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points by bryanlarsen 3 days ago | 0 comments

Agreed, e-fuels seem to be far and ahead the front-runner.

We can get to 95% using just short term storage. I suspect that most jurisdictions will just say "95% is good enough" rather than paying for ridiculously expensive long term storage.

We've got the nat gas capability already; keeping it around for the 5% is quite inexpensive, making the massive investment in long term storage for marginal benefit hard to justify.

OTOH, while the premium for e-fuels seems excessive, if you're only running that nat gas plant 2 days a year that premium is not so scary.

[–] pfdietz 3 days ago

> Agreed, e-fuels seem to be far and ahead the front-runner.

For covering Dunkelflauten? Yes.

For seasonal storage with a prolonged discharge (so, low power/storage capacity ratio)? I think thermal storage wins there.

[–] cyberax 3 days ago

I just don't see it with thermal. You have to pay the Carnot tax, and you also need humongous amounts of material.

E.g. 100 GWh (4 days of 1GW thermal power) with a 100C temperature swing (you can't go down all the way to room temperature) will need around 3.6*10^4 tons of sand. That's a lot.

Honestly, power-to-gas sounds like the only reasonable way for now. In theory, with SOFC (Solid Oxide Fuel Cells) for power generation, it can beat the steam cycle.

[–] Panzer04 3 days ago

36000 tons... doesn't actually sound like that much? That's a ~30x30m cube of dirt.

But I have no reference for the cost of setting up such a storage and how it compares to battery or hydro.

[–] adgjlsfhk1 3 days ago

The ~grid scale design of this for reference is somewhere around 200mx200mx50m for above ground, or ~100m cube (or bigger) for quarry infill. Such a design is going to get crushed by battery/hydro over the hours/days, but wins on cost for seasonal usage.

[–] pfdietz 3 days ago

The cost of putting dirt in a pile is $1-2/ton.

[–] cyberax 3 days ago

The biggest expense will be the pipework inside that pile. That you won't be able to service easily. And you'll also need a LOT of insulation around it to prevent heat from leaking out too quickly.

[–] adrianN 3 days ago

Thermal storage benefits from the square/cube scaling, so the amount of insulation goes down as you scale up the storage.

[–] pfdietz 2 days ago

Even better than that: the thermal time constant is proportional to the square of the radius. There's the square/cube effect that causes a factor of r ratio in the volume to surface area, but there's also a factor of r from the temperature gradient (which, from the center of the pile to the surface, is inversely proportional to r).

[–] pfdietz 3 days ago

The insulation is just more dirt.

[–] cyberax 3 days ago

Homework: compute the heat flux from a 40 meter radius sphere, given the surface temperature 600C and the ambient temperature 0C. Now compute the amount of insulation needed to keep its temperature above 500C for 3 months, given that dry sand has an R value of 0.25 W/mK.

[–] PaulDavisThe1st 3 days ago

so $36-72k for days of 1GW of power? Sounds pretty cheap to me ...

[–] pfdietz 3 days ago

They also state the cost per kWh(th) of storage capacity is about $0.10, or $100,000/GWh(th). This is thermal energy, which is converted to work at maybe 40% efficiency, mind you. There are costs over and above the dirt (for example, pipes laid through the dirt to carry heat in/out).

[–] pfdietz 3 days ago

The temperature swing can be much greater than 100 C. Heat can be extracted as a wave passing through the store, then heat added back by reversing the flow. It would operate like a counterflow heat exchanger.

View another way: if we hook several piles in series, then lukewarm air from a first mostly discharged pile can be directed into later piles for more heating. In the continuous limit this is the concept I described above.

[–] cyberax 3 days ago

> The temperature swing can be much greater than 100 C.

No, because your steam will be at a lower temperature as you draw down the heat, bringing down efficiency.

