OK, you put in electricity and get hydrogen. So where are the efficiency numbers?
Just using an electrolyzer to break up water is about 70% efficient, in terms of energy out / energy in.
critically, the electricity here is not actually directly breaking apart the water into hydrogen, so you can't directly calculate an energy efficiency like you can for electrolysis. this company is using electrohydraulic fracturing to shatter ultramafic rocks and thereby increase the rate of natural geochemical processes like serpentinization or the Schikorr reaction in which rocks react with water to produce geologic hydrogen. the actual source of the free energy that drives the reaction is partially reduced Fe(II) species in the crust (olivine, wüstite, etc) that can absorb an oxygen atom from water to produce free H2 (2 FeO + H2O -> Fe2O3 + H2).
something like 3-6% of the Earth's crust is ferrous Fe(II) so in principle the quantity of hydrogen that could be produced in this manner is something ungodly like 1e15 tons. but the problem is the natural rate of formation is very slow because it's rate-limited by low rock surface areas and slow formation of new rock faces. hence shattering rocks with electricity to increase the reaction rate.
source: I worked on the 2024 us govt research effort that funded these guys
This makes me wonder how many tons of free hydrogen would it take to react with a significant percentage of Earth's oxygen to the point it would harm life.
sure, and I'm saying that it's not a well-defined number because it's not a closed thermodynamic process. it's kinetically limited, so your H2 yield is a function of how much time you're willing to wait. it also obviously will depend on the type of rock, electrohydraulic fracturing parameters (pressure, voltage, pulse sequence) and so on. and in practice one of the biggest challenges in this field is actually capturing and collecting the hydrogen you stimulate, which is difficult to model and can depend on the precise geology and downhole microbiology. you simply can't compare it to electrolytic H2 on an atoms-per-joule basis.
I realize there may not be an ideal oxygen mixture down there but wouldn’t there still be problems with using extreme heat in the presence of hydrogen???
I think there is literally no oxygen down there with the hydrogen. It’s the same with hydraulic fracturing of hydrocarbons. Plenty of methane COULD go boom, but there is no oxygen deep underground.
something like 3-6% of the Earth's crust is ferrous Fe(II) so in principle the quantity of hydrogen that could be produced in this manner is something ungodly like 1e15 tons. but the problem is the natural rate of formation is very slow because it's rate-limited by low rock surface areas and slow formation of new rock faces. hence shattering rocks with electricity to increase the reaction rate.
source: I worked on the 2024 us govt research effort that funded these guys
https://arpa-e.energy.gov/news-and-events/news-and-insights/...