Electronics > Power/Renewable Energy/EV's

Solar panel voltage matching question

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Someone:

--- Quote from: Red Squirrel on August 27, 2016, 11:48:29 pm ---
--- Quote from: Someone on August 27, 2016, 10:48:53 pm ---
--- Quote from: Red Squirrel on August 27, 2016, 08:02:31 pm ---
--- Quote from: cravenhaven on August 27, 2016, 02:04:28 pm ---The MPPT point for a panel will change with the irradiance, so if you have panels at different orientations paralleled into one controller then the mppt point selected by the controller will less than optimal for any of them.
As far as I have seen with solar installations you would always use separate mppt tracking regulators for each array of panels if they are significantly different from each other in either orientation or design.

--- End quote ---

That's what I was thinking too, so each array should be on it's own mppt then?  How would this work for battery charging, would I need to just set them to float, and maybe only have one that charges at a higher/variable rate?  Otherwise they'd all fight each other. Or would it just work?

--- End quote ---
Look at how much the peak power point will vary with temperature/insolation, going from a single voltage on all strings to individual MPPT on all strings will only gain a few % in increased generation. Then you can compare the costs of adding additional MPPT and cabling compared to the additional energy produced, or adding more panels to the strings.

--- End quote ---

Yeah true guess if it's not that much of a difference it may not be worth going through the trouble.  I could probably also put arrays on relays, have the relay of the less optimal one turn on when there is enough sun.   

This is more of a what if scenario anyway, chances are good I'd start with just the roof top array.    What I've been thinking about as well is I need to build a shed, I might make it use solar power for lights etc and it will make a decent solar project as a start to get an idea of what is involved.

--- End quote ---
Disconnecting panel strings when they are only illuminated even from diffuse light would just reduce production capacity, the MPPT would find the peak of all the strings together and none would be "taking" power away unless they were in complete darkness (measure the output under moonlight if you don't believe it!). It's very hard to come up with situations where reverse leakage will be a problem especially if the string voltages are well above the battery voltage and using a buck regulator.

Red Squirrel:

--- Quote from: Someone on August 28, 2016, 09:41:52 am ---
--- Quote from: Red Squirrel on August 27, 2016, 11:48:29 pm ---
--- Quote from: Someone on August 27, 2016, 10:48:53 pm ---
--- Quote from: Red Squirrel on August 27, 2016, 08:02:31 pm ---
--- Quote from: cravenhaven on August 27, 2016, 02:04:28 pm ---The MPPT point for a panel will change with the irradiance, so if you have panels at different orientations paralleled into one controller then the mppt point selected by the controller will less than optimal for any of them.
As far as I have seen with solar installations you would always use separate mppt tracking regulators for each array of panels if they are significantly different from each other in either orientation or design.

--- End quote ---

That's what I was thinking too, so each array should be on it's own mppt then?  How would this work for battery charging, would I need to just set them to float, and maybe only have one that charges at a higher/variable rate?  Otherwise they'd all fight each other. Or would it just work?

--- End quote ---
Look at how much the peak power point will vary with temperature/insolation, going from a single voltage on all strings to individual MPPT on all strings will only gain a few % in increased generation. Then you can compare the costs of adding additional MPPT and cabling compared to the additional energy produced, or adding more panels to the strings.

--- End quote ---

Yeah true guess if it's not that much of a difference it may not be worth going through the trouble.  I could probably also put arrays on relays, have the relay of the less optimal one turn on when there is enough sun.   

This is more of a what if scenario anyway, chances are good I'd start with just the roof top array.    What I've been thinking about as well is I need to build a shed, I might make it use solar power for lights etc and it will make a decent solar project as a start to get an idea of what is involved.

--- End quote ---
Disconnecting panel strings when they are only illuminated even from diffuse light would just reduce production capacity, the MPPT would find the peak of all the strings together and none would be "taking" power away unless they were in complete darkness (measure the output under moonlight if you don't believe it!). It's very hard to come up with situations where reverse leakage will be a problem especially if the string voltages are well above the battery voltage and using a buck regulator.

--- End quote ---

I was thinking it might drive the MPPT curve down for the rest of the panels but guess I'm probably just overthinking this.

Someone:

--- Quote from: Red Squirrel on August 28, 2016, 11:31:18 pm ---
--- Quote from: Someone on August 28, 2016, 09:41:52 am ---Disconnecting panel strings when they are only illuminated even from diffuse light would just reduce production capacity, the MPPT would find the peak of all the strings together and none would be "taking" power away unless they were in complete darkness (measure the output under moonlight if you don't believe it!). It's very hard to come up with situations where reverse leakage will be a problem especially if the string voltages are well above the battery voltage and using a buck regulator.

--- End quote ---
I was thinking it might drive the MPPT curve down for the rest of the panels but guess I'm probably just overthinking this.
--- End quote ---
There is no need to imagine anything when you have data available, strings need to be extremely dark to produce no energy into a parallel array. Attached is an example IV curve floating around the internet annotated with the voltage of the peak power point of the fully illuminated case, and it crosses the 0A axis well below any of the other traces, you could expect that a panel at just 5% illumination of the peak would still be contributing energy. But as panels heat up the peak power point voltage drops which would include more energy from the less illuminated panels.

It all falls apart when you have partial shading of significant parts of a panel/string, then you might consider shorter strings and higher paralleling or optimisers/micro inverters with MPPT per panel. Or positioning the system so it won't be shaded.

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