The battery beneath your feet

How many batteries have you used today?

Energy storage devices have become an integral part of our lives, but they still aren't really a part of our electric grid. There are some good reasons for that—at that scale of storage, batteries become gigantic and extremely expensive. But the lack of storage on the grid has some distinct drawbacks, putting the stability of our electric system at risk and making it harder to add in lots of renewable energy generation.

Because of that, researchers are looking for ways to get the benefits of batteries without some of the detriments. There are lots of different ways to do this, but one solution is particularly awesome to describe. Hint: It involves caves.

Last Friday, I had a guest post on i09 explaining Compressed Air Energy Storage, an old technology that could be one of the most cost-effective ways to store energy at a grid scale.

At any given moment, there must be almost exactly the same amount of electricity being produced as there is being consumed. If the balance tilts either way-even by a fraction of a percent-it could lead to a blackout. To simply keep the lights on, the grid has to be constantly monitored, with controllers predicting demand and making small adjustments, minute-by-minute, to supply. This happens 24 hours a day, 7 days a week.

... That's where CAES comes in. CAES systems store energy underground in the form of compressed air, but to make it work you have to start with the right kind of geology. In particular, you need a space that's airtight. This means that you can't just pump air into the sort of cave you've toured while on vacation. Instead, you have to find a hollowed-out space underground that used to hold something naturally-such as a natural gas reservoir that's had all of the gas pumped out of it.

Read the rest at i09

Learn more about how the grid works and why storage is so important by reading my book, Before the Lights Go Out.

Image: Holes in porous rock, a Creative Commons Attribution (2.0) image from blmurch's photostream



  1. You allude to alternative energy storage mechanisms, but really there’s only one that counts for now: pumped storage hydro.

    Given that PSH can be made to work in most of the scenarios where you might try CAES, what is it about CAES that is likely to be better? Is it cheaper or more efficient?

    1. It’s out of the way, more than anything, I would assume.

      I mean, PSH would require some sort of silo, and more construction.

      CAES takes advantage of the fact that we have all these oil and gas wells — now mostly empty– that can be used for storage.

      1.  Pretty much what Derek Crocker said.  For hydro, you have to build dams and frankly, people are against that plus it can be kinda rough on the environment.  Not to mention the problems associated with hydro in dry years (like this one in California), and the issues with sediment being brought into the resevoir.

        CAES is deep underground and you probably won’t see very large footprints for the facilities.  It may have a lower environmental impact and it’s definitely not dependant on rainfall.  But it would be very highly dependent on geology so it might be located far from the locations the power is needed, reducing it’s efficiency (there are huge transport losses in the electric system iirc).  Of course, that last one is pretty much true of all of our power sources so it might not mean much.

        1. Sorry, I wasn’t being clear. I meant a closed system using, say, wind power during the night to pump water out of one underground cave into a higher one, and then sending it back through a turbine during the day.

    2.  Some concentrating solar plants have heat storage- either hot oil or molten salt. This lets them produce after dark, but could also be used in other contexts.

  2. this is partly a + to Stooge’s question, but why not more storage systems that use gravity to convert the stored energy to electricity?  There are any number of heavy things that could be winched UP with surplus energy and let back down as needed.

    1. Are you sure that’s a piece of porous rock? I ask because it looks exactly like a piece of drift wood that has been attacked by teredo worms. The wood’s cell walls also show up really well.

      1. Hi there Guy!  I am pretty sure it was rock, but it was years ago, so I cannot prove it.  I remember it feeling like rock.  It could have been fossilized drift-wood, but it was really light and felt and looked like pumice stone to me.  Someone left a comment on flickr saying that’s what it was.  I was on the coast in northern Peru and there’s a lot of volcanic activity around there, so it makes sense.

  3. My questions/comments about CAES:

    1.  What pressure are we talking about?  This breaks into the same issue as with fracking, really.  This could be mitigated by having pressure monitoring, and carefully watching for any unintended loss I suppose.

    2. I’m no chemist, but wouldn’t it behoove us to pump only inerts (CO2, for example) downhole, instead of ambient air?  I’m just looking at how Oxygen reactivity changes under pressure, especially in the somewhat unknown environment that would exist downhole.

    This is a GREAT idea, and could even be a way to take all the petro-producers of West Texas and turn them into green energy producers.  Tell them that their old worn out wells can still make them money.  

    Also:  it’s a great reason to get people NOT to frack — it allows their drilling investment to be useful down the road as CAES instead.

