The Case for Hydrogen in Transport

In the last post I looked at the problem of generating electricity, and found that one of the problems is demand smoothing One approach to this is to look at the transport problem, the other major energy demand system. Currently we fill our tanks with petroleum derived products, and everything is set for that. However, battery-powered cars would remove the need for petrol, and if they were charged overnight, they would help this smoothing problem. The biggest single problem is that this cannot be done because there is not enough of some of the necessary elements to make it work. Poorer quality batteries could be made, but there is another possibility: the fuel cell.

The idea is simple. When electricity is not in high demand, the surplus is used to electrolyse water to hydrogen and oxygen. The hydrogen is stored, and when introduced to a fuel cell it burns to make water while generating electricity. Superficially, this is ideal, but there are problems. One is similar to the battery – the electrodes tend to be made of platinum, and platinum is neither cheap nor common. However, new electrodes may solve this problem. Platinum has the advantage that it is very unreactive, but the periodic servicing of the cell and the replacing of electrodes is realistic, and of course recycling can be carried out because unlike the battery, it would be possible to merely recycle the electrodes. (We could also use pressurised hydrogen in an internal combustion engine, with serious redesign, but the efficiency is simply too low.)

One major problem is storing the hydrogen. If we store it as a gas, very high pressures are needed to get a realistic mass to volume ratio, and hydrogen embrittles metals, so the tanks, etc., may need servicing as well. We could store it as a liquid, but the boiling point is -259 oC. Carting this stuff around would be a challenge, and to make matters worse, hydrogen occurs in two forms, ortho and para, which arise because the nuclear spins can be either aligned or not. Because the molecule is so small there is an energy difference between these, and the equilibrium ratio is different at liquid temperatures to room temperatures. The mix will slowly re-equilibrate at the low temperature, give off heat, boil off some hydrogen, and increase the pressure. This is less of a problem if you have a major user, because surplus pressure is relieved when hydrogen is drawn off for use, and if there is a good flow-through, no problem. It may be a problem if hydrogen is being shipped around.

The obvious alternative is not to ship it around, but ship the electricity instead. In such a scenario for smaller users, such as cars, the hydrogen is generated at the service station, stored under pressure, and more is generated to maintain the pressure. That would require a rather large tank, but it is doable. Toyota apparently think the problem can be overcome because they are now marketing the Mirai, a car powered by hydrogen fuel cells. Again, the take-up may be limited to fleet operators, who send the vehicles out of central sites. Apparently, the range is 500 km and it uses 4.6 kg of hydrogen. Hydrogen is the smallest atom so low weight is easy, except the vehicle will have a lot of weight and volume tied up with the gas pressurized storage. The question then is, how many fuel stations will have this very large hydrogen storage? If you are running a vehicle fleet or buses around the city, then your staff can refill as well, which gets them to and from work, but the vehicle will not be much use for holidays unless there are a lot of such stations.

Another possible use is in aircraft, but I don’t see that, except maybe small short-haul flights driven by electric motors with propellors. Hydrogen would burn well enough, but the secret of hydrocarbons for aircraft is they have a good energy density and they store the liquids in the wings. The tanks required to hold hydrogen would add so much weight to the wings they might fall off. If the main hull is used, where do the passengers and freight go? Another possibility is to power ships. Now you would have to use liquid hydrogen, which would require extremely powerful refrigeration. That is unlikely to be economic compared with nuclear propulsion that we have now.

The real problem is not so much how do you power a ship, or anything else for that matter, but rather what do you do with the current fleet? There are approximately 1.4 billion motor vehicles in the world and they run on oil. Let us say that in a hundred years everyone will use fuel cell-driven cars, say. What do we do in the meantime? Here, the cheapest new electric car costs about three times the cost of the cheapest petrol driven car. Trade vans and larger vehicles can come down to about 1.5 times the price, in part due to tax differences. But you may have noticed that government debt has become somewhat large of late, due to the printing of large amounts of money that governments have promptly spent. That sort of encouragement will probably be limited in the future, particularly as a consequence of shortages arising from sanctions. In terms of cost, I rather think that many people will be hanging on to their petrol-powered vehicles, even if the price of fuel increases, because the difference in the price of fuel is still a few tens of dollars a week tops, whereas discarding the vehicle and buying a new electric one involves tens of thousands of dollars, and with the current general price increases, most people will not have those spare dollars to throw away. Accordingly, in my opinion we should focus some attention on finding an alternative to fossil fuels to power our heritage fleet.

The Hydrogen Economy

Now that climate change has finally struck home to at least some politicians, we have the problem, what to do next. An obvious point could be that while the politicians made grandiose promises about it thirty years ago, and then for economic reasons did nothing, they could at least have carried out research so they knew what their options are so that when they finally got around to doing something, they knew what to do. Right now, they don’t. One of the possibilities for transport is the use of hydrogen, but is that helpful? If so, where? The first point is you have to make your hydrogen. That is easy: you pass electricity through water. There is no shortage of water but you still have to generate your electricity. This raises the question, how, and at what cost? The good news is that generating hydrogen merely consumes energy so it can be turned down or off at peak load periods, but the difficulty now is the renewables everyone is so happy about offer erratic loads. As an example, Germany is turning off its nuclear power stations and finds it has to burn more coal, especially when the wind is not blowing. 

