non Optimal Configuration?

solar roller

What does it mean when using the Xantrex sizing calculator and the proposed strings and modules results in the " non Optimal Configuration.
Inverter will regulate the PV voltage to 235Vdc"?

I new to the market and want to make sure the system I’m getting installed is done correctly. The system is claimed to be pre-engineered and consists of 20 sharp 224w panels with a Xantrex G.T 4.0 to be installed in 2 strings of 10 panels.

When i put in the information that i know of on the Xantrex sizing calculator the proposed string to module combination is coming up in the "non Optimal Configuration" (resulting that the Inverter will regulate the PV voltage to 235Vdc). What does this mean and should i be alarmed and stop the installation?

Below is some information copied and pasted from Xantrex's sizing cal.



Sharp nd224uc1 panel
Max Power Voltage - Vmp 29.28 Vdc
Open Circuit Voltage - Voc 36.6 Vdc
Voltage Temp Coeff - Vtoc -0.1318 V/°C
STC Rating - Pmp 224 Wstcdc
Max Power Current - Imp 7.66 Adc
PTC Rating 192.6 Wptc

Xantrex gt 4.0 inverter
Pnom 4000 Wac
Idc max 18 Adc
Vmptmax 550 Vdc
Voc 600 Vdc
Vmptmin 235 Vdc
Efficiency 95.5 % AvgEfficiency






Max Voc Min Vmp
at Min Temp at Max Temp
(Vdc) (Vdc)
******9 Modules 374.87 204.21*****
******10 Modules 416.52 226.9******
11 Modules 458.18 249.59
12 Modules 499.83 272.28
13 Modules 541.48 294.97
14 Modules 583.13 317.66

DC input range 235-600Vdc
DC MPPT range 235-550Vdc
*******non Optimal Configuration.
Inverter will regulate the PV voltage to 235Vdc


This sizing tool uses actual manufacturer

BB.

Re: non Optimal Configuration?

If you assume your high temperatures approach 100F, the Vmp of the panels will drop to ~22.983 volts. Or for 10 panels in series:

10 Modules
412.13 Voc (maximum open circuit voltage with panel = 14C)
229.83 Vmp (minimum optimum voltage under load = 100F)

The 4kW GT inverter (240 VAC model) requires a minimum of 235 volt for "optimum" operation... Basically trying to satisfy this equation:

Pmax=Vmp*Imp

The GT inverter takes a little more current or a little less current and calculates V*I and looks for where that is the maximum power.

In this case, the Imp of 7.66 amps per string, time two because you have two parallel strings:

• Pmax=Vmp*Imp=229.83v*2*7.66 amps= 3,521 watts at 100F ambient

But what you will get will be (just a guess):

• Pnon-max=V*I=235v * 2 * 7.19 amps = 3,380 watts "non-optimum" at 100F ambient

Your average high temperature may be 92F, and your record high is around 100-103F.

At 92F and still air, the String Vmp>235 volts DC (excluding wiring drop).

At 100F and still air, the String V,mp=229.83 volts--a bit below what the controller can use.

The ideal answer would be to install 11 panel strings.

However, if you cannot, the system will still operate just fine, but will generate something like 4% (guess, plus or minus) when you are near 100F.

Note, there are "fudge factors" already included in the Xantrex calculation for high temperatures. They are assuming the panels are mounted pretty much flat to the roof and there is little wind. This is the worst case for Vmp (minimum voltage). So the Vmp calculated is, pretty much, a worst case voltage.

And a loss of "optimum" output may be 4% in the hot, breeze-less afternoons where you are near record temperatures.

Sort of your choice. Running this near the inverters Vmin is the rough equivalent of running 19.2 panels because of the configuration.

My guess--and I try to be conservative (worst case). Others here maybe able to address the issue more exactly for your case.

-Bill

icarus

Re: non Optimal Configuration?

Just to add to Bill's most learned response, remember that panel temps will be significantly higher than any given ambient. For example, from my experience, even at well below freezing, panels will absorb enough heat to melt snow and ice. Air temps ~100f will result in much higher cell temps.

Tony

BB.

Re: non Optimal Configuration?

The Xantrex String Sizing Calculator assumes a 35C/63F cell temperature rise over ambient...

That is probably the worst case rise for a normal installation (a 100F day would be a 163F panel temp). Already getting near the 85C/184F typical maximum panel temperature rating.

-Bill

solar roller

Re: non Optimal Configuration?

Thanks so much for your helpful knowledge.