Hydrogen fuel is sometimes given as a clean, sustainable alternative to petrol and other fossil fuels.
For background, fossil fuels such as oil, petrol and coal mostly consist of hydrocarbons; they are basically made of hydrogen and carbon. These burn – combine with oxygen – to produce water from the hydrogen combining with the oxygen, and carbon dioxide or carbon monoxide from the carbon combining with the oxygen. Plus some energy, that is used to make the car go or the water heat or the fire burn merrily.
Burning hydrogen is, unsurprisingly, like burning a hydrocarbon but without the carbon; you get water and energy. Nice one. No carbon monoxide human poison, no carbon dioxide planet poison, just water and energy.
However there are two issues: There is no hydrogen lying around for us to burn, and it’s not all that easy to move around from where we make it to where we use it. And once we’ve considered the whole lifecycle, from finding an energy source to using it, then hydrogen is by no means the cleanest.
Making a fuel
Hydrogen can be a fuel, in the sense that we can use it to fuel an engine, but it’s not an energy source. There’s no hydrogen gas lying around on the planet. It’s very light, so it tends to to float off up the atmosphere and into space, where there is quite a lot of it but it’s very spread out.
So we need energy in order to make the hydrogen gas, the fuel.
At the moment it’s usually made by splitting oil hydrocarbon molecules to get hydrogen and a smaller hydrocarbon, methane, a stronger greenhouse gas than carbon dioxide. This splitting takes energy, whether that comes from burning fossil fuels or wind farms or nuclear power.
You can also make it from water by electrolysis as you might have done at school; using electrical energy you can break the water molecule into hydrogen gas and oxygen gas. Water molecules are quite ‘strong’, and tearing it apart takes more energy than breaking up oil molecules, though people are working on how to make it easier.
So to make hydrogen you need energy. That energy needs to come from somewhere; and it’s whether that source is clean and sustainable that defines whether your energy system is clean and sustainable.
If you’re making hydrogen out of water by using electricity from a petrol generator for example, you’re even less ‘clean’ than putting that petrol into your car engine directly, because of the extra losses in making the hydrogen gas and moving it around:
Transporting a fuel
Hydrogen acts very much like electricity: that is, it acts as a medium for moving energy about. You can make some in one place and move it somewhere else to use. This means, for example, that you can use it in a car where you want to be moving while using it.
The energy density of hydrogen is pretty good by weight – it’s very light for the amount of energy you can get from it. But it’s pretty poor by volume – it takes up three or four times more space than petrol for example.
Gas is much harder to move around than liquid. To make it liquid you need to either pressurise it or cool it, and that means either strong special containers or more energy for the cooling. This is already routinely done for natural gas, but as hydrogen is a very small molecule, it gradually leaks through container walls. So there are losses in using hydrogen as an energy transport medium: it takes energy to cool it, to make and to move the containers, and you expect to lose some before it gets used.
And it’s a lot less convenient for general use. You can’t just pour some out from a jerry can.
Still, the pressurised containers are much stronger than the thin plastics currently used to hold petrol in a car, and so are less likely to break in an accident. And hydrogen gas escaping from a fuel line would generally disperse upwards rather than lie around near the ground waiting to go bang as petrol does. On the other hand, an airburst of highly flammable gas might not be that much better than a somewhat manageable ground fire.
The whole lifecycle
Once we separate out the concept of transporting energy from the concept of the energy source, we have far more options available.
Ideally, we want a sustainable, cheap energy source, and a convenient, easily transportable, safe fuel. At least, as safe as something that stores a lot of readily available energy can be.
Sometimes these are the same; petrol is (for general purposes) an energy source because we can just dig out of the ground, and it can be conveniently carried around in tankers, pipes, and small tins for dodgy old camping stoves.
Where they are not the same, in principle we can pick the best of both. After all, if we have a clean and sustainable energy source, we don’t just have to settle for hydrogen; we can make ethanol, or methanol, or bigger hydrocarbons such as petrol from water and carbon dioxide, like wot plants do eventually.
Artificially making diesel would keep the overall impact of carbon dioxide neutral, as all the CO2 released when the fuel is burned matches the CO2 broken into hydrocarbons during manufacture. Biofuels are an example of this, using solar power to manufacture hydrocarbons.
So hydrogen offers very little if anything over petrol even as a way of transporting energy, and the advantage of making and transporting petrol instead is that we can continue to use our existing oil infrastructure, our existing cars and car technology, and our old dodgy camping stoves.
All we need to do is build clean sustainable energy sources. Just as soon as we run out of oil.
Addendum Update Thing
We don’t have to limit ourselves to a single energy transport. We might use nuclear power or solar power as energy sources, then transport the energy as electricity over generally safe, fixed power lines to our homes.
There we plug in our new slimy algae farm to generate diesel from electricity, so that we can use convenient diesel to fuel a car.