Hydrogen is all around us, but exactly how (and why) it ends up in a highly-pressurised form at a hydrogen fuelling station – which can be pumped into a hydrogen fuel cell vehicle’s (or FCEV’s) storage tank – is an interesting story: one which we explore here.
The basic problem of ‘extracting’/isolating hydrogen
Hydrogen is the most frequently found element on the planet, however it’s not as easy as just ‘plucking’ this from where it ‘lives’ and storing it away, waiting for a car to use this as fuel. This is because the H2 chemical element (H2 being hydrogen) is abundant on earth, but nearly always as part of another compound such as water (H20 – i.e. a compound which contains two hydrogen atoms and one oxygen atom).
Due to how fuel cells work, hydrogen needs to be in gas form to power a car. So we need some way of separating hydrogen from a multi-atom compound (like water) into just its base H2 compound, and also storing it as in gas form. The next section takes a detailed look at different ways this can be achieved.
Producing hydrogen as a usable fuel source (hydrogen gas)
There’s two main commercially-viable methods for producing hydrogen currently:
- Steam methane reforming: SMR – also called natural gas reforming or gasification – is currently the most common way of making hydrogen. Natural gas contains methane (CH4) and when mixed with a very high temperature steam (close to 1,000oC) and 3-25 bar (43.5 – 362.5 PSI) pressure alongside carbon monoxide (acting as a catalyst), hydrogen is produced. Further reactions are then applied to the carbon monoxide, to produce further hydrogen – but also carbon dioxide. Naturally carbon dioxide is the main gas responsible for climate change. So whilst SMR is currently the more cost effective and efficient way of producing hydrogen, it does produce greenhouse gas emissions; albeit 34-50% fewer carbon emissions than gasoline-powered vehicles end up producing (along the fuel generation chain). Read more at energy.gov.
- Electrolysis: hydrogen can be produced using electrolysis, where an electrical current is passed through water, splitting the water molecules into hydrogen and oxygen. The electrolysis process itself produces no negative externalities, e.g. greenhouse gas emissions. But naturally the source of the electric might produce bad emissions: i.e. if produced by burning coal. However when the electric is produced through renewable energy sources (solar, wind, biomass etc), there are zero harmful global warming emissions. Electrolysis is a fairly light-weight process which wouldn’t require complex equipment, and thus more local generation is possible.
There are then a few other methods of producing hydrogen which are either not yet commercially-viable, or are still in the R&D phase:
- Biomass gasification or renewable liquid reforming: a form of biomass (be it a solid, for example plant-based biomass, or a liquid, for example ethanol) can be reacted with very high temperature steam to produce hydrogen and carbon dioxide. This is a similar process to SMR, but the carbon dioxide here is from a renewable energy source that originated within the Earth’s atmosphere (e.g. entering plants via photosynthesis) and thus the overall carbon footprint is much lower. The equipment required would mean that more centralised production (e.g. in a power plant) would be required.
- Fermentation: certain bacteria can produce hydrogen in certain cases, such as with dark fermentation which requires chemical energy or photofermentation which requires light energy. With this approach (which is currently not commerically viable, but research is aiming to bring down the cost of it), renewable biomass is converted into sugar-based feedstock – which said bacteria can then ferment, producing hydrogen in the process.
- Thermochemical production or high-temperature water splitting: this method involves even higher temperatures than the steam method earlier. These temperatures generated by nuclear reactors would be at 850oC – 1,800oC, and would split water – yielding hydrogen in the process.
- Photochemical production or photoelectrochemical water splitting: this method is similar to electrolysis, in that it involves splitting water into its constituent parts. However it differs in that it uses sunlight and specialist semiconductors in the process.
- Biophotolysis watter splitting: this final way of producing hydrogen involves using green algae (or similar microbes) which ‘consume’ sunlight and water, and produce hydrogen in the process.
Compression (and why this is needed)
Hydrogen is a great energy source by weight, but it is quite a sparse element: meaning that by volume it’s a poor energy source. As H2Tools puts it:
The volume ratio of liquid to gas is approximately 1:850. So, if you picture a gallon of liquid hydrogen, that same amount of hydrogen, existing as a gas, would, theoretically, occupy about 850 gallon containers (without compression).
As a result of this sparseness, hydrogen needs to be compressed down a lot before it is viable to store and distribute. If this wasn’t done, a hydrogen fuel-cell car wouldn’t look nice – or be very practical – with massive hydrogen storage tanks on its roof!
So in its gaseous state, the hydrogen is stored under pressure: that is, within high pressure hydrogen tanks which are between 350-700 bar (5,000 – 10,000 psi).
Once the hydrogen is produced and stored at the production centre, it naturally needs to be distributed to the hydrogen fuelling stations which FCEVs will visit.
Unfortunately it’s not as simple as transporting gasoline in a big tanker on the road (or sea), due to the high pressure the hydrogen is under, and increased flammability that hydrogen has (compared to gasoline). This makes transporting hydrogen more expensive, especially over longer distances. NB: this method (i.e. higher-pressure tube trailers transporting the hydrogen via road or sea) can be used, but it’s worth highlighting that the greater the distance (between the hydrogen generation centre and the hydrogen fuelling centre), the greater the overall cost.
And so the more distributed methods of producing hydrogen (i.e. putting hydrogen generation centres close to hydrogen fuelling centres, or even producing hydrogen at the fuelling centre – in the case of the electrolysis method) are preferred where possible. Where this isn’t possible though, other distribution methods include:
- Hydrogen pipelines: using pipelines is a viable option, but it’s limited by the current infrastructure. America only has around 1,600 miles (2,575 km) of hydrogen pipelines available in total; mostly in California (the main state for hydrogen vehicles). This compares to America’s 2,400,000 (2.4 million) miles of gasoline pipelines.
- Liquefaction: storing the hydrogen in liquid form allows it to be transported easier because it’s denser, and thus more can be transported compared to hydrogen as gas. The hydrogen is cooled by a process known as cryogenic liquefaction, although this is a more energy intensive process than using compressed hydrogen air.
As you can probably tell, there’s no single ‘way forward’ for hydrogen production or distribution just yet. Steam methane reforming is currently the cheapest and most common, but it commonly results in greenhouse gas emissions. Plus SMR needs to be centrally produced due to the equipment required, meaning that more expensive forms of distributing it (high pressure tankers, liquefaction or pipelines) are needed.
Electrolysis – and some other methods – is much more promising in that they can be more regionally created, possibly even at the hydrogen refuelling station itself. But right now the cost to produce isn’t as good as SMR, so the cost savings from not having to transport hydrogen are wiped out by the higher production costs.
So it’s all a balancing act right now, but active research is being done to bring down the cost of producing and distributing hydrogen.