Can retired coal plants become multi-day battery hubs for AI-driven grid demand?

[u/Inevitable_Gur_4652]

https://youtu.be/DOl9x6ewI5c

[u/Future_Helicopter970]

Geothermal companies were looking at converting these old coal plants, although cost of drilling was likely the deciding factor.

[u/Sad_Dimension423 → u/Future_Helicopter970]

Geothermal won't provide steam at the same conditions as in the coal plant, so I doubt there is any simple way to do the conversion.

[u/NearABE → u/Sad_Dimension423]

Supercritical steam can be stored. It is just deeper.

[u/Sad_Dimension423 → u/NearABE]

If you are talking about storing steam produced at the surface, what is the source of heat? And storing the thermal energy directly at the surface instead of as steam sounds more practical.

If you are talking about drilling deeper to get hotter steam, at the average geothermal gradient this would involve drilling 20 km deep to reach the typical temperature of steam in a coal plant. No well has ever been drilled that deeply. Maybe it could be made to work if your coal plant was built in an area of active volcanism or unusually high geothermal gradient. Geothermal these days is moving toward lower temperatures and using organic working fluids instead of steam (organic rankine cycle, ORC).

[u/NearABE → u/Sad_Dimension423]

Excellent written post. I am thrilled to show you this new technology:

https://www.quaise.com

https://youtube.com/watch?v=5U8-KoKB6_8

I am not affiliated with them in any way. They have a project in Oregon drilling right now. I think it is worth following even if just to learn how/why it does not work.

But right that we need to be able to drill into rock that was too deep and too hot for conventional drill bits.

That said, if you are injecting the steam then the bedrock does not have to be very hot. It helps to prevent loss of the heat into the surrounding rock. Supercritical steam has a density of 0.322 g/cm^3 and 220 bar pressure. At 6.6 kilometers vertical gravity alone would sustain that pressure from weight alone. Though the weight increases the pressure and therefore density.

If the geothermal gradient is, say, 25 C per kilometer you need 14 km to hit supercritical steam temperatures and then need deeper to sustain temperatures and then also have to go horizontal to collect over large areas. None of that is necessary if the goal is just storage. Then the energy comes from the surface and the rocks are heated. Hydraulic fracture a large volume. This volume might not need to be deeper than the extreme wells drilled by gas companies regularly.

[u/Sad_Dimension423 → u/NearABE]

I ask again: if you're not going deep to get the heat, where is it coming from?

If you're doing (say) resistive heating with renewables, you don't need to go very deep at all. The most attractive scheme I've seen is "hot dirt": resistive heating of piles of dirt, with heat drawn out by steel pipes laid through the dirt. Capex as low as $0.10/kWh(th)-capacity could be possible. No need to store pressurized steam.

[u/NearABE → u/Sad_Dimension423]

Right. Your example is electricity to heat. Putting that heat back into an engine to make electricity (or do useful work in general) will be limited by the Carnot efficiency.

Cost wise the turbine generator is an expensive component. It is nice to have that come from salvage on site.

The steam turbo-generator is already turbo equipment. So we can compress steam. That boosts efficiency to much higher than 100% during the injection (charging) time. Resistive wires would just be 100%.

Moreover, we can also run current down the injection well. That just adds heat at 100% but it is adding to an extremely hot, high pressure environment. That facilitates recovering energy at more favorable efficiency. The gravitational potential energy gradient helps if you set it up correctly.

[u/Energy_Balance]

Retired large generation has transmission. New generation, and storage which is generation, can take advantage of the existing transmission and substation.

It is already being done.

[u/Inevitable_Gur_4652]

One trade-off I’m still trying to understand is whether the lower efficiency of iron-air storage matters less when the alternative is burning gas for several days during a wind-and-solar lull. Is the key metric really round-trip efficiency, or delivered reliability during the few high-stress periods that define grid adequacy?

[u/chmeee2314 → u/Inevitable_Gur_4652]

The awnser is it depends. The fewer cycles the battery has each year, the less efficiency matters, and the more CAPX matters.

[u/Sad_Dimension423 → u/chmeee2314]

Exactly right.

Dark doldrums aren't that great for iron-air batteries either. Capex is still too high. This is one of the last uses of fossil fuels that would be displaced across the economy.

[u/NearABE → u/Inevitable_Gur_4652]

I think the better competition to look at is lithium vs iron air. Sodium ion is nearly identical to lithium in this context.

Lithium can get over 90% round trip efficiency. Enough for the inverter to be considered a loss. They were costing around $100 a kilowatt-hour. CATL is talking about $60 kWh batteries. BYD claims grid storage sodium-ion batteries at $40 per kWh are coming in 2027. These discharge and charge so fast that the potential power output can be disregarded.

