Liquid Air for Energy Storage? No, it’s not a joke

January 7th, 2013
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Thought that compressed air energy storage was ridiculously inefficient?   Well, it looks like they’ve managed to best it with a new concept in energy storage:  liquid air.

Yes, liquid air, as in cryonic liquid air.  In other words, this is a combination of liquid nitrogen, liquid oxygen and little bit of liquid argon.

Via Discovery News:

Frozen Air ‘Battery’ Stores Wind Turbine Energy

Liquid air, which can be frozen, stored and warmed later, could work better than batteries or fuel cells to store energy from wind turbines or other renewables.

The technology was originally developed by Peter Dearman, a garage inventor in Hertfordshire, U.K., to power vehicles. For the past several years, U.K. tech firm Highview Power Storage has been working to transfer Dearman’s innovation to a system that can store energy for power grids.

Dearman’s idea works like this: electricity generated by wind farms at night is used to chill air to -310 Farenheit — its cryogenic state — turning it into a liquid. The liquid air is then stored in a giant vacuum flask until it time to be used again. This is done at night when demand for electricity is low and the energy from wind would otherwise go wasted

When demand increases during the day, the air can be warmed to ambient temperature. As it vaporizes, it drives a turbine to produce electricity, according to the BBC’s Roger Harrabin.

In July, Highview Power Storage signed a commercial agreement with a German firm to develop “frozen air” plants in Sub-Saharan and South Africa. And it now has a pilot facility near a traditional gas-powered plant outside London. That way it takes advantage of the plant’s waste heat to warm the liquid air, making the entire process more efficient and less costly. Company officials say their energy-storage system is best designed to help smooth out the peaks and valleys of energy production that often occur with wind, solar and other renewable energy project.

And the video…



Liquifying air is a common industrial process.  It is most often used as the first step in certain types of air separation techniques.   Partial liquification of air can be used as the first step for the separation of nitrogen and oxygen, followed by additional liquification to separate out argon, or it can be used to produce liquid air which is then boiled in a series of distillation columns.    Occasionally, air is liquified and used in its mixed state as an ultra-low temperature refrigerant.

The process is very simple.   Air is compressed to extreme pressure, which causes it to heat up.   The heat is removed by passing the air through heat exchangers, which may be actively refrigerated to aid the process.   Once the heat is removed from the highly-compressed air, it is reexpanded back to ambient pressure.  Some of the air boils off in the process, taking additional heat with it and resulting in a super-cooled liquid.   In practice, of course, it’s a little bit more complex than this.  There are additional heat exchangers to reclaim some of the heat from the process stream and the process may be done in stages.   After the air is liquified, it is stored in an insulated tank, often a Dewar container.

Although the principle is simple enough, the process is incredibly energy intensive.  Even when the most modern and efficient equipment is used, the amount of energy required to liquify air enormous.  So much so that plants that liquify atmospheric gasses are normally located near cheap sources of electricity or even have their own generation capability on site.

Most of the energy is lost in the process.  The final product does contain some recoverable energy, by virtue of the fact that it has a much different temperature than the ambient environment and can be expanded when warmed.   However, the best of the best gas refrigeration systems only manage to achieve a Carnot efficiency of about 25-50%. That also does not include the loss that occurs as a result of storing the liquid for any length of time, during which, as a result of imperfect insulation, some inevitably evaporates away.  Thus, for every two joules of energy that goes into producing liquid air, only one joule is actually retained.

In fact, existing large plants are much less than 50% efficient.  The figure comes from a hypothetical proposal of a purpose-built energy storage plant where the cold temperatures of the expanding air is re-captured into some kind of intermediate storage mass and then used to aid in the pre-cooling of air that is being compressed.   Of course, this would vastly complicate the procedure and as yet, it has not been validated as workable.  These ideas are common in various proposals for compressed air or liquid air storage.  Unfortunately, as a consequence of thermodynamics, you cannot keep re-capturing and reusing the same heat (or cold) without losing most of it.

