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# AC power storage

store electricity as AC rather than DC
 (-1) [vote for, against]

Storing power locally (e.g. in the home or office) is useful for UPS (uninterruptible power supply) and for locally produced 'green energy' (photovoltaics, wind energy etc.)

Power is usually stored in batteries and sometimes flywheels. If the power is stored in batteries, it must be converted from AC to DC (for storage) and then from DC back to AC (for use with standard appliances). There are some inefficiencies in these conversions.

I'm wondering whether it would be more efficient to store the energy in AC form. Basically this would involve a bunch of capacitors (and preferably super-capacitors). Each capacitor stores a particular voltage.

As the AC flows into the circuit, the switch shunts the current to the appropriate capacitor in quick succession. When the power is drawn out of the circuit, a switch shunts current from the appropiate capacitor in quick succession.

The switch could be mechanical (e.g. motor spinning at 60 Hz) or electronic (relays or transistors).

 — xaviergisz, Jun 03 2007

illustration http://imgur.com/a/ubflb
[xaviergisz, Dec 13 2011]

Switcheroo Power Supply Switcheroo_20Power_20Supply
As mentioned in an annotation [Vernon, Dec 13 2011]

ring flywheel 2
Shameless plug to AC storage on a flywheel [xaviergisz, Dec 13 2011]

 The biggest hurdle is the relatively high price and bulk of capacitors. You could use your multi-voltage arrangement with batteries rather than supercaps.

 If the system is large enough to make the capital cost of a complicated system worthwhile, you might be able to do something like this:

 1) Charge a series of capacitors to various steady voltages in your fashion (I'd prefer solid-state switches to anything electromechanical, but the voltages have to be high enough to make the forward voltage drop of the electronics negligible).

 2) From each capacitor, run a switchmode voltage converter to the same voltage as all the others, and use all of them to charge the same battery at a single voltage.

 3) Providing AC power from the unit is the reverse process.

Note that the efficiency of switch mode voltage converters can be very high if both voltages are constant - the usual sine-to-square wave inefficiency disappears, because both sides of the device are square waves. Efficient transformers for square waves are INTERESTING, but possible (if you're not careful, the various harmonics behave differently, and you don't end up with a square wave at all).
 — Cosh i Pi, Jun 03 2007

I might be wrong here but don't capacitors have all sorts of toxic chemicals and other not-nice stuff inside them? Do I really want that kind of an oozing liquid mass in or near my house if there was to be a major catastrophe? I'll take DC for 500 please, Alex.
 — Canuck, Jun 03 2007

 [Canuck] You're right about capacitors and toxic chemicals - in general. But the same could be said about batteries. There are less toxic batteries around now, but it's always a compromise between cost, performance, and environmental friendliness. There are less toxic types of capacitors, too, and if the demand was high enough they might be produced reasonably cheaply, and might achieve decent performance.

 One area of the design of both batteries and capacitors where there's some room for manoeuvre for this application is size: both batteries and capacitors tend to be made as small as possible for their performance, and that isn't such an issue in this application. We could trade off a bit of minaturization in exchange for greater environmental friendliness.

 [xaviergisz] One point worth noting is that computers and suchlike usually take the mains and then have a PSU that produces low voltage DC. This makes UPSes look extremely silly and inefficient: take AC mains, produce low voltage DC to store in batteries, generate high voltage AC from the batteries, then convert back down to low voltage DC in the computer.

Why not just run the computers straight off the batteries, without ever recreating the high voltage AC at all? More efficient (so a given size of battery lasts considerably longer running the computers) and more reliable, too. (I did this with a classroomful of BBC micros many years ago - after the UPS provided by the management proved less reliable than the mains!)
 — Cosh i Pi, Jun 03 2007

Sorry for the churn. The image hosting site I was previously using has disappeared, so I've reposted the illustration using a different image hosting site.
 — xaviergisz, Dec 13 2011

 You could store AC more simply as the kinetic energy of a rotating flywheel, via an AC motor/generator.

Also, there are some similarities between this Idea and my older "Switcheroo Power Supply" Idea (linked), although that one was not about storage; it was about AC/DC conversion.
 — Vernon, Dec 13 2011

...or you could use the AC to store potential energy - e.g. by lifting a 500-ton weight up on extending stilts above your house - which can then be slowly released through some sort of gearing and a dynamo to generate electricity when the AC fails.
 — hippo, Dec 13 2011

 Yeah, a flywheel would be better for storing large amounts of power. This idea might be more appropriate as a short-term uninterupted power supply (UPS).

Thanks for the link to the "Switcheroo Power Supply" idea. Great minds think alike and fools seldom differ ;)
 — xaviergisz, Dec 13 2011

 You would want a low-pass filter to smooth the output and create a cleaner sine wave; in fact, the minimum requirement is two capacitors (the Nyquist rate) or one capacitor and a polarity switcher, but that necessitates a far better (= expensive) filter and/or more unwanted harmonics, and reduces this idea to existing solutions.

Anyway, you really only need a quarter wave worth of capacitors (or half, if you don't want to reverse the polarity), as sin(pi/4) = sin(3pi/4) = -sin(5pi/4) and so forth.
 — spidermother, Dec 13 2011

storing energy in capacitors. yes, when we can do that well, and safely, then we will be somewhere. this idea has no other merit. fail.
 — WcW, Dec 14 2011

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