Comment by driest | Hacker News Reader

Comment by driest | Hacker News Reader

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sorry to disappoint, but this article glosses over a pretty significant constraint that prevents this from being long-term storage at all: storing the thermal energy from the compression process.

storing thermal energy over long periods of time is a pretty lossy process, and that 75% efficiency number will be out the window if one tried to use this system for seasonal storage. this system fills the same space as battery storage, which it is also marketed for (over night storage for solar power).

here is a more detailed post on the matter: https://www.rechargenews.com/energy-transition/new-co2-batte...

citing from this: "In Energy Dome’s system, carbon dioxide is compressed at a pressure of 60 bar which heats the gas to 300°C liquid. The heat is then extracted and stored in “bricks” made of steel shot and quartzite for later use, cooling down the CO2 to an ambient temperature. The gas is then condensed into liquid form and stored in carbon-steel tanks.

‘Our lithium-ion battery will have double the energy density of standard Li-ion for same price’

When electricity is required, the liquid CO2 is run through an evaporator to turn it back to a pressurised gas, which is then warmed up back to 290-300°C causing the stored heat."

The trade-off here is these seem to be substantially simpler and cheaper than batteries. No fancy metals, no wear cycles, no risk of spontaneous fire. In fact, the up-front environment impacts seem significantly less than that of battery storage. These are still a good solution when the power supply you're using is in excess - as is common with most natural sources (solar, wind, hydro, etc).

Lastly, it's worth remembering that not everything is competing against an "optimal" solution. This may be a _very_ viable system for infrastructure that simply sheds excess energy. Shedding of excess energy results in 0% efficiency gains. Additionally, this type of storage potentially offsets the need to run a secondary system for peak/off-cycle loads. That in itself can be a massive energy savings.

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Finally, it seems that power plants are actually terribly inefficient at converting an energy source to electric. Coal - 33%, gas - 42%, and combined cycle - 60%.

http://needtoknow.nas.edu/energy/energy-sources/fossil-fuels....

that's all nice and true, but not at all relevant to the point of long term storage.

Long term is defined as 10 hours or more, so that shifting is absolutely within the realm of this solution.

storing the thermal energy from the compression process.

They're not storing heat. They're dumping the heat, and store liquid CO2 near ambient air temperature. Here's some analysis of CO2 liquefaction cost, from an unrelated project.[1] You can have multiple stages of compression, with heat exchangers between them to get the temperature down. How to set this up is a good homework problem in thermodynamics.

It's not clear if this is profitable, but it's a lot better than some of the other ideas. Ones such as the crane and concrete block thing, or the electric trains full of rocks on a hill thing, or the giant rock cylinder with water underneath thing.

[1] https://www.researchgate.net/publication/293044124_Simulatio...

The crane and block thing, by Energy Vault, is an obvious investment scam, already displaced by an equally impractical "hi-rise condo for blocks, with elevators". Probably they will pivot again to something harder to conclusively demonstrate is stupid, such as one with an orbital component. E.g., "Loft mirrors to reflect and focus sunlight onto solar farms at night".

The mine shaft things are not obvious losers, but suffer by inability to re-use the expensive part for multiple mineshafts. There are undersea methods that do better, sharing the expensive, onshore equipment among as many simple undersea units as you like.

The scheme described in TFA has the advantage that the tankage needed for an unlimited amount of storage is cheap, with only the total wattage rate in and out limited by initial investment. Tankage underground (e.g. in salt domes) could store the CO2 and the heat in the same place, without losses; earth is excellent insulator. They specifically say in TFA they are storing the heat of compression by pumping it into iron.

The Chilean project storing energy in liquified nitrogen is similar. They also say they are banking heat, even though boiling the nitrogen with ambient air on the way out, in a "warming tower", seems to me more practical.

75% round-trip efficiency is absolutely fine. Pumped hydro is not better, that way.

