Electronics > Power/Renewable Energy/EV's
Mismatched PV panels in series
Siwastaja:
Hi,
Asking for practical experiences from those of us who know what we are talking about, because Googling this seems to be an infinite source of elementary level (+ often wrong even at that) bullshit.
So I have a 3kWp rooftop system installed by the previous owner of the house. One series chain, one MPPT tracker in the inverter. Ten panels of approximately 300Wp. Last time I checked today with dirty (spring; pollen, no rain for weeks) panels, production with blue skies and nearly optimal solar angle is 2.2kW at 299V so around 7.4A. All-time high peak record is at 3700W.
The system is installed in 2017 and I doubt I could get the exact same panel.
There is room for 5 panels more on the roof (10 -> 15), and besides, the inverter is grossly oversized (5kW) so it would be just obvious to add those 5, for a 4.5kWp system.
My understanding is, of course, that I need to match
* Most importantly, rated current at maximum power point
* But also, temperature coefficient thereof
* And also, having similar response to stray light vs. direct light
The last two bullet points so that maximum power point current would be similar not only in optimal conditions, but over wider operating area. But I guess finding similar enough panels would be a true challenge - so matching rated current is likely the best I can do.
Would you agree so far?
Since 5 < 10 (i.e., less than 100% increase in system size), the old panels should dominate, and it would be a smaller monetary loss to buy panels with a tad larger current rating than the existing ones - only the new ones, 5 out of 15 would be operating at <100% current. If I buy panels with too small current rating, then the old ones - 10 out of 15 would be operating with too little current: more wasted resources. Again, would you agree with this?
MPPT obviously should be able to find the global optimum for the whole chain.
Any other practical comments?
As it is now, both roof area and inverter power capability is wasted resource, and I do want more PV. But of course if 50% of the investment goes to waste, I buy 1500Wp worth of panels but only get 750Wp more due to the whole system operating outside optimal power point, then it does not make sense to extend the system. But my gut feeling says, match the rated current and add maybe 10% and it'll be good enough?
bdunham7:
--- Quote from: Siwastaja on May 25, 2022, 05:07:39 pm ---MPPT obviously should be able to find the global optimum for the whole chain.
Any other practical comments?
--- End quote ---
Some inverters have a MPPT algorithm that can get 'stuck' at a local peak power point and miss that a better peak somewhere else. This most often shows up in systems with shading, but I suppose a mismatched system might cause it as well. So whether it finds the global optimum depends on whether or not it has that feature--and it isn't universal.
Assuming the panels are the same voltage, or at least the same number of similar cells, than as long as the new ones are rated for more current than the old ones I think the worst-case scenario would be an output equal to 15 of the original panels. This is all assuming that all the panels, new and old, are the same basic type--presumably poly-silicon. Although in this case, even mixing poly- and mono- might not be much of a problem. Just make sure they all have operational bypass diodes if you have any shading at all during any time of the day where you care about production.
And yes, I agree that larger is better, especially since you probably won't have to pay much more for them as solar panels keep increasing in output.
Siwastaja:
Thanks for comments, the inverter is StecaGrid 5003 and they advertise "global MPP". It probably does a wider perturbation scan every now and then. I would guess problems with MPPT would happen in partial shadow conditions more easily than just mismatched panels.
Any particular reason you suggest same number of cells per module? Why would the number of cells in a module affect anything, in the end, all cells just end up in series.
I could imagine voltage per cell would be indicative of other possible differences, though.
I totally agree that if Impp_new > Impp_old, then the whole system should act like 15 old panels, and only 5*(Pmpp_new - Pmpp_old) power capability is wasted. So I would need to figure out the exact model of the panel I have - get on the roof and try to get a camera between the roof and the panels to read the labels - and then pick panel type with maybe 5-10% higher Impp. It would be a pity if the new type performed significantly more poorly in low or diffuse light conditions, bringing down the whole system.
My old panels are full black so they might be mono, but might be poly as well. Mixing poly and mono might or might not pose a problem because depending on who you ask, mono/poly is better in low light or diffuse light conditions, than the other one. Found this great resource about the classic myths: http://www.solarblogger.net/2014/12/mono-vs-polycrystalline-solar-cells.html As always with components, only actual data matters, and "rules of thumbs" are usually incorrect.
bdunham7:
--- Quote from: Siwastaja on May 25, 2022, 06:54:45 pm ---Any particular reason you suggest same number of cells per module? Why would the number of cells in a module affect anything, in the end, all cells just end up in series.
--- End quote ---
No, I wasn't really suggesting you need to the use same-voltage panels in a string, just using that assumption to make the rest of the sentence an easy calculation. Of course you have to watch the maximum string voltage--I think the MPPT algorithm will cause the higher-current panels to run very near their OCV at high power levels, depending on how much greater their current capacity is. Here you can find 300W 72-cell mono panels cheaply on the used market, although that probably isn't the case for you.
trobbins:
Definitely need to assess the inverter max DC voltage rating, and the worst case OCV of the proposed array at lowest temp.
Navigation
[0] Message Index
[#] Next page
Go to full version