Practicality of using capacitors as a power source

At first glance it appears that, at least for some applications, employing
capacitors as a means to store energy has considerable advantages over the
need to rely on batteries. Yet, batteries tend to be used almost
universally. Is there a reason for this? Can anyone more knowledgeable on
the subject comment on the practical implications of relying on capacitors
as a power source instead of other standard means such as batteries?


Thanks in advance,
Rui Maciel

At first glance it appears that, at least for some applications, employing
capacitors as a means to store energy has considerable advantages over the
need to rely on batteries.  Yet, batteries tend to be used almost
universally.  Is there a reason for this?  Can anyone more knowledgeable on
the subject comment on the practical implications of relying on capacitors
as a power source instead of other standard means such as batteries?  

Thanks in advance,
Rui Maciel

At first glance it appears that, at least for some applications, employing
capacitors as a means to store energy has considerable advantages over the
need to rely on batteries.  Yet, batteries tend to be used almost
universally.  Is there a reason for this?  Can anyone more knowledgeable on
the subject comment on the practical implications of relying on capacitors
as a power source instead of other standard means such as batteries?  

Thanks in advance,
Rui Maciel

Here are a couple of web sites on the subject of using capacitors as a
power source.

http://en.wikipedia.org/wiki/Electric_double-layer_capacitor

http://www.alexanderbell.us/Initiative/GEL.htm

Alexander Bell's site seems to me a bit biased and doesn't give a fair
representation of the trade offs. Remember the words of the great
Oliver Heaviside. "Some think electricity is energy. Others think
electricity is power. Some manage to think both these things at the
same time."

Capacitive power storage and battery power storage are two different
animals with different applications. Capacitors have high power
density. Batteries have high energy density. So if you need high power
for a very short time, you use a capacitor. Think flash bulb. If you
need low power for a very long time, you use a battery. Sometimes you
need a combination of both. You use the battery to store the energy
and charge the capacitor. Bursts of power are then taken from the
capacitor.

http://www.mpoweruk.com/alternatives.htm

The energy storage ratio is extreme, four or six orders of magnitude.

John Larkin wrote:

The energy storage ratio is extreme, four or six orders of magnitude.

Yes, it appears that those values are coherent with the information which
has been presented in this thread. Yet, in some applications energy density
may not be the decisive property of a energy storage device. For example,
in applications such as small LED flashlights energy density may be less
important than recharging speed and even the ability to recharge them
through simpler means, such as mechanical generators. Is there any reason
that makes this sort of applications impractical?

At first glance it appears that, at least for some applications, employing
capacitors as a means to store energy has considerable advantages over the
need to rely on batteries. Yet, batteries tend to be used almost
universally. Is there a reason for this? Can anyone more knowledgeable on
the subject comment on the practical implications of relying on capacitors
as a power source instead of other standard means such as batteries?


Thanks in advance,
Rui Maciel

John Larkin wrote:
The energy storage ratio is extreme, four or six orders of magnitude.

Yes, it appears that those values are coherent with the information which
has been presented in this thread.  Yet, in some applications energy density
may not be the decisive property of a energy storage device.  For example,
in applications such as small LED flashlights energy density may be less
important than recharging speed and even the ability to recharge them
through simpler means, such as mechanical generators.  Is there any reason
that makes this sort of applications impractical?

Rui Maciel

At first glance it appears that, at least for some applications, employing
capacitors as a means to store energy has considerable advantages over the
need to rely on batteries.  Yet, batteries tend to be used almost
universally.  Is there a reason for this?  Can anyone more knowledgeable on
the subject comment on the practical implications of relying on capacitors
as a power source instead of other standard means such as batteries?  

Thanks in advance,
Rui Maciel

At first glance it appears that, at least for some applications, employing
capacitors as a means to store energy has considerable advantages over the
need to rely on batteries. Yet, batteries tend to be used almost
universally. Is there a reason for this? Can anyone more knowledgeable on
the subject comment on the practical implications of relying on capacitors
as a power source instead of other standard means such as batteries?


Thanks in advance,
Rui Maciel

On Sep 6, 6:33 pm, Rui Maciel <rui.mac...@gmail.com> wrote:

John Larkin wrote:
The energy storage ratio is extreme, four or six orders of magnitude.

Yes, it appears that those values are coherent with the information which
has been presented in this thread.  Yet, in some applications energy density
may not be the decisive property of a energy storage device.  For example,
in applications such as small LED flashlights energy density may be less
important than recharging speed and even the ability to recharge them
through simpler means, such as mechanical generators.  Is there any reason
that makes this sort of applications impractical?

Rui Maciel

Lets just do it.

Here's a supercap from digikey  1 Farad 6.3V around 4 dollars.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=604-....

Lets say we can charge it to 6 volts and discharge it to 2 volts and
we build a constant current circuit to drive 10mA into a 2 volt Led.

Energy in a cap is (1/2)*C*V^2 so at 6 volts the cap has 18 joules and
at 2 volts it has 2 joules so we can supply 16 joules.

The LED at 10mA and 2 volts uses 0.02W.

A joule is a Watt*second so 2/0.02 = 100 seconds = 1.6 minutes

Now a AAA alkaline has about 1000mAh at 1.5v. or 5400 joules. Let say
we boost that to 2v and we're 50% efficient. So, we supply 2700 joules
to the LED. 2700/0.02 = 135000 seconds = 37.5 hours.

So you will need a supercap with lots of Farads of capacitance and a
high voltage rating to compete with the battery.

Wanderer wrote:


Here are a couple of web sites on the subject of using capacitors as a
power source.

http://en.wikipedia.org/wiki/Electric_double-layer_capacitor

http://www.alexanderbell.us/Initiative/GEL.htm

Alexander Bell's site seems to me a bit biased and doesn't give a fair
representation of the trade offs. Remember the words of the great
Oliver Heaviside. "Some think electricity is energy. Others think
electricity is power. Some manage to think both these things at the
same time."

Capacitive power storage and battery power storage are two different
animals with different applications. Capacitors have high power
density. Batteries have high energy density. So if you need high power
for a very short time, you use a capacitor. Think flash bulb. If you
need low power for a very long time, you use a battery. Sometimes you
need a combination of both. You use the battery to store the energy
and charge the capacitor. Bursts of power are then taken from the
capacitor.


Thanks for the links. Although I had already read wikipedia's article on
supercapacitors (it was one of the reasons I subscribed to this group and
subsequently started this discussion), I hadn't read the link from
alexanderbell.us, which was interesting.

Regarding capacitor's rate of discharge, is it possible to release energy
from a capacitor in a gradual manner without much waste instead of releasing
it in intense burts?

Thanks for the help,
Rui Maciel