    1. In the west, there are a lot of natural gas storage providers (Google can probably find them for you).  They basically do this but with natural gas; injecting it into the ground when it’s cheap and bringing it out again when it’s expensive (usually summer and winter respectively).  They are very, very careful to not change the natural formation because that could cause leaks and lead to a very expensive investment becoming useless.  I believe they stay below “discovery pressure,” or the pressure the field was originally found at, by quite a bit in order to avoid cracking the non-porous cap on top of the field.   Sounds like the opposite of fracking to me.

      The downhole conditions are also not unknown.  I’m not a resevoir engineer but I do work with some of them and they’re pretty clear on what’s happening in a given field.  They actually lower equipment down the wells to inspect them and the field.

      1. Yeah.  I realize that.  I actually work on a resistivity/gamma ray tool for that, and am __right now__ coding a routine to analyze the differences between multiple passes, to see if we’ve started having issues with mud invading the borehole.

        The downhole conditions are unknown in terms of things like 

        –OOPS, I didn’t think that fault extended quite this far–  


        –why is there a pocket of shale HERE?–.

        These issues come up far more likely than you think. I know, because I’m the one that gets phone calls about the tool not working correctly.

        These really are not big concerns for  the most part, and I can’t think of a problem that wouldn’t be noticeable by simply monitoring the pressure.

          I just wanted to bring up that, in reality, we don’t have a perfect knowledge of where exactly our drilled well is, due to the problem of error cones in drilling.

         (I’m sure you know, but to reiterate, MWD is a series of measurements BASED ON previous measurements.  An error in measurement at any single spot affects all the measurements for the rest of the well, and thus errors tend to accumulate).

        Publicly this isn’t really discussed, but as of a few years ago at least one major directional company I worked for had an internal expectation of 1 degree of error per 1000 feet drilled, on an EAST/WEST basis.  At the end of a 5000 foot lateral, this means our uncertainty in position is +- 437 feet left or right.  A big difference.

        Subtle differences like that could, and do, lead to chemistry differences in the formation. In drilling this can lead to things not behaving the way you expect, with the mud you are currently using.

        I mean, as you said, I am not a geologist either. I’m just asking if it might be smarter to guard against this. Hope for the best, plan for the worst.

        Edit: I’m sure my concerns would be easily dealt with by a geologist. As you said, they do have a record of the discovery pressure, and if one were to maintain the air at that pressure, it would HAVE to be stable in that formation, or the gas would never have been there in the first place.

        So, in short. That is a good observation I should have thought of myself.

        1. It sounds to me like you work with the tools for drilling holes, yes?  But this sort of thing would probably use fields with existing wells in it (at least I would expect it to) so the chance of surprises should be a little slimmer.  I’ve never heard of chemical issues with the storage fields I’m familiar with but none of them are new.

          Thing I’m curious about, though; why hasn’t this been done before?  Is it a cost issue?  Something to do with safety?  It’s not like this is new technology, really.

          Oh, and re: your first comment about turning petro companies into green energy producers: you can’t produce energy this way.  In order to get pressure to turn the turbines, you have to compress air into the ground, which takes energy.  But it COULD help make solar or wind more useful in terms of having the power when you need it.

          1. Right.  That’s why I talked about West Texas.  The area I grew up in is rich in both wind power and old, dead wells.

            I do work on the exploration side, but all of the work I do is preparation for the geologists and the production side.  The tools I work on and with are for logging operations to give geologists and idea of the composition and character of the well.  They are used for both drilling and wireline (which can be much closer to the production side) or even direct inspection of the casing of old wells.

            For the purposes of getting a productive well, having a few hundred feet of another formation instead of the target one is not always a big deal.  In terms of making  certain the well is drilled successfully, this can even be the best choice, and is routinely accepted in order to keep the well going.

            But when one is using the well to store a high pressure gas or fluid, those boundaries between formations might be really important.

            With that said, there is a standard method of drilling called air drilling (really foam) where one uses really high pressure air/foam mixture instead of mud to wash away your cuttings.  One of those wells would be ideal for this.  However, I am not familiar with any air-drilled wells contiguous to wind power areas.

            Most likely it hasn’t been done before because the O&G industry is full of Grognards.  Gamma Ray sensors and directional drilling have really only taken off in the past 2 decades, but the technology isn’t particularly new.

