E-Fuels: worth their weight in gold

The recent insistence by Germany and others that cars powered by e-fuels should be categorised as zero carbon has generated intense interest. The idea that we could have synthetic petrol powering zero-carbon cars seems too good to be true – so let’s try and separate hype from reality.

Firstly, we need to define e-fuels, also known as synthetic fuels and “power to liquid”. E-fuels are the result of combining pure hydrogen and waste CO2 to create a synthetic hydrocarbon which, with a bit of tinkering, can power anything from a VW Polo to a Boeing 787. It is important to note that e-fuels are not the same as biofuels which are produced from biomass, such as ethanol produced from sugar cane, and risk creating competition for land needed to grow food.

The “e” in e-fuels comes from the fact that the hydrogen is produced by electrolysing water, which is a virtually carbon-free process if renewable energy is used. The origin of the waste CO2 is a bit more variable – in the short term, it could come from industrial processes that produce a lot of CO2 (e.g. a local steel furnace) but the long-term solution is said to be direct air capture, given that heavy industry intends moving to CO2-free hydrogen-based production.

So e-fuels are perfectly possible, but are they practical? That depends on three things: how easy are they to make at scale, how efficient are they and how much would they cost?

Manufacturing

The e-fuel plant that has got everyone excited is part-owned by Porsche and is located in Chile. That location is not as random as it may seem – Chile has long been identified as one of the cheapest places in the world to produce green hydrogen. That is due to fierce winds from the Atlantic enabling very cheap renewable electricity via wind turbines.

The problem is that, while the rest of the world is working out how to make green hydrogen at high volume and low cost by 2030, e-fuel manufacturers are working out how to efficiently combine the cheap green hydrogen they don’t yet have with the captured CO2 that is not yet available at scale - CO2 of course, is all too prevalent, which is the fundamental problem we are trying to solve, but it does not come in a form that we can conveniently access.

Even e-fuel’s most ardent supporters are talking about global production of only 1 billion litres by 2030 – for comparison, the UK alone uses 46 billion litres of petrol and diesel per year for road transport. So in terms of assuming e-fuels will offer a scalable panacea, which will enable internal combustion to carry on as normal, the phrase “trying to run before you can walk” comes to mind.

Efficiency

The Royal Society of Chemistry calculates that the production of 1 kg of kerosene e-fuel (which has similar characteristics to diesel) requires 1.4 kg of hydrogen and 14.5kg of CO2.

Taking hydrogen first: we think of fuel in terms of volume rather than weight, so let’s convert 1 kg of kerosene to its volume equivalent: 1.2 litres. That quantity would take an average car approximately 12 miles, assuming 45 mpg. It is true that we do not drive cars on kerosene, although trucks in cold climates often do mix it with diesel to reduce waxing, or even use neat kerosene (not recommended). Kerosene is half-way between petrol and diesel in terms of energy content, so it is a reasonable basis for comparison.

So, if 1 kg of kerosene e-fuel takes us 12 miles, how far does 1 kg of hydrogen take us when used in a fuel cell car? The Toyota Mirai has a range of 370 miles using a 5.6 kg tank, so the answer is 66 miles.

That is an eye-watering difference, and is partly explained by the difference in thermal efficiency between internal combustion engines and fuel cells. Internal combustion engines (ICEs) typically achieve c.30% thermal efficiency when burning petrol or e-fuels. In contrast, the Toyota Mirai achieves 62% thermal efficiency.  The other reason is that hydrogen is simply a lot lighter than any liquid fuel – 1 kg of hydrogen contains far more energy (33.3 kWh) than petrol (12.8 kWh). Increasing the 12 miles driven on e-fuel thanks to an FECV’s better thermal efficiency gives us 25 miles, and then multiplying by 2.6 for the extra energy density of hydrogen gives us 65 miles – basically the same as the 66 miles from Toyota’s figures.

In terms of engine efficiency, the role of CO2 is already included in the above calculation. However, there is still a lot of energy required to capture CO2 to make the fuel in the first place. Most calculations (e.g. IEA) puts the energy required for Direct Air Capture at 7-8 Gigajoules per tonne of CO2. In simple terms, that equates to over 2,000 kWh of energy – or all the energy from 20 long-range Tesla batteries.  Put another way, every time an e-fuelled car is filled with 50 litres, that amount of fuel will include 600 kg of captured CO2 – at a cost of 1200 kWh of energy. And we worry about the impact on the national grid of recharging EVs with their 60kWh batteries.

Cost

At this point, you may be thinking that the amount of energy required to produce an e-fuel must make it quite expensive. The Royal Society, the world’s longest established scientific academy, undertook a study, and came to the conclusion that 53% of the price of an e-fuel comes from electrolysis, and 47% comes from capturing the carbon and combining it with hydrogen – hence e-fuels are nearly double the price of the hydrogen equivalent.

Source: Royal Society Briefing Paper

Concawe, the oil and gas research body says that, even in 2050, e-fuels will be 30% more expensive than hydrogen – and they are the ones that stand to make money out of e-fuels. As for how much it costs today – well, if you have to ask, you can’t afford it. It is around £45/litre, or £2,000 for a typical fill-up.

The one area where a road vehicle powered by e-fuel would have a cost advantage is the price of the powertrain. An ICE can burn e-fuels without any modification, hence e-fuels being described as a ‘drop-in’ technology.  A fuel cell powertrain currently costs many times as much as an ICE equivalent, not least because the mass production of fuel cells is still to get going. However, fuel cell manufacturers say they will achieve price parity with ICE by 2030. Once price parity is reached, it will be hard to make a case for powering, say, a 44-tonne HGV on e-fuels.

The great attraction of e-fuels is that they seem to offer the elixir of zero-emission transportation with no significant change of technology. The reality is they are inevitably more costly than hydrogen, and production will remain a fraction of current petrol and diesel fuel. E-fuels are best seen as a “fuel of last resort” for those vehicles which cannot be electrified in the foreseeable future.

The prime example of this is probably aircraft: replacing the jet engines in a Boeing 787 with a zero-emissions alternative is going to take a very long time: not least because a new 787 ordered today will still be in service in 2050. For long-haul commercial planes, e-fuels could be the “get out of jail” card.

Hence, there is rarely a single right-or-wrong answer when dealing with alternative propulsion. The following table sets out a framework to consider different alternatives:

Conclusion

E-fuels make sense as a ‘drop-in’ transitional technology, albeit with limited application die to cost, thermal efficiency and scalability.  With these limitations, it would not be sensible to use e-fuel to power a new car when BEV or FCEV alternatives are available. E-fuels would be better used to fuel a long-range airliner for which there is currently no alternative – oh, and of course by deep-pocketed Porsche owners who can’t live without the flat-six wail of a 911 burning hydrocarbons.