Making fuel from fresh air: Sunlight and air create ‘drop-in’ replacement for burning petrol

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The idea of conjuring petrol from nothing but fresh air and sunlight sounds like alchemy but a pilot project in Switzerland has proved that it’s possible.

On the roof of research university ETH Zurich sits a miniature solar refinery, using a unique set of processes to create hydrocarbon fuels.

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Although there are already projects around the world to create synthetic fuels, taking hydrogen from water and carbon from CO2 in the air, they usually require a large amount of electricity. ETH Zurich’s mini refinery, in contrast, uses sunlight as a direct part of the process and doesn’t need a vast amount of electricity.

There are three processes in the mini refinery. First, it has to extract CO2 and water from the air, before both are fed into a reactor where the real magic happens.

A parabolic mirror directs sunlight back onto another reflector, then focuses it onto the solar reactor, heating it to 1500°C. The reactor is made of cerium oxide, and this burst of heat forces it to release oxygen into the atmosphere.

ETH Zurich reactor diagram

Then the CO2 and H2O are added and the reactor strips oxygen from them leaving a mixture of hydrogen and carbon monoxide. This is called syngas and can be used as the basis for a variety of fuels.

Because heat is only needed for the initial step, the prototype has two solar reactors, with a movable mirror to switch the sunlight’s focus back and forth, doubling its efficiency. The prototype makes around 100 litres of syngas per day.

Stage three takes the syngas and runs it through a gas-to-liquid unit, which converts it to methanol. The university say it could be made into gasoline or kerosene using catalysts.

In terms of emissions, it’s carbon neutral, since burning it will only release as much CO2 as was originally used to create it. However, in a combustion engine there are other emissions, NOx, for instance, which comes from the reaction of nitrogen and oxygen in the air during combustion.

How the ETH Zurich reactor works

However, since CO2 is by far the largest proportion of vehicle emissions, neutralising that promises to make a vast difference.

If you’ve got visions of plonking a reactor like this on your own roof and having an endless supply of petrol, think again. The mini refinery can make around a teaspoonful of fuel per day. Filling your tank could take a while.

It’s calculated that to create a solar plant that could completely replace the 414 billion litres of kerosene used in aviation in 2019 (before the pandemic slashed air travel), you’d need 45,000km2 of desert. That’s equivalent to 0.5% of the Sahara.

The cost of the fuel would depend on the industrialisation of the process, but the scientists behind the project reckon that once solar fuels account for 10-15% of the market they’ll be competitive with fossil fuel prices.


Synthetic petrol: How bikes could become carbon-neutral without switching to battery power at all

First published on 3 July 2021 by Ben Purvis

Filling up a petrol powered motorbike

As governments propose firm dates for the end of petrol and diesel powered vehicles you might be forgiven for thinking we’re living in the end days of the internal combustion engine. But the technology that’s literally driven the transport revolution for more than a century isn’t ready to give up just yet and could be thrown a lifeline by the development of synthetic fuels that are nearly as green as going electric.

Synthetic petrol, diesel and aviation fuels are all the subject of heavyweight pilot projects and are gaining interest fast as they eliminate the CO2 emissions from transport without the need for end-users to switch to new tech. Aviation in particular is backing the idea but motorsport is getting in on the action, with the World Rally Championship due to use 100% sustainable fuel as soon as next year.

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While biofuels already offset their emissions to an extent, synthetic fuel takes a more direct approach by stripping CO2 from the air and combining it with hydrogen to create a petrol replacement.

By using wind, solar or hydro-electric to generate the electricity for the electrolysis, the process is sustainable, and while engines using the fuel still emit carbon, the same amount is reabsorbed in the fuel-making process.

The idea of using electricity to create synthetic petrol and then to burn that fuel in combustion engines might seem wasteful but the benefits are also significant, reusing existing infrastructures and giving new life to billions of combustion engines rather than reducing them to scrap. With similar power density to normal petrol, synthetic fuels also mean you don’t need to lug vast batteries around.

At the moment, there are several synthetic fuel projects underway. Porsche – part of the VW Group and hence a sister to Ducati – have invested in Siemens Energy’s Haru Oni pilot project, a prototype plant in Chile that uses wind power to make synthetic petrol. It’s due to make 130,000 litres next year, 55 million in 2024 and 550 million litres in 2026.

BMW, meanwhile, have invested in US firm Prometheus Fuels, which uses a different process but again creates petrol from air, water and electricity.

While many governments seem keen on switching from combustion tech to all-electric vehicles, it’s looking likely that a better approach is to adopt multiple technologies to reach carbon neutrality, applying them where they fit best. And perhaps most importantly of all, synthetic fuel should mean that more than 100 years’ worth of bikes have a future.


Synthetic fuel technology explained:

Siemens Energy’s Haru Oni pilot

  • Methanol-to-gasoline The methanol forms the basis of synthetic petrol, created at the Haru Oni plant using an ‘MTG’ (methanol to gasoline) technology provided by Exxon Mobil.
  • Methanol synthesis Once the factory has extracted CO2 from the air and hydrogen from water, the two can be combined into a synthetic methanol fuel.
  • Direct Air Capture unit CO2 is removed from the air using ‘direct air capture’ technology, which involves chemical filters and large fans to draw the ambient air through the system.
  • Tank farm The resulting fuel is intended to be a direct replacement for fossil-derived petrol, eliminating the need for new engines and infrastructures. Estimates range from as little as £1 per litre to more than £3 per litre by 2030, but that may be worth it as a lifeline for existing engines and an alternative in applications where battery power is unsuitable. 
Ben Purvis

By Ben Purvis