There is an unfortunate habit in the energy sector of promoting ideas in absurdly expensive form, just because that makes it look more "hi-tech", to be taken more seriously by investors attuned to look for that. The form of each idea actually built and used by utilities will be whichever form is cheapest, which will seem too boring for the press to pay it any attention.

What is the issue with the "crane & block thing" ? It certainly seems to be a plausible method for energy storage and recovery.

Even Energy Vault abandoned the moronic crane thing. It is very easy to show that it stores only a minuscule amount of energy, and does not operate at all anywhere there is wind.

Their hi-rise condo for blocks also stores only a tiny amount of energy, and is furthermore extremely expensive.

Likely they will pivot to something else as foolish, but might use their market cap to buy out something that works. Most likely they would then ruin that.

Anyplace where you mean to use gravity storage, and all the height you can muster is expensive stuff you have to build, will be a non-starter. Lots of other things can make a scheme impractical, but one is always enough.

Pumped hydro is practical because you need only a pipe between upper and lower reservoir, but mainly because in most current uses the lower and/or upper reservoir already existed, and often enough the turbines too.

The problem with any block system is that the weights are too expensive and the storage capacity doesn't scale very well. Concrete is already too expensive to be practical.

Then there is the engineering challenge of building a crane system.

It just doesn't get better.

OK then. I stand corrected.

if that were the case, how do you explain this sentence? "The heat is then extracted and stored in “bricks” made of steel shot and quartzite for later use,"?.

or did the source simply explain their process incorrectly?

But heat loss improves as the product is scaled up(heat loss is function of surface area, whereas total heat is a function of volume, which grows quicker than surface area), so at a certain point you could make this big enough to let you store energy efficiently on a seasonal basis?

Also the same reason elephants can't have metabolisms as fast as mice, or else they would spontaneously combust.

It could be that the loss in heat storage can be compensated with heat pumps, powered by solar power, to keep the heat reservoir full.

I mean, the process is simple, clean, does not require special materials, provides lots of jobs and really seems long term.

The holy grail of energy storage will ultimately be a similar form (compressed gas, fuel generated with solar power, heat reservoirs, etc).

No matter how much you root for batteries, they will not provide grid scale power.

Batteries do, and will, provide grid scale power for short periods. Beyond that, they cost more than other methods. New battery chemistries might bring costs down, but it is hard to compete with tankage for cost per stored kWh.

Do we need to store energy seasonally to make big strides in increasing the share of solar and wind on the grid? Or only daily/weekly?

A week or two is the longest almost anybody should need to bank energy. Mostly you just need a few hours of high-efficiency storage to maintain local resiliency. Beyond, say, the 4 hours' worth cycled every night, round trip efficiency matters little.

Beyond a week, burning liquified ammonia imported from tropical solar farms wins. Probably you keep a week's worth of that on hand to burn while you wait.

There is no need for storage (beyond use for load-leveling and peak shaving) until after renewable generating capacity is overbuilt enough to charge it from. (The alternative would be to recharge storage by burning NG: You would better burn that NG and deliver the power to users instead of drawing down and then recharging storage from it.) Until then, capital is better spent building more renewable generating capacity itself, displacing more coal, than on storage.

But building the factories that will build the storage that will someday be needed should happen now, because we will need a lot, and making automated factories takes a long time. And that is happening. Some of those may end up idle as cheaper methods replace theirs, but for at least a few decades, demand will be insatiable.

It depends how much you overbuild. If you build enough to supply half your needed energy for the worst case week and 3/4 for the worst case month, 4 weeks is probably enough.

You continue building out local generating capacity until its maintenance cost less income from grid and synthetic fuel sales exceeds the cost of imported grid power and synthetic fuel imported. All the costs will be changing continuously. Generally, more generating capacity than you have will be reliably better for a long time.

After things begin to stabilize, you might decommission older installations or devote an increasing fraction of capacity to carbon sequestration.

is that relevant? the premise of the article was "long term storage", and my comment aimed at putting that into perspective.

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