Unfortunately, it gets even worse from here, because getting that energy back from the liquid air means even greater loss.  In order to convert the energy back into mechanical and ultimately electrical energy, the liquid air must be heated so that it expands back into a gas.   In principle, this could be done by just exchanging the heat with the atmosphere to return it to the ambient temperature, but doing so is more difficult than it might seem.  For one thing, frost is quick to build up on any radiators used, reducing their ability to exchange heat.   Of course, it could be heated by using a gas flame or some other heat source, but that would also mean a significant amount of the energy would be coming from burning fuel.  You may as well just burn the fuel to begin with and dispense with the ridiculously cumbersome and lossy process of pre-cooling the air to a liquid.

Waste heat may be able to help, but it can only do so much and would limit this to a very secondary method of energy storage, making the claims that it is somehow going to have a major impact impossible. It does, however, make this a great way of adding a “green” addition to a thermal power plant, which always seems to makes people feel good.

Once the air is heated enough to cause expansion back to a gas it would be used to drive a turbine or some other engine.  This is actually a thermal engine, although the method of energy storage is the reverse of how thermal engines are typically thought of, since in this case, it’s the environment that is hot.  As such, it is possible to use basic formulas to calculate the Carnot limit of an engine that runs on a liquified gas.  An engine running on liquid nitrogen and at an ambient temperature and pressure of STP would be expected to have a total Carnot efficiency of about 74%.  Nitrogen composes the majority of air, but since oxygen boils at a higher temperature, a liquid air engine would have a Carnot efficiency of a bit less.

Of course, Carnot efficiency is the theoretical limit of an engine’s efficiency but no engine ever reaches it.  It would presume that the liquid air were warmed all the way to ambient temperatures without loss (which it wouldn’t be) and that there was no fluid friction involved (which there would be) and that all other aspects of the engine were otherwise perfect and free of any resistance.   This never happens.   So, really, the engine would not achieve anywhere near 74% or even 70%.   The best turbines out there can get about 75% of their Carnot limit.   That would mean that realistically, a liquid air engine might be able to achieve about 50% efficiency.

I should note that if we compare this to the actual historical performance of liquid gas engines then these numbers turn out to be exceptionally generous, because such engines have been built and they tend to be very very inefficient.   However, there will be those who claim that they can somehow stop frost from becoming a problem and push everything to its limit to get a 50% efficiency rating.

Note that the efficiency will go down significantly during cold weather.

Therefore, we can approximate how much energy you can get out of a liquid air storage system:

Based on current air liquification technologies and the current standard for small to medium thermal engines:

25% * 33% =8.25%

Best case, if the turbine preforms as well as the best large turbines do and the refrigeration is 50% efficient (Which is highly suspect):

50% * 50% = 25%

Of course, none of this actually considers the other losses, such as the fact that electric motors and generators are only 98-99% efficient and that some of the liquid air will evaporate.  These may seem like small losses, but they compound!

It’s abysmal!

By comparison, the current standard for grid energy storage is pumped hydro.  Pumped hydroelectric storage systems can achieve efficiencies of between 66 and 75%, which blows away the idea of liquid air energy storage.  Utility scale batteries are currently expensive, but a good battery and inverter plant can return more than 90% the energy put in.

And NO, this is not a new idea!

Based on the amount of press out there and the number of pundits jumping up and down and saying this will be the end of batteries and that it’s a revolutionary way of storing energy, you might think that this was actually new and that someone had only recently come to realize that liquified air can be used to store energy.   Unfortunately, it’s an all too familiar pattern.   Journalists and politicians seem to think every tired old idea was just invented and is brilliant and the perfect place to throw some money.

