Paul Keinanen wrote:
On Mon, 12 Apr 2004 13:02:38 -0700, "Watson A.Name \"Watt Sun - the
Dark Remover\"" wrote:


Joerg wrote:


Another option might be to use a different voltage panel,
whatever has a good price, and then use a small switcher to run
the cells at their optimum load.

Regards, Joerg.

Seems foolhardy to me, to use a boost circuit, and waste a lot of power.
Just put more PV cells in series to increase the voltage.


The solar cell operates as a (badly) regulated power supply with
current limiting. At low load currents, the cell operates nearly as a
constant voltage source, but after a specific current (for a given
illumination) it operates nearly as a constant current source and
deliver approximately that current even into a short circuit.

The largest power from the cell (for a specific illumination) is
obtained at the point it switches from constant voltage to constant
current mode, in which both the voltage is quite close (within 30 %)
of both the maximum voltage (as measured at open circuit) and maximum
current (as measured at short circuit).

This maximum power point varies with illumination, but if the switcher
always loads the cell at this maximum power point, the largest
available energy at a specific time is extracted from the cell
independent of illumination.

Even if the losses in the maximum power point tracker is 10-20 %,
usually more energy can be obtained than running the module in some
non-optimal constant voltage or constant current mode.

Paul


Anybody got any real data on this stuff.
I set out to build a constant power solar battery charger.
I was gonna just put a PIC to measure the voltage/current and
ratchet the switcher duty cycle up and down around peak power.

Went out in the yard at noon and plotted some curves. Yep,
there's a pronounced power peak right around 14V.
At lower intensities, the shape of the curve is the same, but
it moves sideways. Ok, my pulse width strategy should track that.
Cool.
Then I turned the panel ever so slightly away from the sun.
I was amazed at how dramatically things changed with just
a small angle. Looks like I'd gain WAY more watt-hours/day
by tracking the sun
than by anything else I could think of.

mike

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