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Jan Steinman


Excellent introduction to the topic, that makes me want to go out and build one!

All your examples appear to use photovoltaic panels, which have the two inefficiencies of photon-to-electricity and then electricity to motion. Do you know of any efforts to harness solar thermal for this purpose, perhaps through Stirling engines?

kris de decker



These ideas are dealt with in the other article: http://www.lowtechmagazine.com/2018/05/history-and-future-of-the-compressed-air-economy.html

Here are the paragraphs & the references:

Some proposed designs follow other approaches to deal with the heat of compression, and these could work for both large-scale and small-scale CAES systems. One interesting idea is a compressed air energy storage system that runs on wind energy as well as solar energy. [24]

Wind energy is stored in the form of compressed air by compressor chain, as in the other CAES plants. However, solar energy from a parabolic dish is stored in an insulated solar thermal tank and used to reheat the compressed air prior to expansion. Because the heat from the compression process is no longer needed to warm the air upon expansion, it is used to produce hot water.

A similar concept for a hybrid thermal and compressed air energy storage design uses electric heating instead of solar thermal power. [25] Because the workload in these systems is shifted from pure conversion to investing partially in thermal storage, energy densities well in excess of traditional CAES can be achieved, and the size of the air storage can be reduced.

[24] Ji, Wei, et al. "Thermodynamic analysis of a novel hybrid wind-solar-compressed air energy storage system." Energy Conversion and Management 142 (2017): 176-187.

[25] Houssainy, Sammy, et al. "Thermodynamic analysis of a high temperature hybrid compressed air energy storage (HTH-CAES) system." Renewable Energy 115 (2018): 1043-1054.



Compressed air is still popular for powering tools. Most powered tools are available in compressed air, battery, or AC variant, therefore I really see this CAES idea ideal for the hobbyist.

My garage for example is not connected to the grid, so no electricity. Battery tools were so far the solution, and I was even thinking to add PV to charge them, but to go all compressed air seems way more intelligent.

Just need to invent micro-expander to light the garage now. LEDs need very little power, and need to be cooled for maximum efficiency and durability anyway ;-)

Israel Walker


An article you and your readers might find interesting on this subject:


Also interesting is the comments which reveal the commenters and to a lesser degree the author, do not understand that Amish are not against electricity: they are against community networks controlled by members outside the community, they are localists not luddites. They have little problem with technology as long as it doesn't make the user dependent on those outside the community.

Also this: https://en.wikipedia.org/wiki/Fireless_locomotive

Fireless locos generally proved themselves more useful than compressed air locomotives, at least in part because of a mixing valve mixed air into the steam jet to create a larger volume of working fluid. If you are already designing a heat pump running on water vapor, water vapor/liquid phase shift, and running high pressures, I suspect some applications would be benefit from steam, rather than compressed air.

Mackey McLelland


Maybe you could do a small series on battery storage of energy - gravity, thermal etc.

I once visited the island of AEro in Denmark. They have a district heating system which is based on a huge insulated tank full of molten salt. They heat it up through the summer with excess electricity from their PV and wind generators and then use it to heat the town's houses and water through the winter. Extremely effective...

Chris harries


For a while I contemplated this as an energy source for a bicycle, the idea being that the (modified) bike frame would act as the containment vessel for compressed air. This all looked good and easy to do and would have higher energy performance than a battery electric-assist bike. The stumbling block, though, is safety. Any highly compressed gas needs a very robust containment vessel. A sudden rupturing of any containment vessel or lines can be disastrous. Weight becomes a problem.

But the idea of using compressed air as a stationery energy storage needs to be pursued further. As it almost certainly will be.



Amish use compressed air systems and have for years. Their Wells and washing machines run on air. It's great and been over 20 years that I've known of it...



Thanks for the article, as always inspiring.
One remark:
At the end it says... Build it Yourself?

In conclusion, .... Simply getting hold of the right components and fittings is a headache, as these come in a bewildering variety and... are only sold to industries....?

Entering "electric air valve" in Ebay seems to give a list of possible valves... Fittings, storage and pipes can be had from a local plumbing store or scrap dump?
Arduinos etc. are fairly simple to program and take very little power.

Bryan H.


Is there any reason why compressed air storage systems aren't kept underwater (i'm thinking a pond, lake or undersea)? It seems like the continuous water pressure may take care of some of the problems related to pressure in the chamber dropping as the air is used (assuming the storage chamber walls are flexible).



This article is assuming the use of compressed air for conversion to electricity...given that premise, is there any way to convert the temperature differential directly into electricity as well, either through modified Peltier coolers or (as a Low-Tech method) a Stirling Engine sensitive enough to run off of the ambient temp changes?

