Capacitor uses

I've been researching capacitors and wonder if they could be used to store and release energy extending solar use after dark. Or with wind turbines to keep energy flowing if the wind quits for a few minutes. Surprisingly there is very little info out there besides using them for HVAC. A run type capacitor seems like it could help a lot. I don't see any drawbacks to their use.
I also wondered about putting an AC capacitor with a breaker box to help stabilize any incoming power rush. maybe someone here is much better educated on this subject.

Ron said:

I've been researching capacitors and wonder if they could be used to store and release energy extending solar use after dark.

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The energy stored is nowhere near enough. We use chemistry for good reason.

Batteries store far, far more energy than capacitors do, even supercapacitors, per weight, and per volume, and per dollar.

There is a use for supercapacitors in a battery application where one needs extremely high surge currents for just a little while, in that a supercapacitor may be significantly smaller, lighter, cheaper, than a bunch of batteries able to supply that same surge load. But for plain old energy storage, it's tough to beat batteries.

The energy stored is:



Where E is in Joules (watt seconds)

Run the numbers for any capacitor you've got and you'll see how much it can store.

Excellent, I've got a spare 50mF, 440V run capacitor for my AC system. It only cost $25. 1/2 * 50e-06 * 440^2 = 4.84 Joules. That works out to 0.00134 Watt hours. Compared to this pathetic little LR44 watch battery which is only 115mAh at 1.5V, or 0.17 Watt hours. Oh that's not a lot.

I use a 6,000uF capacitor bank and it can only store enough useful energy for about 0.4 milliseconds. Half a cycle at 60Hz is 8.3ms.

The best thing about working on RF circuits is that the capacitors only have to store energy for a few, or a few tens of, nanoseconds. 0.1 microfarads is a huuuuge capacitor at 100 MHz.

Needless to say the picture changes at DC.

For comparison sake the AH equivalent of a capacitor is given by AH = F*V/3600

F is farads not mF or uF.

Using the formula a 10 Farad capacitor ( a very large cap ) rated 12 volts and costing about $100.00 will store less than .03 AH.
The same $100.00 will buy you about 50 AH of 12 volts liFePo battery(s).

As metioned above there is no comparison as far as AH storage. The Batteries win hand down.

On the other hand a fully charged 10 F capacitor can deliver an instataneous power of 10,000 watts.

The average Li battery may deliver 100A * 12 volts = 1,200 watts.

A super cap in parallel with a LI battery bank gives you the best of both worlds.

You must pre-charge the Cap before adding it to the circuit and place the cap as close (wire length wise) as possible to the load.
A small inductive reactance between the battery bank and the Cap is helpful. This can be achieved with a long hook up wire

So much for the free lunch. Thanks for the replies.

For entertainment value:

By the @McKravitts formula above, 1.6 Ah. Not much.

With AC, capacitors do not really store electricity, more about filtering and phase shifting. . .

bolhuijo said:

Excellent, I've got a spare 50mF, 440V run capacitor for my AC system. It only cost $25. 1/2 * 50e-06 * 440^2 = 4.84 Joules. That works out to 0.00134 Watt hours. Compared to this pathetic little LR44 watch battery which is only 115mAh at 1.5V, or 0.17 Watt hours. Oh that's not a lot.

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A "run" cap on an HVAC is to create a phase shift for the start winding on the motor. It does not store any energy in circuit, it simply passes it thru slightly delayed. The bigger the capacitor the greater the shift, and the more energy it can pass thru to the winding. I think people get really confused about capacitors in AC circuits, they don't store electricity like in a DC configuration.

ksmithaz1 said:

With AC, capacitors do not really store electricity, more about filtering and phase shifting. . .

A "run" cap on an HVAC is to create a phase shift for the start winding on the motor. It does not store any energy in circuit, it simply passes it thru slightly delayed. The bigger the capacitor the greater the shift, and the more energy it can pass thru to the winding. I think people get really confused about capacitors in AC circuits, they don't store electricity like in a DC configuration.

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I have no idea why it's so hard to explain capacitors & inductors in AC circuits, why it's just a little Ohm's law, complex numbers, phasor math, etc.
Here, let me dig up a basic problem from my EE textbook...

Transmission line can be modeled as a series of those RLC circuits.

bolhuijo said:

I have no idea why it's so hard to explain capacitors & inductors in AC circuits, why it's just a little Ohm's law, complex numbers, phasor math, etc.
Here, let me dig up a basic problem from my EE textbook...


View attachment 230789

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I didn't know it was this easy.

ksmithaz1 said:

With AC, capacitors do not really store electricity, more about filtering and phase shifting. . .

A "run" cap on an HVAC is to create a phase shift for the start winding on the motor. It does not store any energy in circuit, it simply passes it thru slightly delayed. The bigger the capacitor the greater the shift, and the more energy it can pass thru to the winding. I think people get really confused about capacitors in AC circuits, they don't store electricity like in a DC configuration.

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So it would help, in an AC circuit?

