SMA SB7000TL-US-22 Inverter - Accessing individual MPPT performance information?

[MinorDeity]

Are you sure the parallel connection was made in the inverter? There might be a junction box on the roof, saving some home run wiring. In any case, if you intend to swap it, it is safer to do it at night.

Actually, there is a junction box just below the inverter and as coincidence would have it my installer just stopped by and I saw how he did the switch. ...piece of cake! Now I can check for any difference in performance.

[sensij]

Regarding such an experiment, I've been tempted to switch the strings myself but I'm a little hesitant to start playing with the wiring at the MPPT inputs of the inverter. I'm not so much afraid of getting a good shock since I've had a lot of experience working around such voltages, but I am concerned with accidentally shorting out one of the strings in the process. Since the open circuit voltage from such a string is in the neighborhood of 300+v, are the panels protected well enough to prevent damage from such a short?

Are you sure the parallel connection was made in the inverter? There might be a junction box on the roof, saving some home run wiring. In any case, if you intend to swap it, it is safer to do it at night.

[inetdog]

Regarding such an experiment, I've been tempted to switch the strings myself but I'm a little hesitant to start playing with the wiring at the MPPT inputs of the inverter. I'm not so much afraid of getting a good shock since I've had a lot of experience working around such voltages, but I am concerned with accidentally shorting out one of the strings in the process. Since the open circuit voltage from such a string is in the neighborhood of 300+v, are the panels protected well enough to prevent damage from such a short?

Panels are inherently current limited. Instead of Imp you will be drawing at most Isc and the panel will not be bothered at all.
There might be a nasty spark though.

[MinorDeity]

I wish I had the OP's setup at my house to run an experiment on two consecutive clear sky days or however many days it took to get a comparison between the two discussed string configurations. It would be interesting to compare those power plots.

Regarding such an experiment, I've been tempted to switch the strings myself but I'm a little hesitant to start playing with the wiring at the MPPT inputs of the inverter. I'm not so much afraid of getting a good shock since I've had a lot of experience working around such voltages, but I am concerned with accidentally shorting out one of the strings in the process. Since the open circuit voltage from such a string is in the neighborhood of 300+v, are the panels protected well enough to prevent damage from such a short?

[J.P.M.]

I believe the discussions in this thread about array temperatures could benefit from the addition of the concept of an energy balance on the array.

Panels get hot from the sun.

Considering only one panel for now, the sun is usually the only energy input. The outputs from the panel are the electricity produced, with the rest of the energy output taking the form of heat loss to the surroundings. Most of those losses are convective losses from the wind and/or natural convection under still air, and thermal radiation losses to the surroundings which are usually, but not entirely and not always at a lower temp. than the panel (think hot roofs). There are some thermal conduction losses through the frames to the racking.

According to the 1st law of Thermodynamics, the energy input to a body must equal the energy output plus any energy stored in the body.

Very simply, and at the risk of some oversimplification, and assuming steady state or quasi-steady state conditions that don't change very quickly, and no energy storage in the a solar device, the panel temp., at least in theory, and theory has shown to often be a pretty decent 1st approx. to reality, will be such that the solar irradiance (the input) exactly equals the sum of the electricity production and the thermal losses from the panel.

More POA irradiance will increase the panel temp, increase the panel current, and increase the power output. The increased panel temp. will however decrease the panel voltage in a measureable way, increase the current in a very small way that's often ignored, and because of the voltage drop, decrease the panel output an thus decrease the panel efficiency.

BUT, depending on how the environmental conditions of wind and air temp. influence the rate of heat loss and thus the panel temp, and finally the panel voltage and the heat portion of the energy balance, the overall power output will likely increase from increased irradiance (but not as much as if the panel is cooled by higher winds, cooler ambient air temps., artificial cooling methods, etc.).

The decreased efficiency (lower power out as a % of power input) resulting from reduced voltage is the manifestation of the effect of more heat loss from increased panel temperature. The energy of more heat rejection has to come from some place.

In the energy balance, hotter panels reject more heat (energy). That additional heat rejection manifests itself in the form of less electrical production and lower panel efficiency.

[DaveDE2]

Let's reintroduce temperature into the discussion. In the isothermal case, the orientation with less irradiance will have a slightly lower Vmp. However, the orientation with less irradiance will also have a lower cell temperature, which should *increase* Vmp for the lower irradiance array.

Excellent point.

[sensij]

Some of the data you want is already in the thread, post 6. You have the vmp of the SE array alone overlaid onto the Vmp of the compromise SE+SW array.

[DaveDE2]

I wish I had the OP's setup at my house to run an experiment on two consecutive clear sky days or however many days it took to get a comparison between the two discussed string configurations. It would be interesting to compare those power plots.

