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Air compressor as energy storage?

Discussion in 'Home Power and Microgeneration' started by Taunto, Feb 19, 2007.

  1. Taunto

    Taunto Guest

    I'm just trying to figure some efficient, cost effective ways to
    "time-shift" energy usage, for charging at night when rates are cheap,
    and discharging when not.

    Seems like battery storage is to inefficient. Flywheels are expensive,
    and liable to fly apart.

    What about pumping air into a tank, and then running it backwards when
    needed? Would need some sort of motor that runs on compressed air.

    What are the inefficiencies of this route?
     
  2. Eeyore

    Eeyore Guest

    Fairly significant. Using compressed air is lossier than batteries.

    Graham
     
  3. Vaughn Simon

    Vaughn Simon Guest

    Actually, that could be the easiest part. With a proper design, the
    compressor could easily be the motor. By the way, you would need a very large
    storage tank and/or very high pressures.
    Terrible. There are good reasons why compressed air energy storage is not
    common.

    Besides the usual things (friction, motor losses, generator losses) you
    have a special kind of thermal loss. Simply put: When you compress air, it gets
    hot, and you supply the energy to make that heat. Unless you recover that heat
    somehow, it is energy lost to you forever. When you expand your air to generate
    power, it suddenly gets cold. This reduces the expansion (and the power you
    extract) and creates problems for your motor. It can end up encased in a big
    block of ice!

    Vaughn
     
  4. HVAC Guy

    HVAC Guy Guest

    That would only work assuming you are charged based on time-of-day
    use. Not many people are.

    What about having a situation where you would mechanically lift some
    large mass using excess energy (solar, wind, low-cost utility power,
    etc) and then use the stored kinetic energy when you need to convert
    it back to electricity by running a generator as the mass is lowered.
    Efficiency would depend on how much you could reduce friction I guess.

    See these:

    http://www.columbia.edu/~zk30/gravity.html

    http://www.halfbakery.com/idea/energy_20storage_20gravity
     
  5. Eeyore

    Eeyore Guest

    Resistance heaters.

    Graham
     
  6. Guest

    I'd like to double the heat in a 957-gallon 170 F STSS tank.

    Got any brand names?

    Nick
     
  7. Eeyore

    Eeyore Guest

    Googling "wax balls" gives a rather less than useful result !

    Graham
     
  8. Guest

    Something that melts at 100 F might be nice, eg the C20 material on
    the site Anthony found, but their "macro" spheres are 4 mm max...
    I didn't see PCMs among the aerospace and packaging products on their site.
    Maybe they stopped making them.
    Lots of patents and studies and companies who might make custom products,
    but Googling didn't turn up any standard stuff.
    The tank has an EPDM liner. I can picture paraffin clogging the draindown
    "solar collector," 64' of fin-tube pipe near the top of an air heater.

    Nick
     
  9. Guest


    WHAT?!? We can't hear you! The sound of the air compressor is
    too LOUD!!!

    Ever worked in a place with one of those things?
     
  10. HVAC Guy

    HVAC Guy Guest

    Ok, that makes no sense:

    Lake holds water. Water flows down through turbines from lake into
    river.

    During times of low demand, keep allowing water to flow down through
    turbines into river, and then use excess electricity to pump water
    BACK up from river to lake?

    That's dumb.

    During times of low demand, why not throttle back the water flowing
    through the turbines and keep more of it in the lake, where it can be
    used later during high demand?
     
  11. Eeyore

    Eeyore Guest

    That's not what he said.

    And put more in the lake by pumping it there with electricity from baseload
    generation.

    Graham
     
  12. EXT

    EXT Guest

    There are two "pump up" installations just north of Niagara Falls, one on
    the Canadian side and one on the American side of the Niagara River, each
    with their own small lake for storage.
     
  13. Vaughn Simon

    Vaughn Simon Guest


    You are correct that those plants like a steady load, but technically
    speaking the boron does not control the power level in the short term. In a
    PWR, power level is controlled by the negative temperature coefficient of
    reactivity. As the water cools, it becomes denser and serves as a more
    efficient moderator, increasing the power level. As it heats up, the opposite
    effect occurs. Back when I was a reactor operator, I could watch a large change
    in power without ever touching the rod controls.

