Practicality of using capacitors as a power source

[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

 

[Charles]

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

 

[shrtrnd]

The call what you're talking about 'supercapacitors'.
Google them.

 

[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.

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

 

[Rui Maciel]

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

Thanks for the link, Charles. The link on capacitors and supercapacitors[1]
was particularly informative.


Rui Maciel


[1] http://www.mpoweruk.com/supercaps.htm

 

[Rui Maciel]

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

 

[[email protected]]

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?

Yes, energy density. Capacitors just don't hold enough charge to make this
sort of use practical. Batteries aren't all that scary.

 

[ehsjr]

Wanderer wrote:




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?

Yes, but you don't get more energy. A capacitor is a storage
container. It can hold only so much, and it cannot release
more than it can hold. Think of a liter bottle of soda. You
can't get more than a liter of soda into the bottle, and
you can't get more out of it, regardless of how slowly you
sip it. A capacitor is like that.

Ed

 

[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=604-1018- ND

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.

I see what you mean. Yet, the capacitor you've shown is a somewhat smaller
than a AA battery and it appears to be possible to stack around 5 of those
to reach the same height. If I'm not mistaken, with this arrangement it
would be possible to store enough energy to power that LED for around an
hour. Although it still lags behind what can be had with batteries, it is
already a decent amount of time. If it was possible to recharge the
capacitors through some means, such as a mechanical generator, then it
wouldn't be a bad thing to have.


Rui Maciel

 

[Kaz Kylheku]

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=604-1018-ND

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

An easier way to look at this is to consider that an Ampere is a Coulomb
per second (C/s) (how much of a charge of electrons flow past a point
per unit time) whereas a Farad is a Coulomb per Volt (how much charge is
crammed into a capacitor for a given potential difference).

At 6V, our 1F cap stores 6 coulombs, and discharges to 2 coulombs, so we
lose 4 coulombs. These 4 coulombs are being doled out using an active
current limiter at a steady 10 mA trickle (in other words 0.01 C/s), so
we get a linear discharge, rather than the typical RC discharge.

We thus easily calculate the discharge time:

Thus 4C / 0.01 C/s) = 400 s.

We get 6 minutes and forty seconds. (Of course, a real current limiter
will have a voltage drop, such as the VCE(sat) of its transistor or
whatever: it will stop working somewhere above 2V).

From this we can also see how much energy is wasted. We have 16J, right?
But we got to run our 0.02W LED for 400 seconds, which gave us 8J of
energy. Watts is Joules per second, so 0.02 (J/s) * 400s = 8J.

8J of light and heat out of the LED, 8J of heat out of the limiter.

 

[Bob F]

I see what you mean. Yet, the capacitor you've shown is a somewhat
smaller than a AA battery and it appears to be possible to stack
around 5 of those to reach the same height. If I'm not mistaken,
with this arrangement it would be possible to store enough energy to
power that LED for around an hour. Although it still lags behind
what can be had with batteries, it is already a decent amount of
time. If it was possible to recharge the capacitors through some
means, such as a mechanical generator, then it wouldn't be a bad
thing to have.

But batteries would regulate the voltage. A capacitor running a lightbulb would
have a huge varience in light output as the voltage drops. A batterys voltage
would be much more constant. Plus it would last way longer per charge. But then,
the capacitor could be charged/drained thousands of times more.

We just need better capacitors.

 

[Rui Maciel]

But batteries would regulate the voltage. A capacitor running a lightbulb
would have a huge varience in light output as the voltage drops. A
batterys voltage would be much more constant. Plus it would last way
longer per charge. But then, the capacitor could be charged/drained
thousands of times more.

We just need better capacitors.

Isn't it possible to at least limit the voltage output of a capacitor?


Rui Maciel

 

[Winston]

Bob F wrote:

(...)

We just need better capacitors.

The capacitors are all right.
We need better batteries more.


--Winston