> At 09:42 AM 4/28/2011, Olin Lathrop wrote:
>
> >That's backwards. =A0Solar cells are more efficient when cold. =A0At 28 =
deg
> >latitude, the sun isn't going to be that much weaker in winter due to
> >traversing more atmosphere on the way to the panel.

It depends which parameter you design for and whether you
electroically drive the panel to maximum power point AND efficiently
convert the output to suit the load conditions.

Most systems do not use an MPPT (maximum power point tracking)
controller. Depending on type of system, insolation , battery state
and more the above assumption may be less relevant than expected or
wrong. For modern crystalline silicon Vmpp/Vos is about 80%. Higher
efficiency calls give higher percentages.

For crystalline silicon (poly and mono) max power and max voltage have
negative temperature coefficinct BUT max current has a slight positive
tempco. IF you are running the panel as a current source - and this
tends to be what you do when you load a panel below its MPP (max power
point) using eg a battery, then you may depend only on the current
tempco, and it may be positive. ie current per insolation rises with
rising temperature.

So an eg 18V panel is optimised for about 0.8 x 18 =3D 14.4V which suits
lead acid charging but loses about (14.4-12)/14.4 ~=3D 17% efficincy if
the load is actually 12V and no energy converter is used.

A well designed system without MPPT will try to operate the panel near
Wpp at typical insolation, load and battery states.
For small portable devices a battery will almost always load its panel
to below its peak power voltage, resulting in something more like a
current source. So Isc is closer to meaningful than Wpp in many cases.
For 3 x NimH batteries industry practice is to use 12 PV cells. I've
mulled over using 11 rather than 12 which in some cases will give
12/11 output or almost 10% more power into the battery for the same
active PV area. The disadvantage is reduced Voltage at lower
insolation so reduced charging in lower light conditions.



  Russell

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