AirBattery- Seems pretty dodgy to me.

[tszaboo]

Honestly, efficiency of the energy storage system doesn't matter. Really, it is a bold claim, but I have reasons to say that.
What matters is the investment cost of (solar generation+ wind generation+ energy storage) / stored energy
If you have a system, that only has 60% storage efficiency (compared to a 90% system), you can easily offset the losses by installing 50% more solar/wind. Solar and wind is cheap, so what you need is cheap storage. Cheap way to store energy means large energy density, like natural gas. We can store months worth of natural gas for entire countries. Plus one way of the conversion can happen in existing infrastructure, as you can just feed it into the pipelines and burn it in the existing power plant. Or someone house boiler for heating.
Plus you don't need to retrofit houses to use electricity for heating the radiators, boilers.
Power to gas has just so many benefits...

[SiliconWizard]

I agree with the storage efficiency not mattering all that much, as long as it still provides more net energy than without. (Since it's made to store energy that we otherwise would "lose".)
Now, a relatively low efficiency would be OK - the bad thing is bold and false claims, that doesn't bode well for the company promoting this. And, of course, we have to consider the cost of maintenance. If it happens to be on the high side, then it could eventually be a net loss over the whole operating life.

[gnuarm]

This is now the worst possible situation as there is no means to recover that heat. It will dissipate into the surrounding ground and get lost.

Presumably the heat capacity of the water is so much higher than the air that the water temperature won't go up much and everything stays near ambient.

The water is sprayed into the air chamber so why wouldn't it absorb the heat?   The efficiency of this process will depend on how closely they can maintain an ideal isotherm.  If there is a nearby source of waste heat, they might even be able to improve it further by warming the water after the compression is complete.

I had an incomplete picture of the system before, that I have now appreciated.

Broadly speaking, when you put energy into the system by pumping the water and compressing the air, you want to be able to get as much as possible of this energy back again when you reverse the process. This is called the power-to-power efficiency, and you want to get as close to 100% as you can achieve.

Now, the energy put into the system gets distributed into two places: (1) pressure energy of the compressed air, and (2) thermal energy (heat) resulting from compressing the gas. If you don't do anything to recover it, the heat part is energy that would be permanently lost.

Not quiet correct.  The heat is removed intentionally.  This way the PV curve is isothermal.  If you don't remove the heat as it is generated the PV curve is different.  A larger portion of the work input will go into the compression.  If you leave the temperature high during compression, ultimately the pressure drops as the temperature drops and you lose work stored in the PV curve.

Apparently, in this system the heat of compression is to be absorbed by the surrounding ground, and when the system reverses the surrounding ground puts the heat back into the air as it cools down on expansion. In traditional compressed air systems extra heat has to be provided to compensate for the air cooling down, but here they are avoiding the need for that by using the ground heat to do it. (There is some analogy with ground storage heat pump systems.)

Surrounding ground, water, whatever.  The point is the universe is a heat sink and the heat of compression is removed during compression yielding an isothermal PV curve.

What is important as well is that the expansion be isothermal which happens with the heat sink providing energy which ends up compensating perfectly with the heat lost in the isothermal compression curve. 

If the compression is adiabatic, you end up at a lower pressure (after cooling) than in the isothermal case and the resulting work recovered is less. 

So, conceptually, the system can work. Can it work in practice? That would seem to be an engineering design problem, and I cannot say. I do know that ground storage heat pump systems use a system of buried pipes to improve the heat transfer.

I find it amusing that there is still much doubt.  The system is amazingly simple once it is understood.  I can't find any practicalities that would make it a problem.  I'm assuming they have the material issues well in hand.  Considering man has been burying tanks in the ground for a very large number of years I expect there won't be any surprises in that.  The rest of the system is not new technology unless they decide to use some new turbine or pump to get another two percent efficiency improvement. 

A ground source heat pump is nothing like this in that it uses a one way flow of heat and simply uses the ground as a heat sink.  There is no cycle of heat input and output, so not really relevant.  The advantage in the ground source heat pump over using an air heat sink is simply that it uses less energy to transfer the heat (lower thermal resistance) and the ground temperature is much more stable.  An air system gets a double blow from temperature extremes in that more heat is required to be moved and more work is required to move that heat against a larger temperature differential. 