In reality, this idea has been around for a very long time.   In the 1970′s, 1980′s and 1990′s, the term “liquid nitrogen economy” was being thrown around right along side all the other “economies” (methanol, ethanol, sugar, hydrogen etc) that were being proposed.   In the late 1800′s, liquified air was tried for a variety of vehicles from cars to flameless locomotives.  They were even less successful than their compressed air cousins and the concept was dropped entirely as soon as electricity and gasoline engines became available.  One company claimed their car could drive 40 miles at 12 miles per hour using a 18 gallon tank of liquified air. Even by standards of the day, that was not very impressive.

In the late 1800′s, an inventor by the name of Charles Triplet began developing a means for liquification of atmospheric gas on an industrial level.  Among other uses, he promoted liquified air as a possible source of energy.  The systems he developed did prove commercially viable for the purposes such as deep refrigeration and gas storage, but they failed to ever succeed for energy storage.

The image to the left is of a liquid-nitrogen powered vehicle (basically the same concept as liquid air) that was constructed in 1997 at the University of WashingtonAnother similar vehicle was constructed at the University of North Texas that same year.  Many many others have been built, both in the form of vehicles and static engines, powered by liquid nitrogen or liquid air.  Every time one of these vehicles or motors is built, the press seems to gather around it and assume it must be something new and amazing, despite the fact that this has been done for more than a century.

These engines are interesting as scientific curiosities and they certainly do a good job at demonstrating just how diverse thermal engines can be and the energy that can be produced through phase changes.  That’s about their only practical use, however.  Perhaps a winning science fair project but definitely not a practical way of storing energy.


This entry was posted on Monday, January 7th, 2013 at 9:12 pm and is filed under Bad Science, Enviornment, History, Just LAME, Misc. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
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23 Responses to “Liquid Air for Energy Storage? No, it’s not a joke”

  1. 1
    drjim Says:

    This just has EPIC FAIL stamped all over it!


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  2. 2
    Gordon Says:

            drjim said:

    This just has EPIC FAIL stamped all over it!

    If the goal is to propose a “new” way of doing something explicitly to soak up some subsidies or unsavy investors then perhaps it will not fail at its mission.


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  3. 3
    Ray Says:

    Once you have gone to all the trouble to convert the atmosphere to a liquid, it would be stupid to rewarm it and release it. The energy you get back would be far smaller than the value of the liquid gas. You could either distill it out to oxygen (which is more valuable than nitrogen, the LN2 is a byproduct really) or you could sell it as is as an cryo refrigerant.

    It’s stupid to waste all that effort ruining perfectly good liquified atmosphere to get back a tiny bit of energy.


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  4. 4
    Brian Says:

    The issue with cooling and having to heat the air is there with compressed air storage too. This is just compressed air energy taken to the extreme, which makes it worse, of course.

    I feel like there should be a rule that any time a new energy technology comes out it should need to be reviewed by a physicist or engineer to make sure it is actually workable and not a violation of thermodynamics before it gets any public money invested into it.


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  5. 5
    Joffan Says:

            Ray said:

    Once you have gone to all the trouble to convert the atmosphere to a liquid, it would be stupid to rewarm it and release it. The energy you get back would be far smaller than the value of the liquid gas. You could either distill it out to oxygen (which is more valuable than nitrogen, the LN2 is a byproduct really) or you could sell it as is as an cryo refrigerant.

    It’s stupid to waste all that effort ruining perfectly good liquified atmosphere to get back a tiny bit of energy.

    True… so what you then built, instead, is a liquefaction plant that only works say 15% of the time, when there’s some spare wind energy.

    As long as there are feed-in tariffs and priority grid access, though, no wind operator is going to bother with this. They can dump their electricity on the grid, no matter what, for guaranteed returns.


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  6. 6
    wcoenen Says:

    Lightsail claims 70% efficiency for their compressed (but not liquefied) air energy storage: http://lightsailenergy.com/tech.html


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  7. 7
    drbuzz0 Says:

            wcoenen said:

    Lightsail claims 70% efficiency for their compressed (but not liquefied) air energy storage: http://lightsailenergy.com/tech.html

    Interesting. I’m very skeptical, though and it seems that they’ve never actually built a plant, they just have “experimental results” presumably from some scaled down systems in controlled conditions.

