make it – in fact, a little less clean because it costs you more energy to make the hydrogen than you get back when you use it.
That doesn’t stop you using it as a means of storing energy. It can convert an intermittent source of energy such as wind or solar into one that’s available on demand, and if you’re using a clean source like that, it’s clean. Whether this is a sensible solution to the intermittency problems of wind and solar power is an economic question – my guess is that it may eventually have a role, possibly even a big one. Storing energy as hydrogen doesn’t have as good a round trip efficiency as some other technologies, but does have the advantage that there are massive existing facilites for storage of natural gas, and power stations that use natural gas, which could be converted to use hydrogen instead. The poor round trip efficiency is not so important for facilities that would only be used relatively infrequently, during long dark, windless periods.*
There’s also an issue with the round-trip efficiency of the conversion from electricity to hydrogen and back again, which is lower than for most other energy storage methods.
And isn’t hydrogen rather dangerous stuff? Remember the Hindenburg?
Well, no, hydrogen isn’t really particularly dangerous. The dangers are a bit different from the dangers of petrol (gasoline) but they’re not particularly greater. There have been a lot of petrol disasters, too; and there’s been a lot more hydrogen around than folks realize, with quite a few accidents, but not disproportionately many or disproportionatly serious. Until the change over to natural gas, gas piped to houses was half hydrogen. The other half, poisonous carbon monoxide, was a much bigger hazard than the hydrogen!
The real problem with the Hindenburg was that (against the engineers’ advice) the envelope containing the hydrogen was flammable. The hydrogen contributed to the fire, but the fire started in the envelope – and the burning hydrogen rose very rapidly away from the scene, whereas the burning envelope remained.
It’s quite difficult to stop hydrogen leaking slowly, but it also disperses very rapidly if it does leak, so it’s unusual for flammable concentrations to build up unless the leaks are big – and it’s not hard to stop those big leaks. However, it would be important to ensure some ventilation – not an unusual amount, but some – in your garage. (That’s actually quite important with petrol too.) Hydrogen is flammable (or explosive) at a very wide range of concentrations compared with the range for petrol vapour, but as it disperses relatively quickly, those concentrations are rarely reached in practice. The flammable and explosive concentrations of petrol vapour are quite easily reached in any enclosed space, despite good ventilation, and sometimes even in unenclosed areas. Finally, petrol vapour mixtures in air contain a great deal more energy than the same volume of hydrogen-air mixture, so the explosions are much bigger, and the fires generate a lot more heat.
The real attraction of hydrogen is that you can use water and electricity from a non-fossil fuel source to make it. The other ways of making hydrogen use fossil fuels. If the fossil fuel is oil, you produce more carbon dioxide making the hydrogen than using the oil directly in your vehicle would; if it’s gas or coal, you could equally well make a far more convenient liquid fuel rather than hydrogen.
Even if you’re using electricity, it’s possible to make liquid fuel rather than hydrogen. It requires rather more advanced technology, but we have such technologies! It’s also significantly less efficient.
There’s one (admittedly large) niche where hydrogen could be useful, and that’s shipping. Large ships could easily carry the insulation required to store liquid hydrogen. Whether the energy cost of liquefying the hydrogen in the first place would make it uneconomic, I don’t know. It might be better to synthesize methane (which is easier and cheaper to liquefy and keep liquid than hydrogen), or methanol which is liquid at ambient temperatures.
It would also be possible to run trucks, trains or even planes on liquid hydrogen. The problem with liquid hydrogen – a killer for most private cars – is the fact that the hydrogen evaporates relatively quickly, particularly if you try to minimize the bulk of the insulation. This means firstly that you have to vent (or preferably use†) the evaporating hydrogen safely (the quantities involved are inevitably much greater than those slow leaks from pressurized vessels), and secondly that you’re losing (or using) fuel at a significant rate whenever there’s any in the tank.‡
Hydrogen is not a potential saviour for air transport; it would make an already ridiculously profligate user of energy even worse – and while the elimination of carbon dioxide from aircraft exhauset would be a good thing, water vapour in the stratosphere is a significant global warming issue in itself.
Anyway, as long we are burning fuel – fossil fuel or biomass – to make electricity, performing the inverse process must surely be wasteful – both processes are substantially less than 100% efficient. So until we’ve completely, or almost completely, given up fuel combustion for generating electricity, we really shouldn’t be using electricity to make fuels, except possibly in places like Iceland where there is ample renewable energy, a relatively small market for electricity, and the vast majority of vehicles are used mainly for fairly short journeys.
* Long after I wrote this, Scientific American published a far more detailed article making much the same points: Solar and Wind Power Could Ignite a Hydrogen Energy Comeback (Scientific American, February 2020)
† Even if your car actually uses most of its hydrogen in a turbine, you might have a small fuel cell to use the evaporating hydrogen – perhaps to trickle charge a battery, or keep various systems going. At worst you could dump the energy as heat, which is better than venting hydrogen. Hydrogen is an indirect greenhouse gas – not a major one, but you don’t want to produce any more greenhouse gas than can possibly be avoided. See More about Hydrogen.
‡ Unlike the mass of the containers for compressed hydrogen which is proportional to the mass of hydrogen they contain, the volume of insulation required for liquid hydrogen is only proportion to the surface area of the containers (for a given evaporation rate). Thus whereas the energy density of a compressed hydrogen system is no better at large sizes than small, the same is not true of liquid hydrogen, which becomes relatively reasonable at large scales. There is however the energy cost of the liquification process to consider – some but not all of which might be recoverable (as in Storing Electrical Energy: Thermal) on a large vehicle such as a ship.