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Energy Density of a Capacitor

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Bret Cahill

Jan 1, 1970
0
How does a capacitor compare with, say, a lead acid battery,
energy/weight?


Bret Cahill
 
B

Bret Cahill

Jan 1, 1970
0
How does a capacitor compare with, say, a lead acid battery,
Since it takes about 70 watt hours to get a one ton car up to 45 - 50
mph, the _increase_ in conventional battery weight to allow for
regenerative braking is 1 kg.

A conventional capacitor would add three (3) orders of magnitude more
weight, 1000 kg, ONE METRIC TON, to the regenerative system.

It's impossible to double the weight of the vehicle and come out ahead
when the best regenerative systems all operate at less than 50%
efficiency.

And that's ignoring the reduced gas mileage cruising.

And that's ignoring the safety issues.

At least 10,000 psi H2 tanks don't leave a crater in the road when they
rupture.


Bret Cahill
 
E

Eeyore

Jan 1, 1970
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Bret said:
Since it takes about 70 watt hours to get a one ton car up to 45 - 50

1/2 mv2 = 242kJ = 67 Wh - OK. ( used 22m/s)

mph, the _increase_ in conventional battery weight to allow for
regenerative braking is 1 kg.

To 'allow' for it ? I don't understand your thinking. It's not a capacity issue, it's rate of
charge issue. The battery has plenty of capacity but doesn't much like fast discharge or
charge rates and these compromise battery life.

A conventional capacitor would add three (3) orders of magnitude more
weight, 1000 kg, ONE METRIC TON, to the regenerative system.

No-one's using conventional capacitors !

You're barking up the wrong tree.

Let's say your cas has a 400V power bus. To accelerate from zero to 50 mph take that 70Wh.
Let's say the car does it in 15 secs at a constant acceleration. That's 252 kJ in 15 secs =
16.8 kW = 42 A.

Decelartion has to be faster. Maybe 3 secs ? If we do the same sums then with regenerative
braking the battery would have to accept charge at a rate of 210A ( 5x the above ). All the
above obvioulsy assuming 100% efficiency for ease of calculation.

In comparison the car probably cruises using no more than 10kW. Which is 25A.

So the battery's designed to last a long time with say a 25A discharge and the super-cap
'takes the strain' of acceleration and braking. It's really quite simple.

The Mini I mentioned in another post for example uses an 11F 350V supercap.
http://www.greencarcongress.com/2006/08/pmls_inwheel_mo.html
That's 673 kJ. Using the above equations that's good for one burst of acceleration to about 85
mph before taking the battery's contribution into account.

It's 21kWh battery contains 74 MJ. There's no comparison. The battery is the long term power
source.

Graham
 
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Bret Cahill

Jan 1, 1970
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How does a capacitor compare with, say, a lead acid battery,

Two pounds additional conventional battery weight will get a one ton
motor vehicle up to 45 - 50 mph while it takes a ton of conventional
cap to store the same amount of energy.

Since the best regenerative systems all operate at less than 50%
efficiency, it's impossible to EVER recoup the energy wasted in a
system that DOUBLES the weight of the vehicle REGARDLESS of the driving
situation.

And we haven't even gotten to the lower mpg cruising.with a one ton cap
on board.

And we haven't even gotten to the safety issue of releasing 70 watt
hours of energy in microseconds [hundreds of megawatts] in case of a
short.

At least 10,000 psi H2 tanks don't leave a crater in the road when they
rupture.


Bret Cahill
 
E

Eeyore

Jan 1, 1970
0
Bret said:
At least 10,000 psi H2 tanks don't leave a crater in the road when they
rupture.

Just repeating yourself now ? How's that echo in the vacuum of your head going ?

Graham
 
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Bret Cahill

Jan 1, 1970
0
Call up Homeland Security and tell them you want to put 70 watt hour
super capacitors in motor vehicles.

Just don't mention my name.


Bret Cahill
 
E

Eeyore

Jan 1, 1970
0
Bret said:
Call up Homeland Security and tell them you want to put 70 watt hour
super capacitors in motor vehicles.

Why ?

Graham
 
J

jasen

Jan 1, 1970
0
How does a capacitor compare with, say, a lead acid battery,
energy/weight?

unless defective the battery is more that 5 times better,
probably much more.


Bye.
Jasen
 
Bret said:
How does a capacitor compare with, say, a lead acid battery,
energy/weight?

They don't, except in history books.
They energy in capacitors arises from that they
charge/discharge in microseconds.
The energy in lead acid batteries comes
from sponge lead, which is where it's weight comes from.
 
B

Bill Bowden

Jan 1, 1970
0
John said:
Yup. More like 10,000.

John

Honda has an 8 farad capacitor charged to 430 volts that can power an
EV for a mile or more. I don't know how big the thing is, but a 50aH,
12 volt lead acid battery would be 600 watts for one hour (600wH) and
the 8 farad capacitor at 430 volts is 206 watts for one hour (206wH).
Now, unless I screwed up my math somewhere, it looks like the capacitor
has about 1/3 the capacity of a 50aH lead acid battery. But, the
capacitor is probably several times bigger than the average automobile
battery, so the energy density may only be only 1/10 or 1/20 that of
the battery. What do you think?

http://www.caranddriver.com/article.asp?section_id=27&article_id=4217&page_number=1

-Bill
 
Bill said:
Honda has an 8 farad capacitor charged to 430 volts that can power an
EV for a mile or more. I don't know how big the thing is, but a 50aH,
12 volt lead acid battery would be 600 watts for one hour (600wH) and
the 8 farad capacitor at 430 volts is 206 watts for one hour (206wH).
Now, unless I screwed up my math somewhere, it looks like the capacitor
has about 1/3 the capacity of a 50aH lead acid battery. But, the
capacitor is probably several times bigger than the average automobile
battery, so the energy density may only be only 1/10 or 1/20 that of
the battery. What do you think?

