From a thermodynamic point of view, it should be noted that compressed air does not actually store energy! The internal energy of a compressed gas is from the kinetic energy of its molecules, and this is a function only of temperature(*).
What a compressed gas represents is not stored energy, but stored (negative) entropy. It is a resource that allows low grade heat to be converted to work at high efficiency. This is what's to happen in this facility: the heat of compression is separated out and stored, then used to reheat the compressed air at discharge time. The energy is actually being stored in that thermal store.
But there are other ways to do this that don't involve compressed air storage. Instead, after the heat of compression is removed and stored the compressed air could be reexpanded, recovering some of the work. This would leave the gas much colder than when it started. This cold could be stored (heating the gas back to its initial temperature) and the gas sent around again. To discharge, the temperature difference between the hot and cold stores could be exploited.
This is called "pumped thermal storage". I believe Google/Alphabet has/had a group looking at this (called Malta). It has no geographical limitations.
(*) Highly compressed air will store some energy because the molecules become so crowded some energy is stored in intermolecular repulsion, but that should be a small effect in this system.
I understand what you're saying, but your claim seems misleading to me.
Compressing air absolutely does store energy. It takes work to compress it, and work will be done when it is allowed to expand.
Yes, one way of measuring the stored energy is by the resulting increased temperature. Compressing air necessarily raises its temperature. And then if you want to you can transfer that heat elsewhere, go ahead. That's how air conditioners and refrigerators work, after all.
But in the basic case, it seems entirely accurate to say that compressing air does actually store energy. Just like raising a heavy object against gravity stores energy.
It doesn't store energy in the sense that all that work you did compressing it doesn't increase its internal energy. It just has the energy it started with, floating around in the atmosphere. Now, some of that energy can be converted to work, if the air is adiabatically expanded, but particularly at high compression it's not large compared to the energy that went into compressing the air (and that was dissipated as heat when the hot compressed air was cooled.)
>Compressing air necessarily raises its temperature. And then if you want to you can transfer that heat elsewhere, go ahead. That's how air conditioners and refrigerators work, after all.
The oh-so-clever trick is to transfer heat elsewhere while you're compressing the gas, so you actually reduce back-loading on the piston instead of having it 'fight' the temperature rise. This can be accomplished via water spray, or by compressing a gas bubble that's surrounded by water (eg in the trompe).
By continuously removing heat as you compress the gas, it effectively acts like a train of compressors and intercoolers with an infinite number of stages, ie true isothermal compression. No magic, just physics.
I want to note that in this system, where the heat is stored to reheat the gas at discharge time, you want to produce heat during compression. Doing adiabatic compression means you are storing more heat, and therefore can produce more energy at discharge time, than you could with isothermal compression (for a given storage volume and pressure.)
This is unlike traditional CAES, where the air is reheated by burning a fuel. There, fuel use is minimized by isothermal compression.
It’s absolutely misleading. The compressed air absolutely has potential energy. Nail guns, air wrenches, etc convert that potential energy into kinetic energy.
It seems like either this is describing an additional energy storage/extraction method that’s possible with these systems. Either to improve the efficiency or something that’s actually far more efficient than utilizing kinetic energy compressed air can create.
It’s kind of like saying the water in a hydropower plant doesn’t store energy. The turbines aren’t extracting energy from the water itself. They’re extracting energy from gravity acting on water. It doesn’t matter if the water is nearly freezing or nearly boiling, it has roughly the same amount of hydropower available.
Your analogy is nonsense. Water in a hydropower system falls from a higher gravitational potential to a lower. When a pumped hydro system is charged, the water is moved up and energy is stored in its potential.
This is not the case in a CAES system. The internal energy of air, which is the kinetic energy of its molecules, is independent of pressure. It's only a function of temperature. The internal energy of the air is unchanged by a process that increases its pressure without increasing temperature. So, no energy has been stored in the air.
What has been done is reduction of the entropy of the air. So, it is now possible to convert some of that internal energy (which was always there) to work by expanding the air. The air would end up even colder than the initial air.
Is compressed air in itself source of energy ? Or does it depend on the external air pressure as well ? For example I compress air in a canister and move it to outer space or deep underwater, does the energy we can exploit out of it goes up or down ?
Yes, if you take such a container into space, the gauge pressure (pressure relative to ambient atmospheric pressure) goes up. A cold gas thruster is a type of rocket engine that is basically a pressurized tank of gas with a valve connected to a propelling nozzle. The nozzle takes a high-pressure, low-velocity gas and converts it to a low-pressure, high-velocity gas, and directs its output generating thrust (which is a reaction force - Newton's 3rd law). The lower the ambient pressure, the higher the expansion ratio of the nozzle can be, which results in a higher output velocity of the gas, which directly increases the thrust. It would give you a higher specific impulse, which is a measure of fuel efficiency for reaction (thrust) engines.
Funny, when I saw the headline I thought, "I hope they're using a trompe / Canot compressor or similar for efficient isothermal compression and expansion."
How does that have anything to do with what I was saying? The internal energy (per unit mass) of the compressed air at given conditions is not a function of how those conditions were reached.
>The internal energy... is not a function of how those conditions were reached.
How the conditions are reached makes a big difference in efficiency.
I'm baffled at why you focus on minor implementation details like internal energy at a particular snapshot in time, as opposed to highly salient metrics like overall round-trip system efficiency.
Ideally, the round trip efficiency of abiabatic CAES is 100%, as would be the ideal round trip efficiency of isothermal CAES.
What isothermal CAES (with heat storage) would require is a larger storage volume per unit of energy storage capacity, since less heat is generated during the ideal compression. That heat generation is not inefficiency, it's something you want!
Only if the heat is discarded and you reheat with fuel do you benefit from isothermal compression.
If I'm remembering this correctly from physics class then in math terms this is described as U=C*n*R*T where U is the energy, the constant C varies depending on the degrees of freedom the molecules of the gas have, n is the molar amount of the gas, R is the ideal gas constant, and T is the temperature. From that perspective it seems impossible to raise the pressure without also raising n resulting in a higher U even if you maintained a constant T while you did so?
I'm trying to reconcile how a consumer-grade compressor fits into this. Clearly such a device can compress a few gallons of gas, which could be allowed to cool to room temperature, and then used to spin a small dynamo. This action clearly converts some amount of energy, but where does it come from? Perhaps the problem is we're not looking at a closed system at that point.
If you empty that compressed gas, it cools everything it comes in contact with. Room temperature gas still has a lot of thermal energy.
The effect the prior poster was discussing is most apparent at large scales when someone is doing actual efficiency analysis - compressed air is not a very efficient way of storing or transporting energy in most contexts. No one would use compressed air for a power grid, because it would quickly be apparent how terrible it is.
It is however a great way to transfer and use energy in many industrial contexts, as turbines and pistons using high pressure compressed air can be very compact while also being very powerful, and have built in cooling.
At the level of typical household use, the efficiency effects aren’t notable. No one is going to care or even notice if it cost 1/2 cent of electricity (at the compressor) to use that air impact wrench vs 1/10th of a cent to use an electric one. Especially when the electric one is heavier and has less torque.
The room temperature compressed air still has some internal energy, and some of that energy can be converted to work by expanding the air through a turbine. The air at the outlet of the turbine will be really cold. Expanding compressed air through expanders like this is part of how air liquefaction plants operate.
I think you are right about the closed system. When the compressed air spins the dynamo, it will cool, and then absorb heat from the environment. In an ideal, lossless scenario, that heat is equal to the heat given off when it cooled. So maybe you can view it as a battery that stores the energy in the environment?
Would it make sense to compress air, discard the heat generated, and then use the compressed air later to increase the efficiency of a turbine on a combined cycle plant?
