Hydrogen is a clean fuel – it doesn’t produce any carbon dioxide 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.
It doesn’t stop you using it as a transport fuel, either. It can convert a fixed power source into a movable one. The problem is that while hydrogen itself is very energy dense, you’ve got to contain it somehow, and the container is inevitably big or heavy or both. There are various clever ways of making it a bit less big or heavy, but it’s still very big and heavy by comparison with an equivalent tankful of liquid fuel, and these clever ways are EXPENSIVE, too. People are working on even cleverer ways of doing it, and they might manage to develop something a bit more energy dense and a bit less expensive, but it can’t possibly ever come close to matching the energy density or low cost of a liquid fuel system. All these hydrogen concept cars are just that: concept cars. And the concept is a non-starter.
That’s not quite true – but very nearly. There are a couple of possible niches for hydrogen as a road transport fuel. If you’re not going very far on each trip, or you don’t need much performance, a fuel tank containing relatively little energy is usable. So if you live on a small island, or never drive out of town, it works. It works for a vehicle doing nothing but local deliveries, too. But don’t expect to drive intercity at motorway speeds, unless you’re prepared to use all your luggage space and half the back seat for a fuel tank that weighs more than the whole of the rest of the car whether it’s full or empty.
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. 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!
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 a bit less efficient, but you have to weigh that against hydrogen’s ridiculous containment problems on the vehicle.
There’s one (admittedly large) niche where I’m less confident that hydrogen is useless, 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 methanol.
It would also be just about possible to run trucks, trains or even planes on liquid hydrogen. The problem with liquid hydrogen – a killer for small vehicles – 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 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.
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.