[sensij]

Let's reintroduce temperature into the discussion. In the isothermal case, the orientation with less irradiance will have a slightly lower Vmp. However, the orientation with less irradiance will also have a lower cell temperature, which should *increase* Vmp for the lower irradiance array.

[inetdog]

I believe you're accounting for power output loss numbers by only considering changes in voltage but not resulting changes in current when the voltage is not at the MPP..

No. When the voltage is above MPP the current is below MPP and vice versa. Exactly at MPP the slope of the power curve is zero.

[DaveDE2]

Let's take a closer look at your argument on that:

Assume two strings, one producing 1kW at peak sun and facing West and the other producing 1kW at peak sun and facing east.
Look first at the time when the sun is at SW in the sky. The two strings will be producing equal power (less than 2kW, but equal contributions from each string.)
No loss of efficiency at all in paralleling them since they have the same irradiance and presumably same temperature.
Now lets move to the time when the sun is due S of the arrays.
One array is producing 1kW, more or less while the other is at at most 40% irradiance and is producing roughly 400W. The Vmp of the second string may be 3% lower (1 volt out of 30).
If the two strings had equal power output the overall power loss could be as much as .3% as a result. But since one string is producing 1kW and the other is producing only 400W, The loss in total power is more like .15%. and for only a low production point in the solar day. Look at the time when the irradiance is only 20%. The voltage difference is twice as large but the power is half as much. Result is still less than .15% overall. Compare that to the constant loss of 2 or 3% or more through the whole day from mixing a 7 panel and an 8 panel string.
I agree that there is a difference, I just do not think that it is enough of a difference to override other concerns or to pay extra for an additional MPPT input in most if not all cases. Remember that any static solution has to be considered averaged over the whole production day.

Thanks for the example, not sure if I follow your numbers here but I believe you're accounting for power output loss numbers by only considering changes in voltage but not resulting changes in current when the voltage is not at the MPP. Given that, I believe your calculations for loss percentages are optomistically low, but I see where you're going with the argument.

By the way, the OP would not have to pay for another MPPT, just move a few wires in the dc disconnect. Cheers.

[DaveDE2]

Really? Are you sure you aren't confusing temperature effects with irradiance effects, at least for the common panels being installed today? Does 4:1 accurately reflect the ratio of irradiance for a SE and SW array for a meaningful amount of time?

I'm getting this from some plots I've seen on the internet. One such is attached here. In the case of this plot, the power loss from half irradiance to full would be about 1-2% if Vmp were fixed. Having said that, I have not analyzed MPP plots for common panels being installed today and maybe I should and my mind would change if I saw that MPP differences caused insignificant loss at fixed V. According to you and inetdog sounds like this is the case for most panels. I read the SMA article you referenced. 0.25% loss for their case is indeed low and impressive and quite surprising to me. Btw, I also get what you're saying about load balancing possibly being an advantage, not just about possible longevity increase which I'm not sold on yet but more about the possibility of inverter efficiency increase with increased power and *possibly* nullifying any advantage of rearranging the strings in the OP's case.

[inetdog]

...
....They're almost the same, but not exactly. Some vary by several volts from 20-80% irradiance. I think therin lies our argument. Yes, I agree that if 1%ish loss in efficiency numbers are good enough for someone, then go ahead and combine strings with different irradiances. On the other hand, if you're trying to eek out every last 0.5%-1% like so many of the people here are, then I submit that combining strings that are facing the same directions and putting them on the same MPPT makes sense.

Let's take a closer look at your argument on that:

Assume two strings, one producing 1kW at peak sun and facing West and the other producing 1kW at peak sun and facing east.
Look first at the time when the sun is at SW in the sky. The two strings will be producing equal power (less than 2kW, but equal contributions from each string.)
No loss of efficiency at all in paralleling them since they have the same irradiance and presumably same temperature.
Now lets move to the time when the sun is due S of the arrays.
One array is producing 1kW, more or less while the other is at at most 40% irradiance and is producing roughly 400W. The Vmp of the second string may be 3% lower (1 volt out of 30).
If the two strings had equal power output the overall power loss could be as much as .3% as a result. But since one string is producing 1kW and the other is producing only 400W, The loss in total power is more like .15%. and for only a low production point in the solar day. Look at the time when the irradiance is only 20%. The voltage difference is twice as large but the power is half as much. Result is still less than .15% overall. Compare that to the constant loss of 2 or 3% or more through the whole day from mixing a 7 panel and an 8 panel string.
I agree that there is a difference, I just do not think that it is enough of a difference to override other concerns or to pay extra for an additional MPPT input in most if not all cases. Remember that any static solution has to be considered averaged over the whole production day.

