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Comment by cinquemb | Hacker News Reader

> … (thermal) energy storage systems, that are more durable (and more affordable) than electrochemical batteries…

Agree, I think for so long the whole renewable build out has ignored storage costs which will really come to bite electrochemical battery dependent systems in the ass (because banks like UBS have long projected that costs will be harder and harder to bring down and ACS has long talked about the difficult to deal with chemistries with all the different batteries out there [and with the inherit degradation of the system over time]).

I think in the long run (well, unless our political challenges with nuclear get solved), on the utility level (potentially even behind the meter for specific high energy applications that have space to construct/"mine" their own solar field), molten salt storage will take the lead (esp as salt mixtures/chemistries get cheaper, have higher thermal operating ranges, storage and transportation gets better).

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Thermal storage in hot sand seems superior. Sand is cheaper, capable of operating at high temperature, and allowing heat exchange to the working gas by direct contact in a fluidized bed heat exchanger.

https://arpa-e.energy.gov/sites/default/files/2021-03/07%20D...

I deff would like to see a 100MW sand system built out.

I think the sand direction is deff an improvement over the 300 delta_C systems now with (NaNO3, KNO3, NaNO2) at the 900 delta_C in used in the pdf, but idk if it will win out in the long run with the need for direct contact vs some kind of sCO2 system with higher delta_C mixtures (for example a theoretical mixture CaCl2 + MgCl2 with 1500 delta_C, or even engineered metamaterials[0, i've only seen some specific examples of electromagnetic metameterials when i was in school about a decade ago, this post was about a year after i left] in a mixture to have even higher delta_c).

There's only so much one can do when one limits the materials used for heat capacity to naturally occurring ones (i imagine that compounds in the future could be engineered to be at least a order of magnatude higher in delta_c from natural mixtures).

[0] https://news.brown.edu/articles/2012/10/quantum

One issue with that sand system is that the sand is hot enough to generate too much thermal NOx in an open Brayton cycle (or combined cycle) system. Which is sad, because that kind of system could save a lot of complexity. One could add a selective catalytic reduction stage afterwards but that increases complexity again.

A very interesting possibility, I think, is to replace the sand with some other mineral and make the solid part open cycle. That is, instead of retrieving and recycling cooled sand, a new stream of pulverized minerals would be used. This could be useful if heat treating those minerals does something desirable. A particular case would be heat treatment of serpentinite to make magnesium oxide and silica, which would then be used for CO2 capture/sequestration.

Both the salt and the sand option are exciting.

This isn't a thermal option, but the iron-air chemistry has promise too. It's basically rusting and de-rusting iron. Much more environmentally benign than lithium ion, and likely way less expensive. AFAIK, this company is the frontrunner: https://formenergy.com/technology/battery-technology/

Iron-air are just another kind of metal-air batteries. The problem with all of them is that hydrogen-air batteries have existed for decades and is basically the same thing. If it wasn't for 20 years of marketing propaganda we'd realize this and never bothered with reinvesting the wheel with another chemistry.

I hear you that marketing spin is a factor here. But if Form Energy can achieve the $20/kWh price that they're pursuing... iron-air is quite different from other chemistries on the financial front. https://www.utilitydive.com/news/form-energys-20kwh-100-hour... (I have no affiliation with this company.)

Yes, but hydrogen-air is already cheaper than that. On large scales it is <$1/kWh and easily cheaper than anything can even theoretically attain.

By hydrogen-air battery do you just mean a water electrolyzer system paired with some kind of hydrogen fuel cell?

If so, the metal-air systems have a major advantage on the discharging side of things by avoiding the need of a fuel cell and being able to harvest electrons directly via galvanic current.

A hydrogen system is basically the two sides of a battery split into separate components. It's fully possible to build device that does both, although as of today this is less efficient.

The galvanic discharge rate is extremely slow and not really suitable as part of a rechargeable battery system. Any practical metal-air will have some way of discharging faster. The problem is that we have basically solved this for hydrogen-air but not for anything else.

Wow. Okay, thanks. I need to learn.

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