Question about PTC/STC v.s. Real Life PV Generation

bart

The system I helped my Brother install on his home is using SMA-3000 and 16 Solyndra SL-001-173. These are wired as 4 x 4 configuration. The system is up and running fine and waiting for rebate check.

However, the output appears just over 1.5Kw.

So, we want to expand the system (add panels) to bring the output closer to 3K. According to the math this is what was expected:

STC Array Power 2,768W
PTC Array Power 2,605W
CEC Power Output 2,488W
Annual Prod. Est. 4,268kwhr

So Logic tells me if I doubled the Panels to 32, I should be close to 3K and still be able to use the SMA-3000.

Where is my logic wrong? I know the math using STC/PTC shows it too much for SMA-3000 - But what about "Real Life" install?

Bart

Cariboocoot

Re: Question about PTC/STC v.s. Real Life PV Generation

All panel ratings are under laboratory conditions and are "at the moment" numbers. Put them in the real world and, on average, 20% disappears right away. It's important to note that "on average" because sometimes they do put out their rated numbers, or even a little bit more.

Normally a GT inverter will have slightly more array than it is rated for. You might have 20% to 30% more, or up to 3800 Watts on a 3kW inverter. Check the manual and it should say what the maximum array size is. Under these circumstances, and "extra" power the array might be capable of would just be "clipped" (lost).

Of more concern is that from 2768 Watts of array you'd normally expect about 2kW of power: 2768 * 0.77 efficiency (typical) = 2131 Watts. Maybe you should check out that array for insolation, shadows, et cetera. You're down half a kilowatt and that's not good. Definitely check that out before you go adding more panels. It could be a weather thing or there could be a problem with connections.

ggunn

Re: Question about PTC/STC v.s. Real Life PV Generation

bart wrote: »

The system I helped my Brother install on his home is using SMA-3000 and 16 Solyndra SL-001-173. These are wired as 4 x 4 configuration. The system is up and running fine and waiting for rebate check.

However, the output appears just over 1.5Kw.

So, we want to expand the system (add panels) to bring the output closer to 3K. According to the math this is what was expected:

STC Array Power 2,768W
PTC Array Power 2,605W
CEC Power Output 2,488W
Annual Prod. Est. 4,268kwhr

So Logic tells me if I doubled the Panels to 32, I should be close to 3K and still be able to use the SMA-3000.

Where is my logic wrong? I know the math using STC/PTC shows it too much for SMA-3000 - But what about "Real Life" install?

Bart

It's not that simple. You must make sure that the maximum voltage your array will ever present to your inverter NEVER exceeds 600V, even on the coldest winter dawn. To do that you'll either have to consult an SMA string sizing program or do the math. The math - adjust the module Voc for the record coldest temperature for your location by using the published Voc and the module Voc thermal coefficient, and multiply it by your string length. Then, of course, you must keep all parallel strings the same length.

That said, you can overload the wattage on a Sunny Boy by 20% or more without a problem as long as you keep the voltage down.

bart

Re: Question about PTC/STC v.s. Real Life PV Generation

Thanks for the info,

So let's assume there is nothing wrong with the install. The install is located along the beach and suffers from morning overcast almost every day. This might account for lower than expected output.

So is seems the Solyndra 173 outputs PTC 162.8 (I think it might be closer to 100 w each) and keeping the 4 panel in a string as it is now. I'm thinking I could add 1 more 4 x 4 (16 Panels) and still be under the SMA3000 max.

Real Life:

16 Panels => Current: 1.55kw / 16 = 96.9 w per panel each
Add 16 Panels => 96.9 x 32 = 3100 watts

PTC:

16 x 162.8 = 2604 w
32 x 162.8 = 5209 w <= over, but is it realistic?

Bart

solar_dave

Re: Question about PTC/STC v.s. Real Life PV Generation

As the others have said you have to make sure you don't exceed the max voltage. My array is 12.5Kw but I have it hooked up to 2 5200 watt inverters. I did see it clip slightly last winter on a clear cool day.

solar_dave

Re: Question about PTC/STC v.s. Real Life PV Generation

bart wrote: »

Thanks for the info,

So let's assume there is nothing wrong with the install. The install is located along the beach and suffers from morning overcast almost every day. This might account for lower than expected output.

So is seems the Solyndra 173 outputs PTC 162.8 (I think it might be closer to 100 w each) and keeping the 4 panel in a string as it is now. I'm thinking I could add 1 more 4 x 4 (16 Panels) and still be under the SMA3000 max.

Real Life:

16 Panels => Current: 1.55kw / 16 = 96.9 w per panel each
Add 16 Panels => 96.9 x 32 = 3100 watts

PTC:

16 x 162.8 = 2604 w
32 x 162.8 = 5209 w <= over, but is it realistic?

Bart

derate ratios are usually around 77 to 80%

bart

Re: Question about PTC/STC v.s. Real Life PV Generation

ggunn wrote: »

It's not that simple. You must make sure that the maximum voltage your array will ever present to your inverter NEVER exceeds 600V, even on the coldest winter dawn. To do that you'll either have to consult an SMA string sizing program or do the math. The math - adjust the module Voc for the record coldest temperature for your location by using the published Voc and the module Voc thermal coefficient, and multiply it by your string length. Then, of course, you must keep all parallel strings the same length.

