Here’s a Swedish company planning to do a more conventional renewable-electricity-to-fuel process:
They’ll electrolyse water to produce hydrogen, then combine with a waste stream of CO2 to make methanol. One of the commenters (a guy I’ve seen commenting knowledgeably in various places for years) reckons they could add air capture of CO2 and still get 60% efficiency:
Methanol is interesting, because it requires only 3/4 as much H2 to make it from CO2 as methane does. The reaction to methane is highly exothermic (lossy). If you air-captured your CO2 @ 3 MJ/kg you’d need 8.25 MJ (2.29 kWh) plus 21.5 kWh into the electrolyzer @ 43 kWh/kg to make 1 kg CH4 @ 13.9 kWh LHV; that’s 58.4% to get a fuel you can’t even pour. How lossy is methanol?
6 kg H2 reacts with 44 kg CO2 to make 32 kg MeOH and 18 kg H2O. If your electrolyzer consumes 43 kWh/kg, you need 258 kWh input. For a closed-loop system you need to air-capture your CO2. Guesstimate 3 MJ/kg for that or 132 MJ (36.7 kWh) for a total of 294.7 kWh. The HHV of MeOH is 6.39 kWh/kg and the LHV is 5.54 kWh/kg, so your fuel value is between 204.5 and 177.3 kWh. The LHV efficiency is 60.2%, which isn’t half bad for a truly renewable liquid fuel. Best of all, you can thermally crack MeOH back to CO and H2, picking up energy from engine exhaust heat and increasing the LHV to 6.63 kWh/kg. That gives you a whopping 72% efficiency from electric input to the fuel value of your fuel gas.
Yup, methanol is great stuff.