I don't know in detail, but they could do something like this: compress the air, making it hot, then run the hot air through a packed bed of small objects (pebbles, bits of iron). As the air goes through the bed the heat is transferred to the material. This ends with a temperature gradient across the bed (hot at one end, cooler at the other). To discharge, the flow of air is reversed: in at the cool end, out at the hot end.
This sort of design (a counterflow heat exchanger, basically) keeps the delta-T at each point low, so the system has low entropy production.
A similar heat exchanger could be designed with a liquid thermal storage medium, where the air and the liquid are flowed past each other in opposite directions (separated by solid walls, most likely). The hot storage tank would be insulated. An advantage of this design is the pressure of the liquid storage tanks needn't be high, unlike the vessel containing the pebbles in the earlier described system.
All these may need to dump some waste heat to the environment as a consequence of inefficiencies in the system.
I should add that there's also some energy stored lifting water. So this is not entirely a thermal store. I didn't run the numbers, but I guess the thermal store stores > energy than is stored lifting water.
I must have missed something. Why not just use the water without the compressed air, i.e., pumped hydro? There must be some advantage, but they didn't seem to say. I'd guess maybe if your lower reservoir were underground, the water-only would require the generator systems to be down there, too, which would mean access for people as well, and being down a mine with a small lake's worth of water overhead seems pretty hazardous. Whereas by forcing air down to push water up, that whole below ground aspect can be almost entirely passive. Maybe?
The short answer is that pumped hydro is mature technology with a pretty wide range of places it can be implemented (at least in Australia[0], which has a mountain range going down the east coast where everyone lives). A-CAES has three advantages, which are in my opinion aren't very fundamental:
1. It takes up less space on the surface than most PHES. This... is almost always a marginal benefit.
2. It doesn't require building a reservoir or dam. These are very well regulated in Australia and elsewhere, and the downsides are known so they are quite slow to get approval.
3. It's a bit quicker at 2.5 - 3.5 years as opposed to 3-7 years.[1] This is a bigger advantage than it looks if you have some tricky renewable energy targets to hit by 2030 (see our 42% emissions reductions target as well as an 82% renewable energy target)
I can't see this gaining traction outside of a few locations in Australia, at least. I wouldn't be surprised if A-CAES is only briefly viable as a result of subsidies and cheap government financing.
> with a pretty wide range of places it can be implemented
How so? Yes, if you dammed up most valleys you could build a lot more capacity than humanity currently has, but that's because we don't really have that much. A factor of two or so might be well within range. But the total amount reasonably buildable simply isn't enough, except for some very local scopes where demand is low in both energy and in other use for the landscape that would be taken over by reservoirs. And that's before you start considering the geological realities required for actually building a dam, you don't just need the geometric shape of a valley, you need the bedrock to brace the dam against and the impermeability of the ground required to not have leakage wash out pathways for ever increasing leakage. Viable sites for pumped hydro are extremely rare and quite a few have actually been given up decades after building the dam, because the geology wasn't quite as cooperative as hoped.
The promise of deep site water head CAES is that you can just throw money at the problem (excavation) and get as much capacity as you want to buy. The price per capacity is higher than that of a low hanging fruit pumped hydro site, but many of those buildable have already been built.
Most viable hydro sites are gone, yes. But you don't need a viable hydro site for pumped hydro. What you need is water at the bottom of a hill. That's common.
If you just have water at the bottom of the hill you'll need to build a giant tank on the top of the hill. The best spots for pumped hydro have a natural depression elevated above a water source, so you can use that instead.
(I agree that this is different from traditional hydro locations, and so there are many good spots still available)
The best spots already have a reservoir up top, or most of one. Pumped hydro is so much cheaper than other long term storage options you can spend a little extra on the reservoir and still come out ahead of competitors.
The sources I've seen point out plenty of places. They may be overoptimistic. But excavation is really expensive, and in a competitive market, you cannot just throw money at the problem. It's the same reason nuclear fission is dead in many markets.
I've thought for a while that old open-cut iron ore pits could be prime spots for 'inside-out' pumped hydro, like that system that some German researchers were working which pumped water out of submerged concrete spheres instead of up a mountain.
If you have abandoned deep shafts, they may be smaller in volume than an open pit, but they have several hundred metres of head, so every kilogram of water is much more effective than in schemes with less head.
It could make a nice option for huge abandoned open cast mines - build a dam across the pit, flood it and then pump water from one side to the other using solar-generated electricity, allowing it to flow back to generate electricity as and when needed.
A big problem is those mines usually leave behind pretty toxic remains so the water you're pumping around is extremely hostile to the people and equipment you're thinking of putting it through/near. Then there's the chance of a dam collapse releasing that toxic water outside the mine.
1. Dams require water. Australia has suffered water supply challenges as long as I've been alive.
2. Dams can be environmental disasters. Both building, maintaining and one day destroying these has a lot of challenges and expenses that we're not great at measuring.
Not having to build and maintain a dam and find an appropriate area to flood is a pretty large upside. Big thing in the US is that we already have a lot of dammed lakes in mountains in some places so we're really talking about making the water level more variable and adding some infrastructure instead of building net new lakes.
Pumped hydro consumes much less water than evaporative cooling of a thermal power plant with the same energy throughput. Pumped hydro doesn't have to be on an existing watercourse, so it doesn't cause the environmental issues that come along with that.
I think you can store energy here not just in the gravitational potential energy of the water, but also in the compression of the air. I _think_ this means that you can get away with a smaller cavern than you could for just pumped hydro.
You might think that you'd need a large amount of water to make the energy release work, but I think it works like this. The force of the water on the air/water interface is dependent not on the reservoir volume, but on the weight of the water in the column (which depends only on the height of the shaft).
By digging a very deep shaft, you can have a very large force of water on the interface, and moving that interface an equal distance hence releases more energy than it would with a shorter shaft.
This way, you can store an arbitrary (up to your ability to compress air to a sufficient pressure, dig a deep shaft, and keep everything from blowing up) amount of energy.
I think if you have a 1 square meter cross section of shaft, and the shaft is 1 kilometer deep, then at the bottom the force of the water above is the weight of 1 square meter * 1 kilometer, or 1000 cubic meters of water, or 1 million kg.
The force then is 9.8×10^6 newtons.
Pressure is force/area, or 9.8×10^6 newtons/square meter here (since we have a unit area).
It says under this pressure, a 1000 cubic meter tank of air at this pressure stores ~5000 KWh.
The one planned in California is supposed to store 4000 MWh, so I guess they have a tank that is ~a million cubic meters.
A water tank of the same size would store ~2700 MWh of energy (e.g., pumping that much water up a 1KM shaft requires that much energy), so it does seem to be more efficient.
It may also be that hot air turbines are cheaper and easier to maintain than hydropower turbines, but I'm not certain.
> The force of the water on the air/water interface is dependent not on the reservoir volume, but on the weight of the water in the column (which depends only on the height of the shaft).
Just wanted to highlight this, since it's the key insight which caused this to "click" for me.
The difference between a 1km shaft and a 1km deep reservoir, when both are used for pumped hydro, is the amount of energy stored. Running a turbine at the bottom of the shaft (to where?!) would drain very quickly. But using air, which has very different properties, enables you to use the water as a "piston" to keep the air at a certain constant pressure underground. And you can store a larger volume of (more compressible) air in a space which is easier to access.
>I think you can store energy here not just in the gravitational potential energy of the water, but also in the compression of the air. I _think_ this means that you can get away with a smaller cavern than you could for just pumped hydro.
+ they are storing the heat extracted from compressing the air.
Compress to store [potential] energy, take the heat generated during compression and use it, decompress at time of need and let the [hot?] atmosphere supply energy to the decompressing gas.