Probably right. But the reason they're using capacitors
is because they're using hydrogen fuel cells,
So you want an electric system that produce
arcs, light batteries systems do.
 
M

Mike Swift

Jan 1, 1970
0
They don't, except in history books.
They energy in capacitors arises from that they
charge/discharge in microseconds.
The energy in lead acid batteries comes
from sponge lead, which is where it's weight comes from.

Current state of the art super capacitors have an energy density of
about 3.1 Wh/kg about 15% of what current lead acid batteries have.
Their costs are over one order of magnitude greater than current Li-ion
cells. Capacitors for engineered systems are only used for high peak
power and high charge/discharge efficiency. Good applications are
accelerating a garbage truck from zero to ten mph, and back to zero in
the 100 feet between houses. Very little energy, but lots of power.
For energy storage to give long range they are a nonstarter.

--
Mike

Some say we must tax corporations more. What they do not understand is that
corporations do not pay taxes. One of our governments conditions for their
existence is they collect the taxes from their customers and pass them to
the government.
Mike Swift
 
B

Bret Cahill

Jan 1, 1970
0
Current state of the art super capacitors have an energy density of
about 3.1 Wh/kg about 15% of what current lead acid batteries have.
Their costs are over one order of magnitude greater than current Li-ion
cells.

That's on a cost/energy basis?
Capacitors for engineered systems are only used for high peak
power and high charge/discharge efficiency. Good applications are
accelerating a garbage truck from zero to ten mph, and back to zero in
the 100 feet between houses.

Regenerative braking EV makes sense on route fleet delivery vehicles
because the range is known or can be managed. There are no surprises.
All trucks wind up at the same garage at night. A mechanic is often
available. It's politically possible to force USPS to adopt new
technology.

Since garbage trucks are expensive anyway they ought to do them first.
Besides, what's most obnoxious about a garbage truck isn't the smell,
but the engine noise.
Very little energy, but lots of power.
For energy storage to give long range they are a nonstarter.

To have any range, EVs wind up having enough batteries for all the
power they need.

A 70 watt-hr charge/discharge doesn't impact battery life all that much
so a capacitor isn't much of an advantage.
Some say we must tax corporations more. What they do not understand is that
corporations do not pay taxes. One of our governments conditions for their
existence is they collect the taxes from their customers and pass them to
the government.

Just do it with the individual income tax.


Bret Cahill
 
B

Bill Bowden

Jan 1, 1970
0
Bret said:
That's on a cost/energy basis?


Regenerative braking EV makes sense on route fleet delivery vehicles
because the range is known or can be managed. There are no surprises.
All trucks wind up at the same garage at night. A mechanic is often
available. It's politically possible to force USPS to adopt new
technology.

Since garbage trucks are expensive anyway they ought to do them first.
Besides, what's most obnoxious about a garbage truck isn't the smell,
but the engine noise.


To have any range, EVs wind up having enough batteries for all the
power they need.

I think you confuse energy and power. The batteries have the energy,
the capacitors provide the power. You need both.
A 70 watt-hr charge/discharge doesn't impact battery life all that much
so a capacitor isn't much of an advantage.

Yes it is, because the little 70 watt-hr capacitor can deliver all it's
energy in a few seconds. 70 watts for an hour is the same as 252,000
watts for 1 second.At 746 watts per horsepower, that's 252,000 / 746 =
338 horse power. The battery probably can't do that because the
internal resistance is too high.
Just do it with the individual income tax.


Bret Cahill


-Bill
 
B

Bret Cahill

Jan 1, 1970
0
Current state of the art super capacitors have an energy density of
I think you confuse energy and power. The batteries have the energy,

About one mile's worth of energy / hp.

The Tesla can cruise 250 miles.

Tesla batteries can put out 250 hp.

The Tesla is a tiny car so the problem with batteries isn't that their
power density is too low but that they are too much like capacitors:

The energy density of batteries is too low.
the capacitors provide the power. You need both.

You get all the power you need with any battery only EV with any range
over 100 miles.
Yes it is, because the little 70 watt-hr capacitor can deliver all it's
energy in a few seconds.

That might be useful in a short range fleet delivery vehicle. USPS
mail carriers rarely go over 15 - 20 miles/day. In that case a few
cheap deep cycle lead acid batteries and a capacitor might make sense.

For longer trips they can add a portable gen set to convert it to a
hybrid.


Bret Cahill
 
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Dan Bloomquist

Jan 1, 1970
0
Bret said:
The energy density of batteries is too low.

That is because you are 'Bret', the spoiled brat that has no concept of
quads....
 
B

Bret Cahill

Jan 1, 1970
0
The energy density of batteries is too low.
That is because you are 'Bret', the spoiled brat that has no concept of
quads....

I'm a populist, that is, I'm aware of the political aspects of prying
'em out of their SUVs.


Bret Cahill
 
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