[PlainName]

The advantage in the ground source heat pump over using an air heat sink

Slightly off topic, but... can't the ground source heat pump wear out? That is, the ground gets colder and colder as the heat is sucked out? With an air source, there's always a bit more air around the corner even if the temperature varies a bit (usually, what goes up must come down).

[gnuarm]

I agree with the storage efficiency not mattering all that much, as long as it still provides more net energy than without. (Since it's made to store energy that we otherwise would "lose".)
Now, a relatively low efficiency would be OK - the bad thing is bold and false claims, that doesn't bode well for the company promoting this. And, of course, we have to consider the cost of maintenance. If it happens to be on the high side, then it could eventually be a net loss over the whole operating life.

Efficiency does matter, the only question is to what extent.  To compensate for a 10% loss in the storage, the entire system must be 10% larger, generation and storage, to achieve a given amount of energy to be delivered when needed.  So clearly this efficiency is not unimportant.  It just has diminishing returns as 100% is approached.  Running a storage system with 50% energy efficiency would require double the capital and land, etc as a system very close to 100% efficient.  90% is a very good number, 80% is acceptable.  Even batteries are not much better than 80%.

I'm still surprised there are questions of efficiency or maintenance costs.  This is not really different from pumped hydro storage.  The only question I've seen is about the efficiency over the operating pressure range.  We don't know what that range is so it's hard to evaluate efficiency over the range even if we had perfect knowledge of the electrical and mechanical components. 

It would be interesting to see more details on this system.

[gnuarm]

The advantage in the ground source heat pump over using an air heat sink

Slightly off topic, but... can't the ground source heat pump wear out? That is, the ground gets colder and colder as the heat is sucked out? With an air source, there's always a bit more air around the corner even if the temperature varies a bit (usually, what goes up must come down).

"Wear out" is an odd term for this, but not if it is sized correctly.  Heat flows in the ground just like anywhere else.  The ground immediately by the coils will be the major impedance to heat flow, but as you radiate out from the coils the flow spreads out effectively reducing the impedance.  So the amount of heat that can be sunk this way is quite large and is more a matter of flow rate than a total quantity. 

I believe it was here that someone discussed a storage system that collected solar directly as heat and stored it in the ground using vertical pipes.  I can't remember any details really, but it was in a housing community with the collectors on the garages to maintain an appearance to the street and a community storage facility.  The idea was to supplement the home heating system by providing warm air/water to the house rather than having to deal with the temperature outside.  This was in a northern clime so cooling was less important, Canada I believe.  The analysis of this system showed how the heat was conducted very well within the arrangement of pipes.  Hmmm, I may be arguing with myself.  They were storing the heat in the ground, so at some point it became an insulator.  I think this was because the area was large enough that the losses around the periphery were not so large compared to the heat stored in the bulk.  This thing stored energy through most of the winter!  I would also point out that water/air based heat systems are pretty effective and inexpensive.  That's the reason why this system was practical. 

[bdunham7]

I find it amusing that there is still much doubt.  The system is amazingly simple once it is understood.  I can't find any practicalities that would make it a problem.

Well, we were only discussing efficiency, but since you asked, practicalities would include energy density and capital cost.  Unless I screwed up the calculations, if you assume an tank pressurized with air at 10 bar and then pumped up with water to 3X or 30 bar, you get an energy density of about 3 kWh/m³.  So you need to build a corrosion-proof (interior and exterior) tank with a working pressure of 450psi--how much do those cost per cubic meter?  So you have your tanks, lets say they are a bit over meter in diameter and 30 meters long, which is a good way to build a high-pressure tank.  You can now store about as much energy as an EV battery.  Maybe I goofed somewhere.

[IanB]

I had an incomplete picture of the system before, that I have now appreciated.

Broadly speaking, when you put energy into the system by pumping the water and compressing the air, you want to be able to get as much as possible of this energy back again when you reverse the process. This is called the power-to-power efficiency, and you want to get as close to 100% as you can achieve.

Now, the energy put into the system gets distributed into two places: (1) pressure energy of the compressed air, and (2) thermal energy (heat) resulting from compressing the gas. If you don't do anything to recover it, the heat part is energy that would be permanently lost.

Not quiet correct.  The heat is removed intentionally.  This way the PV curve is isothermal.  If you don't remove the heat as it is generated the PV curve is different.  A larger portion of the work input will go into the compression.  If you leave the temperature high during compression, ultimately the pressure drops as the temperature drops and you lose work stored in the PV curve.