I wouldn’t call it a closed loop, but maybe a lack of deficit.
Is there any pump efficiency advantage to multiple pressure vessels instead of one large?
You could probably move the compressed air in a GPE gravitational potential energy storage system, or haul it up on a winch and set it on a shelf; but would the lateral vectors due to thrust from predictable leakage change the safety liabilities?
Air with extra CO2 is less of an accelerant, but at what concentration of CO2 does the facility need air tanks for hazard procedures?
> "Solar energy can now be stored for up to 18 years [with the Szilard-Chalmers MOST process], say scientists"
> [...] Though, you could do CAES with captured CO2 and it would be less of an accelerant than standard compressed air. How many CO2 fire extinguishers can be filled and shipped off-site per day?
> Can CO2 can be made into QA'd [graphene] air filters for [onsite] [flue] capture?
> FWIU China has the first 100MW CAES plant; and it uses some external energy - not a trompe or geothermal (?) - to help compress air on a FWIU currently ~one-floor facility.
Salt water is a nightmare to include in any sort of mechanical system. It is super corrosive itself, enables galvanic corrosion, and is so “fertile” biologic fouling is a big issue.
The water's not there to directly store gravitational potential energy, it's there to provide a variable sized containment vessel under nearly constant pressure conditions. This has two advantages:
1) You're not losing thermal energy from the main body of air (assuming their thermal recovery at the compressor works well enough) because the change in air pressure only occurs at the compressor.
2) You get constant pressure and constant power output for the entire volume of your compressed air storage, instead of both dropping rapidly as air is released. This gives you far better bang for buck per unit volume of pressurized air.
I haven't done the calcs but I'd guess that if they're bothering doing all this extra stuff the energy stored in a cubic meter of compressed air at these pressures is significantly higher than the energy you'd get from just lowering a cubic meter of water down to the reservoir.
Compressed air and water head are in a balance, my physics instincts strongly suggest that the hypothetical sum is 2x the amount of energy stored by raising the water alone. But just like you I do hope that my instincts are missing something!
Hmm.. the more I think about it the more I agree with your intuition. If the air's at constant pressure then the work it's doing as it exits the chamber is coming directly from the water entering the chamber. There's no difference (in terms of energy) between having a turbine at the bottom of the column of water vs. having the water push the air and then the air spin a turbine. The whole thing seems like they started with "let's do compressed air energy storage" and realised half way through that the best way to do compressed air energy storage is for it to actually be pumped hydro.
It's more capacity than just pumped hydro of identical head x volume: even if we ignore the "compressed spring" of the air involved, charging would require at least the energy for lifting the water plus the energy for heating up their heat source. If you close a valve in the water connection, then pop open the air pressure vessel letting all energy still in the compressed gas you to waste, you'd still have the heat storage, and you could let the water rush into the lower reservoir through a turbine, harvesting the entire energy contained in the water (minus inefficiency). So it's definitely more, the heat and the gas discharge we just wasted.
The question is how much more, whether the symmetry suggested by the balanced state implies 2x energy storage or not.
Another mental model to further separate the two storage media (water lift and air compression) is this: you could charge separately, first raising the water with a pump, with the dry cavity at ambient pressure via an open connection to daylight, then seal the cavity an charge it as a compression vessel in the non-isobaric way. I still can't decide between trusting the "balance implies 2x" intuition or not, but it should be easy enough to calculate. At least the thought experiments show that it can't be 1x the energy stored by lifting water alone.
I consider starting with "let's do compressed air energy storage" as a given, the "do it by pumped hydro" does not come into play because pressure vessels are hard, it comes into play because building compressors and turbines that have good efficiency over a wide range of pressure is hard.
You need to get replacement air to the cavern when you pump the water up, and let the air to make room for the water when it's going down. So you need a separate shaft for airflow anyway. I assume the rest is just economics, it's probably a lot cheaper to have all the moving parts easily accessible on the surface.
Edit: Also the fact, that if the pump was at the very bottom and it failed, you'd need to have an alternative way to clear the cavern of water in order to go down and fix the pump.
Edit: Also the fact, that if the pump was at the very bottom and it failed, you'd need to have an alternative way to clear the cavern of water in order to go down and fix the pump.
Put the pump on a chain and hoist it up with a crane, like they do with pumps in lift stations.
Exactly, they used the wrong kind of TBM to match soil conditions, and land owners are furious over the transmission line easements. Mostly rural property.
I think your answer is the correct one, rather than the replies. By keeping all the machinery at the top, initial build and maintenance costs are cheaper, they can use a cavern of any shape and at any depth, and they only need to use drilling techniques not mining techniques, which will be far cheaper .
I suspect the maximum stored energy for the compressed air solution is significantly higher.
Not only can you store the energy required to lift the water to the surface, but after all the water is lifted you can keep compressing the air in the closed reservoir to store even more energy.
If you only used water you could of course build an open reservoir with more storage capacity instead. But maybe these will be built in areas where that’s not feasible. It’d take much more space on the surface, and you need to deal with evaporation.
I do wonder, regarding the hydrostor approach, if the stored energy (theoretical best, excluding all losses) is 2x the amount that would be stored if it was just the water head (mass x height), with air displacement served by an open connection to the surface, or if there can be more to it:
Constant volume CAES would store energy without any water lift involved, and with water mediated constant pressure CAES the lifted water is added to the amount of energy contained in the "air spring". But that's balanced at equal force in the force x surface area x underground reservoir level system. My bird's eye view understanding suggests that it would come down to 2x the amount of energy stored in either (minus losses, of course) due to the balance. Is that the maximum energy a water-column mediated constant pressure A-CAES could hold, per water displacement volume x head height, or am I missing something? That 2x would surely be an improvement over conventional mineshaft pumped hydro, but it would also define a somewhat sobering limit to the amount of theoretical best case capacity.
Minor but perhaps very relevant detail: afaik, or rather as far as I don't know, hydrostore aims at a shaft depth equivalent to a pressure either right below of where pure CO2 would liquefy, or right above where that happens (no idea about the boiling points of N2, O2 and all those other main components of ambient air). I think the target depth is not quite that deep, as in carefully avoiding the "transliquid" range (as in transonic).
Yes this is normal even in normal residential wells. Boreholes are just a few inches across so obviously nobody ever goes down them!
I have an injector pump that sits at the top of the borehole and has a pipe that pushes water down to pump water up through a second pipe, but it's more common to have a submersible pump down at the bottom of the bore.
I've wondered how well the inject-pump style works! Do you have experiences to relate? All the electronics are up top where you can get at them. Has the mechanism at the bottom ever needed to be serviced? Can it be retrieved easily?
You can pull the parts out of the hole anytime you want to - special equipment is normally used, but a rope and a tripod to hold a pulley over the hole works (might not be safe).
What is at the bottom of the hole is a "injector" which is basically a U shaped pipe and a small jet to push water back up. If the well water is only 25 feet below the ground a pump at the top along works. This jet system gets down to 60 feet. The pump down the hole gets to 600 feet (check the pump specs - many are rated to only 250). After that you need oil well type pumps where the motor is at the top of the hole but the pump is lowered down.
(I can't find any English lit on it, but my Grundfos Ejektorpump (had to go out and look at it to see what it's called; perhaps the correct translation is ejector pump rather than injector pump?) is in a 85m deep borehole and works great. It's 40 years old, heavily used and never serviced and quite a puzzle how it's still going. I have no idea if they can pump from deeper than that)
Two normal plastic water pipes descend down to the bottom of the borehole. At the bottom is a brass u-shaped fitting with a inlet with a non-return valve.
