Their Battery is Full of Air

Storing electrical energy is a huge problem. A lot of gear we use every day use some form of battery and despite a few false starts at fuel cells, that isn’t likely to change any time soon. However, batteries or other forms of storage are important in many alternate energy schemes. Solar cells don’t produce when it is dark. Windmills only produce when the wind blows. So you need a way to store excess energy to use for the periods when you aren’t creating electricity. [Kris De Decker] has an interesting proposal: store energy using compressed air.

Compressed air storage is not a new idea. On a large scale, there have been examples of air compressed in underground caverns and then released to run a turbine at a future date. However, the efficiency of this is poor — around 40 to 50 percent — mainly because the air heats up during compression and often needs to be prewarmed (using energy from another source) prior to decompression to prevent freezing. By comparison, batteries can be 70 to 90 percent efficient, although they have their own problems, too.

The idea explored in this paper is not to try to store a power plant’s worth of energy in a giant underground cavern, but rather use smaller compressed air setups like you would use batteries to store power at the point of consumption. The technology is called micro-CAES (an acronym for compressed air energy storage).

Although the article compares them to batteries, they seem more akin to fuel cells, to us, even though the technology is quite different, of course. Batteries usually have a fairly limited lifespan and often produce just a few times the amount of energy required to manufacture them. CAES and fuel cells typically have very long lifespans, so they produce a lot more energy over that lifespan than was used in their construction. There’s nothing very exotic or toxic either. An air storage tank, a compressor and a generator is all you need.

Keep in mind “micro” here is in context of giant underground storage caverns. The article estimates that a system for a typical residence would cost about $10,000. More than batteries, but with a lower total cost over time because batteries wear out. No one is suggesting your laptop or cell phone will run on compressed air.

This sounds great, but there are two major problems. First, the amount of air you have to store to be practical is an issue. The other problem is the system efficiency is low. Worse, these parameters are interrelated. So storing at a higher air pressure to get more air in a particular volume will also reduce efficiency.

The main extra proposal to help make micro-CAES practical is to take advantage of the heat produced on compression to heat water and living space. On decompression, the heat absorbed can be used for air conditioning and refrigeration. This reduces the electric demand for those tasks, making the overall system more efficient. In other words, if you could reduce electric usage by half because of the cooling and heating properties of the micro-CAES, that will offset the low efficiency of the unit.

The other way to possibly make micro-CAES practical is by using low pressure so that there isn’t much heat produced or consumed. You can read all the pros and cons of this approach in the original post.

This is an area of active research around the world. It struck us that this would be an area where a citizen scientist could make a real impact. There’s nothing super exotic about it. Air compressors and tanks are easy to obtain. Generating with a turbine is easily accomplished, too. We have a feeling a little hacker ingenuity could go a long way in making real advances in this area. Your tough choice? Do you publish in a journal or do you submit your work to the Hackaday tip line? We vote for the tip line.

45 thoughts on “Their Battery is Full of Air

  1. Another way to try and boost efficiency is to utilize a multi stage turbine system, with a low pressure turbine downstream of a high pressure turbine. But of course that brings back the issues of the high pressure in the first place. I like this idea from the onset though and I’m curious as to what kind of advancements could come from it.

    1. Stirling engines are notoriously low torque. Beyond that Carnot takes over, high pressure high temperature is prohibitively expensive for the hobbyist and probably affects compressor efficiency.

  2. I think the primary energy source of the word will end up being PV solar. In most situations that means you would want to store energy and cool things at the same time, and extract energy and heat things when there is little sunlight, so the exact opposite of the solution presented here. You would have to store the heat and cold to use this solution, which results in more losses and even more space requirements.

    It has become very difficult to beat the efficiency, price, size, simplicity and maintenance of a li-ion battery, which still have potential to improve even more, without running into any limits imposed by laws of physics/nature.

    1. It seems reasonable to think that PV will use batteries and other things like wind, geothermal, wave, etc will use a variety of other storage systems, or even different storage systems during different seasons. Especially if one season’s preferred option is really cheap and doesn’t require special materials, as here.

