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Panel array design
#1
Hello all,

My first post though I have been lurking for a while.

I have been planning a small off grid system for a while and have now started chasing bits.

I have a mpp solar pip 5048GK on order eta two weeks hopefully.

4 x slightly used Seraphim Eclipse 320W panels VMP= 36.4V, IMP= 8.9A, VOC= 44.85, ISC= 9.11A, MAX FUSE 20A.

4 x new Trina Honey 330W panels VMP= 33.8V, IMP= 9.76A, VOC= 40.6V, ISC= 10.39A, MAX FUSE 20A.

I had also planned on building my own "HBPowerwall" after spending hours watching his youtube channel. But this has become increasingly frustrating as I have been waiting months for holders and have only sourced 15 used laptop packs so far.

So I have ordered 2 x PowerPlus Eco 48V 4.0kWh LiFePO4 Modular Batteries to get be started with the hope of adding a diy pack in the future.

So my first question is how to connect my panels. My north facing roof has two levels, the higher level gets first full sun at 7.15am this time of year and only has space for 4 panels while the lower level doesn't get full sun until 8.30am and I loose full sun on both levels around 4pm (trees).

According to the 5048gk specs it can handle 5kw at 500v so stringing all panels together in series would give me 341.8 VOC but half would be in shade for the first hour. Would I still harvest all the energy from the higher 4 panels in that first hour or will the shaded panels drag the the whole array down?

I am guessing that as the panels have a 20 amp max current I can only have two panels in parallel.

Any guidance from the forum on how to setup this array would be greatly appreciated.

I am a retired hazardous locations electrician so keeping the installation safe will be my top priority.

Thanks for your time :)
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#2
Maybe consider having the better 4x Trina panels on the top roof so they get the most sun for the longest.
I'd have the panels as two separate strings each with it's own MPPT charger.

If you series them all, the lowest current Imp values would limit the peak current, so lower array max power output. It would also stress the reverse diodes + loose a lot of power in partially shaded times.
If you parallel the Seraphim's with the Trina's there's quite a difference in the Vmps so again you'd loose array efficiency by not running the panels at the best Vmp/Imp points.
If you decided to parallel the two strings (accepting some loss) you would need a series blocking diode on each string.

This means you'd need MPPT chargers rated for Voc of 4x 44.85 = 179.4V (+ margin + cold day correction) so say 200V & good operation efficiency at around Vmp of 145.6V,
Eg the Victron Smart Solar 250/xx models
I see the Pip unit needs approx 150V to start MPPT operation, this isn't much above the panels Voc, but once started, the units then run MPPT down to 120V, so just OK?
The Victron units only need battery (ie ~nominal 53V) + 5V to start MPPT

You could run the Pip unit from one string & a Victron on the other.

+1 for LiFePo4 batteries :-)
Running off solar, DIY & electronics fan :-)
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#3
I would agree with @RedPacket's suggestion.   I have 3 arrays and 1 of them is moving-shade till 11am'ish.   I have that 3rd array (3s5p) on it's own MPPT Controller and I can see the significant affects of shade.  For example, right now (8am) its about 50% shaded (corner of house is blocking full sunlight) and is producing 550w where the unshaded array is producing 1025w. You don't want shade to pull down unshaded if you can compartmentalize them.
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#4
Thanks guys. Exactly the real world experience I was looking for.
  I will put the Trina panels on the higher roof. Wow, I just checked fleabay for a price on a victron 250/60. at around $1k AUD its a significant extra investment for an extra approx 1kwh per day. So just thinking out loud. What if I used a few dc rated contactors (haven't priced them yet) to switch in the four Seraphim panels in series with a timer once they are in full sun? Yes, there would be losses from mixing the two types of panels but wouldn't I then gain from the higher voltage input to the PIP once I switched in the extra panels?

Thanks again for your advice :)
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#5
(05-04-2020, 11:01 PM)jimbo1890 Wrote: Thanks guys. Exactly the real world experience I was looking for.
  I will put the Trina panels on the higher roof. Wow, I just checked fleabay for a price on a victron 250/60. at around $1k AUD its a significant extra investment for an extra approx 1kwh per day. So just thinking out loud. What if I used a few dc rated contactors (haven't priced them yet) to switch in the four Seraphim panels in series with a timer once they are in full sun? Yes, there would be losses from mixing the two types of panels but wouldn't I then gain from the higher voltage input to the PIP once I switched in the extra panels?

Thanks again for your advice Smile
Can you do 2 strings of 4 panels each in parallel - e.g. 4s2p.  Put the 4 in the sun in 1 series and the other 4 (shaded) in another string.   Use 2 Seraphim + 2 Trina for 1 string and then 2 Seraphim + 2 Trina for the other string - e.g. keep the voltages the same.   

The voltage should be pretty close for both sun and shade sets but the sun 4s will have more amps and the shade 4s less amps...  which combine pretty well - much better than 8s where 4 of them are in shade. 

And, if you do the above, perhaps wire things (do the combining) close to your existing controller so you could add a 2nd controller at some point in the future if you're not satisfied.
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#6
Yep, Sounds like the way to go.

Thanks again for the input.
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#7
I would sell some of the panels. Combining is not optimal. You Will loose alot due to shading and you loose some due to not same voltage/current.

So best is to get same type panels in same mppt. Shading is only sorted with optimizers or microinverters.

Above Will work but dont expect more than max 80% in the combinations that have been recomended. Best is to just add an extra mppt Charger here
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#8
Summarizing:
- don't mix panel types in a string
- put the 4x Trina's in 1 string on the top roof > one MPPT
- put the 4x Seraphim's in 1 string on the lower roof >a separate MPPT (or prefered to optimizers or microinverters)

You could also look for "1/2 cell" panels which are better in shade
Running off solar, DIY & electronics fan :-)
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#9
Thanks for your input Daromer, My installation is not totally off grid so the pip will have 15 amps at 240volts of utility input I can call on when the batteries get low. As OffGridInTheCity suggested I will run the feeds from both strings back to separate 2pole 20A breakers next to the pip so if I'm still using too much utility power I can bite the bullet and install a victron without too much drama.
On average I use 6 kwh per day so even at 80% I should see a significant reduction in my utility usage.
when I smooth the bank account curve I will get the victron and rematch the panels.

Thanks ;)

Hey guys the lower panels are only shaded for an extra hour or so early morning 7am to 8am and both levels loose full sunlight in the afternoon at the same rate.

unless my calcs are incorrect we are talking about roughly one kwh per day.
I have just ordered some 25A 1000V BLOCKING DIODES from fleabay for the lower string of panels
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