Yeah, I have to wonder how it compares to pumped hydro operationally too. They talk about capital costs but didn't explore what opex would be like at all. I would think with heat, air and water (as opposed to water in pumped hydro), maintenance and operations will be more expensive.
I know of some pumped hydro sites that have been given up due to opex, with no clear change of mind due to changing circumstances in the decarbonisation age yet in sight. What they do have in common, I think, is low head. There's just a lot of water to carry traces of sediment into the mechanism, and so much reservoir ground to keep from leaking for a given amount of capacity.
But you don't arrive at isobaric storage looking at mineshaft pumped hydro wondering if it could be improved upon. You start looking at constant volume (A-)CAES, either in high pressure tanks at/near the surface or in salt caverns, and go from there, what of we did not have to deal with a very wide range of operation pressure.
> The next project would be Willow Rock Energy Storage Center, located near Rosamond in Kern County, California, with a capacity of 500 megawatts and the ability to run at that level for eight hours.
Their California battery will be 4GWh capacity with a $1.5 billion cost, which is $375/kWh. Their Australian one will be 1.6GWh for $415 million USD, working out to be $260/kWh. Both are more expensive than lithium ion, so I wonder what the case is for it.
"VanWalleghem said there is room to push costs down as the company gains experience from these first few plants. The storage systems have a projected lifespan of about 50 years, which is an important data point when comparing it to battery systems, which have much shorter lives"
So both longevity and working towards reduced costs for future plants. I guess someone thinks the long-term costs will end up below Li-ion, and likely lower environmental impact, at least compared to the battery component.
One thing that hasn't been mentioned in siblings yet: the bulk of the money, more if you lean further towards capacity in the capacity vs throughput decision, is in the excavation. This is not only a "forever" investment, it's also money spent on the local economy. This isn't the case at all for battery cost, unless you happen to be the world center of the battery business, all the way from mine to recycling.
Another aspect, completely unrelated and I'm not sure hydrostor already makes that part of the design (but it could totally be introduced later, without invalidating any of the excavation investment): some of the energy stored in an A-CAES system is stored as heat. When you do need heat, for a district heating system, for a swimming pool site or whatever, you can decouple some of the heat from the pressure storage. Worst case some of the joules repurposed end up missing in discharge, but it's also possible that they are simply joules not lost to cycle inefficiency. And if you happen to need cooling (datacenter on site?), at the time of discharge you can just keep some of the stored heat untapped, substituting with energy from the warm end of the coolant cycle that you want to freeload on the A-CAES. Compared to other waste heat/cold coupling schemes, at hydrostor pressure levels you would get considerably higher heat/cold deltas to work with. Huge potential, and with the reservoir shaft having very few site requirements, coupling opportunities should be plentiful (as compared to e.g. opportunities that only ever arise in remote valleys)
How do cycles and lifetime compare? More cycles over long time would lower final unit cost. That is price of kWh at time of release. Which I don't think will ever go down for load shifting.
In the end 3 figures really matter total capacity, power output and cost per "generated" kWh on average over lifetime.
The compressor/turbine part will have predictable wear rates, not substantially different from what you see in fossil plants.
The reservoirs will at some point see noticeable sediment buildup, but not at all comparable with surface pumped hydro based on blocking valleys, due to the cyclic nature of the water flow in the hydrostor facility. And occasional cleanout (measured in decades or centuries?) will be trivially cheap compared to construction cost. Very much unlike cleanout cost behind valley dams, which are would-be cleanout costs because that never ever happens as it would utterly dwarf the cost of the original dam. There's been an article linked here a few months ago (can't find it, unfortunately) about how the total capacity of pumped hydro is getting increasingly smaller each year, despite new sites getting built. This is because sedimentation already outpaces the buildup, and it will only get worse the more we build. The volumes accumulating behind a dam are just too big, unless you have zero natural flow from rainwater (and then just getting the working volume of water to the site would be prohibitively expensive, per capacity, even before you factor in evaporation - the cycle capacity per unit of water is just so much lower than what a hydrostor site would achieve with its much larger head plus the energy stored in air compressionand heat)
Surely to prove and improve the technology? Being able reuse gas technology as the article says, in Australia would be a boon - there’s an enormous CSG industry
The economics of batteries are a function of cycle limits (none in this case, probably) and how much energy you can store and discharge over time and the price difference between charging and discharging. All that minus the upfront installation cost.
The article says this thing should last at least fifty years. There's no good reason it couldn't last longer as it shouldn't really degrade over time. If you assume daily charge/discharge cycles, that would be about 18250K cycles over 50 years. Times 4GWh is about 73TWh of energy sold to the grid at, hopefully, some profit. Of course that all depends on demand, utilization, and whether there are any cheaper ways to store energy. It's probably going to end up some percentage of that. But best case that's energy you buy cheap and sell at a higher price. Even a few cents difference starts adding up to billions pretty quickly. And that's before you consider the alternatives (buying energy on the open market from another provider, investing in more energy generation, etc.).
The prices you cite are just the purchase price. And of course lithium batteries don't last forever. So you'd be writing them off at some point. But in fairness, there are some battery chemistries that are getting quite good cycle times. So, the comparison might become a bit more fair over time.
Are you comparing battery cell cost vs battery pack + structures + electronics + lines + land + installation + different continent + N other things I have no idea about?
BloombergNEF reports a cost of $115 per kWh for turnkey energy storage systems (in China) so their comparison is likely to hold up for complete systems in Australia and the US.
That is not cheap. And we have very high pressure here and not only cave and rock, but technic around it. And pushing air in and letting air out again will have degradation of that expensive equipment.
Yeah, also it takes 3 years to build. I predict that there will be so much more lithium-ion batteries deployed by the time this is finished that it will change the economics of operating it.
I always wondered why compressed air storage systems work against ambient pressure instead of having two tanks at high and higher pressures. This would greatly increase the density of the gas, as well as lowering the temperature differential.
It would take a long time to get it up to initial pressure, as there would be a lot of heat to dissipate, but then it differential mode, the gradient would be much better.
Does having an increase in the density of gas present an advantage over having a higher pressure delta by dumping to ambient for a given volume of compressed-gas storage?
Why would I want one tank at "high" pressure, and another tank at "higher" pressure, when I could just have one tank of "higher" pressure to begin with? Or, better: Two tanks of "higher" pressure in even less space than one of "high" and one of "higher" pressure?
(If the answer is "Because turbines work better with higher densities," then: Do they work more-betterer-enough to make up for the size and complexity?)
The first is fundamental: density is not really helpful for this sort of application. The work done in expanding material (including gas) at a given pressure is P dV, and the work done in moving material across a pressure difference is V dP. Notably, mass doesn’t appear at all here, so adding more mass or density doesn’t add energy storage capacity in and of itself.
You can compute this more explicitly, and, for an ideal gas, the useful energy extractable from a tank of gas at pressure P (under ideal, isothermal conditions) is proportional to the change in log P. So it’s actually rather more important to achieve low pressure than high pressure.
On top of this, there’s a practical consideration: the atmosphere is an effectively infinite source of gas at 1 atm. If you are working between high and higher pressure, you need two reservoirs.
All you’re gaining is less temperature change per unit pressure change, but it’s probably the same amount of heat for any practical purpose, so you still need an intercooler of some sort for good efficiency.
I would have thought this is like the Carnot cycle: the greater the difference across which you're generating energy, the better. So you want the low side at as low a pressure as possible.
Because with the tanks are not infinite size. That means as you release pressure the differential between the two tanks equalizes in the middle. Mean while in the current system the low pressure vessel is effectively infinite and so you have more usable volume to work with. Plus of course you can use both vessels to store energy instead of one.