I'm not sure what part you think is not correct here. To maintain isothermal conditions, heat has to flow from the gas into the surroundings (tank walls, soil, water, whatever). Whether intentional or unintentional, the heat still flows.

Now if we treat air as an ideal gas (which under these conditions it practically is), then the enthalpy of the gas is independent of the pressure. That means the heat that flows into the surroundings did not come from the gas itself, it came from the work of compression. Some of the work done pumping water into the system is lost as heat into the surroundings. Isothermal compression (and expansion) can only be achieved by heat transfer.

The interesting bit for a complete compression/decompression cycle is if you can get that heat back again. Because when a gas expands it cools down, and to maintain isothermal expansion you have to put heat back into it. Which is the next paragraph...

Apparently, in this system the heat of compression is to be absorbed by the surrounding ground, and when the system reverses the surrounding ground puts the heat back into the air as it cools down on expansion. In traditional compressed air systems extra heat has to be provided to compensate for the air cooling down, but here they are avoiding the need for that by using the ground heat to do it. (There is some analogy with ground storage heat pump systems.)

Surrounding ground, water, whatever.  The point is the universe is a heat sink and the heat of compression is removed during compression yielding an isothermal PV curve.

What is important as well is that the expansion be isothermal which happens with the heat sink providing energy which ends up compensating perfectly with the heat lost in the isothermal compression curve. 

If the compression is adiabatic, you end up at a lower pressure (after cooling) than in the isothermal case and the resulting work recovered is less.

True, but the parallel problem occurs with adiabatic expansion too, and we need an isothermal process both ways.

Here is where we go beyond thermodynamics and into heat transfer. To maintain isothermal conditions in the gas during compression and expansion, there has to be very effective heat transfer between the air and the surroundings. Very effective means large surface area or high heat transfer coefficient. Static air contained inside a walled vessel may not exhibit a high enough heat transfer rate to maintain the isothermal conditions that are desired. This is where the engineering comes into it.

So, conceptually, the system can work. Can it work in practice? That would seem to be an engineering design problem, and I cannot say. I do know that ground storage heat pump systems use a system of buried pipes to improve the heat transfer.

I find it amusing that there is still much doubt.  The system is amazingly simple once it is understood.  I can't find any practicalities that would make it a problem.  I'm assuming they have the material issues well in hand.  Considering man has been burying tanks in the ground for a very large number of years I expect there won't be any surprises in that.  The rest of the system is not new technology unless they decide to use some new turbine or pump to get another two percent efficiency improvement. 

A ground source heat pump is nothing like this in that it uses a one way flow of heat and simply uses the ground as a heat sink.  There is no cycle of heat input and output, so not really relevant.  The advantage in the ground source heat pump over using an air heat sink is simply that it uses less energy to transfer the heat (lower thermal resistance) and the ground temperature is much more stable.  An air system gets a double blow from temperature extremes in that more heat is required to be moved and more work is required to move that heat against a larger temperature differential.

The relevance of the ground source heat pump is that it needs good heat transfer between the working fluid and the surrounding ground. If our system being considered here is to operate isothermally, it also needs good heat transfer. Nothing in the video talks about that. I know it is a marketing thing and doesn't have much technical content, but still the matter must be considered.

[gnuarm]

I find it amusing that there is still much doubt.  The system is amazingly simple once it is understood.  I can't find any practicalities that would make it a problem.

Well, we were only discussing efficiency, but since you asked, practicalities would include energy density and capital cost.  Unless I screwed up the calculations, if you assume an tank pressurized with air at 10 bar and then pumped up with water to 3X or 30 bar, you get an energy density of about 3 kWh/m³.  So you need to build a corrosion-proof (interior and exterior) tank with a working pressure of 450psi--how much do those cost per cubic meter?  So you have your tanks, lets say they are a bit over meter in diameter and 30 meters long, which is a good way to build a high-pressure tank.  You can now store about as much energy as an EV battery.  Maybe I goofed somewhere.

Why do you want to debate made up numbers?  An EV like battery is not a cheap device.  They are running around $125 per kWh so 100 kWh would be $12,500, not installed, plus the conversion electronics, so say $25,000 as a wild guess.  Do you expect the 30 meter tank would be significantly different?  I have no idea.  Installing 100 of these may end up fairly inexpensive from a factory that does 1,000 such builds a year. 

