@dagb #efuel that uses variable renewable power that we already have is just a shell game for increasing fossil fuel use. Unless it uses newly added VR energy, it can not be green.

So green #efuel intrinsically uses an #energy supply that we do not yet have. It must, in order to be green.

@dagb The question is how effective it is for its core use case. At the extremley high capital cost of batteries for 4 cycles/year seasonal storage, efficiency wouldn't matter.

In a pedal to the metal low carbon transition, sometimes we will have to prioritize emissions reduction efficiency over energy efficiency.

So we should start w/the strongest use cases, not the weakest. ...

@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.

@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.
{*}