with a Classic 250 would you guys do six in series or not? (HyperVoc marginal)

[alyaz]

PV Array 
Number Of Modules In Series:  6 
Number Of Parallel Strings:  1 
Total Modules 6
Rated PV Array Power:  1500 Watts
Anticipated Array Power @ 34C:  1445 Watts
Rated PV Array Current:  8.22 Amps
Battery Charging Current @ 28.8 V:  52.1 Amps
VMP (Maximum Power Point Voltage) :  181.8 Volts
VOC (Open Circuit Voltage):  224.4 Volts
VMP @ -15 C°:  211.8 Volts
VOC @ -15 C°:  253.2 Volts
Charge Controller Classic250   
Max Operating Voltage  250
Max Non operating VOC (HyperVOC) @ 24V Nominal Battery Voltage  274 
Maximun Number Of Modules In Series 5 
Max Number Of Modules In Series (Using HyperVOC)  6 
Max Allowable Output Current Per Classic
Based On This Current Configuration  60
Max Allowable Wattage Per Classic
Based On This Current Configuration  1722
Present PV Array Wattage Of This Configuration  1500
Design Check 
Max VOC  MARGINAL
(HyperVOC) MARGINAL
(HyperVOC)
Temperature The Classic Will
Enter HyperVOC  -11 C° 
Array Power (Wattage)  OK 
Classics Required  0.9 

[boB]

alyaz,

I would think you'd probably be OK with this.  How often does it get to less than -10 or -12 degrees C there ??
253V is nuthin' for HyperVOC for 24V battery.  Now, -40C might be getting real close.

And as far as anticipated output...  One rarely gets the full rating out of their array.  Not that it can and
does happen...  Just not all that often.  You might be lucky to get 90% out SO that puts you, I would
say at an good spot with the 250.

Nice job  at listing the particulars !

My guess is you should go for it.

boB

[toothy]

Hello

I have a theory about the "record cold" we all use when doing these calculations, this theory is worth less than you paid for it, but here it is.

The record cold for most locations is, I would guess, recorder at night when there is little solar risk.

Just my theory.

Wade

[TomW]

Hello

I have a theory about the "record cold" we all use when doing these calculations, this theory is worth less than you paid for it, but here it is.

The record cold for most locations is, I would guess, recorder at night when there is little solar risk.

Just my theory.

Wade

Never thought of it that way.  It is just too obvious.

I guess I never understood if these voltage limits are hard limits or there is a bit of headroom?

Tom

[vtmaps]

I have a theory about the "record cold" we all use when doing these calculations, this theory is worth less than you paid for it, but here it is.

The record cold for most locations is, I would guess, recorder at night when there is little solar risk.

The coldest hour is usually right around sunrise.  This is because temperature is usually measured a few feet above ground level.  The coldest temps are at ground level... at sunrise the sun causes convection and stirs the air so that the coldest air circulates by the recording thermometer.

The panels themselves will be cooled by radiative cooling and may be several degrees below the air temperature at the same elevation as the panels.

Anyway, it takes almost no light to raise the panel voltage high enough to damage electronics if you exceed their Vmax.

--vtMaps

[alyaz]

alyaz,

I would think you'd probably be OK with this.  How often does it get to less than -10 or -12 degrees C there ??
253V is nuthin' for HyperVOC for 24V battery.  Now, -40C might be getting real close.

And as far as anticipated output...  One rarely gets the full rating out of their array.  Not that it can and
does happen...  Just not all that often.  You might be lucky to get 90% out SO that puts you, I would
say at an good spot with the 250.

Nice job  at listing the particulars !

My guess is you should go for it.

boB

it may get into the - 5 C to - 10 C temperatures maybe once or twice a winter for a day or two or three...

[vtmaps]

Alyaz, why do you want to run six panels in series?  Your Classic will be more efficient and run cooler if you configure the panels in a series/parallel configuration.

--vtMaps

[alyaz]

Alyaz, why do you want to run six panels in series?  Your Classic will be more efficient and run cooler if you configure the panels in a series/parallel configuration.

