Comment by kragen

Yeah, biofuels are laughable, but fully synthetic fuel is a plausible contender, if cheap solar energy lets us make cheap fully synthetic fuel.

Your mention of aviation fuel is relevant. That's a context where batteries are not pretty good already, although they are viable for short hops; in aviation, as in shipping, heavier batteries create more energy consumption, so a carrier with a higher specific energy per kilogram is very valuable. Conventional jet fuel is 43MJ/kg (https://en.wikipedia.org/wiki/Energy_density#Chemical_reacti...) while lithium-ion batteries are up to about 0.8MJ/kg (https://en.wikipedia.org/wiki/Energy_density#Electrochemical...).

Very crudely, an airplane whose weight is mostly jet fuel can fly about 50 times as far as a rechargeable-battery-powered airplane if both are going the same speed. Rechargeable batteries are 50 times worse than jet fuel. And they're improving slowly; the last time that difference halved was with the invention of lithium-ion battery, which went mainstream about 30 years ago. The previous major improvement was the lead-acid battery 120 years earlier.

However, promising alternative synthetic fuels other than paraffin include aluminum, magnesium, and zinc, which can be burned in aluminum-air batteries, magnesium-air batteries, and zinc-air batteries, respectively. Aluminum and magnesium are 31.0 and 24.7MJ/kg, respectively, but burning them in metal-air batteries instead of in heat engines allows you to extract about twice as much useful energy from the reaction, so they are in practice higher in energy density than current jet fuel.

Current aluminum prices are US$2828/tonne (https://www.lme.com/Metals/Non-ferrous/LME-Aluminium#Overvie...) which works out to 9.1¢/MJ. A barrel of oil is 6 gigajoules (https://en.wikipedia.org/wiki/Barrel_of_oil_equivalent) and US$58.78 (https://www.cmegroup.com/markets/energy/crude-oil/light-swee...) which is about 0.98¢/MJ. So the price of aluminum would have to drop by about a factor of 5 to make this appealing. It's plausible that this will happen, but not before 02040.

Magnesium is 17050RMB/tonne (https://tradingeconomics.com/commodity/magnesium), which is US$2410/tonne, working out to 9.8¢/MJ.