It's hard to find prices on such tanks, but the prices I found were in the ballpark of the lithium battery without the electronics.  The price goes down considerably with quantity.  I expect in the end the cost is as much installation as the tank.  I bet in these are very price competitive with batteries.  Batteries may come down in price with time, but that's some years away. 

Vanadium flow batteries are the likely grid storage device if you want to stay electrical.  They are still new, but the first installations are being done now.  They have the property of storing electrolyte to be run through the battery not entirely unlike a fuel cell, but reversible.  So the storage capacity is not determined by the actual cell, rather the size of the tank.  I wonder if vanadium is a difficult to mine mineral? 

[gnuarm]

I had an incomplete picture of the system before, that I have now appreciated.

Broadly speaking, when you put energy into the system by pumping the water and compressing the air, you want to be able to get as much as possible of this energy back again when you reverse the process. This is called the power-to-power efficiency, and you want to get as close to 100% as you can achieve.

Now, the energy put into the system gets distributed into two places: (1) pressure energy of the compressed air, and (2) thermal energy (heat) resulting from compressing the gas. If you don't do anything to recover it, the heat part is energy that would be permanently lost.

Not quiet correct.  The heat is removed intentionally.  This way the PV curve is isothermal.  If you don't remove the heat as it is generated the PV curve is different.  A larger portion of the work input will go into the compression.  If you leave the temperature high during compression, ultimately the pressure drops as the temperature drops and you lose work stored in the PV curve.

I'm not sure what part you think is not correct here.

This...

If you don't do anything to recover it, the heat part is energy that would be permanently lost.

We are intentionally tossing this heat in order to optimize the energy going into the PV of the gas. 

To maintain isothermal conditions, heat has to flow from the gas into the surroundings (tank walls, soil, water, whatever). Whether intentional or unintentional, the heat still flows.

Ok.

Now if we treat air as an ideal gas (which under these conditions it practically is), then the enthalpy of the gas is independent of the pressure. That means the heat that flows into the surroundings did not come from the gas itself, it came from the work of compression. Some of the work done pumping water into the system is lost as heat into the surroundings. Isothermal compression (and expansion) can only be achieved by heat transfer.

If you say so.

The interesting bit for a complete compression/decompression cycle is if you can get that heat back again. Because when a gas expands it cools down, and to maintain isothermal expansion you have to put heat back into it. Which is the next paragraph...

You can say you are "getting that heat back", but the reality is you aren't.  You are getting heat from the isothermal expansion and the surroundings.  It's not the heat you lost in the isothermal compression, it's just heat from the environment.

Apparently, in this system the heat of compression is to be absorbed by the surrounding ground, and when the system reverses the surrounding ground puts the heat back into the air as it cools down on expansion. In traditional compressed air systems extra heat has to be provided to compensate for the air cooling down, but here they are avoiding the need for that by using the ground heat to do it. (There is some analogy with ground storage heat pump systems.)

Surrounding ground, water, whatever.  The point is the universe is a heat sink and the heat of compression is removed during compression yielding an isothermal PV curve.

What is important as well is that the expansion be isothermal which happens with the heat sink providing energy which ends up compensating perfectly with the heat lost in the isothermal compression curve. 

If the compression is adiabatic, you end up at a lower pressure (after cooling) than in the isothermal case and the resulting work recovered is less.

True, but the parallel problem occurs with adiabatic expansion too, and we need an isothermal process both ways.[/quote]

Yes, that's what I said, no?

Here is where we go beyond thermodynamics and into heat transfer. To maintain isothermal conditions in the gas during compression and expansion, there has to be very effective heat transfer between the air and the surroundings. Very effective means large surface area or high heat transfer coefficient. Static air contained inside a walled vessel may not exhibit a high enough heat transfer rate to maintain the isothermal conditions that are desired. This is where the engineering comes into it.

Humpty Dumpty said, a word "means just what I choose it to mean—neither more nor less."  "Effective" depends on the context and in this context we don't need large surface areas or radiators or high thermally conductive materials because the rate of heat generation is limited.  The tanks aren't filling in 10 minutes.  They are filling in a work day or maybe a full day or maybe a week.  So how hard is it to allow the heat produced or absorbed to be exchanged in 8 hours or more?

So, conceptually, the system can work. Can it work in practice? That would seem to be an engineering design problem, and I cannot say. I do know that ground storage heat pump systems use a system of buried pipes to improve the heat transfer.

