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