the power of capacitance

[Dr_Doggy]

hi, I was just wondering what size of (hypothetical) capacitor i would need to produce one watt hour of energy

 

[crutschow]

The energy in a capacitor is ½CV² joules (watt-seconds) so it involves both voltage and capacitance. And to extract all the energy you need to discharge the capacitor to zero volts.

A watt-hour is 3600 joules.

 

[bychon]

1/2 CV^2 = watt-seconds
1/2 CV^2 = 3600
CV^2 = 7200
V^2 = 7200/C
for 1000 microfarads,
V = 268.3 volts

So, it's possible, but for what purpose is a mystery to me.

 

[EternityForest]

personally i think capacitors are a great way to store power because they are made of much less toxic materials then batteries and can be recharged millions of times. the only problem is that they are rather large and expensive right now for storing
watt-hours of power.

there is such a thing as an ultracapacitor that stores a lot of farads at low voltage, this would be the cheapest way to store power in a cap.

the most cost effective capacitor i know of is digi key part number 493-3316-ND
which costs 15$ for one and has 100 farads with a maximum voltage of 2.5 volts.

i have also heard of people making their own ultracapacitors with activated charcoal and water. looked pretty easy to do and there's a thread about it on ultracapacitor.org

also keep in mind that the discharge of a capacitor is linear not like a battery which has an almost flat voltage till its almost run out. From wikipediea:

A farad is the charge in coulombs a capacitor will accept for the potential across it to change 1 volt. A coulomb is 1 amperesecond. Example: A 47 mA current causes the voltage across a capacitor to increase 1 volt per second. It therefore has a capacitance of 47 mF

So if you take a capacitor of 1 farad charged to 2.5 volts, and it supplies a current of one amp for one second, it will then only have 1.5 volts in it.

this puts a practical limit on how far you can discharge it before there is not enough voltage to be useful.
but, if you have a good step up regulator and enough capacitors you can store energy this way.

another note the leakage current from an ultracapacitor is more then a normal cap so take that into consideration

 

[JimB]

1/2 CV^2 = watt-seconds
1/2 CV^2 = 3600
CV^2 = 7200
V^2 = 7200/C
for 1000 microfarads,
V = 268.3 volts I think you slipped a decimal place here, I make it 2683 volts. JimB

So, it's possible, but for what purpose is a mystery to me.

I now have to write something here to make the minimum character count.
JimB

 

[bychon]

Bad calculator! Go stand in the corner and think about what you're done!

 

[Dr_Doggy]

so to power a 100watt lightbulb for one hour i would need :

360000/2 = C 120^2
180000/14400 =c
12.5

12.5 Farads @ 120volts ??????????????????????????????????????????

so whats the closest thing out there, not worried about price, also what would be the value on a cap this sized?

 

[Nigel Goodwin]

so to power a 100watt lightbulb for one hour i would need :

360000/2 = C 120^2
180000/14400 =c
12.5

12.5 Farads @ 120volts ??????????????????????????????????????????

so whats the closest thing out there, not worried about price, also what would be the value on a cap this sized?

Batteries are FAR smaller, FAR cheaper, and FAR more useful.

Capacitors are pretty crap as energy storage - for a start their voltage decays rapidly, and exponentially.

To build capacitor storage of 12.5F at 120V is going to be VERY expensive, and VERY large. Use 10 car batteries in series, for a fraction of the cost, a fraction of the size - and many, many times the storage capacity.

 

[Voltz]

so to power a 100watt lightbulb for one hour i would need :

360000/2 = C 120^2
180000/14400 =c
12.5

12.5 Farads @ 120volts ??????????????????????????????????????????

so whats the closest thing out there, not worried about price, also what would be the value on a cap this sized?

You've done it the wrong way round, its
100*3600 = 360000
36000 = ½CV²
if V = 120 then
360000*2/(120²) = 50F
if V = 2.5 then
360000*2/(2.5²) = C = 115200F
If V = 200
360000*2/(200²) = C = 1.8F

The problem is, no electrolytic capacitor is rated at 200V which leads on to the fact that, no capacitor has a value of 1.8F except electrolytic capacitors, so the capacitor you need doesn't exist and as you can see it's not a good way to store energy unless your power draw is tiny

 

[Nigel Goodwin]

The problem is, no electrolytic capacitor is rated at 200V

Electrolytics of 200V (and higher) are common place, and always have been.

 

[Voltz]

Electrolytics of 200V (and higher) are common place, and always have been.

Hmm, true - well as far as I can find there are no commercially available electrolytics rated 200V that have a capacitance not measured in uF

 

[tvtech]

Electrolytics of 200V (and higher) are common place, and always have been.

True, SMPS's running off 220VAC mains normally have their main smoothing cap/s (DC rating obviously) @ around 350 to 450 Volts. And they can give you a nice burn and a shock if you neglect to discharge them (mains unplugged obviously) before you start working on the PSU. Talk about burning your fingers....

