Solar Panels? - Why PWM Works
Wagner Lipnharski email (remove spam text)
Hi Eric, thanks for the explanation.
I am just learning about solar cells, and it looks like you
have much more experience. So I need to ask you:
What you are saying, is that the *delivery power* of a solar
cell changes with the current drained, is that right?
Nothing to do with internal impedance, is that correct?
Note that *power* means voltage x current, so according to
what you are saying, suppose:
1) A pack of cells can supply 100V with open output.
2) You connect a load and the current goes to 10mA, while
the voltage drops to 10V. The power delivered is 100mW.
3) If you increase the load to drain only 5mA, the voltage
will be higher than 20V (more than 100mW supplied).
It is natural that *any* voltage cell increase the voltage
at the output connections when load is removed since the
voltage drop at the internal impedance is reduced.
If you connect a capacitor in parallel with any battery
with high internal impedance, it will delivery a higher
peak of power at the initial connection since the cap
has an impedance lower than the battery.
But according to what you are saying, this is not what
happens to a solar cell, and that there is an "optimum"
point in the power curve (current versus voltage) where
it is higher. Is that right?
> But if you pulse load this then the average voltage
> delivered is some higher.
It works the same way if you apply a capacitor in parallel
to any power cell, reducing the internal impedance. In
real, a solar cell may act somehow as a capacitor, since
the large surface area.
In the analogy to the accelerating car it doesn't work;
A car with the traction tires lifted in the air, then
accelerating at high rpm, creating mechanical inertia,
and then releasing the car into the track, it will jump
a little bit because the mass inertia of the rotating
mechanics, but it has no torque, just inertia, and the
acceleration process will reduce the rpm according to
the friction so it needs to create the torque and at
the end of the track it consumed more fuel/distance/time
than if not doing that. The fuel consumed to create the
initial inertia (in the air) has a terrible low
productivity. In a videogame for the Nintendo64 has a
SuperMario race game where this effect allows you to
speed up more than your competitors, jumping the car
continuously along the race, this is not true in the
real world. If yes, a simple reduction in the gear box
would have the same effect. I have a friend that once
told me that a hammer-drill can penetrate the concrete
not because the hammering effect (that breaks the
crystallized concrete surface), but because when the
hammer retracts it gains more speed... can you imagine?
Wagner Lipnharski - UST Research Inc. - Orlando, Florida
Forum and microcontroller web site: http:/http://www.ustr.net
Microcontrollers Survey: http://www.ustr.net/tellme.htm
Eric Borcherding wrote:
See also: www.piclist.com/techref/io/pwm/index.htm?key=pwm
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