"Paul Keinanen" wrote in message
...
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

Yeah, I see what you mean, sort of like an impedance match, but at DC.
But at the beginning or end of the day, or cloudy day, you can't pull
any more energy out of the cells than there is there. What it looks to
me is that you're adding circuitry to give a better match at the ends of
the day or a cloudy day, and in return sacrificing a few percent
overall.

My attitude is that rather than try to do this (and in the process lose
reliability), it's better to go supersize on the cells, add more area
and overall capacity to get you thru the cloudy days, and have a higher
capacity overall.