proper string inverter sizing

[sensij]

sensij, here is a quote from an NREL engineer:

"You're right to question whether the efficiency is any good for DC voltages below 270V. If you look at the plot of efficiency that is shown in the datasheet for the 5000TL, when you go to a low DC input voltage of 175V, the efficiency has dropped from 97.5% to 96%. In order to maintain a weighted average CEC efficiency of 96%, SMA has changed the 'Rated' MPPT voltage range minimum from 125V to 270V for the 7700TL. The only reason to have two different MPPT input ranges is to game the CEC efficiency rating.

"As far as how low the actual efficiency is going to be, I would expect that anything much lower than 93%-94% would start to lead to problems in the heat dissipation and lifetime of components in the inverter. The fact that they even allow the 7700TL to connect at these low voltages indicates that the efficiency can't be too catastrophic. It's impossible to know without actually testing the inverter at these low input voltages, but I would bank on operation around 96% efficient (max) at 270V input, and possibly down to 92-93% efficient around the minimum of 125V. If you really need to operate at these low input voltages, you might be better off with two SB4000TL inverters."

I went with the 3.8kw inverters that you recommended. Thanks for your advice!

Thank you very much for following up on this thread. What the NREL engineer is saying makes a lot of sense to me. I think you'll be happy with the design flexibility of the 3.8's, and can be confident you'll be getting the performance you are looking for.

[inetdog]

Speaking very generally, by how much does MPPT increase inverter output?

For a typical insolation profile corresponding to clear sky and with the panels aimed for on solar noon, my SWAG would be that:
1. If your inverter did not have MPPT it would have to be customized specifically for whatever your personal peak panel power was.
2. Given that, you would lose about 1/3 of the total available power over the course of the full day.
3. For a constant impedance design, anything that cut the maximum panel output by a factor of two would drop the inverter output by a factor of four. Etc.

[charles2]

sensij, here is a quote from an NREL engineer:

"You're right to question whether the efficiency is any good for DC voltages below 270V. If you look at the plot of efficiency that is shown in the datasheet for the 5000TL, when you go to a low DC input voltage of 175V, the efficiency has dropped from 97.5% to 96%. In order to maintain a weighted average CEC efficiency of 96%, SMA has changed the 'Rated' MPPT voltage range minimum from 125V to 270V for the 7700TL. The only reason to have two different MPPT input ranges is to game the CEC efficiency rating.

"As far as how low the actual efficiency is going to be, I would expect that anything much lower than 93%-94% would start to lead to problems in the heat dissipation and lifetime of components in the inverter. The fact that they even allow the 7700TL to connect at these low voltages indicates that the efficiency can't be too catastrophic. It's impossible to know without actually testing the inverter at these low input voltages, but I would bank on operation around 96% efficient (max) at 270V input, and possibly down to 92-93% efficient around the minimum of 125V. If you really need to operate at these low input voltages, you might be better off with two SB4000TL inverters."

I went with the 3.8kw inverters that you recommended. Thanks for your advice!

[charles2]

Speaking very generally, by how much does MPPT increase inverter output?

[charles2]

If you look at the "specific energy yield" for those two SMA designs, the single inverter produces 1449kW/kWh and the dual produces 1450kW/kWh. so all the extra expense of doing a dual install is 1 point better. KISS.

By that metric the dual install is actually worse if you consider that the single inverter system used 1 less module (26) than the dual (27). What accounts for this?

[sensij]

Solarix, glad you jumped in. The question here is how the inverter handles operating within the dc range, but outside the mppt range. I'm not sure I would trust the simulation results without knowing the answer to that question. Do you know?

[solarix]

If you look at the "specific energy yield" for those two SMA designs, the single inverter produces 1449kW/kWh and the dual produces 1450kW/kWh. so all the extra expense of doing a dual install is 1 point better. KISS.

[charles2]

sensij, what do you make of the attached simulations? Do they clarify the issue or further obfuscate?

[charles2]

If the SMA tech so goes for it, who am I to say otherwise? The attached CSI data doesn't show below 270 V, but I'll take your word for it that the jpg shows data from this inverter.

Well, I'd say you have a keen eye for details and that official pronouncements are not always correct. I've certainly learned not to trust the claims of our incompetent and corrupt local government officials.

[sensij]

I should add... inverter efficiency measurements are not likely to be the right way to tell how the inverter will operate outside of the MPPT range. Efficiency generally refers to the conversion of the DC to AC. The MPPT function is what helps the panels produce to the maximum ability. So if the panels operate at some point other than the maximum, it doesn't really matter that the DC to AC conversion is still 95% or better... the losses are upstream of that. The penalty for operating at something other than the maximum might be small, or might be significant. It just depends on what the inverter is doing in place of MPPT.

[sensij]

sensij, SMA tech support addressed my question today and they say the single 7.7kw inverter will only suffer a slight (1%) efficiency loss if operated below the Rated MPPT voltage range lower limit of 270V. In other words, not enough to justify the expense and trouble of 2 inverters.

As support, they cited data obtained from http://www.gosolarcalifornia.org/equ...-22_240Vac.pdf . I've attached the actual graph they sent me.

Your thoughts?

If the SMA tech so goes for it, who am I to say otherwise? The attached CSI data doesn't show below 270 V, but I'll take your word for it that the jpg shows data from this inverter.

[charles2]

sensij, SMA tech support addressed my question today and they say the single 7.7kw inverter will only suffer a slight (1%) efficiency loss if operated below the Rated MPPT voltage range lower limit of 270V. In other words, not enough to justify the expense and trouble of 2 inverters.

As support, they cited data obtained from http://www.gosolarcalifornia.org/equ...-22_240Vac.pdf . I've attached the actual graph they sent me.

Your thoughts?

[sensij]

Agreed. But I'm still wondering if it's possible to make a single 7.7kw work well. How do you like dropping the odd panel to reach 26, then have 2 strings of 9 panels and 1 string of 8 panels? I guess I'm asking, how much do the different panel orientations mess things up?

Different panel orientations on a single string will mess things up. The current will be limited to that of the lowest performing panels, unless they underperforms by enough to just get bypassed (through their diodes) completely.

sensij, are you sure you are looking at this correctly? Isn't the Rated MPPT voltage range the sum total of both strings on the MPPT? For example, suppose I have 2 strings and each is running at 150V. They both meet the start up Voltage requirements (125V/150V) per string. They combine for 300V total, putting them above the 270V MPPT range minimum. Can you confirm or disconfirm?

Strings in parallel do not sum voltage. They only sum current.

[charles2]

I think the problem is 13 panels are on one roof face, and 14 are on the other. Splitting them into parallel strings doesn't work great. A string of 6 * 37.0 Vmp = 222 Vmp, which is too low for this inverter (low MPPT range is 270 V).

sensij, are you sure you are looking at this correctly? Isn't the Rated MPPT voltage range the sum total of both strings on the MPPT? For example, suppose I have 2 strings and each is running at 150V. They both meet the start up Voltage requirements (125V/150V) per string. They combine for 300V total, putting them above the 270V MPPT range minimum. Can you confirm or disconfirm?

[charles2]

You might actually be better off going for a pair of the SB3800TL, with a 1.20 ratio. You can use a 40 A breaker to protect two of those, which in some cases will help avoid the expense of a panel upgrade, in addition to the savings from going with the smaller inverter.

Agreed. But I'm still wondering if it's possible to make a single 7.7kw work well. How do you like dropping the odd panel to reach 26, then have 2 strings of 9 panels and 1 string of 8 panels? I guess I'm asking, how much do the different panel orientations mess things up?