Power to Liquid: The potential for “excess” electricity


Since there has been a recurring debate lately about synthetic fuels (“Power to Liquid”, PtL) as an alternative to battery-electric cars, here’s a little calculation.

First of all, it is undisputed that the direct use of electricity for driving is many times more energy-efficient than the detour via chemical substances. If the electricity has to be generated in addition, PtL is quite a waste of energy.

But one can, argue PtL proponents, use “excess” green electricity for it. After all, when there is a lot of wind and little demand, wind turbines are always switched off. This means that the operators have to take their systems off the grid, but still receive remuneration for the electricity that is not generated. So it makes more sense to use it to produce flammable hydrocarbons.

In principle, that is correct. But how much “surplus” electricity is there anyway? What scale are we talking about here?

According to the Federal Network Agency, around 6,482 GWh were generated in 2019 “Failure work“, Mainly in wind power. The reason for this was not a lack of demand, but a bottleneck in the network – namely “around 83 percent in the transmission network or at the network level between the transmission and distribution network”.

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Now that alone is not an argument against SynFuels. However, you have to make it clear that you would then have to build the factories in the immediate vicinity of the wind turbines and distribute the finished product to the rest of the country by pipeline or tanker – which puts a corresponding burden on the energy balance.

Is that really more efficient than network expansion or local electricity storage? And couldn’t something smarter be done with electricity through sector coupling? Let’s leave this aside and turn to the question of how much e-fuel can be produced with the existing downtime.

The basis for this is provided by a study the Ludwig Bölkow system technology for the Federal Environment Agency. In 2016, using the example of a factory with an annual capacity of 100,000 tons, she calculated how much electricity is needed with which methods to produce synthetic kerosene. The figures for diesel and petrol should be comparable.

According to this, the most efficient path is high-temperature electrolysis in conjunction with the Fischer-Tropsch synthesis. Where the required carbon dioxide comes from also has a decisive influence. If it is already available in concentrated form – for example from the processing of biogas, from the separation of fossil power plants or from cement production – the study comes to an efficiency of 64 percent in the best case. Specifically: 613 MWh for 32.8 tons of kerosene, i.e. 53.5 kg / MWh. Does the CO2 are only filtered out of the air, it is only 39 kg / MWh.

(For a well-to-wheel balance, of course, you also have to use the Efficiency of the internal combustion engine, approx. 35 to 45 percent. Even without the expenses for transporting the fuel, a maximum of one third of the electrical energy used would reach the axle of a car, even in the most optimistic scenario. Significantly lower values ​​are found in the literature. In an e-car, the well-to-wheel efficiency should be in the 80 percent range.)

If you put all of the green electricity, which was regulated in 2019, into optimal PtL production (and you have enough concentrated CO2 available), at best you get almost 350,000 tons of kerosene. The Kerosene consumption the German airlines alone amounted to around 9.1 million tons (11.4 billion liters) in the same year. With PtL you could cover a little more than 3.8 percent of the demand.

The rate is even lower if you look at road traffic: in 2019, around 38 million tons of diesel and 28 million tons of gasoline were produced in Germany consumed. With PtL from the failure work, just 0.6 percent of it could be generated. It can of course be done, but it does not make a significant contribution to the decarbonisation of transport.


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