IMO "green" hydrogen really has no future apart from "green...

IMO "green" hydrogen really has no future apart from "green washing". The cost of generating H2 via renewables. PEM (or alkaline) electrolysis from cold water is simply prohibitive. Australian utilities say that here in Australia we can get 3 contiguous days of low renewables generation 4 times a year. Also renewable power from wind (in May) is only available ~42% of the time and solar ~22% (see HERE) - solar is NOT random (i doesn't work at night) and wind varies a lo with season (i.e mostly in spring and autumn when electricity demand is low). The time it's in surplus (i.e curtailed) is less than half these number (i.e most of this time and even during "curtailment" renewables can more economically supply electricity to the grid rather than producing "green" H2).- this means that the capex over H2 production from "cheap" (or zero cost) electricity is about 10% of the time - so over 20 years life you only get about 2 years of H2 production and the capex has to be amortized over just 2 years (even though it may be siting there (and staffed) for 20 years). The write off of the capex (electrolizers etc) and staffing (staff are usually sitting idle) then has to be taken over about 2 years in 20 years). The cost/tyear of H2 operation is thus about half the capital costs + half accumulated 20 year of staffing & maintenance

Once H2 is produced its got to be "stored"; "freezing to -252C uses about 30% of the generated energy and keeping it at -252C a lot more. H2 also leaks everywhere old town's gas systems (CO + H2) lost about 20% in pipes and joints. (i.e you lose about 50% in leaks and freezing (including keep the H2 at - 252C).

You then have to re-generate the electricity again from the H2 (that is stored) - in a turbine maybe you get 40% back as electricity for fuel cells you get a bit more but fuels cells do not have any kinetic energy (this is also true for solar PV with an inverter) and thus are very poor at frequency stabilization. If the grid frequency is not held very close to 50c/s the whole system falls to pieces and the lights go out - this means turbine generation (with kinetic energy) are preferable. Again capex (and maintenance/running expenses) are a problem since they are only required maybe 4 times and year (during a renewable deficit) assuming batteries are used for most diurnal demand swings (also immense cost for 8 hrs of battery).

The end result is the "green" H2 for occasional backup "firming" (with batteries/inverters used for normal diurnal swings) and also providing power during the 3 consecutive days four times a year of major renewables outage is outrageously expensive and he equipment mostly idle (but essential during outages otherwise the lights go out).. Also without turbines (with kinetic energy) generating around 40% of the demand the frequency control will be unstable causing frequent short duration power outages - it would be a nightmare.

The grid costs (to connect remote renewables to the grid) would also be immense as against generating power at the current locations js also a large costs which can be avoided. The fact is that our current grid infrastructure needs power to be generated at (or near) existing locations - costs for connecting remote renewables will cost a lot.

1. My conclusion is that H2 back up and "firming" is really only a pipe dream (the cost of the "green" H2 from curtailed power and it's recovery back to electricity is ridiculous due amortization of capital and running costs, freezing, leakage and thermodynamic losses
2. Renewables to about 60% (with at least 4 hrs of battery/inverter) is about all that's reasonably achievable unless cost is not a factor (i.e electricity bills go through the roof and/or heavy government subsidies)
3. Turbines are essential for "firming" in order to retain frequency stability on the grid; batteries and inverters just do not provide the stability necessary for a reliable grid.

The conclusion is that we need gas with combined cycle gen sets) in the interim for "firming" which are eventually replaced by SMR nuclear reactors. High temperature Helium reactors (with 700C/800C steam) and capacity factors of 93% (as against 10% for curtailed renewable power) using high temperature Solid Oxide electrolizers can produce "emissions free" H2 (for smelting, fertilizers etc) much more efficiently and cost effectively than "green" H2 from cold water electrolysis and renewables. The other option is pyrolysis using CH4 to generate C (to improve soil carbon levels) and 2H2 for emissions free H2 - Hador Topsoe (Denmark) and BASF (Germany) have interesting developments in this area.