If I connect my 20V open circuit PV panel to a discharged 20V cap

bank, I assume I get the short circuit current avail from the panel

until the cap charges up closer to 20V? Does this technique harvest as

many or more coulohms from the PV panel as trying to seek a max power

point from the panel? Seems like max current trumps max power when

charging the cap bank? What am I missing here?

wrote:

*>If I connect my 20V open circuit PV panel to a discharged 20V cap*

*>bank, I assume I get the short circuit current avail from the panel*

*>until the cap charges up closer to 20V? Does this technique harvest as*

*>many or more coulohms from the PV panel as trying to seek a max power*

*>point from the panel? Seems like max current trumps max power when*

*>charging the cap bank? What am I missing here?*

Charging up a cap doesnt look like a short cct other than when you

start.

Once the cap voltage rises above 0V it no longer looks like a short

cct.

The only way find the max power point of a Solar panel is to do what a

MPPT controller does, which is progressivley load the solar panel with

an ever increasing resistive load, whilst measuring the current draw

and the load voltage.

There are a number of common techniques for doing this , but most

involve a switch mode controller controlled by a small microcontroller

which continuously juggles the effective load on the Solar panel to

maintain max power at varying levels of sunshine.

You ,issed on very important fact.

Current does not make power and voltage does not make power.

In DC the product of voltage and current make power.

IOW: Lots of current times 0 volts = 0 watts.

*> If I connect my 20V open circuit PV panel to a discharged 20V cap*

*> bank, I assume I get the short circuit current avail from the panel*

*> until the cap charges up closer to 20V? Does this technique harvest as*

*> many or more coulohms from the PV panel as trying to seek a max power*

*> point from the panel? Seems like max current trumps max power when*

*> charging the cap bank? What am I missing here? *

*> If I connect my 20V open circuit PV panel to a discharged 20V cap*

*> bank, I assume I get the short circuit current avail from the panel*

*> until the cap charges up closer to 20V? Does this technique harvest as*

*> many or more coulohms from the PV panel as trying to seek a max power*

*> point from the panel? Seems like max current trumps max power when*

*> charging the cap bank? What am I missing here?*

Someone just answered your question, but someone has found interesting to

use the cap charged at open-circuit voltage as a startup current deliverer

for electrical motors.

This means: high peak power for a while, but low average power (that is what

we have at its maximum with a MPPT circuit)

Bye

CG

BobG wrote:

*> If I connect my 20V open circuit PV panel to a discharged 20V cap*

*> bank, I assume I get the short circuit current avail from the panel*

*> until the cap charges up closer to 20V? Does this technique harvest as*

*> many or more coulohms from the PV panel as trying to seek a max power*

*> point from the panel? Seems like max current trumps max power when*

*> charging the cap bank? What am I missing here?*

Bob:

All right, I _am_ an electrical engineer.

Let's have a little fun. To a first order approximation, a PV panel with

sun shining on it looks like any other "real" power source: It can be

modeled as a voltage source with a series resistance or a current source

with a parallel resistance.

So, remembering that this is all first order approximation stuff, the

open circuit voltage of the panel (no load) gives you the open circuit

voltage; divide this by the short circuit current and you have the

source resistance.

The circuit would then look like:

-(V)--(R)--(C)--

| |

----------------

Sophomore EE201 has the equation for current, assuming that the cap

starts off with 0 charge at t = 0:

I(t) = V/R * (exp(-t/(R*C)).

Voltage on the cap would be:

V(t) = V*(exp(-t/(R*C)) - 1.0)

At t = infinity the current is zero; at t = 0 the current is V/R (your

short circuit current). Let "tau" be R*C; when t = 5*tau, you're pretty

much done. When t = tau, you're about 63% of the way there.

Oh, yeah: "exp(x)" is e raised to the x, where e is 2.718.

The reason that this is a first order model is that there's a ton of

second order effects. Your standard PV panel consists of a whole bunch

of P-N junctions wired up in series and parallel. So, the resistance of

the whole thing looks like the bulk resistance of all the silicon plus

the voltage drops of the P-N junctions, which goes I = Io *

(exp(q*Vd/(eta*K*T) - 1), where q = 1.609e-19 (electron charge), K =

Boltzman's constant, T is the temperature in Kelvin, and Vd = ln(I/Io -

1)*eta*K*T/q. Io is the leakage current the diodes when they're reversed

biased; natch, you've got diodes in parallel and diodes in series, so

life gets even more interesting. Oh, yeah: The diodes themselves are

capacitors, too, and the capacitance goes up as the P-N voltage goes

down. Lots of non-linear fun.

And, if you really want to get nasty, capacitors themselves are rarely

ideal. They have internal resistance at DC (leakage, especially

electrolytics) and odd things can happen when the ripple current into

one gets large. Capacitors have Q and D factors for first order loss

analysis; it gets even more fun when the frequencies begin to climb.

Now, I don't work with PV panels much, other than having fun looking at

the specs on the ones on my roof, so I haven't tried (or needed to) do

any modeling and simulation, but, for rather bigger caps (couple hundred

microfarads) I'd expect the first equation up there to work relatively

well; that is, the current curve you'd get would be within 10%-15% of

reality. For small values, all the nonlinear diode stuff would probably

through that curve right off.

Now, I notice in the EE trade rags that people are starting to talk

about supercapacitors as a storage mechanism. We're talking here about

capacitors with values up in the farads. If that's what you're thinking

about, it's not quite clear to me if just hooking a PV panel up to a

supercap is necessarily the best way to charge said cap. I could be

wrong about this, but typical power sources have max power transfer when

the external load has the same resistance as the internal resistance of

the source. Hence, some kind of switcher that draws the max power from

the PV panel (that is, ISC/2 at Vo/2 or something) and dumps said power

into the cap until the switcher runs out of steam at the higher

voltages, would probably be a better bet than just hooking said cap up

directly across the PV panel; more power in the batter, and faster, too.

Have fun!

KAB

>If I connect my 20V open circuit PV panel to a discharged 20V cap>bank, I assume I get the short circuit current avail from the panel>until the cap charges up closer to 20V? Does this technique harvest as>many or more coulohms from the PV panel as trying to seek a max power>point from the panel? Seems like max current trumps max power when>charging the cap bank? What am I missing here?