[–] adgjlsfhk1 3 days ago

Realistic designs for this can achieve 450C delta t. Your energy store can store at ~1000C (slightly below rock melting point), and run steam turbines with a hot side of 600C and a cool side of 150C.

Over the season, your sand drops from ~1000 to ~600 (which is fine since your power needs drop in the spring)

[–] cyberax 3 days ago

Sorry, but 1000C is wildly unrealistic. There are no bulk materials that can be used to construct heat exchangers for this kind of environment. 600C is already a stretch.

At 1000C steel will react with _water_.

To be clear, I'm not talking about cooling a small crucible but a structure that is at least 20 meters in size.

[–] pfdietz 2 days ago

At 600 C one can use stainless steel, but things rapidly become expensive and problematic beyond that. 1000 C would require Inconel or something.

[–] pfdietz 3 days ago

That is wrong, and I already explained why.

[–] bryanlarsen 3 days ago

Sure, thermal is great where it's possible. But there aren't that many places where district heating is viable.

[–] pfdietz 3 days ago

I'm not talking about district heating. I'm talking about high temperature storage for conversion back to electricity. Round trip efficiency could be around 40%, better than green hydrogen burned in combined cycle plants.

[–] bryanlarsen 3 days ago

Standard Thermal's website's front page only covers heat delivery; did you mean to link to somewhere else?

Those are 2 very different use cases.

Thermal is low rate extraction. It's 1 cycle per year, but the extraction is performed over months so is low rate.

Electricity long term storage is high rate extraction. It's only used a few times a year, but when it is used it needs to cover roughly 100% of the entire grid's needs. I doubt thermal storage is suitable for high rate extraction, but I'd love to be wrong.

Edit: rereading your parent comment you seem to think thermal is suitable for a different use case? Could you elaborate further? When is "prolonged discharge" useful?

[–] pfdietz 3 days ago

That's what they're focusing on to start with, but the temperature is high enough to drive heat engines to make power. There's more on the concept at the seed company's (Orca Sciences) web site and at Austin Vernon's blog.

https://www.orcasciences.com/articles/standard-thermal

https://austinvernon.substack.com/p/building-ultra-cheap-ene...

They talk about temperature up to ~600 C, which I believe is limited by the desire to use cheap steal (much hotter and creep becomes a problem). This is also perhaps not coincidentally about the temperature of steam from a coal fired boiler, so one of the eventual goals is to repower retired coal fired power plants with this storage providing the heat instead of coal combustion.

You're right this is not good for something needed "a few times a year" for relatively short periods, for example to cover Dunkelflauten. Combustion of an e-fuel is better for that. But it's well suited to seasonal storage of solar energy, as that would be extracted over months, more or less continuously. The resistive heaters should also be much cheaper per W than electrolyzers.

[–] bryanlarsen 3 days ago

> But it's well suited to seasonal storage of solar energy, as that would be extracted over months, more or less continuously.

But what is that useful for? During almost all of those months it would be a lot cheaper to use renewables & short term storage. And it's not fast enough for the few days it's not beaten by something cheaper.

[–] pfdietz 3 days ago

It's useful for storing cheap solar energy for use in winter. Sizing the solar collectors for winter would require them to be much larger. Basically, this lets 2.5 W of solar in summer deliver 1 W of solar in winter. For a high latitude where the change in insolation is higher than that, this is superior. In Hamburg, for example, insolation varies by more than a factor of 10 from July to December.

Put another way: if you oversize the solar to be adequate in winter, you have enormous amounts of surplus solar in summer. So why not store it?

[–] skybrian 3 days ago

I thought that post was interesting when I read it over a year ago, but the startup seems to have gone dark since then.

[–] pfdietz 3 days ago

They appear to be focusing on Europe, and have raised $11M (or maybe 11M euros, I'm not sure which). There is some favorable buzz in Germany, which makes sense because natural gas has become quite expensive there.

https://www.cleanthinking.de/thermischer-erdspeicher-erdhueg...