I've been running several configurations of PV panels through the MPPT calculator, with encouraging results. However, going through the PV panel specs I note there is a dramatic difference between the maximum rated power at Standard Test Conditions (STC) and Normal Operating Cell Temperature (NOCT). The former will never be achieved in the real world (1kW/m2 solar energy; cell temperature 25 deg. C.; Air Mass 1.5). The NOCT temperature of the (JA Solar) panels I'm looking at is 45 +/-2 degrees C which seems far more likely when the sun is up.
I'm at the South African south coast where the temperature never drops to freezing point; about 5 deg. C is the coldest it's ever gotten. When the sun comes up every surface exposed to it quickly heats up to at least 45 deg. C.
Which leads me to wonder: if I plug the Pmax @ STC into the calculator it prescribes an MPPT that is about twice the cost of what I need for the Pmax @ NOCT, even given that I'm using the specified voltage and current temperature coefficients.
To illustrate: the specs for the panel I'm looking at give a Pmax @ STD of 500W but at NOCT it's only 378 (which seems much more realistic).
Seeing as STC is a theoretical lab situation never achievable in practice, shouldn't I work on the basis of Pmax @ NOCT rather than Pmax @ STC? I mean, why worry about whether or not the MPPT can handle a power rating you'll never achieve anyway?
(Needless to say I am still working on the basis of Voc and Isc @ STC since these are the highest values and the only way to ensure the voltage and current limits of the MPPT will never be exceeded.)
Your thoughts?
// F