I find it amusing that there is still much doubt.  The system is amazingly simple once it is understood.  I can't find any practicalities that would make it a problem.  I'm assuming they have the material issues well in hand.  Considering man has been burying tanks in the ground for a very large number of years I expect there won't be any surprises in that.  The rest of the system is not new technology unless they decide to use some new turbine or pump to get another two percent efficiency improvement. 

A ground source heat pump is nothing like this in that it uses a one way flow of heat and simply uses the ground as a heat sink.  There is no cycle of heat input and output, so not really relevant.  The advantage in the ground source heat pump over using an air heat sink is simply that it uses less energy to transfer the heat (lower thermal resistance) and the ground temperature is much more stable.  An air system gets a double blow from temperature extremes in that more heat is required to be moved and more work is required to move that heat against a larger temperature differential.

The relevance of the ground source heat pump is that it needs good heat transfer between the working fluid and the surrounding ground. If our system being considered here is to operate isothermally, it also needs good heat transfer. Nothing in the video talks about that. I know it is a marketing thing and doesn't have much technical content, but still the matter must be considered.

I believe that is a fallacy, the comparison between the two systems.  A heat pump has to exchange the full heat flow into the ground or air.  This system has to exchange the heat from compression/expansion which is only a part of the total energy flow.  Also, you seem to only see A tank.  The system is composed of many tanks with each one handling a small part of the total power flow.  So each very large tank only needs to handle a small heat flow.  An earlier estimate per tank was 100 kWh.  Over an 8 hour day that would be 12.5 kW.  I think the air in a 30m³ tank can exchange the level of heat produced without a significant temperature rise.

[Kim Christensen]

I find it amusing that there is still much doubt.

Well, it is simply more efficient to have doubt than belief. 

When you think of how many times we are bombarded with false claims of new battery technology, cold fusion, and other miracle solutions, is it any wonder that people are skeptical?

[bdunham7]

Why do you want to debate made up numbers?  An EV like battery is not a cheap device.  They are running around $125 per kWh so 100 kWh would be $12,500, not installed, plus the conversion electronics, so say $25,000 as a wild guess.  Do you expect the 30 meter tank would be significantly different?  I have no idea.  Installing 100 of these may end up fairly inexpensive from a factory that does 1,000 such builds a year.

Well I do have a bit of experience with tanks, so a 100-foot long tank with a working pressure of 450psi, so a test pressure of at least 600psi.  Corrosion resistant, safety certified (it's a BIG bang when something like that fails) and suitable for underground.  Hmmm.  Would that cost more than $12,500?   Hmmmm.  Yeah, I'm pretty sure it would.  Just the postage might be more than that, actually.  But feel free to shop around.  Or just click 'Investors' if you're sold.  I'm sure they will respond.  Maybe they'll even give you some more details! 

Here's a 200 cubic meter tank on AliBaba for propane (about half the pressure, above-ground and won't tolerate water) for $80K.  Perhaps there's a volume discount, although I doubt they have free shipping.

https://m.alibaba.com/product/62415230410/ASME-200000l-propane-gas-tank-200.html?__detailProductImg=https%3A%2F%2Fs.alicdn.com%2F%40sc04%2Fkf%2FH41f306ee0add40f29d0b4f9b3902d343Q.jpg_200x200.jpg

[gnuarm]

I find it amusing that there is still much doubt.

Well, it is simply more efficient to have doubt than belief. 

When you think of how many times we are bombarded with false claims of new battery technology, cold fusion, and other miracle solutions, is it any wonder that people are skeptical?

Skepticism is good when you don't understand or are not given the information to consider something new properly, but so far there is little about this that is not clear and actually well established principles.  There's no rocket science involved, no dark matter, no new technology really.  It is existing knowledge, technology and principles that we simply need to understand. 

The economics are not known for this system.  But I can't see anything to even suspect without gathering some information.  It's not like pressure tanks aren't already very commonly produced, installed and operated over decades. 

They are presently building systems.  If the idea is not so good, we will find out soon enough.

Someone once told me that increasing the production quantity of a thing ten fold cuts the cost in half.  I'm not sure this is an absolute rule, but if these things take off I expect to see "gigafactories" producing them at very affordable prices.  I really like the fact that they are low tech.  Every problem doesn't require high tech.