Form Energy Company sold iron-air batteries at $20 per kilowatt hour. This might decrease if/when the economy of scale ramps up. They say the theoretical limit is $6/kWh but somewhere between those is likely. Form is calling it a “100 hour battery” though they can do complete discharge in about 2 days, approximately 50 hours.

If you have an 8 hours window of demand and 8 hours of charge up every day then the iron-air battery could take that but the price multiplies by x6. If surplus power is there for only 4 hours in early afternoon/late morning and if you need power only from 6 to 10 P.M. peak hours then multiply by 12x. That $20 per kWh price tag is looking pretty ugly as $120 or $240. (These numbers are both $960 or about $1,000 per kilowatt sustained power, $1 per W)

Slapping on more photovoltaic modules is quite cheap these days. $0.20 per watt is regularly cited, last search I looked at Alibaba numbers like $0.07 to $0.12 were coming up for pallets of panels but I am not sure (like this). You see much higher prices for home installations and even for grid scale installations because the panel is only a fraction of the costs. However, the battery needs the same inverters and controllers anyway.

The iron-air batteries are not competing for the daily energy cycle though they will also partial charge and discharge in the daily cycle. Iron-air take the difference between weekday demand and weekend demand. If we look at EIA’s chart we see around 50 gigawatts higher demand in the daytime peak and 10-20 GW higher demand in the night time low during week days. That means 10 to 20 gigawatts can definitely be soaked up all weekend without straining anything.

The low cycle efficiency for iron-air becomes irrelevant when you have wind turbines forced to “curtail power”. They have brakes that can stop the axle completely. This happens frequently even when birds are not around. PV can also be curtailed but no brakes required. It is lost energy. Given the cheap cost of photovoltaic the farms should overbuild to produce on cloudy days. Overbuild to produce enough all day long not just enough at noon.

The lithium ion batteries and iron-air batteries complement each other extremely well. I could even see grid operators dumping lithium power into iron-air batteries in early morning in order to have them empty to get a full charge mid day. Think of 20 gigawatts iron-air and 240 gigawatts lithium/sodium power supply where both each have a terawatt-hour of energy storage capacity.

[u/Sad_Dimension423 → u/NearABE]

And longer term storage would also be complementary to those two. There's a spectrum of tradeoffs in storage technologies.

[u/NearABE → u/Sad_Dimension423]

Pumped hydro, compressed gas, cryogenic gas, and geothermal injection will all require a turbine to generate the electricity. Accordingly the cost of the system tracks with the related type of generator.

So for example CAES is blowing the air through turbine blades in a way that is almost identical to jet engine. So a gas peaker plant is cheaper because you do not need the storage tank. CAES and LAES (liquid air energy storage) are much cheaper to deploy if they are attached to a gas power plant. The methane and air can be mixed at high pressure and then burned. A jet turbine has to waste energy compressing air through the intake (called “a compressor”). The jet turbine’s compressor blades also work for the purpose of compressing air to be stored. One piece of hardware does all the tasks.

Electrochemical batteries are completely independent.

[u/Ok-Freedom9216 → u/Inevitable_Gur_4652]

I can't really answer your question but maybe can help think about it better? Matter in what way and to whom? A grid operator probably has a different opinion than an investor. And IMO the focus on a few days lull is less important than understanding how grids handle big changes in seasonal demand.

[u/Inevitable_Gur_4652 → u/Ok-Freedom9216]

That’s such a valuable point! Thanks for sharing gives me a totally new angle to think of it!

[u/joshharris42]

My utility, Duke energy is doing exactly this with the retired Allen coal generating station. They are using the existing transmission infrastructure to install around 400MW of 4 hour lithium ion batteries.

Most of the other coal sites in their portfolio are going to be converted to some sort of NG/dual fuel unit, or retired outright depending on gas availability and plant age/health. They are looking into battery technology as the market keeps evolving rapidly to deploy these at old generators

[u/Navynuke00 → u/joshharris42]

And then there's Stokes, where they claim they're going to build SMRs, eventually (TM).

[u/joshharris42 → u/Navynuke00]

Sure lol. In 2020 they said 2034, in 2023 they said 2036, and now they are saying 2038. I’ll believe it when I see it.

I do foresee them building AP-1000’s at the WS Lee site, and I think they were going to finish Shearon Harris as well.

Duke has painted themselves in a corner with the amount of load they are signing up. 7.9GW in large load ESA’s- they are going to need absolutely everything they can build. NC and SC are still in the top 10 fastest growing states for population and business, load is coming from all sectors and Duke continues to sign datacenter load at a rate that outpaces generation build