@dagb

The strongest efuel use cases require high energy density, perhaps aviation, perhaps CNG efuel for ocean shipping but certainly not long haul land freight (which should be electric rail http://SolutionaryRail.org ).

So the size of the real effective demand could well lead to efuels being a useful niche fuel rather than a major player, just as landfill biogas is a useful niche energy supply, but likely below 1% of generation share.

@dagb The first choice for hourly storage down to spinning reserve may be pumped hydro, perhaps fixed cathode/anode batteries in the daily task alongside thermal, flow batteries and molten metal coming into the frame for weekly, and possibly seasonal, storage.

The stack on newly added generation would be:
[ efuel ]*
[ molten metal / flow batteries ]
[ fixed cathode/anode batteries ]
... with falling cost per kW capacity as you go down the priority & rising incremental cost per kWh capacity.
{*}

@dagb

However, #efuels intrinsically cannot be the SOLE consumer of the "otherwise-redundant or non-economic" VRE ... the economics only work for it consuming a firmed share of that generation. (eg, steady consumption for 20hrs a day and turned off during the ducks belly).

As the cost of firming per kWh rises, there will be a point where some of that energy must go into some storage where the capital cost of the flow capacity is much lower, so it can be used as a flexible demand.