Why not use capacitors to store power instead of batteries ?

[Mike90250]

not a crime to use old threads

Caps have internal inductance and resistance, and the cables connecting them would need to be same beefy cables as battery cables. Caps are 50x size of battery for amount of USEABLE power.
example : Only using 12.75 to 12.73V = just a .02v difference. Then the easily useful power has been consumed from the cap and it's all on the battery.
Ripple caps can be found in large inverters, and they do supply this peak demand, via low inductance internal traces in the inverter.

[FreeBTU]

I hope it's not against the forum rules to reply to the old thread...

Perhaps peak loads are a good use of CAPS.
Many devices have power on peaks that are much higher than the normal load. Such peaks last only a second or less. Not much of an issue with many small appliances but those 1kW devices may cause a serious peak load,
Instead of over sizing a the battery system a CAP may be a good way to handle the peak and keep the voltage stable.
An added benefit is that caps smooth out the voltage during non peak use.

Just a thought.

[FunGas]

Super capacitors have a lower power density than batteries (they take up far more space).

Repeated high current charge / discharge is what they're best at.

I have a string of six 10F supercapacitors charging from a small solar photovoltaic string, it closes a relay using a Zener diode when the string reaches 10.5V
The relay discharges the supercapacitors into extractor fans until the Voltage drops to about 4.5V, then the relay opens and the cycle starts again.

The whole cycle takes around a minute and runs when there is sunlight on the panels. Been running for about 6 months now with no maintenance (apart from trimming the trip Voltage).

The fans wear out eventually, but I have an endless supply of free ones and made it easy to change them out...

[Sunking]

My citizen solar watch is operated by some type of capacitor NOT a battery.

Wrong, it is a lithium battery, not a Super Cap

[SunEagle]

My citizen solar watch is operated by some type of capacitor NOT a battery.
It only must be charged once every 6 months. I know this for sure because I haven't worn it for several years and keep it in the house in a drawer. I have to charge it in the sun about twice a year. It continues to run the rest of the time without charge. The citizen company and many other experts in the field say this "capacitor" should work well for 20 years or more. OK, now enlarge this unit and adapt it to residential solar applications. Sell a bunch of them to get the price down. How can this be wrong?

Well for starters that Lithium Ion "cell" inside the watch is a battery. The advertisement states you will never need to replace a battery. Maybe because that "cell" can't be replaced. Once it stops holding a charge you replace the entire watch. Or maybe you send the watch back to the factory and they change out the "cell". So technically the owner never has to change a battery.

As for enlarging that type of "cell" for bigger uses. That is already being done in most Electric Vehicles. The problem is that those type of batteries are very expensive

[Mike90250]

My citizen solar watch is operated by some type of capacitor NOT a battery.
It only must be charged once every 6 months. I know this for sure because I haven't worn it for several years and keep it in the house in a drawer. I have to charge it in the sun about twice a year. It continues to run the rest of the time without charge. The citizen company and many other experts in the field say this "capacitor" should work well for 20 years or more. OK, now enlarge this unit and adapt it to residential solar applications. Sell a bunch of them to get the price down. How can this be wrong?

Not sure what the sales droid told you, but it's a BATTERY, with about a 20 year life:

http://www.citizenwatch.com/en-us/eco-drive-technologies/eco-drive-how-it-works/

Citizen Eco-Drive technology harnesses the power of light – from any natural or artificial light source – and converts it into energy which is stored in a permanently rechargeable lithium-ion cell.

[BBobbert33]

What about my watch capacitor

Super caps are available for a price.
There are a many reasons they are not good as replacement for batteries for storing energy in solar systems .Here just a few.
1. EXPENSIVE compared to lead acid batteries.ie about $50,000 to equal about $1000 of lead acid batteries
2.Very limited lifetime in charge discharge cycles compared to lead acid batteries.
3.There are no suitable solar chargers available to charge them.

My citizen solar watch is operated by some type of capacitor NOT a battery.
It only must be charged once every 6 months. I know this for sure because I haven't worn it for several years and keep it in the house in a drawer. I have to charge it in the sun about twice a year. It continues to run the rest of the time without charge. The citizen company and many other experts in the field say this "capacitor" should work well for 20 years or more. OK, now enlarge this unit and adapt it to residential solar applications. Sell a bunch of them to get the price down. How can this be wrong?

