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# DIY Capacitor

05/10/2010 5:55 AM

I'd like to know how much energy (watt-hours) can be stored in a capacitor made from two rolls of aluminum foil. The foil is the kind one buy's at a restaurant supply. The ones I have are 18"x500ft x about .001 inches thick. If I rolled two of these together with a dielectric between them and another dielectric separating them, how much energy storage could I get? Would 6 mil polyethelene be ok as a separator?

Would someone knowledgeable in this field give me some guidance?

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#1

### Re: DIY capacitor

05/10/2010 7:25 AM

Not very much, I fear.

I may've slipped up (if so, I apologize in advance), but back-of-envelope stuff gets me to about 50μF for the capacitor (parallel plate approx).

Taking a breakdown voltage of 500V, that gives maximum possible stored energy of about 7 Joules. (1 Joule = 1 Watt.second).

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#2

### Re: DIY capacitor

05/10/2010 8:26 AM

Thanks. That's not much storage. I was hoping to get about 100watt hours.

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#7

### Re: DIY capacitor

05/11/2010 3:04 AM

Parallel_plate_model

.

Where ε is the relative permittivity.

As others have pointed out you can nearly double this because with a roll you're using both sides of the foil.

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#11

### Re: DIY capacitor

05/11/2010 12:12 PM

Then use a rechargeable battery instead.

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#3

### Re: DIY Capacitor

05/10/2010 9:13 AM

I experimented with something similar. I rolled one roll of foil through the laminator and then laminated another piece directly on top. I was terribly disappointed it didn't store much charge at all. I made some leyden jars that worked much better.

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#4

### Re: DIY Capacitor

05/10/2010 1:34 PM

I've made several of these for home-built Tesla Coils, though not quite this large.

18 [in] = 0.457 [m]
500 [ft] = 152.4 [m]
6 [mil] = 152.4e-6 [m]

e0 = 8.854e-12 [F/m]
er = 2.25

C = e0 * er * Area / spacing

C = 8.854e-12 [F/m] * 2.25 * 0.457 [m] * 152.4 [m] / 152.4e-6 [m]

C = 9.1e-6 [F] or 9.1 [uF]

If the poly is PREMIUM quality (uniform thickness with no voids or defects) and/or is laid up in layers (like 3 x 2 mil), you can raise the voltage up to 500V/mil. I've pushed it to 1kV/mil, but that won't last long in a home-made capacitor.

If V = 3000 [V]

Energy stored is

E = 1/2 * C * V^2

E = 1/2 * 9.1e-6 [F] * 3000^2 [V^2]

E = 41 [J]

As noted in post #1, this is 41 [watt-seconds].

That is enough energy to really HURT you if you accidentally shock yourself, but not very useful for any practical energy storage. I believe the calculations above are good for a simple parallel plate capacitor. Corrections for any errors are welcome. There is 2x capacitance if you properly "roll" up the plates which could increase energy 2x. Still not enough practical storage, but it makes a very nice Tesla Coil display.

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#5

### Re: DIY Capacitor

05/10/2010 7:30 PM

- I missed the "6 mil" bit - I'd taken the dielectric thickness as 0.001".

By my revised calcs[1], I get C~9μF, and the breakdown voltage as ~3kV, giving ~40J max.

Still not very much .

[1] I set up a spreadsheet (and saved it in my CR4 directory ) - so all I had to do was change 0.001 to 0.006 for the thickness. I took the breakdown voltage as 20MVm-1 from Wikipedia.

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#6

### Re: DIY Capacitor

05/11/2010 12:51 AM

I was once associated with an electrolytic capacitor manufacturing process by electrolysis process. The capacitor foil was etched in hot salt solution (I think) and it consumed nearly 60% of the power required for the entire company. The turnover was low, margins were low compared to other products.

The area of a foil is computed as length x width - by school maths- not so with etched foil. The etching process gives hills and valleys at micron level and gives a multiplying factor - of about 10 to 20 in 1978-80. Now it must be 100 to 200 or more- without the foil getting pierced through or tearing. Foil is 99.99% or better in purity. All this is not applicable for kitchen foil. The capacitance made from kitchen foil is for kids to understand the principle- not an industrial level product. Hope you understand. Further the dielectric gives additional improvement is capacitance- which was vacuum impregnated into the wound foil with dielectric absorbing paper foil- then!!!

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#8

### Re: DIY Capacitor

05/11/2010 7:33 AM

So, even with a 200 times capacity using special designed foil and dielectric, I could only get about 2 watt-hours of storage. That's a long way from 100 watt-hours. I wish there was a way to DIY a capacitor that would hold 100 watt-hours of energy. Then, make 20 of them to have 2kwh of storage. Then, install a 4kw solar panel system. The 4kw solar panel would charge the capacitors during the day and provide power for a home. At night, the home would get power from the capacitors alone. My home uses a max. of 4kwh per day during the summer. A system like this would have a long life cycle. And, using multiple capacitors would reduce the seriousness of a short. What would be the equialent amount of TNT of 100 watts? There's a downside to nearly every good thing. The down side for this scheme is the severity of an explosion and the initial cost of the system.

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#9

### Re: DIY Capacitor

05/11/2010 9:22 AM

If you are looking for something cheap (that is likely to fail within a short time just a home made storage capacitor), you could recycle old electrolytic capacitors from junk motor drives etc... Easier than winding your own.