    What the boron does is allow the rods to be withdrawn more fully, which gives
    more even fuel burn and (incidentally) allows a greater shutdown margin. After
    a sudden shutdown, the boron can indeed prevent restart until it is either
    removed or poisons (mostly xenon) allowed to decay. That is an effect called
    "xenon precluded startup".

    (Just thought I would add a bit to the group's base of useless knowledge)

    Vaughn
     
  14. HVAC Guy

    HVAC Guy Guest

    A water turbine can presumably change it's output rather quickly to
    meet demand, morseo than a reactor or coal/gas plant.

    If a small storage lake is situated to feed a water turbine, then the
    turbine's water input must be able to draw from both the main source
    of water as well as the storage lake. Which means there is (or there
    can be) a connection between the main water source and the storage
    lake.

    If demand is low, then water could be diverted into the storage lake
    instead of going down the turbine.

    Instead, what your saying is that the water is allowed to go down the
    turbine to generate electricity which is then used to pump that same
    water back up to a storage lake. That again sounds stupid.
    A hydro-electric plant is always generating power at the same "cost"
    all the time. It's the market that puts additional premium on that
    power.
    cost to who?

    The consumer, or the operator of the plant?

    But that still doesn't address the issue of why let the un-needed
    water drain down the turbine in the first place instead of diverting
    it into a storage lake. Why let it go down, then turn around and pump
    it straight back up?
     
  15. Taunto

    Taunto Guest

    Cost of electricity to the utility.

    Dude, I didn't invent it, I'm just citing to installations that use it.
    If you have questions, I think you should ask it of the people who
    make it work.
     
  16. Eeyore

    Eeyore Guest

    Indeed. That's the whole point. It's used for peaking generation.

    No. Why did you introduce such a silly complication ?

    Nightime typically....

    He's not saying that at all.

    When you're pumping water 'uphill' the turbine is off. You're using baseload
    generation to do it.


    Graham
     
  17. Eeyore

    Eeyore Guest

  18. HVAC Guy

    HVAC Guy Guest

    What do you mean no?

    If you have a storage lake for excess water, then of course that water
    must have a way to get to the intake side of the turbine when called
    for. How else is it going to be used?

    And if you're going to recharge it, then the most efficient way is to
    let water flow from the main source into that lake directly.
    Where is that baseload coming from?

    Why do you need to pump water ->up<- into the storage lake when you
    can simply divert it from the main source which (presumably) is at the
    same altitude as the storage lake?

    And remember that these pumps contribute to base load. If they aren't
    used, then baseload would fall.

    The energy the pumps use is not free, and by rights there should be no
    pumps and no excess base-load to run them.
     
  19. Eeyore

    Eeyore Guest

    No connection between 2 supposed sources of water.

    Why do you think you need 2 sources ?

    Good Lord. You just don't get it do you. The 'main source' has limited flow
    therefore a limit on power capacity. By pumping water up into the lake /
    reservoir you generate extra capacity for peak demand. It's all about peak
    demand you see which is more expensive to provide.

    Pumped storage is way to provide peaking power for little more than the price of
    baseload.

    Coal and nuclear mainly.

    In the examples I know of the 'main source' *is* the lake.

    A reduced baseload isn't much use since large power stations (esp nuclear) like
    working at fixed and a high percentage of full output power.

    Peaking power generation is significantly more expensive than baseload. The
    whole point is that it doesn't result in 'excess' baseload. It takes advantage
    of the lower cost baseload generation when it's not fully stretched.

    Graham
     
  20. If you by energy means heat, to heat your house. Something as easy as
    a timer on your boiler would be a good start. Then insulate the boiler
    for even better effect.

    If you have central heating, you should be able to save quite a bit by
    doing this.

    Even more if you could get a realy big and rely well insulated tank.

    And even better still if you use a heatpump (air/water* or geothermal)
    to heat the water at night.

    *This may depend on the COP at night compared to the COP at daytime.
    Since the outside temperature is lower at night - The price-difference
    may be smaller than with geothermal..
     
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