Can solar fuels ever be viable?

The idea of producing fuels from air, water and the sun has great appeal and the chemistry and technology to do so exists. Electrolysis (using renewable electricity) of water to make hydrogen and air capture of CO2 are both done today, albeit on a relatively small scale (compared to other ways of making hydrogen) in the case of hydrogen and very small scale in the case of air capture. Hydrogen and carbon dioxide can then be combined in different ways to produce fuels.

  • The Sabatier reaction takes place at elevated temperatures (optimally 300–400 °C) and pressures in the presence of a nickel catalyst to produce methane and water. Synthetic methane can substitute for natural gas.
  • Using a palladium and copper catalyst the production of methanol is possible, which can be used as a liquid fuel or can be a building block for other fuels or petrochemicals.
  • Activation of the carbon dioxide to carbon monoxide (e.g. using hydrogen, but CO2 electrolysis is also a possibility), then combining this with more hydrogen (as synthesis gas) to synthesize hydrocarbon liquids for use as fuels using the Fischer-Tropsch process (this process exists at large scale in Qatar and South Africa).

As an end to end process the above doesn’t exist, other than at pilot plant scale. All the technologies have been proven, but building a facility that at least matches the scale of the Shell Gas-to-Liquids synthesis plant in Qatar is likely many years away. For example, the largest air capture facility in the world announced so far is a 500,000 tons per year facility due to start up in 2023. This is the equivalent of 136,000 tons of carbon or some 160,000 tons of Jet A-1, but the facility in Qatar produces ~8 million tonnes per year.

The reason for wanting to synthesize hydrocarbons from air and water is threefold;

  • To mitigate the need for fossil sourced hydrocarbon fuels, but continuing to offer the convenient energy dense carrier they represent;
  • To meet the needs of activities that depend on hydrocarbon fuels, but cannot find alternative options. For example, aviation depends on Jet A-1 and while there is talk of some electrification of short haul flights and the long term possibility of fuels like hydrogen, there is no line-of-sight to a viable alternative. We should expect at least some planes to still be using Jet-A1 at the end of this century even if a comprehensive viable alternative does eventually emerge.
  • To avoid using synthetic fuels derived from biomass, which is technically easier and more cost effective but may face social issues relating to the use of biomass (e.g. the food vs. fuel debate).

We might also imagine building a very large scale synthetic hydrocarbon industry to meet all sorts of requirements, thereby taking the pressure away from the need to find solutions for all the current uses of fossil fuels, so effectively lowering the bar of difficulty from just aviation to many other sectors and uses. But to do this, the manufacture of synthetic hydrocarbons needs to be competitive with alternatives, even allowing for policy instruments such as robust carbon pricing.

A recent paper on synthetic fuels tackles the cost issue head on and looks at what we need to believe for so-called ‘solar fuels’ to become a reality. The paper focuses on the five principal elements required to manufacture solar fuels; solar PV, electrolysis to produce hydrogen, direct air capture of CO2, hydrogen activation of CO2 to CO and Fischer-Tropsch synthesis. Based on current costs of this array of technologies, the end-to-end cost of product from this process approaches $900 per barrel, or around $5 per litre. Much of this cost sits with the newer technologies, namely solar PV, hydrogen electrolysis and direct air capture. But these are also the areas where sharp cost reductions are either being seen or are anticipated.

Solar Fuels Cost Estimates

  • There is certainly abundant evidence that solar PV costs are falling, so a shift from $50 /MWh to $15/MWh is plausible. This alone equates to $60 per barrel.
  • There have been several recent articles outlining potential pathways for big cost reductions for direct air capture.
  • In its recent report on the future of hydrogen, the IEA examined the rapidly falling cost of electrolysis.

In addition, significant relative cost reductions (i.e. 50-70% cost improvement) are also required in already mature technologies, such as Fischer-Tropsch synthesis.

The five technologies required are also at very different stages in their respective development. For example, although electrolysis remains quite expensive, it is far ahead in terms of commercial deployment when compared to Direct Air Capture. The latter is still in the early pilot phases of development, but is nevertheless making progress.

With all these factors aligning and imagining a policy regime which injected a carbon price into the mix (to bridge the final gap), it is just feasible to see synthetic fuels becoming an option. However, commercial considerations will doubtless prevail. For example, if direct air capture costs fall dramatically, it could well make more sense to continue using legacy infrastructure (refining crude oil) to make Jet A-1 and then capturing and storing carbon dioxide to balance the emissions. Such a route forward would also be one that supplied numerous other products where alternatives remain elusive.