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Comment by BenoitP | Hacker News Reader

At scale means with mining requirements as well.

With current and projected reserves we can manage intra-day requirements for the whole world. This is the we mine it all option.

But we need intra-year storage capacity.

Nuclear is clearly the easier and only option (and the most environment-friendly to me, considering the mining requirements)

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Mining of what? Hydrogen as storage medium for example doesn't require all that much mining. Are you assuming storage will be in the form of Lithium batteries?

I'm assuming currently proven tech of batteries. And we need to act now for climate change, with current tech.

Electricity to H2 to electricity has horrendous efficiency (about 20% at most IIRC). We'd be much better off CO2-speaking using it to replace coal in steel production, and replace gas in fertilizer production. Both of which are proven tech (but not globally deployed, as coal and gas are cheap)

Where are you getting that 20% number from?

https://www.irena.org/-/media/Files/IRENA/Agency/Publication...

https://www.sciencedirect.com/topics/engineering/round-trip-...

The round trip efficiency is 30% or more and rising with better electrolysis and reverse electrolysis being developed.

Power2Gas2Power has low round trip efficiency (say 40%), but as only a fairly small fraction of the energy flow goes through it that doesn't matter that much at the end of the day. If it did, that just means heavy industries would migrate to less seasonally variable places.

Inefficiency just requires extra capex, not opex. And the capex is capex is still falling at an exponential rate. So, no.

Personally, I expect something made with iron or sodium to take over.

For hydrogen, we have to solve the horrible cycle efficiency of its storage. It's not so bad for use as a fuel (e.g. for airplanes), but as static storage hydrogen is currently a very bad choice.

Cycle efficiency is trivially resolved by feeding in more zero-opex power.

But hydrogen efficiency is improving as fast as all renewable tech.

I agree that hydrogen efficiency doesn't matter here, but the capital expenditure to build a giant electrolyzer with 5% utilization doesn't really make sense. The cost of the electrolyzer is the things that needs to greatly fall if hydrogen is going to be used for long-term storage.

Electrolyzers driven by otherwise curtailed renewables would have a capacity factor much higher than 10%. If I go to https://model.energy/ and solve for the optimal solution for the US for 2011 weather and 2030 cost assumptions, the capacity factor of the electrolyzers is 43%.

That is more of a what-if. If there was more wind and solar built out there would be enough curtailed energy to run electrolyzers at 43% capacity factor. I am saying today it doesn't make sense because there isn't a lot of over-provisioning taking place. There should be but what investors are going to jump first with a curtailed energy electrolyzer when curtailed energy is low.

It's a demolition of your nonsensical 5% number (which is also what-if).

In the state with the most solar built out in the US the curtailment is 5% https://www.eia.gov/todayinenergy/detail.php?id=49276.

Which is of course meaningless, since curtailment increases with the amount of installed wind/solar. What matters is how much curtailment there would be in an optimal all-renewable energy system.

I tell you what: show me a cost optimized renewable energy system in which the electrolyzers have a capacity factor of 5%.

Hydrogen has important industrial uses. Currently we get it from natural gas. That has to change in the future.

Hydrogen produced using NG will be deeply undercut by electrolysed hydrogen. Extracting NG will become unsustainably expensive as zero-opex synthesis surges.

Demand for hydrogen for all uses will drive elecrolyser use to as close to 100% as available energy permits. Ultimately, other cheap storage will be used to drive them.

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