Solar Panel Voc-MPPT?

Salvo wrote:
You're beginning to see the negative sides of mppt. From the get-go, mppt has about 30% advantage over pwm. But much of that advantage evaporates. The killer of mppt is heat. The PV cell voltage decreases 2 mV for every rise in degree Centigrade. A 24V panel may have 68 cells. A 10 deg C rise in panel temperature, which by the way is way less than typical, reduces panel voltage (as well as panel power) by:

V_drop = 68 cells * 10C - 2mV/C/cell = 1.36V

That's a 4% loss in voltage and output power.

Sal

BFL13 wrote:
So that makes it more difficult to do better with MPPT than with PWM? Rats.

not really. MPPT will do better, but how much does depend on your location and temperature. with PWM the maximum output current is the panel current. with MPPT power out = power in. So yes at the panel heats up the power will go down, since output voltage goes down. But I doubt seriously if the max output power ever drops to the level of PWM even with 12V nominal panels. It certainly hasn't for me.

Now on the upside, below 25C which is where panels are typically rated, the max panel output power will go up since Vmp will be higher than at 25C.

so fall and winter full sun MPPT may actually give you a suprise in power if it is cool enough to keep the panel below 25C (75ishF).

In my almost 2 years using my panels with a morningstar sunsaver 15A MPPT controller I have never seen peak clear sky sun output currents into a battery that will accept full current to be less than or even as low as the Imp that a PWM controller will give. It usually is at least a 1.5-2A gain, 10A peak vs. 8.5 Imp from my original PWM controller. At times I have seen over 11A.


However, the gain over PWM is going to be lower in the arizona summer sun than it would be in northern US fall, that's for sure. And the 2m/C drop is why panels in Oregon with the many cloudy days will often actually outperform full sun arizona panels on an annual basis for total power.

And if I was strictly using a panel in the summer in the southern US in high temps, not sure that MPPT would be enough gain to justify the expense.