Some questions for the hydrogen economy
We urgently need to decarbonise hydrogen production, is transportation the right use case?
Hydrogen is the most abundant element in the universe. It is also the lightest and simplest, consisting of a single proton and electron in its most common isotope. Fusion of hydrogen into helium within the core of stars like our sun is also responsible for virtually all the energy available on the earth and hence it is not an exageration to say that it is the most important element in the universe.
There have been a lot of words written about hydrogen over the years, in particular around its potential use as a sustainable fuel to replace our addiction to fossil hydro-carbons. In this post I want to discuss where I think hydrogen will and won’t play a role in our energy future. I will discuss the following key aspects:
Current hydrogen production and uses.
“Green” hydrogen.
Potential as a sustainable fuel for transportation.
First let’s discuss how hydrogen is produced. The world produces a lot of hydrogen, some 90 million tons in 2020. How much is 90 million tons? Well, if it were stored at 0 deg C at standard atmospheric pressure, it would occupy a volume of 22,444 million million litres, or 22,444 cubic km - that’s a giant cube approximately 28 km per side.
Virtually all hydrogen production in the world today is made either by steam methane reforming or as a by-product of some other fossil fuel intensive process. The figure below from the International Energy Agency (IEA) shows production in 2020. Collectively the production of hydrogen was responsible for 2.5% of the world’s CO2 emissions, equivalent to the emissions of the UK and Indonesia combined.
Notice how there is no “slice” in the graph for electrolysis, which is the process for making green hydrogen from water through an electrolyser. According to the same IEA report, this made up just 0.03% of hydrogen production in 2020. Today hydrogen is produced almost exclusively from fossil fuels and as a result has a high carbon footprint.
Why do we produce so much hydrogen today? It has four main uses:
As feedstock for production of ammonia based fertilisers (27%)
Oil refining (33%)
Methanol production (11%)
Steel production via direct reduction of iron ore (3%)
Focusing in on the production of fertiliser - this is perhaps its most critical use case. It is estimated that the world’s population would be 50% smaller without synthetic fertiliser produced by the Haber-Bosch process, for which hydrogen is a key ingredient. Put another more direct way:
“Half of the world’s population today relies on hydrogen to eat”.
Circling back on the production methods, why is so little hydrogen produced by electrolysis today? Simple, economics. Fossil fuels are cheap. Electrolysers and electrolytic cracking of hydrogen are not.
It takes a lot of electrical energy to turn water into hydrogen and oxygen. How much? Approximately 50 kWh / kg. So how much energy would we need to produce just the hydrogen that we need to feed half the world’s population? 27% of 90 Mt = 24.3 Mt. How much energy would be needed to produce this by electrolysis? 50 kWh * 24.3 Bkg = 1,215 TWh. That is probably a meaningless number to most people, however, it is a meaningful fraction of the world’s total electrical energy supply, about 5%. Or to put another way, this is roughly equivalent to the half the electrical energy production from all the world’s windfarms in one year.
We need a long term solution to produce green hydrogen without producing CO2 because the world needs to eat. It will be an enormous task to build the required green energy infrastructure just for this use case.
What about hydrogen for transportation? At first glance, hydrogen has a lot of nice properties as a transportation fuel. It is very light and contains about three times the energy of oil by weight. You can produce it cleanly by cracking water. When combusted, or used in a fuel cell, it has only water as emissions and no CO2 is produced. What is not to like?
Unfortunately, it’s weight and molecular size is also a big downside. In order to store it usefully, it needs to be stored in specialised tanks at very high pressure. The fuel tanks in the Toyota Mirai store hydrogen at 700 bar - that is 700 atmospheres of pressure. Even when stored at high pressure, it takes up a lot of room. That same Mirai fuel tank is about 200 litres, 3-4 times the size of a typical vehicle fuel tank. On this basis, there is no plausible proposal to fuel a trans-continental jet with hydrogen as there is just nowhere to store the required volume of fuel, whilst still being able to take a reasonable amount of cargo and passengers a long distance.
The problems with hydrogen as a transportation fuel do not end with its poor energy density. It is also notoriously difficult to store. Its small molecular size means that it tends to easily escape typical pressure vessels. Moreover, it can also ruin steel containment vessels in a process known as hydrogen embrittlement. Hence why the Mirai’s fuel tanks are carbon fibre; a very expensive storage vessel.
The biggest downside of all though is an economic one. Hydrogen will never be a cheap transportation fuel because producing it using electrolysis and then using it for vehicles has very poor efficiency. The graph below illustrates. For every 100W of input energy, you only get 38W to the vehicle drive train. This is less than half the efficiency of an electric vehicle.
Even if you assume that your electrolyser costs you nothing (a bad assumption), you are still going to be paying at least twice as much to “fuel” your hydrogen car compared with your EV. This is playing out in the real world. Hydrogen prices over USD 20/kg at the pump have been seen recently in California. This translates to roughly four times the cost of charging an equivalent EV. There are other problems too, like recompression times at the pump and fuel bowsers freezing over.
In summary, we urgently need to decarbonise hydrogen production so we can cleanly produce the fertiliser the world needs to feed everyone. The physics and real-world evidence suggests that using hydrogen for a transportation fuel is an expensive waste of a valuable resource when better solutions exist.