Olin Lathrop wrote:
> > The question is do you lose 100% of the unused electricity, or perform a
> > scheme like this and only lose 40%-70%  (I've not seen figures, only
> > guessing).
> >
> > That is a question for a cost-benefit analysis - does building the
> > compressor and turbine generator cost less than the savings in energy,
> > and given the cycles that people use electricity in does it make sense
> > to use such a shaft as a battery to supply the greater demend during the
> > day, even if the cost-benefit isn't, by itself, a good plan.  It may be
> > cheaper to build this project than to upgrade a power plant to provide
> > for the peak demand, when that newer portion of the plant will not be
> > used 90% of the time, or perhaps the plant costs about the same to
> > produce a lot of energy as it does to produce a little, so it's best to
> > run it at its peak efficiency and then store the excess elsewhere.
>
> Yeah, it may still be worth it, but thermally uninsulated compressed air
> just doesn't sound like a good way to store energy.  (Maybe they are
> insulating it).  There's a power plant around here that uses excess capacity
> to pump water into a reservoir at higher altitude.  During peak demand the
> reservoir is drained to run turbines as with any hydro power plant.  At
> least this scheme doesn't suffer from large inherent losses due to basic
> physics.  I sorta remember efficiencies around 80%, but I could be off.
> I'll try to find a reference if anyone is interested.

There is a company in the US (in Texas) that stores natural gas in big
underground caverns. They actually make the caverns themselves out of
salt deposits. The salt removal process involves a lot of water. Since
they have all of the water on hand they also use it to maintain a
constant pressure in the caverns. Gas goes in salt water comes out, gas
comes out water goes in. Supposedly that scheme is to keep the pressure
in the cavern high enough that it doesn't collapse, but I would imagine
that it helps with energy loss as well (constant temperature).

It seems to me that if you're going to compress and store something for
later use, natural gas would be more energy efficient than air. Any
energy lost or gained during compression/extraction would be small
compared to the energy from burning the gas.

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