            Caveat:  I’ve never worked on the storage side.  That’s a peculiar area that has grown dramatically recently, and the little bit of work I have done in natural gas had to do with efficient extraction and compression, not compression and storage.

    1. Nope. It’s a completely different thing. They both involve the ground, but the stuff that causes problems with fracking is not applicable to CAES. 

      The point of fracking is to break rock deep below ground. That’s not at all what you’re doing here and, I believe, would actually be counterproductive. 

      Fracking involves pumping lots of liquid (some mixtures of which include toxic chemicals) into the ground. There is none of that involved in CAES. 

      With fracking, you have lots of used toxic liquid that has to be disposed of. Again, not an issue with CAES because there is no toxic liquid to begin with. 

      1. This a wee-bit disingenuous given the way foobar asks the question.  fracking causes earthquakes (as well as other environmental problems to which you allude).  The amount of subterranean area (really: square meters) subject to pressure that wasn’t there previously can have the same effect as fracking when it potentiates earthquakes — not exactly the same causes (fracking includes some “lubrication” for lack of a better term) but still pressurizing underground voids can cause seismic events.   (see mythbusters and alka-seltzer and prison breaks if you don’t get the “area” issue)

        1.  Actually, the pressure WAS there before.

          “Instead, you have to find a hollowed-out space underground that used to hold something naturally-such as a natural gas reservoir that’s had all of the gas pumped out of it.”

          You never take a resevoir above its “discovery pressure” (the pressure it was at when it was found) because that might crack the cap, rendering it leaky and useless.

          If you’re interested, you might look up natural gas storage.  They basically do the same thing that this article is talking about but with a different purpose.  That sort of thing has been going on for 30 years or more.

  4. Main problem with CAES which you don’t mention are the losses.

    Your compressors will operate at maybe 85% (axial’s get up over 90% but nowhere near the pressures you need, so you’d have an axial inlet compressor then multistage centrifugal) but are connected to a 98% motor and a 98% gear, so your down to 81% even before you account for anything else.  The turbo-expander is similar, so your down to around 60%.  

    1. You’re right about the losses but it could be less of a problem than you think.  One of the major problems facing the electric grid is that power consumption isn’t constant.  It spikes during the day from about noon to 6pm when it’s hottest and drops low at night but there’s always at least some load and when that load spikes up, production must also spike up.  Wind and solar can’t do that because you can’t control the wind or the sun.  But you could store the electricity you didn’t need this instant and use it later.  Yes, you’ll lose some, but if it was extra already it was going to be wasted anyway (I don’t know how they control the production of wind or solar facilities, but I assume they can to a certain extent) then you just came out ahead.

  5. Do any of you actually work with compressed air?   I do…every day…and while it is convenient for delivering a lot of  power to light weight tools, it is a horribly inefficient power storage medium.   You’ve got water to deal with…large amounts that condense out of all those cubic feet at atmospheric pressure when you compress the air.  Just in my small woodworking shop, we have to drain the 60 gallon tank of about a quart of water every day.  Then there is the heat build-up and loss; compressing air builds up a lot of heat, and unless you’re going to then add some sort of heat-to-electricity exchanger, that heat is just going to heat rock.   Then there is the issue of even tiny leaks in the system, and the higher the pressure, the worse that issue will become as little fissures blow out into major cracks.  And I don’t want to be anywhere near that in an earthquake unless it’s way, way underground.

    And don’t forget those line losses in even very high voltage transmission lines.   Centralized power generation, storage, etc. is just not efficient enough, and it leads to Enron-like monopolization and empowers the oligarchs of our country who love having power nodes over which they have total control. 

    Distributed micro-power sources and a smart grid will go a long way to solving our problems.   Every south facing roof should have solar panels for both electricity and for hot water.   My friend Susie Bright has the more or less south facing roof of her Santa Cruz Victorian completely covered in solar panels.  Her utility bills are practically nothing, and are only something because she chooses not to be obsessive about electricity use.

    For the power “flywheel effect”, think in terms of pumping water up hill, and then releasing it as needed for hydro power.   I have a feeling there would be far fewer losses in such a system.   That can even be done on a fairly localized level.   Reservoir high, and reservoir low. 

    I think the whole power grid should be nationalized and administered much like the Interstate highway system.  Then whoever wants can pump power into the system and be paid accordingly.  The more efficiently they can produce kilo Watt hours, the more money they make.  Capitalism is saved, but the ripoffs of monopolization is neutered.

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