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.

The Hydrogen Economy to solve Climate Change?

One of the interesting aspects of climate change is the number of proposals put forward to solve it that do not take into account adverse consequences. There is a strong association of wishful thinking with some of these. On the other side are the gloomy ones, and maybe I fall into that category. What brought that thought to the fore was I have seen further claims for hydrogen as a solution. Why? Well, there are wild claims that wind and solar will solve everything. One problem with these is they tend to deliver their energy in pulses: solar during the day, wind when it is blowing. The net result is that if these can deliver adequate power for all times, there is serious overproduction required at other times. The problem then is how to store this energy. One way is to pump water uphill, but that requires large storage. In a country like New Zealand, where much of the electricity is hydro generated, you would just turn off that generation and use the hydro to manage power demand. However, that assumes there is not a large increase in electricity demand. One proposed solution is to generate hydrogen by electrolysing water. This is a well-understood technology, with no problems, given the power. There are, however, significant economic ones.

This is claimed to solve another problem; a very significant amount of domestic heating is obtained from burning gas. Now, all we have to do is burn hydrogen. We could also use hydrogen in vehicles. My big problem, having worked with hydrogen before, is that it leaks, and is extremely flammable. According to Wikipedia, the flammability range of hydrogen in air is between 4% and 75%; to detonate, the limits are 18.3 – 59% (each by volume), and a leak can support combustion at flow rates as low as 4 micrograms/second. Mixtures can ignite with very low energy input, 1/10 of that needed to ignite gasoline/air, and any static electric spark can ignite it.

The leak problem is made worse by the fact that hydrogen can embrittle metals, and thus create a way for it to escape. It is lighter than air, so it tends to accumulate at the ceilings of buildings, and its very wide explosive range is a broad hazard. The idea that hydrogen could be piped into houses to provide heating is not something I would want to see. The problem is made worse in that you might be sensible and cautious and not take it up, but your neighbour might. The consequences of that can impinge on you. Recently, in Christchurch, a house blew up, and reduced itself to a collection of boards, roofing material, etc., with only the foundations remaining more or less where they started. Several neighbours houses were severely damaged, and made effectively unliveable, at least without major repairs. By some miracle, nobody was killed, although a number had injuries. What apparently happened was a registered gasfitter had done some work on the house’s gas system, there was a natural gas leak, and something ignited it. My guess is, he has some explaining to do, but my point is if this can happen to a registered tradesman, what will happen if there is widespread use of something that leaks with orders of magnitude more ease?

There is a further irony. The objective behind using hydrogen is to help the greenhouse effect by reducing the amount of carbon dioxide we emit. Unfortunately, leaked hydrogen also magnifies the greenhouse effect. At first sight, this does not look right because greenhouse gases work because there is a change of dipole moment in the vibrational mode. This is needed because unless the transition involves a change of electric moment, it cannot absorb a photon. Hydrogen has only one vibrational mode and no electric moment, and no change of electric moment when it is stretched because of its symmetry, end of H2 is exactly the same as the other end. There is a minor effect in that the molecule can be polarised for an instant in a collision with something else, but that is fairly harmless.

The problem lies in downstream consequences. One of the important greenhouse gases is methane, emitted by natural gas leaks, farm animals, other farm processes, anaerobic fermentation, etc. Methane is about 35 times more powerful than carbon dioxide as a greenhouse gas, and worse, it absorbs in otherwise transparent parts of the infrared spectrum. (The otherwise does not include other hydrocarbon gases.) However, methane is not as serious as it might be because it is short-lived. UV radiation in the upper atmosphere breaks water, directly or indirectly, into hydroxyl radicals and hydrogen radicals. The hydroxyl radicals rapidly degrade methane, and the hydrogen radicals react with oxygen in the air to make peroxyl radicals that also degrade methane. Molecular hydrogen reacts with both these sort of radicals, and thus indirectly preserves the methane.

There are, of course, other ways of using hydrogen, such as in chemical reactions, including upgrading biofuels, and it can be stored in chemical compounds. Hydrazine (N2H4) is an example of a liquid that could make a very useful fuel. (In the book, and film “The Martian”, the hero has hydrazine from the fuel tank of a rocket, so he catalytically converts it to hydrogen to burn to make water, and blows up his “dome”. It would have been so much easier to burn hydrazine, as it was, after all, from a rocket fuel tank.) Other options include storing hydrogen as hydrides, e.g. borohydrides, or as ammonia, which is cheaper to make than hydrazine, but it is also a gas, unlike hydrazine. The problem is usually how to deliver the hydrogen at a regular and controllable rate.

The use of hydrogen in a chemical manufacturing plant, or when handled with expertise, such as when used by NASA, is no problem. My concern would be for the average person doing repairs themselves to pipes conveying hydrogen, or worse still, plumbing incorrectly. As for having hydrogen as a fuel to be delivered at refuelling stations, I used this concept in my ebook “Puppeteer” to illustrate the potential danger if there are terrorists on the loose.