Jim Baerg


Bryan: I think that is perfectly possible IF you have quite deep water close to shore. It takes 10 m water depth to give 1 bar of extra pressure, so for 10 bar pressure you need a water depth of 100 m. For 100 bar 1 km water depth is needed. An exercise for the student is to determine where these conditions are available.

I'm a bit surprised that the technology from this company (http://www.lightsail.com/ ) wasn't mentioned in this article or the previous one. The trick of spraying water into the air being compressed or expanded to absorb or release heat & so keep the compression/expansion near isothermal sounds like a good way to increase the efficiency of CAES. I recall that the claimed efficiency was at least as good as pumped hydro, but I can't find the efficiency figure on that website now.

Energy storage as good as claimed by the Lightsail company, would greatly reduce my skepticism about the worth of wind energy. Since energy demand is higher during daylight hours, solar is still of considerable use without storage in the large part of the world where seasonal variation in sunlight is modest.

kris de decker


@ Bryan

So-called "Underwater CAES" is a research topic. The advantage is a steady pressure, but anything that's built underwater, especially in salt water, is going to have a shorter lifespan. And it makes CAES dependent on geography again.

@ order99

Sounds possible to me. The Stirling engine is a way to convert solar energy to mechanical energy, allowing to drive the compressor directly.

@ Wim

That's not my experience, but it may be different depending on where you live. Maybe I was looking in the wrong places.

@ Jim

I did not mention LightSail because quite a lot has been written about them already, and because they seem to have advanced little recently. By the way, there are other approaches I did not write about. There is so much research going on at the moment that I had to make a selection.



I was left thinking that a passive solar approach to heating tanks (and thereby increasing the pressure) could be used as a way to boost output if the energy usage was cyclical. Day and night temperature variances would seem to be a natural component of any rural design along with direct solar heating.

Oliver Evans


wind powered air compressors are built by the Amish in Maine. I have seen them at an Amish cabinet shop in Choteau Oklahoma. They pump air into a large propane tank that is used to power the air tools in the shop. They work great!



I can't access the original paper [27] but 410Wh energy from 570l of stored air is higher than I would expect. It is about 6.5x what the formula at the Wikipedia page on Compressed Air Energy Storage says is the theoretical maximum in the case of isothermal storage.

Why is that not an appropriate formula to apply in this case? What am I misunderstanding?

Here's my units session, with the Wikipedia formula giving the maximum as being 63Wh:

You have: 5 bar 570 l ln(1 atmosphere / 5 bar) + ((5 bar - 1 atmosphere) 570 l)
You want: watt hour
* -63.248223
/ -0.015810721

Cristián Lávaque



Great article, again!

About heat during compression/decompression, this could be used directly, avoiding the need of electricity later for those same uses. Most homes would have a water heater and a refrigerator.

The compression part could have a heat exchanger to heat up a water tank, and the decompression part have one to cool an insulated box/room. They'd avoid energy losses and improve the overall efficiency of the system.

But that's assuming that the home would have used electricity for cooling and heating, for which there are other efficient approaches.

kris de decker


@ Karl (#15)

I have sent you my copy of the paper you can't access.

In short, a higher energy density is obtained by using multiple interconnected storage tanks instead of one large tank. This allows more effective heat transfer to take place, because every air tank acts as an additional heat exchanger. Also, discharging the vessels sequentially allows longer discharge times and solves the problem of pressure drops. These factors are not taken into account in the Wikipedia formula.

kris de decker


@ Oliver Evans (#14)

Great link, thanks !

This link goes straight to the product page: http://windcompressor.com/html/compressors.html

And here is another wind-powered compressor: https://www.cottagecraftworks.com/wind-compressor-wind-driven-air-compressor-alt-energy

The same website has several air powered kitchen tools for sale, as well as fans:



If a windmill compresses air directly, without converting it to electricity, and the devices are powered directly by compressed air, a small-scale CAES system can become even more energy efficient than the articles states. This strategy, which also works on a larger scale, is explained in the other article: http://www.lowtechmagazine.com/2018/05/history-and-future-of-the-compressed-air-economy.html

Additional links found via Hackernews: https://news.ycombinator.com/item?id=17143163

Pr. Théodose


Combining a windmill-powered rope pump [1] and a recirculating trompe (with the air reservoir on the ground) could give us a low-tech, high-efficiency system at moderate pressures. Since both the maximum pressure achievable and the wind power available increase with the height of the windmill, the whole system can scale up pretty well.

[1] https://web.archive.org/web/20090521195033/http://www.gamos.demon.co.uk/just%20gamos%20homepage/henkfnl2.htm">http://www.gamos.demon.co.uk/just%20gamos%20homepage/henkfnl2.htm">https://web.archive.org/web/20090521195033/http://www.gamos.demon.co.uk/just%20gamos%20homepage/henkfnl2.htm

Logan Simmering


"So-called "Underwater CAES" is a research topic. The advantage is a steady pressure, but anything that's built underwater, especially in salt water, is going to have a shorter lifespan. And it makes CAES dependent on geography again"

If your already building off shore windmills, it seems like a good idea though.



