A couple weeks ago helped organise a panel event on behalf of the Oxford Scientist magazine along with the Oxford Climate Society looking at the use of hydrogen as a fuel, focused on using hydrogen in commercial aviation and the emerging industry of geologic hydrogen. Dr Andrew Pilkington, a postdoc in the engineering department, and Dr Anran (Annie) Cheng, who used to lecture me in the Earth Science dept but is now working for a geologic hydrogen based company, were both interviewed and brought contrasting expertise into the discussion. The panel was incredibly interesting and brought up discussion surrounding areas of hydrogen fuel usage I had never come across before, so I thought I would share some of what I learnt on here.
Image credit Luka Slapnicar via Unsplash
What is hydrogen?
It makes sense to start off with a quick background into what hydrogen is and how it can be used as a fuel. As I'm sure you are aware, hydrogen (H2) is a gas that only makes up 0.00005% of our atmosphere but is found in much higher abundances on Earth combined with other elements. As a fuel, hydrogen would be used in one of two ways. Firstly, it can be burned, much like natural gas is burnt, to release energy. Alternatively it can be used in a fuel cell where it is combined with oxygen to create water and this gives you energy in the form of electricity.
Where does hydrogen come from?
In order to use this hydrogen as a fuel we first have to acquire it. This can also be done in a number of ways. Firstly, you can steam reform methane (CH4) to produce hydrogen which is essentially just reacting hot steam with the methane. This is what is currently done to produce much of the hydrogen for the fertiliser industry. Another option to produce hydrogen is via electrolysis, which is a very expensive technique that passes an electrical current through water to split it up into hydrogen and oxygen. Finally, the industry of geologic hydrogen is emerging. There is lots of naturally produced hydrogen in cratons (very old stable bits of continental rock) made via the decays of U and Th splitting up water molecules. It is also produced in mafic and ultramafic rocks such as bits of the ocean floor that have been obducted (pushed onto the continents) that are rich in Fe II. If the Fe II is oxidised by water to Fe III then it releases hydrogen. This is, as I said, a very new industry so sites of extraction are few and far between. One of the key localities Dr Cheng mentioned was the natural hydrogen reserves in Mali. Around 10 years ago a well was found which had a gas phase of 98% hydrogen in the subsurface (this is very pure in hydrogen) and this is very exciting. Issues, however, arise with extraction due to the lack of infrastructure, both physically and policy wise, in Mali.
The hydrogen rainbow
Different types of hydrogen reserves all have their own colour assigned to them depending on where they came from. This has lead to the formation of the 'hydrogen rainbow'. For example, black and grey hydrogen comes form burning fossil fuels, and blue hydrogen is the term used for hydrogen produced by fossil fuels but has it's CO2 emissions used up or stored in the subsurface via carbon capture and storage methods. Green hydrogen is produced using renewable sources such as wind and solar, red hydrogen is produced using nuclear energy, and as natural hydrogen is so new, it is yet to formally have it's own colour but people have put forwards suggestions of white, gold, and even colourless.
How is hydrogen stored geologically?
Storing hydrogen underground is very similar to storing natural gas or hydrocarbons underground. You need a porous reservoir rock (one with lots of pore space to keep the hydrogen in) underlying a impermeable cap rock. The cap rock does not allow the hydrogen to flow through it so traps it in the reservoir rock. This is a highly simplified version of hydrogen/petroleum systems as many other aspects of structural geology come into play but that gives you the basics.
A further issue with storing hydrogen underground (say we put it there to use later rather than found it there in the first place) is biodegradation. Turns out, microbes love to eat hydrogen (use it as an energy source)so if we leave it in the ground long enough it will begin to be used up by the microbes and none will be left for us. This is less of an issue for short term storage but for long term it needs to be considered.
What might we use hydrogen as a fuel for?
The obvious answer here is trying to use hydrogen to power commercial aircraft. It is, according to Dr Pilkington, the best way to decarbonise aviation and this is an area of much research. Hydrogen is also already used as a fuel in some space craft, being used for the first time back in NASA's Apollo Programme.
What are the challenges with using hydrogen in aviation?
So there are two possible ways of using hydrogen to power aircraft, both come with their own issues. The main problem overall is that hydrogen is a very low density gas with very small molecules. These molecules are very good at leaking out of containers such as steel.
In order to get enough hydrogen in one place for it to be worth it (to overcome the low density) the gas has to be stored at very high pressures. To get it up to high enough pressures (700bars is what is currently used in hydrogen powered cars and this about 700 times atmospheric pressure) it needs to be stored in very strong, and therefore very heavy, containers. The weight of these containers is obviously not ideal for aviation as it would take a lot more energy to fly the aircraft.
Another option to overcome this is storing it as a liquid. This massively increases it's density so means that you can have a lot more of it in a smaller space, it also won't escape out of the container. However, in order to get hydrogen to be in a liquid state it needs to be at very low temperatures. By very low temperatures I mean 35K or -238ºC, which is incredibly cold! This involves the use of cryogenics. Using liquid hydrogen is the only viable option for long haul flights that can't afford the extra weight of heavy containers but much research is still being done to figure out aspects such as keeping the hydrogen cool and then heating it up again before burning it to release the energy.
Flying smaller, slower aircraft means that generating energy via a fuel cell rather than by burning the hydrogen is a possibility. This is a more efficient process but requires a much heavier system of infrastructure on board the plane and results in an electric motor running a turbine rather than a conventional burning engine.
Could we use hydrogen to heat our homes?
In theory yes, but in reality there are many issues. Firstly, as mentioned above, hydrogen leaks very easily out of metal pipes so a whole new set of piping infrastructure would need to be out in place to transport the gas to peoples homes. In addition to this, the energy density of hydrogen is about a third of that of natural gas. This would mean that 3 times as much hydrogen would be needed to get the same amount of energy natural gas currently gives. This would require more energy to pump more gas through the pipe network. In addition to this, as hydrogen is a lower density gas, it takes more energy to pump it through the pipe system anyway. The density difference also means that our current burners, designed for burning natural gas, could not run on 100% hydrogen. At best they could run on a mixture of 20% hydrogen, 80% natural gas without too many issues but this, essentially diluted, mixture would give you less energy. So, along with the issues of transporting the hydrogen, at the moment heating homes via hydrogen is not really worth it.
What is the future of hydrogen as a fuel?
Hydrogen will be vital in decarbonising society, especially when it comes to aviation if we want to keep flying at the rate and distances that we do today. It's looking like getting hydrogen as a wide spread fuel in commercial aircraft won't happen until around 2050 but the work is being done now to begin to make this a reality.
In terms of natural hydrogen, geologic exploration is only just the beginning, and reservoir exploration is in it's infancy. Depending on how this goes then maybe hydrioen could become the fuel of the future?
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