[gnuarm]

Why do you want to debate made up numbers?  An EV like battery is not a cheap device.  They are running around $125 per kWh so 100 kWh would be $12,500, not installed, plus the conversion electronics, so say $25,000 as a wild guess.  Do you expect the 30 meter tank would be significantly different?  I have no idea.  Installing 100 of these may end up fairly inexpensive from a factory that does 1,000 such builds a year.

Well I do have a bit of experience with tanks, so a 100-foot long tank with a working pressure of 450psi, so a test pressure of at least 600psi.  Corrosion resistant, safety certified (it's a BIG bang when something like that fails) and suitable for underground.  Hmmm.  Would that cost more than $12,500?   Hmmmm.  Yeah, I'm pretty sure it would.  Just the postage might be more than that, actually.  But feel free to shop around.  Or just click 'Investors' if you're sold.  I'm sure they will respond.  Maybe they'll even give you some more details! 

Here's a 200 cubic meter tank on AliBaba for propane (about half the pressure, above-ground and won't tolerate water) for $80K.  Perhaps there's a volume discount, although I doubt they have free shipping.

https://m.alibaba.com/product/62415230410/ASME-200000l-propane-gas-tank-200.html?__detailProductImg=https%3A%2F%2Fs.alicdn.com%2F%40sc04%2Fkf%2FH41f306ee0add40f29d0b4f9b3902d343Q.jpg_200x200.jpg

Why would you look at a tank that isn't suitable for the application when you don't even know details of the application???  I found tanks at much lower prices that aren't in Mozambique.  Not sure why you are looking at 200m³ when we were talking about 30m³.  Given that this tank would be equivalent to 7-100 kWh batteries, $80,000 is not bad at all.  The one new thing in this system is the tank actually.  Well, it's not metal anyway.  They don't give specifics, but its "polymer" based. 

I suppose there's no point in discussing this further.  We've uncovered all the information available and people are inventing information.  There's also the call for me to invest, so essentially "ad hominem".  Are we all good?  Or is there something else factual to discuss?

[Kim Christensen]

Well I do have a bit of experience with tanks, so a 100-foot long tank with a working pressure of 450psi, so a test pressure of at least 600psi.  Corrosion resistant, safety certified (it's a BIG bang when something like that fails) and suitable for underground.  Hmmm.

It looks like they are using inflatable polymer tanks which are placed in a hole, surrounded by rebar, and covered with concrete. Claimed pressure is 40bar. (580 psi)
PV magazine article.

[IanB]

Look, I'm not trying to prove anyone right or wrong here, I'm just pointing out certain details that have to be considered in the design of such a system.

We are intentionally tossing this heat in order to optimize the energy going into the PV of the gas.

That is a problem. Because for an ideal gas, increasing the pressure isothermally does not increase the energy stored in the gas. For an ideal gas, both internal energy and enthalpy are a function only of temperature. Reference here, but you can find others:

https://www.ecourses.ou.edu/cgi-bin/ebook.cgi?doc=&topic=th&chap_sec=03.4&page=theory

Furthermore, this has an interesting consequence for isothermal compression. All of the work done compressing the gas will be lost as heat to the surroundings. If you pump 100 kWh into your storage system isothermally, 100 kWh of heat will flow through the walls of the tank and into the ground. This comes from the thermodynamic relation that dU = Q + W, and if dU = 0, then Q = -W. Reference here:

https://en.wikipedia.org/wiki/Isothermal_process#:~:text=It%20is%20also,of%20ideal%20gases.

You can say you are "getting that heat back", but the reality is you aren't.  You are getting heat from the isothermal expansion and the surroundings.  It's not the heat you lost in the isothermal compression, it's just heat from the environment.

See below.

What is important as well is that the expansion be isothermal which happens with the heat sink providing energy which ends up compensating perfectly with the heat lost in the isothermal compression curve.

This is true, but it is also a matter of accounting. If I lend you $100 today, and you pay me back $100 tomorrow, is it the same $100? Is it important?

If I store 100 kWh into my system and 100 kWh disappears as heat lost to the surroundings, then if I have any hope of getting some of my 100 kWh back again, I need to get at least some of that heat back. Is it the same 100 kWh? Yes, from an accounting point of view it is, or the books won't balance.

Humpty Dumpty said, a word "means just what I choose it to mean—neither more nor less."  "Effective" depends on the context and in this context we don't need large surface areas or radiators or high thermally conductive materials because the rate of heat generation is limited.  The tanks aren't filling in 10 minutes.  They are filling in a work day or maybe a full day or maybe a week.  So how hard is it to allow the heat produced or absorbed to be exchanged in 8 hours or more?