How do they work around the ”heat problem” with compressed air storage. When you compress the air, it heats up, and actually there's a big part of the energy that get stored in the form of heat, not pressure. When you want the energy out the pressured air cools down during depressurization.
If you were able to keep the air hot the whole time the process is almost symmetrical so that's not an issue, the “heat problem” as I call it is how do you store this heat for an extended period of time? At scale, it's much harder to keep than just the pressurized air.
The prototypes I've seen in the past were not storing the heat, but relied on industrial fatal heat (that was lost anyway) but this also has scale problem as you don't have that much available power except near very specific industries (NPP are an option, as are other heavy industries, but the supply is necessarily limited)
If the energy input is free/renewable they can ignore this to some extent. Yes they lose a lot of energy, but who cares if the wind is blowing/sun is shining making more energy than you need right now - the other option is turning those systems off - either way they cost the same $$$.
The problem is that you need this energy when you want to provide electricity from the storage, which isn't the moment where energy in general is cheap.
The positive aspect of such a system is that the thermal energy you need is not subject to Carnot's law, so the temperature of the heat source doesn't matter unlike most use of thermal energy (and that's why you can use waste thermal energy in the first place) but you still need a way to get that energy.
Yes, but without the thermal energy you cannot depressurize it in a turbine, because it gets so cold the air will start condensing, and then you'll have droplets of nitrogen destroying your turbines!
If the the air is compressed to higher than storage pressure and routed through a heat exchange, they could extract all the compression heat before letting the air into underground storage. The compression heat could simply be kept in liquid storage on the surface. But that's just me guessing.
No mention of how efficient the energy cycle is? Without checking sources, I think I've read that batteries and pumped hydro end up in the 80-90% range round trip? Without knowing what this method produces it's almost pointless to consider.
One advantage this has (I assume) is almost limitless cycle lifespan.
Compressed air energy-storage is notoriously low-efficiency compared to the alternatives.
The idea is that this is a hedge: if it turns out that solar/wind become "too cheap to meter"? Then "efficiency" is meaningless and what matters is cost-per-unit-storage and the hope is that compressed-air will be able to store and output more joules-per-dollar than any other storage method (regardless of how many more joules you had to put in first).
Compressed air is notoriously low-efficiency when you do it like in ye olden days, by venting the compression heat to the environment. A-CAES means capturing that heat, storing it separately and transferring it back into the decompression stream in discharge. Yes, this means that there will be some trickle discharge loss when using the storage scheme for long duration, but heat storage is a happy square-cube law thing, at a certain scale it even become viable for seasonal storage. A-CAES usually claim about 70% round trip efficiency.
But you are right in that this number really isn't all that important in the renewables age: when we are anywhere close to getting to 100% renewables on a point of median supply and median demand, production capacity (conversion capacity from sunshine and air movement) at times when sun coincides with wind will so far outpace demand that any energy sink will do that can still pay a positive rate. We're a long way from fully renewable were I live, and some days I see two thirds of turbines stopped in nice wind. It's really all about capex per W and per Wh. Admittedly, A-CAES isn't necessarily excellent in this right now, but it might scale quite nicely with routine.
Recently there was a gravity storage scheme linked here on hn about lowering mining refuse back into old mines to discharge, and digging it back up to charge, which comes with the curious property that it never really reaches a point of saturated capacity: in theory you could keep digging new tunnels forever when energy surplus keeps coming in.
> the system extracts heat from the air and stores it above ground for reuse. As the air goes underground, it displaces water from the cavern up a shaft into a reservoir.
> When it’s time to discharge energy, the system releases water into the cavern, forcing the air to the surface. The air then mixes with heat that the plant stored when the air was compressing, and this hot, dense air passes through a turbine to make electricity.
By "releases water into the cavern", do they mean simply opening the air valve to let the air (pressured by the water) come back out?
They don't really release water into the cavern, they release air out (through the turbine stages). The water is never really held back. It's a floating counterbalance for keeping the air pressure constant (or mostly constant, considering surface reservoir level differences which would never exceed a tiny fraction of the total water head, unless the facility runs into extreme water shortage)
I couldn't see in the article - is this using natural caverns or did they excavate? I know the pilot versions of this tech generally use old salt-caverns, like the one in Germany.
I'm actually pretty excited about this tech - it seems like solar and wind are getting cheap faster than batteries will be able to meet the needs for grid-scale energy storage, so a cheap-but-inefficient energy storage tech is an exciting prospect. Massively overbuild the solar/wind and use these things to defer the overflow.
This is a good solution (storing in rock) to get around the heat and corrosion (from water) that above-ground tanks go through when storing compressed air.
How does the air "push the water up"? What mechanism prevents the air from simply bubbling up through the water column? I'm assuming some kind of valve or piston, but would be interested to see what it looks like.
Put the air inlet/outlet at the top of the cavern, and the water inlet/outlet at the bottom. Then just stop pumping air while the water level in the cavern is still above the water outlet. A little water has to remain in the cavern at all times.
To build a trap, the pipe would have to go almost all the way up to the reservoir, then down, and then up again to connect to the reservoir: https://en.wikipedia.org/wiki/Trap_(plumbing)
But there might another way to avoid bubbling which I have missed. Maybe the compressed air is under so much pressure that it becomes denser than the water?
It doesn't need to be denser than the water because the incoming air is always above the water.
Imagine an cavern shaped like the great pyramid of Giza, full of water. They dig two pipes, an air entry pipe to the point at the top, and a water exit pipe to one of the corners at the base. Air pumped in at the top pipe will almost evacuate the cavern, before the water level drops to the point where air could get to the exit pipe.
What this means is that depending on the shape of the cavern, they may not be able to utilise its whole volume. In the worst case, if its roof were entirely flat, they would not be able to use any. They can use the volume of a section, bounded by horizontal planes, from the air entry pipe either down as far as the exit pipe, or (if the roof dips down between the two) down as far as the lowest point on the highest possible path between the two.
That's assuming totally vertical pipes. I know they can curve them a bit, but I don't know if they can curve them enough to enter a cavern from below. If they could, then they can utilise the entire volume, as long as they can identify the lowest and highest points
I found another technique quite fascinating.. When electricity is surplus, spin large disk-shaped rocks levitated by magnets in a vacuumed enclosure. Use this spinning motion to create electricity when required.
What happens when you get a leak in a random seal in the chamber deep under ground, does all the compressed air escape? How do you ensure such a large reservoir stays air-tight for 20 years?
A company called Energy Vault[0] is (was?) working on this. I think it is relatively capital intensive compared to what the company in the article is doing. Of course storing underground requires particular geology.
I also remember reading about a system that moved dirt/rock up a mountain on a train. Can’t find a link, but that also seems capital intensive and requires different geology.
There is also pumped hydro storage that works via gravity. That’s been around a while. My dad worked as an engineer on one in the 80s [1].
What a compressed gas represents is not stored energy, but stored (negative) entropy. It is a resource that allows low grade heat to be converted to work at high efficiency. This is what's to happen in this facility: the heat of compression is separated out and stored, then used to reheat the compressed air at discharge time. The energy is actually being stored in that thermal store.
But there are other ways to do this that don't involve compressed air storage. Instead, after the heat of compression is removed and stored the compressed air could be reexpanded, recovering some of the work. This would leave the gas much colder than when it started. This cold could be stored (heating the gas back to its initial temperature) and the gas sent around again. To discharge, the temperature difference between the hot and cold stores could be exploited.
This is called "pumped thermal storage". I believe Google/Alphabet has/had a group looking at this (called Malta). It has no geographical limitations.
(*) Highly compressed air will store some energy because the molecules become so crowded some energy is stored in intermolecular repulsion, but that should be a small effect in this system.
