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Fuel cell vehicles

Fuel cell vehicles

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Background

A fuel cell electric vehicle (FCEV) is an electric vehicle (EV)—that is, a vehicle propelled using only electric motors—powered by the electricity generated by passing fuel and oxidant through a fuel cell. In practive they usually have an additional small battery for storing extra charge from regenerative braking, which can also be used to power the motors to improve range.

So what is a fuel cell?

A fuel cell creates an electric current from the catalysed reaction of a fuel and oxidant, usually O₂ from air. First demonstrated in the early nineteenth century, modern fuel cells are based on the hydrogen-powered alkaline or "Bacon" fuel cells invented in the 1930s by English engineer Francis Bacon.

Fuel cells typically produce low voltages, typically less than one volt, so they are usually arranged in series into fuel cell "stacks" for practical high voltage use such as powering vehicles.

Fuel cell efficiency

Fuel cells can have high thermal efficiencies of up to 85% in combined heat and power (CHP) applications, despite having lower electrical efficiencies, but this means running at very high temperatures. Conversely, the smaller fuel cell designs suitable for vehicles have a higher electrical efficiency and operate at more manageable temperatures.

The fuel cell stack powering the 2019 Hyundai Nexo has a reported 60% electrical efficiency.

Cost

Fuel cells are very expensive. This is because they rely on platinum-group metals as catalysts for the REDOX reactions they rely on to extract electrons from the oxidation of the fuel without overtly combusting it.

Platinum is insanely expensive.

In contrast, the principal limiting metal in the production of battery electric vehicles, lithium, is dirt cheap.

Hydrogen as a fuel

Hydrogen is an attractive fuel for many reasons. It can be produced from water through electrolysis, and when combusted produces the same amount of water it was derived from. There are however a lot of problems to overcome for it to be a sensible option for powering private transportation.

Power density

Hydrogen is the lightest element, so while its energy-to-mass ratio is impressive, its very low density works against it. To store hydrogen at the sorts of high power-to-volume efficiencies that we are used to with liquid hydrocarbon fuels, we need to compress it in tanks capable of extremely high pressure (70 MPa), or as a cryogenic liquid kept at -252°C, which is achievable only with space programme budgets.

For example, travelling 600 km in a modern FCEV requires only 4-5 kg of H₂ gas to power the fuel cell, but to store this seemingly modest amount of fuel on-board requires an 80 litre tank pressurised to 70,000 kPa, three times higher than a typical CNG tank.

Safety

As you can imagine, not everyone wants to drive around sitting on a tank of explosive gas compressed to 700 atmospheres. Reliable safety data for how such storage tanks perform in accidents and collisions is not available.

With a detonation velocity at standard pressure of about 3-4 km/s, hydrogen is explosive and thus much more dangerous in failure situations than petrol, given that at the point of any detonation, it will already be a rapidly decompressing gas mixing with air across a large expanding surface area. In short, car go boom, very bad.

The safest and most power-dense way to store hydrogen remains bonding it to carbon; hydrocarbons have a very high energy-to-volume ratio, are stable liquids at room temperature, and are relatively safe when handled correctly, even in failure situations.

Loss from leaks, transportation and storage

As already mentioned hydrogen is the lightest element, its nucleus consisting of a single proton. It is very difficult for anything made of matter to be any smaller. This means that molecular hydrogen gas, H₂, being the smallest molecule that can exist, will inevitably escape through the walls of all storage devices, given sufficient time. This means that loss through the process of generating, pumping, transporting and storing hydrogen is both measurable and significant.

Damage to the ozone layer

Since H₂ is also far too light to be held by Earth's gravity, it escapes upwards through the atmosphere and into space. On its way to space it passes through and damages the ozone layer, where it permanently reacts with ozone to form water:

3H₂ + O₃ → 3H₂O

This never gets mentioned anywhere, and seems to only be known to meteorologists.

Hydrogen interests are fossil fuel interests

But most significantly of all, over 95% of the world's hydrogen is produced from fossil fuels. Specifically, from the steam reformation of natural gas, which is approximately:

CH₄ + H₂O → 3H₂ + CO

The carbon monoxide is used in further industrial/chemical processes, or burnt to produce energy, which produces CO₂.

This means of course that currently, powering your car with hydrogen is effectively powering it with natural gas, but in a much less efficient and far more expensive manner than if you had just powered it with CNG.

Hydrogen is being researched and hyped by the fossil fuel industry, desperate to re-purpose their colossal global infrastructure assets, which are about to be ruinously stranded over the next 20 years by the disruptive combination of exponential growth in solar, electric vehicles and battery technology advances.

Hydrogen is thus a massive waste of energy, resources, investment and time, for very little gain.

Overall efficiency

To measure this we have to take into account where the hydrogen comes from. Currently, hydrogen fuel is just a refined fossil fuel, with the CO₂ emissions moved upstream where you can't see or smell them. But this analysis takes the best-case scenario, which doesn't exist in the world yet.

For a fuel cell vehicle to play the sustainability game, it needs to source its hydrogen from water electrolysis, powered with renewable electricity. So let's take a unit of electricity and call it e. Given water hydrolysis results in about 70% of the input energy available as hydrogen (including electricity conversion to DC), we will produce 0.7 units of energy as hydrogen fuel.

Note that already, before the hydrogen has even been siphoned, pumped, stored, and transported to a nearby hydrogen filling station, the battery-powered electric vehicle is winning because it can use all of the electricity generated initially.

After generation of electricity and hydrogen fuel:

  • FCEV: e = 0.70
  • BEV: e = 1.00

We now have hydrogen gas which needs to be pressurised and transported to your local filling station, each at about a 10% loss (either through actual loss of gas or the consumption of extra energy). Generously, let's assume this is all done using more of the same renewable electricity for the sake of argument, which leaves us with only half of the energy we started with available as hydrogen.

Meanwhile for the battery EV, the full unit of electricity is transmitted to your garage at about 5% transmission loss and stored in your EV battery with about 85% efficiency; there is some loss from charger equipment AC/DC conversion and as heat.

After transportation and fueling/charging the vehicle:

  • FCEV: e = 0.51
  • BEV: e = 0.80

Now we consume the on-board energy to propel the vehicle. The FCEV converts the H₂ fuel into electricity at 60% efficiency at best, which powers the electric motors which are about 90-95% efficient. Meanwhile the BEV simply dumps the charge into the motors directly, at about 90% efficiency.

At the point of propelling the vehicle forward:

  • FCEV: e = 0.29
  • BEV: e = 0.72

Here we see that compared to a fuel cell EV, a battery EV is more than twice as efficient, and this is its best imaginable scenario. In reality, the compression and transportation of the hydrogen fuel will be done with existing ship rail and road infrastructure, which currently involves high usage of fossil fuels.

Comparison with petrol-powered cars

The battery EV still wins even when we burn all of the petrol to generate electricity, because while the combustion emits CO₂, it is still far more efficient to dump the electricity into an EV battery directly than to use trucks to transport petrol to filling stations, and burn it at a still terribly poor 20% efficiency in an internal combustion engine.

Many pundits think we are truly witnessing a revolution in transport not seen since the disappearance of steam traction engines and horses a hundred years ago; it's just that nobody in 2020 has noticed yet, the same way nobody noticed the internet in 1995.

References

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