PV perromance in differing light conditions

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I have what I believe an interesting observation on PV performance on my boat these past ten days. I have 158W of panels regulated by an MPPT controller. In the clear skies that we’ve recently experienced with full uninterrupted sunshine overhead I have typically seen 7.5A passing to the batteries. However today with grey could with the sun completely obscured (albeit still bright enough to need sunnies) I have seen 9.2A regularly. Prima facie solar PV panels prefer diffused sunlight? Has anyone else experienced this?

Rob

 

[AntarcticPilot]

I don't have Solar PV on my boat, but I do have 2.2 kW worth on my roof. Certainly the ones on my roof only generate full power when the sun is shining from a clear sky. They do generate a useful amount of power - perhaps half to three-quarters of the maximum - in thin overcast conditions, but for the maximum output, they need full sun. You get a bit of power out of them even if there's thick cloud!

Of course, my solar panels simply connect directly to the house mains supply via an inverter and if I'm not using it all, it goes back into the electricity grid. So, my panels always generate at the maximum rate they can.

As you're feeding batteries through a controller, the state of charge of the batteries will have an impacty on the current that is flowing; if your batteries are well-chaged, you may well not get as much current flowing as you would when they are less fully charged. Whatever is actually happening, you can't judge the efficiency of the solar panels in your set up without knowing what else is happening to restrict the current flowing.

 

[BAtoo]

IIRR from previous threads on PV the temperature of the panel has an effect - higher temps giving lower output.

 

[xeitosaphil]

I have what I believe an interesting observation on PV performance on my boat these past ten days. I have 158W of panels regulated by an MPPT controller. In the clear skies that we’ve recently experienced with full uninterrupted sunshine overhead I have typically seen 7.5A passing to the batteries. However today with grey could with the sun completely obscured (albeit still bright enough to need sunnies) I have seen 9.2A regularly. Prima facie solar PV panels prefer diffused sunlight? Has anyone else experienced this?

Rob

Hi Rob,

I have just in the last week fitted a 100W PV panel fitted to a Morning Prostar PWM controller, which over last week has been regularly been putting out 4.2A – 4.6A in full sun clear skies Panel angle 35 degrees off horizontal pointing southwards. However today, grey cloudy, limited sun and overcast conditions at times PV panel was putting out 5.0A - 5.4A. Sounds a similar sort of thing you have observed?

I also wondered if it might be the panel temperature, or could it be something else?

Philip

 

[Nigel-in-Oxford]

Panels are less efficient when hot, and a bit of cloud absorbs IR more than UV. The UV also produces electricity, less so the IR, that just heats things up.

 

[William_H]

I think more likely it is a question of battery state. As said. To confirm you would connect the PV panels diectly to a battery without controller but with an amp meter. Or altenatively put a big load on the batteries to ensure that controller will be doing its best to put power into batteries. I feel confident that you would get more current from PV in bright sunshine than overcast. olewill

 

[xeitosaphil]

I think more likely it is a question of battery state. As said. To confirm you would connect the PV panels diectly to a battery without controller but with an amp meter. Or altenatively put a big load on the batteries to ensure that controller will be doing its best to put power into batteries. I feel confident that you would get more current from PV in bright sunshine than overcast. olewill

Interesting theory William, I wondered this myself but with my controller, it has a digital meter display which I assumed the charge from panel to be measured as PV panel output amps before going through the controller box regulation system which is what the literature state (Panel Output measured in amps). What it doesn’t say is whether that is measured on the battery feed side, or the solar panel output side?
I thought rightly or wrongly that the panel would produce a fixed amount of amps relative to the available light, and when the battery state varies the panel still produces the same amount of amps, but it is regulated or disconnected, and the charge which is converted to heat is dissipated through the heat sink.
It seems that the heat of the panel does affect the output of amps from the panel ( if you believe what you read on the web) but battery voltage also has a part in panel output as well. I think this is where a MPPT controller comes into its own.
My understanding is that the Panel is rated at a maximum output using possibly 18v. Most yacht batteries will be 12v -13v. This could mean that with my battery maybe only being 12.6v and not 18v, the charge output from the panel will be reduced by the same ratio. From what I read this is where MPPT controllers become effective. They somehow fool the PV modual into thinking that the battery voltage is 18v which then increases the amp output, something to do with Ohms law? To get really good results and best added advantage from MPPT I thought you need lots of PV panel watts 200+,which is something I don’t have room for so went for a standard PWM controller.
I may be completely wrong on this, as my knowledge of such things is relatively small, but increasing daily, no doubt others in the know will put me straight if I’m wrong.