That said, you can overload the wattage on a Sunny Boy by 20% or more without a problem as long as you keep the voltage down.

OK, so using VOC for / from Solyndra 173: VOC - 95.2 x 4 in string = 380 v. Way under SMA300 max DC volts of 500.
So would it be better to go to 5 in a string (95.2 x 5 = 357v)? or 6 in a string (96.2 x 6 = 577v)?

Bart

bart

Re: Question about PTC/STC v.s. Real Life PV Generation

so I derate PTC by 77 to 80% or STP?

Bart

solar_dave

Re: Question about PTC/STC v.s. Real Life PV Generation

bart wrote: »

OK, so using VOC for / from Solyndra 173: VOC - 95.2 x 4 in string = 380 v. Way under SMA300 max DC volts of 500.
So would it be better to go to 5 in a string (95.2 x 5 = 357v)? or 6 in a string (96.2 x 6 = 577v)?

Bart

95.2 X5 = 476V to close to max in my opinion. Strings of 4 sound right to me.

Cariboocoot

Re: Question about PTC/STC v.s. Real Life PV Generation

If he paralleled an identical array the input Voltage would not be changed.

If the peal output of the 2768 Watt array is only 1500 Watts then the panels are running at a mere 54% of their nameplate rating. That would suggest an array of 5555 Watts to achieve the 3kW output of the inverter.

I'd check that array very carefully before spending money on doubling its size.

ggunn

Re: Question about PTC/STC v.s. Real Life PV Generation

bart wrote: »

OK, so using VOC for / from Solyndra 173: VOC - 95.2 x 4 in string = 380 v. Way under SMA300 max DC volts of 500.
So would it be better to go to 5 in a string (95.2 x 5 = 357v)? or 6 in a string (96.2 x 6 = 577v)?

Bart

You cannot just use the nameplate Voc for max voltage calculation. You absolutely must correct that voltage for low temperature.

Vmax is the most critical number in a PV array design. Inverters are very unforgiving when it comes to DC overvoltage; many (all?) of them have "black box" recorders in them which keep track of the highest DC voltage the inverter has ever been exposed to. Even if an inverter survives an overvoltage condition, if it is ever submitted for a warranty issue, for any reason, even years later, if that register shows it having ever experienced a DC input voltage of even a little bit over its published maximum, the manufacturer can and usually will deny the claim.

How much that correction is depends, of course, on where you are, but unless you are deep in the tropics, wiring your array at Voc = 577 is practically a guarantee of disaster.

I would heed 'coot's advice about checking the array thoroughly before buying any more modules (if you can even find any - Solyndra supplies have pretty much dried up). Solyndra wiring isn't as straightforward as most; you can screw it up where you have modules canceling each other out.

Cariboocoot

Re: Question about PTC/STC v.s. Real Life PV Generation

I doubt California sees the super cold temperatures we get up here. At most a factor of 1.2 on the Voc, or 457 Volts for the four panels in series. I agree with ggunn that five in series would be flirting with disaster.

However I'm still under the impression that the OP's intent was to add a similar string or strings of panels (same Voltage) in parallel to the existing array, increasing the current and Wattage.

bart

Re: Question about PTC/STC v.s. Real Life PV Generation

Cariboocoot wrote: »

I doubt California sees the super cold temperatures we get up here. At most a factor of 1.2 on the Voc, or 457 Volts for the four panels in series. I agree with ggunn that five in series would be flirting with disaster.

However I'm still under the impression that the OP's intent was to add a similar string or strings of panels (same Voltage) in parallel to the existing array, increasing the current and Wattage.

That's correct. I'd be keeping the 4 in series for the additional panels. I did do a voltage reading after the install and each string was putting out equal voltage. Frankly, I don't remember the amount now. Going there Saturday to check it and will report back. I know the connector were kind of confusing on that panel. We had to a volt meter to double check we used the correct plug. I think we are good there, but I'll double check anyways

Thanks Guys,

Bart

ggunn

Re: Question about PTC/STC v.s. Real Life PV Generation

bart wrote: »

That's correct. I'd be keeping the 4 in series for the additional panels. I did do a voltage reading after the install and each string was putting out equal voltage. Frankly, I don't remember the amount now. Going there Saturday to check it and will report back. I know the connector were kind of confusing on that panel. We had to a volt meter to double check we used the correct plug. I think we are good there, but I'll double check anyways

Thanks Guys,

Bart

Check the wiring thoroughly on the existing array. I forget the exact particulars, but on a large Solyndra installation a friend of mine was debugging, one string had much too low DC voltage. IIRC, it turned out that a module had been wired into the system backwards, which is apparently possible for a Solyndra module with its four conductors and two male and two female Tyco connectors. This caused the string voltage to be two modules low because the one backwards module effectively canceled out the voltage of one other module. You said that the voltage was the same on every string, but was it the voltage you expected to see?

If you decide to add to the array, I encourage you to run a simulation through Sunny Design or SMA's online string calculator (it's an SMA inverter, right?) before you buy any more modules. If you can find any, that is. I saw something on line about all of Solyndra's inventory and equipment, right down to the office furniture, having been sold at auction a few days ago. Expect to pay a premium for any Solyndra modules that are still out there on the market.

BB.

Re: Question about PTC/STC v.s. Real Life PV Generation

Yep. There is nothing left of Solyndra as a company.

-Bill