In the TV game this happens always when a set is stuck in standby or the Line Stage does not start up as normal.

Main smoothing cap/s keep their energy stored for a while as there is no other drain path available...been bitten many times.

Cheers

 

[Noggin]

I recently helped design a professional flash unit, it used 4 caps of 1200uF charged to 520v. At maximum flash power the caps would discharge to about 30 volts.

I think that comes out to roughly 650 watt-seconds, or 0.180 watt-hours? That doesn't seem like much

 

[Voltz]

True, SMPS's running off 220VAC mains normally have their main smoothing cap/s (DC rating obviously) @ around 350 to 450 Volts. And they can give you a nice burn and a shock if you are stupid enough not to discharge them (mains unplugged obviously) before you start working on the PSU. Talk about burning your fingers....

In the TV game this happens always when a set is stuck in standby or the Line Stage does not start up as normal.

Main smoothing cap/s keep their energy stored for a while as there is no other drain path available...been bitten many times.

Cheers

If your stupid enough not to bleed PSU caps before working with it you'll get a hell of a lot more than burnt fingers

 

[tvtech]

If your stupid enough not to bleed PSU caps before working with it you'll get a hell of a lot more than burnt fingers

Stupid is kind of a harsh word don't you think??

I work on between 5 to 20 TV's a day. Depending on what comes through the door for repairs. I don't purposely try and shock myself. I know the rules.

Being fixing TV SMPS for ever and a day. And have fixed some of the very first successfully. The Thorn 4000 was the first set South Africa saw that had a SMPS. A piece of junk that was imported to our shores early 1974....and was a POS.

I know the rules about working safely with SMPS.....(I probably wrote part of the book). I get busy and sometimes forget.

Anyway...back on Topic:

As pointed out above (maybe)...Electrolytic caps are basically a reservoir, not a battery. Electrolytic caps were not designed/invented for charge storage purposes. Batteries excel at that.

Over and out guys. Have a great evening.

 

[Nigel Goodwin]

Being fixing TV SMPS for ever and a day. And have fixed some of the very first successfully. The Thorn 4000 was the first set South Africa saw that had a SMPS. A piece of junk that was imported to our shores early 1974....and was a POS.

The worlds first domestic use of a SMPSU was in the Thorn 3000 series, which was a great TV - the 4000 was 'OK', and that's about it - not a patch on the 3000.

 

[Nigel Goodwin]

Hmm, true - well as far as I can find there are no commercially available electrolytics rated 200V that have a capacitance not measured in uF

You never made any mention of values, just the voltage.

You won't get farad sized single capacitors because of the size and the price.

But you could use 1000 capacitors of 1000uF in parallel to give 1F?, like I said huge and expensive

 

[tvtech]

The worlds first domestic use of a SMPSU was in the Thorn 3000 series, which was a great TV - the 4000 was 'OK', and that's about it - not a patch on the 3000.

Good Man you Nigel. Now we are more or less on the same wavelength and all.

Cheers mate....we are talking years of stuff here.

Great Moderator comes to mind. And that's you.

 

[EternityForest]

if my calculations aren't wrong:
48 of these http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=493-3326-ND
in series would be 2500 farads / 48 in series=52 farad. at a voltage rating of 2.5*48=120 volts.
so 120 volts at 52 farads. if money really isn't an issue you could build that , but the peak current if you shorted it would be insane because each cap has an equivalent series resistance of 4.0 milliohms.
you'd want to be real careful

so it might be better to connect them in parallel for 120,000 farads at 2.5 volts.
if you discharge from 2.5 to 1 volt that's 50 amp hours! That would be an awesome capacitor bank for sure! the price would be around 8k$ though. i think it could put out around 40,000 watts peak too, and according to E=(C*V^2)/2 would store abut 104 watt hours if you could discharge it completely.

These appear to have the same lifetime rating as electrolytic capacitors(2000hrs @ 60C), which can last longer then batteries by far under certain conditions. assuming the same formula for electrolytic capacitors to calculate lifetime(a factor of two improvement for every 10C reduction) that should be 2 years minimum at 30C, or 8 years at 10C. if you were doing something like a high altitude sensor or something where it didn't get to hot (or you insulated the capacitors and had a thermoelectric cooler cooling them) they could last a really long time.

so they are definately useful for energy storage for some uses. one great feature is that being capcitors as long as you stay under the rated voltage you can't over charge them, they just stop drawing power. this makes them great for when you don't want to build charge control circiutry. they also have very good peak current and cycle life (500,000+).

so i would say they are useful for some things but you should always use the right tool for the job.

 

[Nigel Goodwin]

So 100 of them is over $14,000 (so assuming 100 rate, $7000 odd for 48), how much would 10 car batteries cost? - which would supply more power for longer, a MUCH more stable voltage, and last longer as well.

Certainly in my experience of super capacitors they have always been very unreliable - a common cause of failure in the cheap VCR's that used them.