Compressing air absolutely does store energy. It takes work to compress it, and work will be done when it is allowed to expand.
Yes, one way of measuring the stored energy is by the resulting increased temperature. Compressing air necessarily raises its temperature. And then if you want to you can transfer that heat elsewhere, go ahead. That's how air conditioners and refrigerators work, after all.
But in the basic case, it seems entirely accurate to say that compressing air does actually store energy. Just like raising a heavy object against gravity stores energy.
Obviously if you let the compressed air cool, then you're letting the energy dissipate. So if you want to preserve all the energy, don't do that.
The oh-so-clever trick is to transfer heat elsewhere while you're compressing the gas, so you actually reduce back-loading on the piston instead of having it 'fight' the temperature rise. This can be accomplished via water spray, or by compressing a gas bubble that's surrounded by water (eg in the trompe).
By continuously removing heat as you compress the gas, it effectively acts like a train of compressors and intercoolers with an infinite number of stages, ie true isothermal compression. No magic, just physics.
https://news.ycombinator.com/item?id=40278128
This is unlike traditional CAES, where the air is reheated by burning a fuel. There, fuel use is minimized by isothermal compression.
It seems like either this is describing an additional energy storage/extraction method that’s possible with these systems. Either to improve the efficiency or something that’s actually far more efficient than utilizing kinetic energy compressed air can create.
It’s kind of like saying the water in a hydropower plant doesn’t store energy. The turbines aren’t extracting energy from the water itself. They’re extracting energy from gravity acting on water. It doesn’t matter if the water is nearly freezing or nearly boiling, it has roughly the same amount of hydropower available.
This is not the case in a CAES system. The internal energy of air, which is the kinetic energy of its molecules, is independent of pressure. It's only a function of temperature. The internal energy of the air is unchanged by a process that increases its pressure without increasing temperature. So, no energy has been stored in the air.
What has been done is reduction of the entropy of the air. So, it is now possible to convert some of that internal energy (which was always there) to work by expanding the air. The air would end up even colder than the initial air.
https://www.pvh2o.com/faq
https://www.youtube.com/watch?v=50fJ8Av_g7Q
https://www.youtube.com/watch?v=uvf0lD5xzH0
https://news.ycombinator.com/item?id=27066295
Sad to see people are misinterpreting the cleverness of isothermal compression, and thinking it's "just" thermal storage.
How the conditions are reached makes a big difference in efficiency.
I'm baffled at why you focus on minor implementation details like internal energy at a particular snapshot in time, as opposed to highly salient metrics like overall round-trip system efficiency.
What isothermal CAES (with heat storage) would require is a larger storage volume per unit of energy storage capacity, since less heat is generated during the ideal compression. That heat generation is not inefficiency, it's something you want!
Only if the heat is discarded and you reheat with fuel do you benefit from isothermal compression.
The effect the prior poster was discussing is most apparent at large scales when someone is doing actual efficiency analysis - compressed air is not a very efficient way of storing or transporting energy in most contexts. No one would use compressed air for a power grid, because it would quickly be apparent how terrible it is.
It is however a great way to transfer and use energy in many industrial contexts, as turbines and pistons using high pressure compressed air can be very compact while also being very powerful, and have built in cooling.
At the level of typical household use, the efficiency effects aren’t notable. No one is going to care or even notice if it cost 1/2 cent of electricity (at the compressor) to use that air impact wrench vs 1/10th of a cent to use an electric one. Especially when the electric one is heavier and has less torque.
http://www.eseslab.com/ESsensePages/CAES-page
do you know how they do it?
This sort of design (a counterflow heat exchanger, basically) keeps the delta-T at each point low, so the system has low entropy production.
A similar heat exchanger could be designed with a liquid thermal storage medium, where the air and the liquid are flowed past each other in opposite directions (separated by solid walls, most likely). The hot storage tank would be insulated. An advantage of this design is the pressure of the liquid storage tanks needn't be high, unlike the vessel containing the pebbles in the earlier described system.
All these may need to dump some waste heat to the environment as a consequence of inefficiencies in the system.
1. It takes up less space on the surface than most PHES. This... is almost always a marginal benefit.
2. It doesn't require building a reservoir or dam. These are very well regulated in Australia and elsewhere, and the downsides are known so they are quite slow to get approval.
3. It's a bit quicker at 2.5 - 3.5 years as opposed to 3-7 years.[1] This is a bigger advantage than it looks if you have some tricky renewable energy targets to hit by 2030 (see our 42% emissions reductions target as well as an 82% renewable energy target)
I can't see this gaining traction outside of a few locations in Australia, at least. I wouldn't be surprised if A-CAES is only briefly viable as a result of subsidies and cheap government financing.
[0]: https://re100.anu.edu.au/#share=g-fa5a20c9c63f6ed6343a7e7573...
[1]: https://www.csiro.au/-/media/Do-Business/Files/Futures/23-00...
How so? Yes, if you dammed up most valleys you could build a lot more capacity than humanity currently has, but that's because we don't really have that much. A factor of two or so might be well within range. But the total amount reasonably buildable simply isn't enough, except for some very local scopes where demand is low in both energy and in other use for the landscape that would be taken over by reservoirs. And that's before you start considering the geological realities required for actually building a dam, you don't just need the geometric shape of a valley, you need the bedrock to brace the dam against and the impermeability of the ground required to not have leakage wash out pathways for ever increasing leakage. Viable sites for pumped hydro are extremely rare and quite a few have actually been given up decades after building the dam, because the geology wasn't quite as cooperative as hoped.
The promise of deep site water head CAES is that you can just throw money at the problem (excavation) and get as much capacity as you want to buy. The price per capacity is higher than that of a low hanging fruit pumped hydro site, but many of those buildable have already been built.
(I agree that this is different from traditional hydro locations, and so there are many good spots still available)
50 year old example: https://en.wikipedia.org/wiki/Ludington_Pumped_Storage_Power...
https://www.whitepinepumpedstorage.com/
For example: https://www.news.com.au/technology/environment/abandoned-min...
1. Dams require water. Australia has suffered water supply challenges as long as I've been alive.
2. Dams can be environmental disasters. Both building, maintaining and one day destroying these has a lot of challenges and expenses that we're not great at measuring.
You might think that you'd need a large amount of water to make the energy release work, but I think it works like this. The force of the water on the air/water interface is dependent not on the reservoir volume, but on the weight of the water in the column (which depends only on the height of the shaft).
By digging a very deep shaft, you can have a very large force of water on the interface, and moving that interface an equal distance hence releases more energy than it would with a shorter shaft.
This way, you can store an arbitrary (up to your ability to compress air to a sufficient pressure, dig a deep shaft, and keep everything from blowing up) amount of energy.
I think if you have a 1 square meter cross section of shaft, and the shaft is 1 kilometer deep, then at the bottom the force of the water above is the weight of 1 square meter * 1 kilometer, or 1000 cubic meters of water, or 1 million kg.
The force then is 9.8×10^6 newtons.
Pressure is force/area, or 9.8×10^6 newtons/square meter here (since we have a unit area).
There's a formula for the energy in compressed air, it's... involved. I downloaded an excel file from here: https://ehs.berkeley.edu/publications/calculating-stored-ene...
It says under this pressure, a 1000 cubic meter tank of air at this pressure stores ~5000 KWh.
The one planned in California is supposed to store 4000 MWh, so I guess they have a tank that is ~a million cubic meters.
A water tank of the same size would store ~2700 MWh of energy (e.g., pumping that much water up a 1KM shaft requires that much energy), so it does seem to be more efficient.
It may also be that hot air turbines are cheaper and easier to maintain than hydropower turbines, but I'm not certain.