    If they can then great, because grid energy storage is an issue, but I have my doubts. Their whole scheme seems to be based on the idea that they can recapture and store the heat from compression and then use it to heat the air back up. That kind of thing is very hard to do in practice.

    Pneumatic technology is very mature and widely deployed. Compressed air is used for energy all the time, whether its for air brakes, power tools, valve actuators, jackhammers etc etc. The efficiency is always low. It’s not used for efficiency in that respect, but because pneumatic systems are very lightweight, reliable and simple and because it can provide a lot of power quickly.

    So much energy is used to compress air and so much is lost in the process that companies routinely have their penumatic systems audited for ways of improving them to squeeze out some savings. When possible, older compressed air devices are replaced with small electrical motors.m

    It has always been understood that compressed air is just a very expensive form of energy and that it only makes sense in circumstances where the other advantages outweigh the inherent expense and inefficiency. Of course, there have been some improvements over the years with better compressors, but it does not change all that much

    When someone comes along and claims they have re-written all the rules and turned one of the least efficient means of storing energy into one of the most efficient, I have to be very skeptical. That’s like someone saying that they tweaked the design of the internal combustion engine and built one that has 60% thermal efficiency in their garage.


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  8. 8
    Sigivald Says:

    Air is compressed to extreme temperatures, which causes it to heat up.

    I’m pretty sure you wanted “pressures” there.


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  9. 9
    drbuzz0 Says:

            Sigivald said:

    Air is compressed to extreme temperatures, which causes it to heat up.

    I’m pretty sure you wanted “pressures” there.

    Thanks. Corrected.


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  10. 10
    DV82XL Says:

    When will people learn that the Laws of Thermodynamics always bat last? When I was young, this was just about the biggest thing we were expected to take away from high school physics, and this knowledge of the way the universe works has served me in many different ways down through the years. I am frankly appalled at how this information, which should be seen as one of the supporting pillars of commonsense, is a mystery to so many these days.


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  11. 11
    Engineering Edgar Says:

    Sorry I do not have a reference to back this up, but to say that the process of refrigerating a gas down to liquid is 25% efficient sounds very generous to me. 50% is just ridiculous. Refrigeration never really gets anywhere near the true Carnot limit for all kinds of reasons (gasses are never ‘idea’, waste heat is always an issue)

    Atmospheric liquification ranks right up there with aluminum smelting and cell electrolysis when it comes to high energy industrial process.

    I don’t know what the world is coming to sometimes.


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  12. 12
    Anon Says:

            Engineering Edgar said:

    Atmospheric liquification ranks right up there with aluminum smelting and cell electrolysis when it comes to high energy industrial process.

    That gives me an idea.

    How about we smelt some Al using excess renewable energy, then burn it during peak (Al burns).


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  13. 13
    Engineer-Poet Says:

    The energy return on this might not be quite as bad as a simplistic analysis suggests.  For instance, the exhaust of the first-stage of expansion might be cold, but it’s still much warmer than liquid air.  If you use it to pre-heat the incoming compressed liquid air, it gets colder yet.  You can re-compress it, re-heat it with ambient heat, and go for another cycle with it.  How well these schemes work depends on how light and efficient you can make heat exchangers.  You definitely want to use LN2 for this; allowing liquid air to distill leads to LOX, which is a fire and explosion hazard.

    Also, compression and expansion of gas in a liquid can be made close to isothermal.  There is a company, SustainX, claiming to have a scheme for high-efficiency energy storage using such a method.  I suspect that other methods might work better, but if they are using water at a maximum temperature less than 100C they can use some mighty cheap insulated tanks to hold it.  They might even be able to use flat-plate solar collectors to boost their expansion temperatures.


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  14. 14
    KitemanSA Says:

            Anon said:

    That gives me an idea.

    How about we smelt some Al using excess renewable energy, then burn it during peak (Al burns).

    It is called an Aluminum-Air battery and is being worked on.


