DIY Capacitor

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

Parallel_plate_model

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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.

Then use a rechargeable battery instead.

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.

- 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.

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.

Thanks for your comment

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.

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.

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/

"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]).

Dear Papadoc,

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

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

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

He may be thinking of an electrolytic cap.

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.

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.