Just wanted to highlight this, since it's the key insight which caused this to "click" for me.
The difference between a 1km shaft and a 1km deep reservoir, when both are used for pumped hydro, is the amount of energy stored. Running a turbine at the bottom of the shaft (to where?!) would drain very quickly. But using air, which has very different properties, enables you to use the water as a "piston" to keep the air at a certain constant pressure underground. And you can store a larger volume of (more compressible) air in a space which is easier to access.
+ they are storing the heat extracted from compressing the air.
Compress to store [potential] energy, take the heat generated during compression and use it, decompress at time of need and let the [hot?] atmosphere supply energy to the decompressing gas.
I wouldn’t call it a closed loop, but maybe a lack of deficit.
You could probably move the compressed air in a GPE gravitational potential energy storage system, or haul it up on a winch and set it on a shelf; but would the lateral vectors due to thrust from predictable leakage change the safety liabilities?
Air with extra CO2 is less of an accelerant, but at what concentration of CO2 does the facility need air tanks for hazard procedures?
FWIU, you can also get energy from a CO2 gradient: "Proof-of-concept nanogenerator turns CO₂ into sustainable power" (2024) https://news.ycombinator.com/item?id=40079784
And, CO2 + Lignin => Better than plastic; "CO2 and Lignin-Based Sustainable Polymers with Closed-Loop Chemical Recycling" (2024) https://news.ycombinator.com/item?id=40079540
From "Oxxcu, converting CO₂ into fuels, chemicals and plastics" https://news.ycombinator.com/item?id=39111825 :
> "Solar energy can now be stored for up to 18 years [with the Szilard-Chalmers MOST process], say scientists"
> [...] Though, you could do CAES with captured CO2 and it would be less of an accelerant than standard compressed air. How many CO2 fire extinguishers can be filled and shipped off-site per day?
> Can CO2 can be made into QA'd [graphene] air filters for [onsite] [flue] capture?
"Geothermal may beat batteries for energy storage" (2022) https://news.ycombinator.com/item?id=33288586 :
> FWIU China has the first 100MW CAES plant; and it uses some external energy - not a trompe or geothermal (?) - to help compress air on a FWIU currently ~one-floor facility.
Also, 1000m3 in air is just a cube of 10m. It seems like a good illustration of the convenience of air vs water.
1) You're not losing thermal energy from the main body of air (assuming their thermal recovery at the compressor works well enough) because the change in air pressure only occurs at the compressor.
2) You get constant pressure and constant power output for the entire volume of your compressed air storage, instead of both dropping rapidly as air is released. This gives you far better bang for buck per unit volume of pressurized air.
I haven't done the calcs but I'd guess that if they're bothering doing all this extra stuff the energy stored in a cubic meter of compressed air at these pressures is significantly higher than the energy you'd get from just lowering a cubic meter of water down to the reservoir.
(see nephew comment https://news.ycombinator.com/item?id=40273030 )
The question is how much more, whether the symmetry suggested by the balanced state implies 2x energy storage or not.
Another mental model to further separate the two storage media (water lift and air compression) is this: you could charge separately, first raising the water with a pump, with the dry cavity at ambient pressure via an open connection to daylight, then seal the cavity an charge it as a compression vessel in the non-isobaric way. I still can't decide between trusting the "balance implies 2x" intuition or not, but it should be easy enough to calculate. At least the thought experiments show that it can't be 1x the energy stored by lifting water alone.
I consider starting with "let's do compressed air energy storage" as a given, the "do it by pumped hydro" does not come into play because pressure vessels are hard, it comes into play because building compressors and turbines that have good efficiency over a wide range of pressure is hard.
Edit: Also the fact, that if the pump was at the very bottom and it failed, you'd need to have an alternative way to clear the cavern of water in order to go down and fix the pump.
Put the pump on a chain and hoist it up with a crane, like they do with pumps in lift stations.
Pumped hydro done on a large scale needs a reservoir that is at a considerable elevation gain.
example: https://en.wikipedia.org/wiki/Taum_Sauk_Hydroelectric_Power_...
Not only can you store the energy required to lift the water to the surface, but after all the water is lifted you can keep compressing the air in the closed reservoir to store even more energy.
If you only used water you could of course build an open reservoir with more storage capacity instead. But maybe these will be built in areas where that’s not feasible. It’d take much more space on the surface, and you need to deal with evaporation.
Constant volume CAES would store energy without any water lift involved, and with water mediated constant pressure CAES the lifted water is added to the amount of energy contained in the "air spring". But that's balanced at equal force in the force x surface area x underground reservoir level system. My bird's eye view understanding suggests that it would come down to 2x the amount of energy stored in either (minus losses, of course) due to the balance. Is that the maximum energy a water-column mediated constant pressure A-CAES could hold, per water displacement volume x head height, or am I missing something? That 2x would surely be an improvement over conventional mineshaft pumped hydro, but it would also define a somewhat sobering limit to the amount of theoretical best case capacity.
Minor but perhaps very relevant detail: afaik, or rather as far as I don't know, hydrostore aims at a shaft depth equivalent to a pressure either right below of where pure CO2 would liquefy, or right above where that happens (no idea about the boiling points of N2, O2 and all those other main components of ambient air). I think the target depth is not quite that deep, as in carefully avoiding the "transliquid" range (as in transonic).
I have an injector pump that sits at the top of the borehole and has a pipe that pushes water down to pump water up through a second pipe, but it's more common to have a submersible pump down at the bottom of the bore.
What is at the bottom of the hole is a "injector" which is basically a U shaped pipe and a small jet to push water back up. If the well water is only 25 feet below the ground a pump at the top along works. This jet system gets down to 60 feet. The pump down the hole gets to 600 feet (check the pump specs - many are rated to only 250). After that you need oil well type pumps where the motor is at the top of the hole but the pump is lowered down.
https://www.vvsbutiken.nu/product.html/ejektorer-grundfos?ca...
Two normal plastic water pipes descend down to the bottom of the borehole. At the bottom is a brass u-shaped fitting with a inlet with a non-return valve.
In contrast, there is underground everywhere on Earth.
But you don't arrive at isobaric storage looking at mineshaft pumped hydro wondering if it could be improved upon. You start looking at constant volume (A-)CAES, either in high pressure tanks at/near the surface or in salt caverns, and go from there, what of we did not have to deal with a very wide range of operation pressure.
Their California battery will be 4GWh capacity with a $1.5 billion cost, which is $375/kWh. Their Australian one will be 1.6GWh for $415 million USD, working out to be $260/kWh. Both are more expensive than lithium ion, so I wonder what the case is for it.
So both longevity and working towards reduced costs for future plants. I guess someone thinks the long-term costs will end up below Li-ion, and likely lower environmental impact, at least compared to the battery component.
Another aspect, completely unrelated and I'm not sure hydrostor already makes that part of the design (but it could totally be introduced later, without invalidating any of the excavation investment): some of the energy stored in an A-CAES system is stored as heat. When you do need heat, for a district heating system, for a swimming pool site or whatever, you can decouple some of the heat from the pressure storage. Worst case some of the joules repurposed end up missing in discharge, but it's also possible that they are simply joules not lost to cycle inefficiency. And if you happen to need cooling (datacenter on site?), at the time of discharge you can just keep some of the stored heat untapped, substituting with energy from the warm end of the coolant cycle that you want to freeload on the A-CAES. Compared to other waste heat/cold coupling schemes, at hydrostor pressure levels you would get considerably higher heat/cold deltas to work with. Huge potential, and with the reservoir shaft having very few site requirements, coupling opportunities should be plentiful (as compared to e.g. opportunities that only ever arise in remote valleys)
In the end 3 figures really matter total capacity, power output and cost per "generated" kWh on average over lifetime.