A capacitor on the power of a motor will correct the power factor. It can also provide some filtering. Once upon a time the grid was filed with incandescent lamps. These absorbed a lot of spikes. Now spikes travel down a wire and reflect back in ringing. I always have a RC network to absorb spikes on the line.

ricardocello said:

For entertainment value:

https://www.amazon.com/XJPOWER-Maxwell-Capacitor-Battery-Rechargeable/dp/B09P8JGMHJ

By the @McKravitts formula above, 1.6 Ah. Not much.

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Note that the vendor is XJPOWER but the brand is "Maxwell Technologies". There actually is a Maxwell Technologies in the US, which does indeed make capacitors; this borders on trademark infringement and fraud. But, reasons ...

outsider said:

Note that the vendor is XJPOWER but the brand is "Maxwell Technologies". There actually is a Maxwell Technologies in the US, which does indeed make capacitors; this borders on trademark infringement and fraud. But, reasons ...

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Unexpected entertainment value!
i certainly wasn’t endorsing the product, but just to note that these things exist.
I’d be worried about short circuiting one of these and being subjected to the wrath of 10000A.
Thanks for the link.

outsider said:

Ultracapacitor Overview - Maxwell Technologies

Maxwell Technologies leading global supplier of ultracapacitors. Backup Power + Regenerative Power + Burst Power + Quick Charge + Cold Starting

maxwell.com

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In case anyone gets the great idea to put one of their 48V ultracap modules in parallel with a battery, unfortunately, they have a rather limited max voltage of 48.7V, so that won’t play well.

Ron said:

So it would help, in an AC circuit?

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No. Think of a capacitor in an AC circuit as a dual diaphragm pump with a lever between. The input side pushes and pulls the electrons in and out of the chamber on one side of the lever and the chamber on the other side pushes and pulls electrons in and out of that side, back and forth, and back and forth, and ...

The bigger the chamber the more electrons you can push and pull, but you are not really storing anything. There are obviously complexities surrounding reluctance, blah, blah, you can read up on it, but that is the basic idea. A capacitor "inline", in series blocks DC and passes AC. A capacitor in parallel on AC will in practicality do nothing, on DC it will store a charge and thus are generally used to protect against short disruptions in DC supply, generally voltage drop / current loss during short term high load moments, for example on an amplifier that momentarily sinks a lot of current into a speaker on a big bass note.

I had some real nice PABST muffin fans from Germany. Trouble was they were 220V and I wanted to use them on 120V. I used a little trick. A capacitor was placed in series with the fan's power lead. Suddenly I had more voltage on the fan. Go figure.

ksmithaz1 said:

No. Think of a capacitor in an AC circuit as a dual diaphragm pump with a lever between. The input side pushes and pulls the electrons in and out of the chamber on one side of the lever and the chamber on the other side pushes and pulls electrons in and out of that side, back and forth, and back and forth, and ...

The bigger the chamber the more electrons you can push and pull, but you are not really storing anything. There are obviously complexities surrounding reluctance, blah, blah, you can read up on it, but that is the basic idea. A capacitor "inline", in series blocks DC and passes AC. A capacitor in parallel on AC will in practicality do nothing, on DC it will store a charge and thus are generally used to protect against short disruptions in DC supply, generally voltage drop / current loss during short term high load moments, for example on an amplifier that momentarily sinks a lot of current into a speaker on a big bass note.

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If it does almost nothing why do they put start and run capacitors on AC units?

Ron said:

If it does almost nothing why do they put start and run capacitors on AC units?

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Capacitors are used in single phase AC motors to start them rotating in the desired direction. In this application the capacitor is placed in one motor winding circuit to slightly shift the timing of the current that generates the magnetism that drives the rotor. All that crazy math we posted up there can be used to calculate the size of the capacitor needed to generate the correct time shift.

Ron said:

If it does almost nothing why do they put start and run capacitors on AC units?

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If you ignore all the differential equations posted above and just look at the sign of the capacitive reactance you'll notice that it is negative and the inductive reactance is positive and their values are a function of frequency.

In a purely capacitive circuit (one where the resistive component is zero), the current through the capacitor leads the voltage by 90 degrees.
In a purely inductive circuit (one where the resistive component is zero), the current trails the voltage by 90 degrees.
The magnetic field generated by a coil (the windings of a motor), is a function of the instantaneous current.
The windings of a coil in a motor have both reactance and resistance (referred to as impedance). In a motor both the resistance and reactance are positive and hence the magnetic field trails the phase of the applied ac voltage.
If you add a carefully chosen capacitor to the circuit it's reactance is the opposite sign of the inductive hence the net reactance is smaller, and may be positive or negative and the current magnitude and phase are shifted.
So if you add a capacitor to only the starting (usually stator) windings it causes the magnetic fields to be out of phase and applies a torque to the rotor windings and causes the motor to spin.

You can demonstrate this effect with two bar magnets both aligned to lie on same longitudinal axis. Hold one South Pole close to a north pole and the magnets simply pull each other. Next rotate one magnet a few degrees and you'll feel a torque develop.