h a l f b a k e r y
Inexact change.
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The biggest hurdle is the relatively high price and bulk of capacitors. You could use your multi-voltage arrangement with batteries rather than supercaps. |
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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: |
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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). |
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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. |
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3) Providing AC power from the unit is the reverse process. |
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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). |
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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. |
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[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. |
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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. |
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[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. |
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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!) |
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