How about hydrogen cars?
In this article, we will be continuing the discussion on the topic the gas family – this time talking about hydrogen. If you haven’t seen the first article in the series, you can see it here.
So, after we have covered what LPG is, let’s see another gas – hydrogen.
To begin with, the use of hydrogen in the transport system is very often seen as the long term solution, overcoming the problem of storing energy on-board and having a vehicle with no emissions at the exhaust, meaning that it would be an electric vehicle.
But why are we seeing only a few hundreds of hydrogen vehicles on our roads?
To answer this question, let’s see what we have to do to have a hydrogen car…and also to drive it!
Where does hydrogen come from?
Since hydrogen is not an energy source (as natural gas) but an energy carrier (like electricity), it has to be produced.
Already, approximately 94% of hydrogen is produced from natural gas (steam reforming process) or from coal (gasification process). Both processes “extract” hydrogen, but by doing so, produce CO2 emissions. This is important to keep in mind!
The sustainable way to produce hydrogen is to use renewable electricity and to produce it from the ‘water electrolysis process’. What may sound like a very complicated technical process could be explained in a simple matter by saying that we can ‘break’ the water (H20) molecule into Hydrogen and Oxygen by introducing electricity into it. In this case, to make it sustainable, the electricity would be coming from solar or wind energy.
In the future, considering a growing production of renewable electricity from wind and solar power, there would also be a growing interest to convert surplus intermittent electricity from renewables into hydrogen. This could serve as an energy storage system.
Storing energy through hydrogen…Yes, but…
It is certainly possible to store energy in hydrogen. We are already doing it, however, it is important to understand how challenging this is.
Let’s start by talking about energy content. 1 kg of hydrogen has an energy content that is 2,4 times that of methane… yet, the critical aspect is its density, which is approximately 9 times lower than that of methane.
What does that mean?
In simple terms, the result is that the energy contained in 1 m3 of hydrogen is 30% of the one contained in 1 m3 of methane.
Furthermore, when we look at transport applications, the energy density is a fundamental parameter.
For this reason, while standard CNG applications operate at 200 bars, hydrogen applications require higher values – very often around 700 bars. Unfortunately, the energy needed to compress hydrogen up to 700 bars doesn’t make the whole operation economical.
For this reason hydrogen is transformed in a liquid phase, similarly to LNG, to increase its energy density.
The liquefaction process needs to reach a very low temperature – down to -253°C (whereas it the same requirement for LNG is -161°C). In practical terms, then, this means that liquid hydrogen requires a high amount of energy and a specific cryogenic gas tank technology to ensure the best thermal insulation of the fuel.
That’s why today’s applications of hydrogen cars are based on compressed hydrogen.
A hydrogen car is an electric car
The hydrogen stored on board is used to produce electricity (but remember what we already have said about where it usually comes from) in the so-called ‘fuel-cell’.
What is a fuel-cell? In practice, a fuel-cell functions similarly to a battery but has an ‘external’ source of energy.
Electricity is generated thanks to an electrochemical process between hydrogen and oxygen, present in the air and introduced to the fuel cell. The reactions in the cell generate electricity, water and heat.
Usually, in automotive applications, PEM (Protons Exchange Membrane) fuel cells are used as they operate at a low temperature – around 80°C, such that can be easily controlled.
Once electricity produced, it is generally stored in an intermediate battery (sometimes also rechargeable from the plug) or directly used by the electric drivetrain.
As you can see, this simple explanation of the process provides an approximate idea of the steps needed to run a fuel-cell vehicle.
Nevertheless, the main advantage of this technology is to overcome the range of electric vehicles due to the limited energy density of the batteries (Li-ion batteries today have an energy density of about 250 Wh/kg, which is 50 times lower than conventional fuels!), but the current applications are still limited at a niche level.
There is a way to use more hydrogen in transport and this can be achieved by blending hydrogen with methane. This is exactly what we will be examining in the next article so stay tuned for that!
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