If you place enough in parallel you will store enough energy to create an electric bomb!...

Joke aside, Capacitors are a dangerous form of energy storage as they can release their energy within micro-seconds. It becomes similar to a lightening strike. A good storage medium has a controlled rate of energy release that is just above the needs but not by much. This makes it intrinsically "safer".

There a safety rule that high voltage capacitor banks should (must) self discharge within 10 minutes to minimize the chances of accidents after powering off a device. This goes against energy storage applications. You will also have terrible arc flash issues.

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#15

### Re: DIY Capacitor

05/12/2010 12:36 AM

Some things to consider:

one stick dynamite = 2,000,000 J
one 12V lead-acid car battery = 2,000,000 J
100 W-hrs electric = 360,000 J
2000 W-hrs electric = 7,200,000 J

Approximate volume energy density comparison...

gasoline = 33,000,000 J/Liter
car battery = 800,000 J/Liter
supercapacitor (EDLC) = 80,000 J/Liter
electrolytic capacitor = 8,000 J/Liter
film capacitor = 800 J/Liter

You can see why we like fossil fuel. The energy density is just better than anything else currently available. Even when we throw away 75% of the energy in gasoline (as waste heat) it is still one of the best energy storage mediums. Chemical energy storage wins for now, but don't let that stop you from trying to find better alternatives.

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#17

### Re: DIY Capacitor

05/12/2010 2:19 AM

GA

Another good way to store energy is to heat up water. Although it takes up a bit more space, you can get old plastic storage tanks very cheaply, and, you can store over 100,000,000 J by heating one m³ of water from 25°C to 50°C.

Water has the highest specific heat capacity (per unit volume) of any substance.

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#18

### Re: DIY Capacitor

05/12/2010 4:35 PM

Unfortunately fossil fuels are a limited resource and don't we all wish it would last forever. We need far more research to develop alternate sources of energy before the diminishing supply of OIL triggers a world war. The gulf war , was the first over oil. Capacitor technology has come a long way. I've seen supercapacitors rated at 5000 Farads and 2.7 VDC. Great for temporary storage of power, but lethal. Here's an interesting article on super caps http://www.supercapacitors.org/

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#19

### Re: DIY Capacitor

05/12/2010 4:54 PM

"Great for temporary storage of power ..."

Unfortunately not all that great - the voltage capability needs to get much higher to store really useful amounts of power (as I assume you know, the energy stored is proportional to the square of the voltage). 5000F @ 2.7V will only store 18225J (about 5Wh [5 watt-hours]).

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#10

### Re: DIY Capacitor

05/11/2010 9:59 AM

You already have several practical answers, but you did not ask just how to make the capacitor. In general energy is stored on the dielectric material which you have suggested but not specified. You also did not say if you want to create a DC or an AC capacitor. The energy stored is directly proportional to the dielectric constant of the insulator and its area and inversely proportional to the plate spacing. The maximum voltage which you have not specified is also required since the energy stored is proportional to the square of the voltage. To maximize energy storage you will need a quality material with uniform thickness and a high dielectric constant and high breakdown voltage. If you want to get the most bang for your aluminum foil you would form a thick aluminum oxide on its surface and roll it with a thin sheet wick material inside a conductive container filled with an electrolyte solution that maintains the oxide coating. Wet capacitors generally have higher capacity per unit volume at a given voltage than dry. You must maximize the voltage to store the most energy.

Good Luck,

Luther M

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#12

### Re: DIY Capacitor

05/11/2010 4:09 PM

AC capacitor? I thought only DC current can charge a capacitor? Won't the cap discharge with each alternation?

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#13

### Re: DIY Capacitor

05/11/2010 5:07 PM

different materials and arrangements have different properties.

AC caps have faster charge and discharge properties, but still create the RC wave shape.

Any difference of electrical potential will charge or discharge.

Chris

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#14

### Re: DIY Capacitor

05/11/2010 7:39 PM

He may be thinking of an electrolytic cap.

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#16

### Re: DIY Capacitor

05/12/2010 12:37 AM

The target application you have in mind to say power your house for 100 w for atleast 10 hours during night time- just from a charged capacitor is not practical with known technologies- and worse from home made, crude capacitors.

That is one issue I had discussed in another blog - how do batteries work? They do not follow the same equation like capacitor depending on area, distance between plates and relative dielectric constant. It store more energy than capacitor for same money and much smaller size.

Well think of alternate ways to achieve your objective.

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#20

### Re: DIY Capacitor

06/12/2010 6:12 PM

Batteries would be a much better solution, solar panels produce a dc current and generally put out a lower voltage which isn't very useful for capacitors or powering homes. Lead acid batteries are made of just that; Lead and (sulferic)Acid, both of which are easy to come by if you are dead set on this being a "do it yourself" project. The easyest way to accomplish this would be to just use several deep cycle batteries for power storage, as they will last longer than regular car batteries, and a power inverter to step the voltage up to a 110volt ac supply. The solar panel charges the batteries, the batteries dump current into the inverter and the inverter puts out a electric source that is usable in your home. Much cheaper than capacitors and all the components are readily available at relatively low costs(with perhaps the exception of the solar panel). I am unsure of how many batteries you would need, how many solar panels it would take to charge them, or what wattage inverter would meet your needs, but speaking realistically this is probably the best way.

Great project, I hope you follow through with it.

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