    2. The article comments that batteries don’t last that long, whereas this technology does.
      My concern is that although batteries ‘don’t last’, they also don’t have moving parts (except on a chemical/atomic level). Any system that has moving parts such as a compressor or generator will require regular maintainence, or it too will wear out just as quickly.

      1. “Any system that has moving parts such as a compressor or generator will require regular maintainence”.

        When was the last time you maintained your fridge?

  3. Paraphrase: direct (waste) heat to things you want warmed, and use cooling to cool things you want chilled

    In other words, design homes to be more energy smart. I’ve often thought how to incorporate a “heat chimney” in a house and have all the heat pumps (fridge, freezer, etc) dump their heat there to be used by appliances that want heat (hot water pre-warming, etc)

    1. As soon as you’ve dumped an appreciable amount of heat into your “heat chimney”, your fridge compressor has to work harder to cool your fridge. I think you need to do it more directly.

    2. Have appliances that always produce unwanted waste heat, dump that heat into a chimney where convection of rising air can be put to work. Use it to draw intake air for HVAC, with inlets on the north, south, east and west of the house. Which inlets gets used depends on the season and time of day.

      If it’s winter you want to use the south facing inlet (in nothern hemisphere) and the east inlet in morning, west inlet after noon. Reverse that for warm weather so the inlet air is drawn from the shade of the house.

      A air to air heat pump is a problem because in winter it’s attempting to air condition the outdoors, so you don’t want the outside coil in your heat chimney. You’d want a system with two outside coils, one optimized for heat output in the heat chimney for air conditioning and a second, on the equator side of the house, optimized for heat absorption, for heating in winter.

      That’s likely going to be the next big thing in heat pumps, replacing using a single coil for heating and cooling. There has to be compromises with using the same coil for both.

      A clothes dryer vent would be easy to run to the heat chimney. Same for the exhaust and inlet for a gas water heater. Venting the heat from an oven, refrigerator and other large appliances wouldn’t be so easy. I have heard of some people (in more southern latitudes) having a hole in the side of their house with the fridge backed up to it and sealed around so the heat from the condenser coil on the back goes outside instead of adding to the burden on the air conditioner. Most fridges these days don’t have a large, exposed condenser on the back.

      Without dumping exhausts from gas appliances into the heat chimney, the top could be closed off in winter so the heat could be captured and directed inside the house.

  4. I have to wonder how noisy a setup like this would be. Compressors outside of very expensive screw-type units are loud, sometimes obnoxiously so. Air motors are not quiet, either. How does the turbine being proposed compare in noise level to something like, say, an air die grinder?

      1. About halfway down there’s a section titaled “New Types of Compressors and Expanders” which talks about using “scroll compressors, which are the types of compressors that are now used in refrigerators, air-conditioning systems, and heat pumps.”
        So it sounds like it doesn’t necessarily have to be loud. None of those things sound anything like the compressor I have in my garage.

  5. In the mid 90’s there was a place in Joplin Missouri that was running cars on compressed air. Pneumacar or something like that. Little Geo Metro with a big high pressure tank on the roof. Refilled with a huge compressor powered by a Chevy V8 in the back yard. Grant money ran out from what I heard is what killed it. And it was horrible. Inefficient, slow, short range.

  6. A bit off topic, but I opened an ultrafire 18650 li-ion and it was full of air too, with storage capacity measured at a typical NIMH AAA cell (700 mAh, despite a 9000 mAh label).

    I have some air-hogs toy cars with compressed air piston engines. Kinda cool. Also, piston air compressors for fish tanks can be run in reverse on compressed air (or steam).

  7. If you want efficient energy storage and distribution then use the solar/wind energy to convert H2O and CO2 into a liquid hydro-carbon. Besides the investment in finding the right catalysts no investment in new infrastructure is needed.

      1. They also use this thing called photosynthesis, which very inefficient (can use only a part of the visible light spectrum), you’ll get literally an order of magnitude more power from PV for the same used up area. Not only that, you can bolt PV panels to buildings and other areas unsuitable for farming, you don’t need fertile soil or lots of water, the cherry on top is that far less care/maintenance is needed for PV.

        Leave the plants for food, we can’t eat electricity.