The reservoirs will at some point see noticeable sediment buildup, but not at all comparable with surface pumped hydro based on blocking valleys, due to the cyclic nature of the water flow in the hydrostor facility. And occasional cleanout (measured in decades or centuries?) will be trivially cheap compared to construction cost. Very much unlike cleanout cost behind valley dams, which are would-be cleanout costs because that never ever happens as it would utterly dwarf the cost of the original dam. There's been an article linked here a few months ago (can't find it, unfortunately) about how the total capacity of pumped hydro is getting increasingly smaller each year, despite new sites getting built. This is because sedimentation already outpaces the buildup, and it will only get worse the more we build. The volumes accumulating behind a dam are just too big, unless you have zero natural flow from rainwater (and then just getting the working volume of water to the site would be prohibitively expensive, per capacity, even before you factor in evaporation - the cycle capacity per unit of water is just so much lower than what a hydrostor site would achieve with its much larger head plus the energy stored in air compressionand heat)
The article says this thing should last at least fifty years. There's no good reason it couldn't last longer as it shouldn't really degrade over time. If you assume daily charge/discharge cycles, that would be about 18250K cycles over 50 years. Times 4GWh is about 73TWh of energy sold to the grid at, hopefully, some profit. Of course that all depends on demand, utilization, and whether there are any cheaper ways to store energy. It's probably going to end up some percentage of that. But best case that's energy you buy cheap and sell at a higher price. Even a few cents difference starts adding up to billions pretty quickly. And that's before you consider the alternatives (buying energy on the open market from another provider, investing in more energy generation, etc.).
The prices you cite are just the purchase price. And of course lithium batteries don't last forever. So you'd be writing them off at some point. But in fairness, there are some battery chemistries that are getting quite good cycle times. So, the comparison might become a bit more fair over time.
A battery UPS under my desk doesn’t really affect my rent or mortgage. Buildings not only need to get built they also need to be maintained.
And how long do lithium batteries last?
Are you comparing battery cell cost vs battery pack + structures + electronics + lines + land + installation + different continent + N other things I have no idea about?
https://about.bnef.com/blog/global-energy-storage-market-rec...
That there is a finite supply of Lithium available on Earth?
1. no degradation
2. cheap to expand? - simply expand the cave
That is not cheap. And we have very high pressure here and not only cave and rock, but technic around it. And pushing air in and letting air out again will have degradation of that expensive equipment.
It would take a long time to get it up to initial pressure, as there would be a lot of heat to dissipate, but then it differential mode, the gradient would be much better.
Does having an increase in the density of gas present an advantage over having a higher pressure delta by dumping to ambient for a given volume of compressed-gas storage?
Why would I want one tank at "high" pressure, and another tank at "higher" pressure, when I could just have one tank of "higher" pressure to begin with? Or, better: Two tanks of "higher" pressure in even less space than one of "high" and one of "higher" pressure?
(If the answer is "Because turbines work better with higher densities," then: Do they work more-betterer-enough to make up for the size and complexity?)
The first is fundamental: density is not really helpful for this sort of application. The work done in expanding material (including gas) at a given pressure is P dV, and the work done in moving material across a pressure difference is V dP. Notably, mass doesn’t appear at all here, so adding more mass or density doesn’t add energy storage capacity in and of itself.
You can compute this more explicitly, and, for an ideal gas, the useful energy extractable from a tank of gas at pressure P (under ideal, isothermal conditions) is proportional to the change in log P. So it’s actually rather more important to achieve low pressure than high pressure.
On top of this, there’s a practical consideration: the atmosphere is an effectively infinite source of gas at 1 atm. If you are working between high and higher pressure, you need two reservoirs.
All you’re gaining is less temperature change per unit pressure change, but it’s probably the same amount of heat for any practical purpose, so you still need an intercooler of some sort for good efficiency.
If you were able to keep the air hot the whole time the process is almost symmetrical so that's not an issue, the “heat problem” as I call it is how do you store this heat for an extended period of time? At scale, it's much harder to keep than just the pressurized air.
The prototypes I've seen in the past were not storing the heat, but relied on industrial fatal heat (that was lost anyway) but this also has scale problem as you don't have that much available power except near very specific industries (NPP are an option, as are other heavy industries, but the supply is necessarily limited)
The positive aspect of such a system is that the thermal energy you need is not subject to Carnot's law, so the temperature of the heat source doesn't matter unlike most use of thermal energy (and that's why you can use waste thermal energy in the first place) but you still need a way to get that energy.
One advantage this has (I assume) is almost limitless cycle lifespan.
The idea is that this is a hedge: if it turns out that solar/wind become "too cheap to meter"? Then "efficiency" is meaningless and what matters is cost-per-unit-storage and the hope is that compressed-air will be able to store and output more joules-per-dollar than any other storage method (regardless of how many more joules you had to put in first).
But you are right in that this number really isn't all that important in the renewables age: when we are anywhere close to getting to 100% renewables on a point of median supply and median demand, production capacity (conversion capacity from sunshine and air movement) at times when sun coincides with wind will so far outpace demand that any energy sink will do that can still pay a positive rate. We're a long way from fully renewable were I live, and some days I see two thirds of turbines stopped in nice wind. It's really all about capex per W and per Wh. Admittedly, A-CAES isn't necessarily excellent in this right now, but it might scale quite nicely with routine.
Recently there was a gravity storage scheme linked here on hn about lowering mining refuse back into old mines to discharge, and digging it back up to charge, which comes with the curious property that it never really reaches a point of saturated capacity: in theory you could keep digging new tunnels forever when energy surplus keeps coming in.
> When it’s time to discharge energy, the system releases water into the cavern, forcing the air to the surface. The air then mixes with heat that the plant stored when the air was compressing, and this hot, dense air passes through a turbine to make electricity.
By "releases water into the cavern", do they mean simply opening the air valve to let the air (pressured by the water) come back out?
I'm actually pretty excited about this tech - it seems like solar and wind are getting cheap faster than batteries will be able to meet the needs for grid-scale energy storage, so a cheap-but-inefficient energy storage tech is an exciting prospect. Massively overbuild the solar/wind and use these things to defer the overflow.
They probably won't even have a Thunderdome. =(
Nothing personal to the commenter I'm replying to, but I love watching HN re-engineer long existing basic systems.
But there might another way to avoid bubbling which I have missed. Maybe the compressed air is under so much pressure that it becomes denser than the water?
Imagine an cavern shaped like the great pyramid of Giza, full of water. They dig two pipes, an air entry pipe to the point at the top, and a water exit pipe to one of the corners at the base. Air pumped in at the top pipe will almost evacuate the cavern, before the water level drops to the point where air could get to the exit pipe.
What this means is that depending on the shape of the cavern, they may not be able to utilise its whole volume. In the worst case, if its roof were entirely flat, they would not be able to use any. They can use the volume of a section, bounded by horizontal planes, from the air entry pipe either down as far as the exit pipe, or (if the roof dips down between the two) down as far as the lowest point on the highest possible path between the two.
That's assuming totally vertical pipes. I know they can curve them a bit, but I don't know if they can curve them enough to enter a cavern from below. If they could, then they can utilise the entire volume, as long as they can identify the lowest and highest points
https://en.m.wikipedia.org/wiki/Flywheel_storage_power_syste...
I also remember reading about a system that moved dirt/rock up a mountain on a train. Can’t find a link, but that also seems capital intensive and requires different geology.
There is also pumped hydro storage that works via gravity. That’s been around a while. My dad worked as an engineer on one in the 80s [1].
[0] https://www.energyvault.com/ [1] https://en.m.wikipedia.org/wiki/Helms_Pumped_Storage_Plant