The article states a overall electric-to-electric efficiency for the 3-cylinder set-up reached a peak of 85% at 3 bar pressure.

But if you have a look at [27]. System efficiency is defined as: The efficiency of the system consists of the conversion efficiency of the pressure potential energy within the cylinders into kinetic energy in the discharged air, and also the mechanical efficiency of the air turbines handling the ultimate energy conversion into electricity.

So it's actually not e-to-e only air-to-e. The used compressor (PowerPlus-POWX1730) is not in the efficiency equation!


kris de decker


@ luk

You are right. So if we assume a compressor efficiency of 70% then electric-to-electric efficiency is 77.5%, not 85%. I will correct the article.



Energy density (not efficiency, I think) can be ages better if the working fluid is stored in liquid state, but worked in gaseous state. The issue is that you need to either store, exhaust, or immediately use the low pressure gaseous exhaust; and you need a way to source it.

CO2 has a useful working range as a refrigerant, and is somewhat benign. Not sure how easy it is to directly harvest it though, as needed. If you can efficiently separate it from atmosphere, you can store pumping energy as a phase changed liquid. When using that battery, you get usable refrigeration and pressurized gas for mechanical work. The CO2 could be vented afterward (Neutral to atmosphere) or directed to a greenhouse operation.

Lots of hydrocarbons (propane, butane, etc) that have useful working ranges and the expanded gas can be used as fuel. Not sure if there is a practical way to source it though, especially renewably. Sadly methane is not in a useful temperature/pressure range, so biogas is out.



doing a bit more reading, dimethyl ether (DME) may be a promising working fluid, although on a municipal scale due to complication of process to refine from methane.

John Newman


And now there's this:

Tony Martin


So we have a device that at some point produces heat which could perhaps preheat water before it goes to a hot water tank and then later produces cold which could perhaps be used to help a refrigerator. This might also improve the efficiency of the device by lowering the inlet air temp of the compressor but obviously needs some integration with household appliances.



Is it possible to control expander/generator efficiencies with variable input pressure by controlling the load applied to the generator?

For instance - if the expander/generator is spinning too quickly due to high pressure can you somehow put load on the generator in order to slow do the speed at which the expander is spinning?

Jean seite


This is a track I had worked on a few years ago. I was working at that time for a solar panel company. This article is giving me news ideas to increase heat recovery for home use with a simple principe.

i intend to implement the experiment 27 In the coming months and to show you my results if you are interested. But it is impossible to access to the complete document.

patrick dufour


What about a very large balloon? The presure of expansion and contracting might have some effect? If the air was hot in the ballon maybe it would rise and cool then fall increasing pressure?



What type of turbine / generator is used in a large scale CAES system that is being reference in the article?



Dear All,

Please suggest me some tips for Excess Solar Energy Storage:

I have installed Solar Power system at my home which has the following details:
PV Modules Installed capacity= 2160KW (08modules with 270Watt/panel)
Off/On grid Inverter (Has multiple functions including Net-metering option) = 3.5KW
Deep Cycle Batteries= 04 (120Amp each)
Average Energy consumption (Load) = 15KWH (60% of this is used at night-from Grid)
Net-metering option is not available in our area. What I am asking here, Can I store excess Solar Energy as compressed air during the day time and use it at night?
What will be the compressor specifications/type for providing 9KWH during night hours?

How it can be coupled with an A.C. generator?
Is there any system already in market that meets my set-up demand?



I wanted to research if it were possible to increase the efficiency by cycling the compressed air between the tank.

peter budd


can someone help me and direct me to where i can read about large scale energy heat storage in rocks or fluids to be used at the point when air is exiting the storage reservoir and the turbine needs to have the air warmed on exit into the turbine? thank you.

H. Stamper


Ditch the tank and just turn the coal fire generators turbines pneumatically?



Kris- since publishing your article, have you any information, examples, and products that you could publish to update the article?

bobby mobby


Quick question, according to the [27] paper

"with cylinders operating fulfilling the job of four 24 V batteries for 20 consecutive hours (the bench mark battery is a Trojan U1-AGM,

When checking the specs of the batteries it mentions each stores 0.47 kWh.

Does it mean a 3 x 10l cylinder setup @ 5 bars holds 1.88 kWh ?

If yes, I'm confused with this part from the above article:

"Interestingly, the storage capacity is 410 Wh, which is comparable to the 360 Wh rural system noted earlier, which requires an 18 m3 storage vessel – that’s thirty times larger than the modular storage system."