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  15. 15
    KitemanSA Says:

    By the way folks, you are giving this a bad rap. It may not be the most efficient way to store energy but you can store a hellaciously large amount of it cheaply. It is among the few storage media that has a shot at making renewables almost economical.


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  16. 16
    drbuzz0 Says:

            KitemanSA said:

    By the way folks, you are giving this a bad rap. It may not be the most efficient way to store energy but you can store a hellaciously large amount of it cheaply. It is among the few storage media that has a shot at making renewables almost economical.

    Well, let them prove me wrong then.


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  17. 17
    Anon Says:

            KitemanSA said:

    It is called an Aluminum-Air battery and is being worked on.

    Primary cell. Has it’s uses but probably too expensive for general purpose use unless we can get electricity really cheaply (which the unrealibles can’t do).

            KitemanSA said:

    By the way folks, you are giving this a bad rap. It may not be the most efficient way to store energy but you can store a hellaciously large amount of it cheaply. It is among the few storage media that has a shot at making renewables almost economical.

    There are no storage media we could develop that can make non-hydro renewables cheaper than well-regulated nuclear (when you need over-optimistic assessments of wind power that ignore storage costs to get it cheaper than nuclear even free energy storage won’t make it economical).


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  18. 18
    I'mnotreallyhere Says:

            drbuzz0 said:

    Pneumatic technology is very mature and widely deployed. Compressed air is used for energy all the time, whether its for air brakes, power tools, valve actuators, jackhammers etc etc. The efficiency is always low. It’s not used for efficiency in that respect, but because pneumatic systems are very lightweight, reliable and simple and because it can provide a lot of power quickly.

    Not a key part of discussion, but as something of an FYI : compressed air circuits and pneumatics are also often used where an equivalent electrical system would be a safety and/or fire hazard.

    For certain industrial processes there’s also a little more of a fail-safe to them, it’s easier to install compressed air accumulators than the equivalent batteries and the accumulators will (almost) never need replacing.


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  19. 19
    DV82XL Says:

            KitemanSA said:

    By the way folks, you are giving this a bad rap. It may not be the most efficient way to store energy but you can store a hellaciously large amount of it cheaply. It is among the few storage media that has a shot at making renewables almost economical.

    Compressed air as an energy carrier/storage medium is not cheap. In fact it is the most expensive form energy in any industrial shop that uses it, and one only uses it for the reasons I’mnotreallyhere and drbuzz0 mention as well as the fact that most of the heat is dissipated at the compressor and not the tool-head. Even in a new, tight system the losses are considerable and are only tolerated as they are offset by the other advantages.


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  20. 20
    maarten van der burgt Says:

    A way to make liquid air storage more attractive would be to pump the liquid air to say 300 bar and then expand and reheat it in stages. Double reheat cycles have made the Rankine cycle for steam more efficienct. The exhaust air from the last turbine cycle can unfortunately not be used as clean air for the liquid air production because the vaste quantities of air cannot be stored.


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  21. 21
    test Says:

    Your analysis is correct except for 2 things:
    1) the system can use waste heat which means Carnot efficiency is higher, and more energy is extracted than you suggested
    2) the cold can be used to offset refrigeration costs if used in a heat-exchanger (ie for industrial A/C, natural gas liquefaction, etc)


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  22. 22
    test Says:

    Also, the alternative in some locations is to have a peaker plant that sits idle most of the time. That’s economically inefficient although technically it is 100% efficient at doing nothing while it’s idle.


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  23. 23
    Jason J Says:

    Weird concept. I am sure the heating from the waste of a powerplant helps, but it still can’t be a very good overall ratio of input to output.

    What I do not understand is why use the equipment for this. If you build a perfectly good liquid air plant and put all that effort to make it a liquid, why vaporize it? Better idea is to just sell it as is or separate it into liquid oxygen and liquid nitrogen and sell those as is.

    It would be a better return. Liquid air is too valuable to just vaporize for energy.


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