  8. Maybe use some of the inflation energy into forming concrete structures with multiple compressed air bladders.

    Basically look for industries that already, have or could possibly, use compressed air systems in a energy storage way.

    I also like the Underwater Compressed Air Energy Storage (UW-CAES) – lots of things going for it, the much much higher water pressure keeps the compressed air at a constant pressure.until the bladder is fully deflated. Maintenance and repair deep under the sea could be an issue.

  9. The question of pneumatic energy storage is a question of keeping the system isothermal. large heat exchangers to cool down the compressed air and heat up the decompressing air result in efficiencies closer to 100% by circulating the lost heat through the atmosphere. This is the point and principle behind the “recuperator” of a stirling engine – it traps part of the compression heat during the operating cycle and returns it on the return stroke.

    If you have something like a cubic metre of water to surround the pressurized hose to/from the tank, you get pretty close to isothermal conditions as long as you’re not trying to push too much power through the system. The slower you compress/decompress the air, the higher your efficiency.

    1. Water is the main issue when using compressed air. Not a big problem when it’s in a big cave where there is already water. But on a tank it’s becoming a major issue.

  10. What if you just compressed air slightly near sea level and then transported it va car/train to a higher altitude? With a solar powered locomotive that might be an interesting efficiency pattern.

    1. You’d be gaining a few psi on a high pressure system. 4000psi +/- 5 psi won’t make the difference. Even at 100 psi you wouldn’t notice it. That’s all if you aren’t using a normal regulator that is open to atmospheric pressure.

  11. Maybe dual chamber pressure storage. Preliminary hot side and cold, pressure reduction side- stored heat with molten salt or whatever for pressure recovery. Put some batteries inside the cold chamber to recover the thermal cycling.

  12. How about replacing the compressor with a water pump/turbine, and the air tank with an expansion vessel? This would negate the temperature differences at the pump, but limit the storage capacity of the tank. The energy will still be stored in the “air-spring”. Temperature differences in the tank due to (charging/discharging) pressure changes will still occur, so not all expansion/compression losses are covered by this idea.

  13. If we are looking at a whole house integrated system, let’s use the compressed air to move things that need motion directly rather than via a conversion to electricity first….

    Consider using the compressed air to drive the compressor on the A/C and fridge, the fan for your hvac*, rather than converting it back into electricity and then that back into motion… just go straight from compressed air to motion, it’s a long solved problem (or so the air tools would have me believe).

    We’ve just taken care of some of the larger electrical loads and we’ve probably increased efficiency since we are removing components that have loss to them…

    *maybe even just use the compressed air in something like a dyson bladeless fan…

  14. All of the comments above completely missed the point. I read them all. Each one. Every one.

    I have an exceptionally good BS detector. The first paragraph of the article describes a significant thermodynamic challenge to compressed air energy storage (that part was awesome by the way).

    Then, the article changes gears completely by saying ok, massive power storage underground causes air to liquify, but; and then follows a bunch of hand wavey chit chat about how we can make this work by doing such and such with this waste heat, or maybe we can make it work by scaling it up or down.

    It sounds like every argument for free energy I’ve ever heard. It sounds like Nichola Tesla’s plan for wireless energy transmission. It sounds like BS.

    The fact that smart people can’t look at something like that and tell it’s BS scares me terribly.

    To be pedantic, if you hear a plan of this form:
    Step 1: Start with an interesting physical phenomenon.
    Step 2: identify a noble goal.
    Step 3: ???
    Step 4: Profit.

    You should do one of two things; either set the person straight by careful discussion if you can, or run away fast. Killing the person might help to protect others; it’s probably going to depend upon the situation. Would I travel back in time to kill Tesla and live without his induction motor, if I never ever had to hear some sophomoric dimwit tell me that “all our dreams would have come true had the government just let Tesla continue his work brah.”? Maybe…

    1. This man had a secret, click here to buy plans to Bedini’s magnetmotor and wirelessly power your phone for free, forever. Please submit CC# to continue.

      too bad the same “plans” were invented by differing people depending on what year/month you happened to browse upon it. :P oh wait, it’s on kickstarter and indiegogo? lolz

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