See, I don't know. Maybe yes, maybe no. Without doing some engineering calculations and running some numbers, I couldn't say. However, I can say it is something to be considered in the system design.

I find it amusing that there is still much doubt.  The system is amazingly simple once it is understood.  I can't find any practicalities that would make it a problem.  I'm assuming they have the material issues well in hand.  Considering man has been burying tanks in the ground for a very large number of years I expect there won't be any surprises in that.  The rest of the system is not new technology unless they decide to use some new turbine or pump to get another two percent efficiency improvement.

It's not a matter of doubt. Clearly the system can work with some level of efficiency. I don't think any of us know the actual numbers, though.

I believe that is a fallacy, the comparison between the two systems.  A heat pump has to exchange the full heat flow into the ground or air.  This system has to exchange the heat from compression/expansion which is only a part of the total energy flow.  Also, you seem to only see A tank.  The system is composed of many tanks with each one handling a small part of the total power flow.  So each very large tank only needs to handle a small heat flow.  An earlier estimate per tank was 100 kWh.  Over an 8 hour day that would be 12.5 kW.  I think the air in a 30m³ tank can exchange the level of heat produced without a significant temperature rise.

I really don't know without doing some calculations. Aren't you doing what you complained about with others, taking some numbers and supposing results without supporting analysis? Do keep in mind that for heat transfer to occur, there has to be a temperature difference. If the temperature does not rise in the tank there can be no heat exchange with the surroundings. So the temperature must rise at least somewhat.

[PlainName]

Efficiency does matter, the only question is to what extent.  To compensate for a 10% loss in the storage, the entire system must be 10% larger, generation and storage, to achieve a given amount of energy to be delivered when needed.

If the alternative is 0 storage then being only 50% efficient isn't that bad, is it? And on the plus side, there is plenty of scope for improved versions later on.

[Haenk]

Funny thing about their business model:
They quote a typical deal size of 250-350k USD. For an industrial installation, that sounds almost like "free". (For comparison, that would be the same cost as a small flat (50m2) in a low-medium attractive city in some larger building complex in Germany. I don't think building costs will be cheaper in Israel...)

[gnuarm]

Efficiency does matter, the only question is to what extent.  To compensate for a 10% loss in the storage, the entire system must be 10% larger, generation and storage, to achieve a given amount of energy to be delivered when needed.

If the alternative is 0 storage then being only 50% efficient isn't that bad, is it? And on the plus side, there is plenty of scope for improved versions later on.

Zero storage is not the alternative.  There are many forms of energy storage available today.

That's also not the way engineers and businessmen would look at it.  The spec would be X kWh of energy to be delivered at Y kW rate.  With a 50% efficiency you would need twice the size of everything making the project cost twice as much including both the storage since you need a fixed amount of energy output and the generation since you lose half of this in storage so you must make twice as much to start with.  As a result the system costs twice as much and the energy costs twice as much per kWh.  This is a major issue for converting to renewables.  They need to be priced in the ball park with fossil fuels for many to accept them. 

If your car got half the fuel mileage, would you think that was ok?  You'd be shopping for a new car.  No?

[gnuarm]

Funny thing about their business model:
They quote a typical deal size of 250-350k USD. For an industrial installation, that sounds almost like "free". (For comparison, that would be the same cost as a small flat (50m2) in a low-medium attractive city in some larger building complex in Germany. I don't think building costs will be cheaper in Israel...)

I don't get your point?  I didn't see the quote you mention, but it makes sense they would start out with fairly small installations as they are a new company and no one is going to give them a $100,000,000 contract to start.  Where did you see these numbers?  Ah, I found it.  That is for the Airsmart system which is a different product for storing compressed air in a factory.  So yeah, that's not going to be as large a system as grid storage.   Under the Airbattery they list Typical deal size: $18M-$22M

Does that suit you better?

https://www.google.com/search?client=firefox-b-d&q=250+site%3Aaug-wind.com

[Kleinstein]

The cost for a tank are quite substantial. A common pressure for compressed air is some 10 bar. This would give around 1 kWh/m³, maybe a bit less if part of the volume is occupied with water and maybe 90% efficiency for the conversion back. I somewhat doubt one could get a 10 m³ tank for less than $1500, which would be about the price for Li Cells.  The costs for the tank would not significant change with scaling to a different size or pressure. For the isothermal concept to work, one may even have to keep the diameter small, so more like a stretch of gas pipeline than a conventional tank as a comparison. Even than it would be isothermal only when used slow.