Bert van Brakel


How about using liquified air? It's about a 700:1 volume ratio, can be stored at ambient pressure, only requires a container with some good insulation, can be built in most local machine shops.

I'm not in any way affiliated with these guys, but a US crowd is working on combining this with their rimdrive wind turbines to directly liquify the air (see https://keukaenergy.com/products/)

The upside is if having this supply locally it could also be used to provide refrigeration (either by direct air release or during rewarming of the liquid), and possible heating during the compression/liquification phase.

I don't know what the smallest feasible size is, but maybe these could be installed in small local clusters of a few houses if individual household size isn't feasible.

Vincent Bourquin


Pray allow myself to indicate you the remarkable research undertaken by Prof. Alfred Rufer at EPFL regarding this topic of energy storage using compressed air. To my knowledge, he and his staff have put a significant scientific effort to understand the way to achieve the best results for this approach. Certainly very useful to develop simple efficient devices.



@bobby mobby I think the battery comparison in the quoted article is broken!

* The reverence battery has a voltage level of 12V not 24V as stated
* 1 cylinder has 0.32 Wh in 0.026h max!
* 4 batteries have 1632 Wh in 20h
* 57 cylinder produce 18.57 Wh in 0.3h max!
* sidefact: ηoverall is higher as ηmech at two different setups ;)

PS. Almost two years later :) https://www.lowtechmagazine.com/2018/05/ditch-the-batteries-off-the-grid-compressed-air-energy-storage.html#comment-6a00e0099229e88833022ad37cb70a200d-content

Energy Fan


In the October 2020 issue of Energy and Building, someone modeled an analysis of a residential system with High Temperature Thermal Storage, Low Temp Thermal Storage, and Micro CAES (analyzed a system that stores 181 kg of air at 50 bar, seems high pressure!)




i read this article a while ago and it has kind of stuck in my brain. yesterday i've wondered about if the turbochargers of cars could be used at the generator side of the setup as cheap available part? the pressure of the exhaust of cars seems to be ~5 bar, which is about the pressure of the small setup presented here. a problem might be that the volume-flow isn't big enough in small setups to use a turbocharger turbine, though.

the reasoning for this idea is to avoid custom manufactured parts, reusing whats available (maybe even as used part).

Seth K


--> "However, to store 360 Wh of potential electrical energy, the system requires a storage reservoir of 18 m3, the size of a small room measuring 3x3x2 metres. The authors note that “although the tank size appears very large, it still makes sense for applications in rural areas”."

I was considering the options for fences include sprayed concrete, earthbags, cob, and stabilized rammed earth when I thought what if:

Two masonry fences were built next to each other.
Capped with a masonry arch or other cap.
Possible vapor lining inside if needed.

Would such a fence be strong enough to work as low pressure compressed air energy storage? Mimicking an artificial cave and serving as the fence that a homeowner would want anyways. It would also be strong enough to support solar or wind power on top of it.

The perimeter of an acre is 254.5 meters. A double fence 2 meters high with hollow space of 1 meter would equal 509 m3 of low pressure compressed air storage. This would store ~10kwh, or current US average 1/3 daily use.

Rodrigo Fernández


Hi Kris. I was thinking that, in case as much cold or heat was not needed as the excess of the CAES, the freshly compressed hot air, before entering the high pressure tank, could be cooled by heating a stirling engine, which would act as a heat exchanger with the atmosphere. The stirling, which would be geared to the compressor itself, would help it compress more air, recovering some of the energy.
For its part, the cold exhaust air would be used in the opposite area of ​​the stirling engine, to extract energy from the heat of the surrounding air.
I know more about letters than numbers, so I don't know how practical it could be.

Brian C


This is all about air handling. I didn't pick up on anything that would allow compressed air to be used to generate electricity. No doubt direct mechanical use is more efficient, but if you are replacing chemical batteries you will need some ability to produce electric power from the storage medium, whatever it is.

George Fleming


There is one insurmountable problem with storing compressed air deep underwater. If the air and the water are in direct contact, the water will absorb some of the air. The absorbed air will make its way around the container and be lost, thus allowing the water to absorb more of the air. This process would continue until all of the compressed air is lost.

Storing the compressed air in balloons would prevent direct contact between the water and the air. Several years ago, Hydrostor indicated that they were trying it, but they do not mention it now. Nor does anyone else, so far as I can tell. Apparently there is a problem with using balloons in this way.

Hydrostor is getting huge investments from Goldman Sachs and others to build a water-compensated CAES plant in California. This idea was thoroughly investigated more than forty years ago, when there was great interest in CAES. The investigators discovered a serious problem with this design - the Champagne Effect. You can learn about it on the internet. I see no indication that Hydrostor is aware of it.

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