Assume we have the electricity and we have hydrogen, now what? The hydrogen could be burned directly in a compression motor, or used to power fuel cells. The latter is far more energy efficient, and we can probably manage about 70% overall efficiency. The reason the fuel cell is more desirable than the battery is simply that the battery cannot contain the desired energy density. The advantages of hydrogen include it is light and when burned (including in a fuel cell) all it makes is water. Water is a very powerful greenhouse gas, but the atmosphere has a way of promptly removing excess: rain.

However, hydrogen does have some disadvantages. A hydrogen-air mix is explosive over a rather wide mix ratio. Even outside this ratio, it has a clear flammability and an exceptionally fast flame speed, it leaks far faster than any other gas other than, possibly, helium, and it is odourless and colourless so you may not know it is there. But suppose you put that behind you, there are still clear problems. A small fuel cell car would need approximately 1 kg of hydrogen to drive 100 km. Now, suppose we need a range of 500 km. The storage of 5 kg of hydrogen would take up most of the boot space if you use a tank that is pressurised to 700 bar. (1 bar is atmospheric pressure.) That requires a lot of energy to compress the gas, and it adds a significant weight to the reinforced tank, which you most certainly do not want to rupture. The volume is important for a small car. You wish to go on holiday, then find your boot is occupied by a massive gas tank. However, this is trivial for very large machines, and a company in the US makes hydrogen powered forklifts. Here, a very heavy counterballancing weight is required so a monstrous steel tank is actually an asset. I previously wrote a blog post on hydrogen for vehicles, here.

There are different possible ways to store hydrogen. For those with a technical bent, the objective is to have something that absorbs hydrogen and binds it with an energy of between 15 – 20 kJ/mol. That is fairly weak. If you can mange that range you can store hydrogen at up to 100 bar with good reversibility. If you bind it in metal hydrides, you get a better density of storage at atmospheric pressure, but the difficulty is then to get the hydrogen back out. Most of the proposed metal organic absorbers bind it too weakly and you can’t get enough in. The metals that strongly absorb can be made to release it easier if the metal is present as nanoparticles, and to prevent these clumping, they can be embedded into carbon. There is an issue here, though, that the required volume is starting to become large for a given usage range because there are so many components that are not hydrogen.

There is another problem with hydrogen that most overlook: how do you deliver it to filling stations? Pressurizing won’t work because you can’t get enough into any container to be worth it. You could ship liquefied hydrogen, but it is only a liquid at or below -253 degrees Centigrade. It takes a lot of energy to cool that far, a lot to keep it that cold, and the part that most people will not realize is that at those very low temperatures for very light atoms, there are some effects of quantum mechanics that have to be taken into account. One problem is that hydrogen occurs as two isomers: ortho and para hydrogen. (Isomers are where there are at last two distinctly different forms with the same components, that may or may not readily interconvert.)  These arise because the hydrogen molecule comprises two protons bound by two electrons. The protons have what we call nuclear spin and as a consequence, have a magnetic moment. In ortho hydrogen, the spins are aligned; in para they are opposed. At room temperature, the hydrogen is 75% in the ortho form, but this is of higher energy than the para form. Accordingly, if you just cool hydrogen to the liquid form, you get the room temperature mix. This slowly converts to the para form, but it gives off heat as it does so. That means a tank of liquid hydrogen slowly builds up pressure. To be used as liquid hydrogen it is probably best to let it switch to the para form first, but that takes a lot more energy maintaining the low temperatures while the conversion is going on. Currently, liquefying hydrogen takes 12 kWh of power per kilogram of hydrogen, which is about 25% that of what you get from a fuel cell. In practice, you may need almost that much again to keep it cold, and since this power has to be electrical, we have an even greater demand for electricity.

So, is there an answer? My feeling is still that hydrogen is not the most desirable material for a fuel cell, from the point of view of usage in transport. The reason it is pursued is that it is easiest to make a fuel cell work with hydrogen. There are alternatives. Two that come to mind are ammonia and methanol. Both can drive fuel cells, and ammonia reacts to give water and nitrogen while methanol reacts to give water and carbon dioxide. Currently, the ammonia cell may be more efficient, but ammonia is somewhat difficult to make, although there is evidence it can be made from hydrogen and nitrogen under mild conditions. The methanol fuel cell has a problem that too much of the methanol sneaks through the membrane that keeps the two sides of the cell separate, and carbon monoxide tends to poison electrodes. Methanol could be made by the reduction of carbon dioxide from the air with solar energy.

So where does that leave us? In my opinion, what we need more than anything else is progress on better performing methanol or ammonia fuel cells, or some better fuel cell. My preference for the fuel cell is simply an issue of weight and power density, and I do not see hydrogen as being useful for light vehicles. The very heavy machines are a different matter, and batteries will never adequately power them. The problem of energy production in the future is a real one, and I feel we need to do a lot more research to pick the better options. We should have been doing this over the last thirty years, but we didn’t. However, there is no point in moaning about time wasted; we are here, and we have to act with a lot more urgency. However, it is not right to use the easiest but not very good options; we need to get these problems right.