What do you mean natural gas infrastructure? Pneumatic storage?

Lots of German households and industries use natural gas for heating, hence we've got something like 400,000 km of pipeline as well as storage facilities sitting around. Instead of buying natural gas from Russia, we could fill them with synthetically produced methane and/or hydrogen, generated during off-peak hours.

Synthetic methane production has a theoretical maximum efficiency of 30-40%. So we'd need to build 3x as much power and is actually needed, and a bunch of electrolysis and Sabatier reaction plants on top of that.

Or we can just follow France's example and build nuclear plants.

Those numbers are outdated. Round-trip efficiencies of up to 80% have been claimed:

https://www.sciencedirect.com/science/article/pii/S036054421...

If that's legit and can be made cost-effective, the case for nuclear is largely gone...

Efficiencies of over 100% have been claimed. Over 50% of the US patent office's applications are for entropy-reversing devices.

Come back to me once these claimed figures are actually implemented.

Reverse fuel cells aren't magic, though. I agree that 80% seems rather ambitous...

...in the form of methane. How do you store wind and solar?

Uhm, well, electrolysis and Sabatier reaction, and then you clean it up and compress it or something. Unfortunately, you need a source of carbon dioxide (no, not air, extracting a trace gas is rather impractical), and then the whole process has a round trip efficiency of certainly no better than 20%. Looks like we need a 4x or so overbuild of unrealiables so that methanation can keep the lights on in winter.

What confuses me is that there are much more practical chemical storage methods nobody talks about. Ammonia comes to mind. It's easier to make and easier to store. I can't help but think that the whole methanation idea is a PR stunt by the gas industry, intended to positively associate renewables with fossil gas in the minds of the unwashed masses.

German Wikipedia lists round-tip efficiencies of 30-38%, and 43–54% if you cogenerate heat.

I've already linked a paper which makes promises of efficiencies of up to 80% using reverse fuel cells.

Here's an older one that promises 'only' 70% efficiency, using caverns for CO2 and CH4 storage:

https://pubs.rsc.org/en/content/articlelanding/2015/EE/C5EE0...

Not sure how much of these claims will survive after contact with reality...

Heat cogeneration doesn't really improve efficiency of the methane generation process itself. Cogeneration refers to using the waste heat of a thermal plant to assist in some other facility. Using the waste heat of a gas plant to heat water in desalination is an example of cogeneration. So it does improve overall energy efficiency, but it assumes that there's a convenient source of heat next door. Wind and photovoltaics don't generate any significant amount of heat though, so there's no opportunity for cogeneration.

Cogeneration happens at the gas-to-power side of things. Doesn't help you with electricity generation, but that's ok as the goal is reduction of emissions across all sectors.

The source calculates somewhat optimistically. They assume storage at 80bar, while mentioning that actual storage is at 200bar. They also assume 60% efficient conversion from methane to electricity, while using 55% in other parts of the paper. There is no accounting for transmission losses or the energy needed to procure the CO2. Cogeneration is again creative accounting. We're talking about supplying electricity, and heat isn't electricity.

Those reversible fuel cells... I'll believe in them when I can buy them. And a round trip efficiency of 80% is unbelievable when simple electrolysis of water, which is only half the round trip, isn't that efficient.