[Sunny Solar]

If its representative of all Maxwell super caps I don't know . But the 100,000 f x12vbank we have at work lost about 30% of their capacity after about 6 months of not continuous use .They are now also harder to charge.(takes longer) Only used mostly for destructive purposes. They were discharged to "dead flat" with each use. It would be interesting to know how fully charged and discharged they get when used in vehicles to recover regenerative braking power.

[Mike90250]

Maxwell Super caps have lifetimes:

http://www.maxwell.com/products/ultracapacitors/docs/1014627_boostcap_product_guide.pdf

7.8 Life

Ultracapacitor life is predominantly a
ffected by a combination of operating
voltage and operating temperature.
The ultracapacitor has an unlimited
shelf life when stored in a discharged state. When referring to
ultracapacitor life the data sheets
reflect the change in performance,
typically decrease in capacitance and
increase in resistance. The life
specified by industry standards is a 20% decrease in capacitance and/or
200% increase in resistance. The ultracapacitor does not experience a true
end of life rather the performance continually degrades over the life of the
use of the product. End of life will be when the ultracapacitor performance
no longer maintains the application requirements. This may be different
from that specified on the data sheets.
The typical degradation behavior of the ultracapacitor resembles that of an
exponential decay. The majority of the performance change occurs during
the initial use of the ultracapacitor and this performance change then levels
off over time. The most dramatic effect of the life degradation is on the
internal resistance of the device.

7.8.1
Capacitor replacement

[Sunking]

Capacitors have limited cycle lifetime. In solar applications, they are used as ripple filters, but the problem is at 60 Hz, hundreds of thousands of cycles keep adding up, and in less than 5 years of 24/7 use,

Mike he is talking about Super aka Ultra Capacitors in place of batteries (DC). Ultra Caps have almost infinite cycle life. In modern EV's they are used for regenative braking and act as a buffer between battery and more to capture the huge energy surge from braking. Then released back into either the batteries or drive train.

[Mike90250]

Capacitors have limited cycle lifetime. In solar applications, they are used as ripple filters, but the problem is at 60 Hz, hundreds of thousands of cycles keep adding up, and in less than 5 years of 24/7 use, you are approaching the end of their design life. So they (smaller caps) are used at a few key points, and hopefully properly de-rated, and 10 years life should be achievable.

[Sunking]

On the positive side they are:

• Almost unlimited cycle life into 100's of thousands.
• Very low Internal Resistance
• Very high charge/discharge rates.

On the Downside:

• Extremely low specific and volumetric energy density on the order of 1/5 to 1/10th of lead acid batteries
• Very high $/wh cost on the order of 10 times more than LiPo
• Extremely high self discharge rates
• Cannot use all the energy stored in them.
• Must use Balanced Chargers when there are two or more used.

So pick anyone of the disadvantages you want and it eliminates them for Renewable Energy application.

[russ]

Super caps are available for a price.
There are a many reasons they are not good as replacement for batteries for storing energy in solar systems .Here just a few.
1. EXPENSIVE compared to lead acid batteries.ie about $50,000 to equal about $1000 of lead acid batteries
2.Very limited lifetime in charge discharge cycles compared to lead acid batteries.
3.There are no suitable solar chargers available to charge them.

Hi John, Outside of those minor points there aren't any other problems?

[Sunny Solar]

Super caps are available for a price.
There are a many reasons they are not good as replacement for batteries for storing energy in solar systems .Here just a few.
1. EXPENSIVE compared to lead acid batteries.ie about $50,000 to equal about $1000 of lead acid batteries
2.Very limited lifetime in charge discharge cycles compared to lead acid batteries.
3.There are no suitable solar chargers available to charge them.

[digimart]

Large CAPS are available and we do use them to store energy, problem is that all CAPS discharge very quickly and then they are done until recharged, they are designed for a different purpose than a battery and thus are ill suited to deliver slow discharge over long periods like we need a battery to do.

Thanks for reply.
First, allow me to say that I am not an engineer, so its possible, my question(s) seem irrtational to you.

I understand your point that CAPS discharge quickly. But is this quick discharge through supply of stored current ? And if thats the reason, then say we have a 10A load to a CAPS bank of 400AH. So with this 10A demand, will the CAP still discharge quick ? If yes, then where will that additional electric current go ?