Efficiency is a factor, but not that dramatic when for longer time storrage and if the costs per kwh are low. After long time storrage the energy put into the system is only a part and maybe a smaller part of the costs. It is like getting the energy when the price is lowest and using the energy when the price is highest. Currently the ratio between the highest and lowest costs is quite large, even if no counting the short time extremes. So 50% efficiency could still be OK if the storrage itself is cheap.

A more expensive system for more short time storrage should be reasonable efficient to use it more on a daily bases, whith less difference in the price.
In the end the available storrage systems would set that ratio.

[bdunham7]

It looks like they are using inflatable polymer tanks which are placed in a hole, surrounded by rebar, and covered with concrete. Claimed pressure is 40bar. (580 psi)
PV magazine article.

That's a more interesting article and it looks like their primary expertise is compressed air.  Their tanks are interesting too and they've solved the corrosion problem, but they still look fairly expensive to construct.  It looks like their niche is combined systems for facilities that use a lot of compressed air, where recovering and saving power is an issue of big money.  That makes sense.  I guess we'll see if they can successfully branch out into straight-up energy storage.

[IanB]

That article is indeed illuminating, but it contains some interesting contradictions.

It contains lots of discussion about heat dissipation and heat absorption in the water, and in the concrete surroundings, which I would expect, but it also contains contradictory statements about "generates minimal heat", "minimal heat produced in the isothermal compression process", and "compress the air without generating any heat", which are thermodynamically inaccurate.

These statements are contradictory because of Boyle's Law of gases. The pressure energy stored in compressed air is approximately equal to pressure times volume, and Boyle's Law for isothermal compression states that pressure times volume is constant (P₁v₁ = P₂v₂). This effectively says that you do not store "pressure energy" in air by compressing it isothermally. Instead, if you pump 10 MWh of energy into the system isothermally, the 10 MWh of electrical energy gets turned into heat. This is not "minimal heat" in my book, it is "maximal heat", since all the supplied electricity becomes heat in the system.

(Thermodynamically, you have the first law of thermodynamics which says that ΔU = Q − W, where ΔU is the change in internal energy of the system, Q is the heat added to the system, and W is the work done by the system. Since ΔU is zero for an isothermal change in an ideal gas, it follows that compression work is exactly balanced by heat generation.)

When you want to get your energy back out of your battery, you have to turn that 10 MWh of heat back into mechanical energy to turn the turbine. This effectively turns the whole system into a heat engine, and there are well known thermodynamic principles that apply to heat engines which likely have relevance here. (In particular, there is the Carnot efficiency, which is the maximum theoretical efficiency of any heat engine, given by \$\eta = 1 - \sqrt{T_c / T_h}\$ . If the hot and cold temperatures are close to each other, as would be implied by an isothermal system, then the maximum efficiency would approach 0%. That would not be encouraging, and really has me thinking.)

Overall, I think it is fair to be skeptical about the system. The company may have expertise in compressed air storage, but does it have sufficient expertise in energy storage?

[PlainName]

If your car got half the fuel mileage, would you think that was ok?

Half mileage of what? A better question is whether the cost of the fuel is more bearable than the time taken to walk, or the danger and messiness of cycling. If I already have a car and then someone brings out one with better economy, I might well look at it (but still think the exchange of mileage vs gadgets not worth it).

So, for this airbattery it's pointless to go on about efficiency because there is not alternative similar system to compare it against. The efficiency here is what it is - you want an air battery, that's what you have to pay.

Instead, you should be asking whether the costs of the airbattery are better or worse than, say, a Lion stack, a hydrogen generator, whatever kind of storage might be an alternative. But then you also have to factor in that this is underground and you can then use the surface for other projects, which you couldn't do with many alternatives.

I think you have succumbed to that complaint of running numbers without considering what they mean

Edit: forgot to mention that one big alternative, which was already stated as a reason for having this setup, was lack of storage space so 100% loss of energy when the sun or wind goes down.

[IanB]

So, for this airbattery it's pointless to go on about efficiency because there is not alternative similar system to compare it against.

But there is, though:

https://energystorage.org/why-energy-storage/technologies/compressed-air-energy-storage-caes/