Also, exist prototype power to gas facilities are ~50% efficient, just for the power-to-gas step. Even with a 66% efficient combined cycle gas turbine, which is the best we have, net efficiency is ~33%.
If you store the resulting hydrogen directly, no carbon source is needed. That makes storage more location dependent, because the cost-competitive options are salt-caverns, but we are talking about long term storage here...
Production of SynGas is more important to replace the current use of ground-pumped methane with CO2 neutral variants.
> Also, exist prototype power to gas facilities are ~50% efficient. Even with a 66% efficient combined cycle gas turbine, which is the best we have, net efficiency is ~33%.
Which is why there are proposals to go purely via hydrogen and reversible oxidation cells. That way you can get up to about 70-80%.
But again, we are talking about long term storage. ALL other options of energy storage are more expensive when you reach the "weekly to monthly" storage timeframe. At that timeframe, the low efficiency becomes irrelevant, as storage cost ($/kWh) is dominating, and we are talking about renewables anayway.
We do have a rather good idea. There are several studies and large test facilities both in Europe and the US. Specifically in the area I was mentioning (salt caverns). From wikipedia on hydrogen storage: "Underground hydrogen storage is the practice of hydrogen storage in caverns, salt domes and depleted oil and gas fields. Large quantities of gaseous hydrogen have been stored in caverns by ICI for many years without any difficulties." "Another study referenced by a European staff working paper found that for large scale storage, the cheapest option is hydrogen at €140/MWh for 2,000 hours of storage using an electrolyser, salt cavern storage and combined-cycle power plant."
There is more here: https://en.wikipedia.org/wiki/Hydrogen_storage
You can also turn CO2 into synthetic oil (Methane, gasoline, Jet fuel, motor oil - your choice of what you want) via known processes. Again the process isn't very efficient, but when the inputs are free who cares. The sum of is carbon neutral.
In order to convert elecricity to methane you first have to REMOVE CO2 from the enviroment. If you subsequently burn that gas, you arrive back at the CO2 level you had before the P2G process.
That of course only works out to net zero when you use co2 that was recently captured, like from waste-biogas or trees.
The big problem with fossil fuels is not directly that they release CO2, it is that they release CO2 that was captured over millenia in the comparatively short timespan of decades.
You can avoid that problem entirely with P2G systems.
Only if you don't look at all available solutions: https://en.wikipedia.org/wiki/Power-to-gas "P2G is often considered the most promising technology for seasonal renewable energy storage"
I dunno why this is such an unknown technique, natural gas storage is a very old and proven technology. Yet it is ommitted from almost any discussion about the subject...