--vtMaps

Guess that's one of my questions too.  I thought it was best to use the higher voltage ability of the controller to run as many panels in series as possible.  The project is only about 40 feet from the Classic controller, so not overly concerned about wire sizing/cost.  Is the best configuration 2 strings of three panels in series? or 3 strings of two panels in series, etc? 

[mtdoc]

If your panels are only 40 feet from the controller, you don't need that high of voltage.  Use a Classic 150.

3 parallel strings of 2 panels each would be best for a 24 volt system.  If you went to a 48V system you'd want to do 2 strings of 3 panels each.

[vtmaps]

The project is only about 40 feet from the Classic controller, so not overly concerned about wire sizing/cost.  Is the best configuration 2 strings of three panels in series? or 3 strings of two panels in series, etc?

Your controller will be happier with 3 strings of two panels.  For a 40 ft run with three strings of two panels and a #6 AWG cable, you will have a voltage drop of 1.31% when you are producing full STC power.  You could use a Classic 150 rather than a Classic 250.  The 150 has a higher power rating if you ever want to expand your system.

--vtMaps

[Halfcrazy]

Alyaz, why do you want to run six panels in series?  Your Classic will be more efficient and run cooler if you configure the panels in a series/parallel configuration.

--vtMaps

Guess that's one of my questions too.  I thought it was best to use the higher voltage ability of the controller to run as many panels in series as possible.  The project is only about 40 feet from the Classic controller, so not overly concerned about wire sizing/cost.  Is the best configuration 2 strings of three panels in series? or 3 strings of two panels in series, etc?

From an electronics point of view it is always better to keep the In and Out voltage closer together. I usually recommend 1 nominal voltage above battery for an MPPT controller. Of course long wire runs can push us to use higher voltages when needed.

Ryan

[boB]

Alyaz,
Since your wire run is only 40 feet away, and you already have a 250 (I think),
I would just wire the array for 2 strings of 3 modules.

That way your Vpv is nice and high but not 200+ volts high and should work just great.

The Classic will run cooler, too which means you will get even more out of it.
High Pv voltages are great if you have long wire runs...  And also if you have
a lot of partial shading in parts of the day.  But even 3 in series will be plenty high
enough voltage to help in cases of partial shading, if you have some of that.

boB

[mtdoc]

Great points boB. I didn't realize he already had the 250 and hadn't thought about the partial shading issue.

How much efficiency loss is there as string voltage (input) rises far above charging voltage (output)??   I see this issue raised a lot but never quantified.  Is there some sort of a curve or equation one can use to estimate this loss?   

Weighing all the factors -  efficiency lost from bucking the voltage down versus gains with partial shading versus voltage drop with wire run distance is a tricky balance.  It would be nice to see a more advanced string size calculator that could incorporate these various factors.

[alyaz]

Alyaz,
Since your wire run is only 40 feet away, and you already have a 250 (I think),
I would just wire the array for 2 strings of 3 modules.

That way your Vpv is nice and high but not 200+ volts high and should work just great.

The Classic will run cooler, too which means you will get even more out of it.
High Pv voltages are great if you have long wire runs...  And also if you have
a lot of partial shading in parts of the day.  But even 3 in series will be plenty high
enough voltage to help in cases of partial shading, if you have some of that.

boB

thanks a lot for all the input!  :-)

[boB]

Weighing all the factors -  efficiency lost from bucking the voltage down versus gains with partial shading versus voltage drop with wire run distance is a tricky balance.  It would be nice to see a more advanced string size calculator that could incorporate these various factors.

One thing that does give an idea of how much you lose is the power graphs on our web site.  It shows the maximum output
power and current with varying input voltages.  I'm not sure exactly how much less efficient it is at the different voltages
but the Classic can dissipate around 75 watts I think.  Less than 100 I'm pretty darn sure, so you only lose maybe around
50 watts at the worsetest and at the hottest Classic temperatures...  This is approximate so don't quote that but it
should be in the ball park.

I like your idea of putting this balance on the string sizing tool.  We should definitely look into that !

boB