[DaveDE2]

Your are looking at it wrong. Let me reiterate what sensij said but in different words.

You can either look at the MPPT as providing the right load resistance to let the panel operate at the MPP or as regulating the panel voltage to Vmp by regulating the current being drawn. Either is valid but one or the other may be more useful in analyzing certain situations.

In the case we are considering, as long as the array temperatures are similar the Vmp for each string will be the same, contradicting the quoted text in bold. By regulating the total current drawn from the two strings in parallel the MPPT algorithm can cause the voltage to go to Vmp for both strings. They will just deliver different amounts of current depending on their irradiation.
If we look at the variable load resistance, you can figure out what the ideal load resistance, R1, is for string one and the ideal load resistance, R2, for string two.
All that the MPPT algorithm then needs to do is to figure out that for the two strings in parallel the correct load resistance is the parallel combination of R1 and R2. It is not necessary to have the inverter input impedance match either string directly, just the parallel combination.
Of the two analyses, I find the one that says the MPPT algorithm needs to seek the same voltage for both strings to be easier to follow.

If the two strings had different lengths or some of the panels of one string were shaded, then the two Vmp values would not be equal and running them in parallel into the same MPPT input would not work as well.

But even with a difference in the string voltages the effect on the overall efficiency may not be as great as you might expect. Because of the slope of the total power versus voltage (or current) curve of a silicon panel is by definition flat at the MPP, small variations from the MPP will not have a proportional effect on the output power.
The accepted rule of thumb is that if the two string voltages are matched within 5% and the strings are then put in parallel the loss of power will be less than 1%. Maybe only .5%?
Similarly if two panels have the same Imp within 5% they can be put in series without losing more than 1% of the output power.

Although I believe we are all in violent general agreement about all of this, I respectfully disagree with your assertion that "as long as the array temperatures are similar the Vmp for each string will be the same". I'll say it again, different irradiances cause different cell currents and voltages and different MPPs. Look at any solar panel data sheet and the family of I-V vs irradiance curves and MPP plots. The MPP voltages vs irradiance are not the same value.They're almost the same, but not exactly. Some vary by several volts from 20-80% irradiance. I think therin lies our argument. Yes, I agree that if 1%ish loss in efficiency numbers are good enough for someone, then go ahead and combine strings with different irradiances. On the other hand, if you're trying to eek out every last 0.5%-1% like so many of the people here are, then I submit that combining strings that are facing the same directions and putting them on the same MPPT makes sense.

[inetdog]

My thinking is that the way the system is wired now neither of the two combined strings are operating at their MPP except at around high noon when they are equally illuminated. At mid morning and afternoon there may be a severe mismatch. For example if one string sees 600w/m^2 while the other is seeing 300w/m^2 then the first string will be putting out roughly twice the current as the second whilst both have about the same voltage. This means the MMP (or input impedance of the inverter, R=V/I) for each array will differ by a factor of two. Since the inverter can only operate at one input impedance, it won't be optimal for either array, but something in between. On top of that, since both strings want to put out different voltages, there will be current flow from one string into the other which is wasted power. It's like connecting two batteries of slightly different voltages in parallel. Granted, leaving the configuration as it is now will work one of the MMPTs less hard. Whether that has a significant impact on inverter life is as you say difficult to know. It depends on design specifics of the inverter.

Your are looking at it wrong. Let me reiterate what sensij said but in different words.

You can either look at the MPPT as providing the right load resistance to let the panel operate at the MPP or as regulating the panel voltage to Vmp by regulating the current being drawn. Either is valid but one or the other may be more useful in analyzing certain situations.

In the case we are considering, as long as the array temperatures are similar the Vmp for each string will be the same, contradicting the quoted text in bold. By regulating the total current drawn from the two strings in parallel the MPPT algorithm can cause the voltage to go to Vmp for both strings. They will just deliver different amounts of current depending on their irradiation.
If we look at the variable load resistance, you can figure out what the ideal load resistance, R1, is for string one and the ideal load resistance, R2, for string two.
All that the MPPT algorithm then needs to do is to figure out that for the two strings in parallel the correct load resistance is the parallel combination of R1 and R2. It is not necessary to have the inverter input impedance match either string directly, just the parallel combination.
Of the two analyses, I find the one that says the MPPT algorithm needs to seek the same voltage for both strings to be easier to follow.

If the two strings had different lengths or some of the panels of one string were shaded, then the two Vmp values would not be equal and running them in parallel into the same MPPT input would not work as well.

But even with a difference in the string voltages the effect on the overall efficiency may not be as great as you might expect. Because of the slope of the total power versus voltage (or current) curve of a silicon panel is by definition flat at the MPP, small variations from the MPP will not have a proportional effect on the output power.
The accepted rule of thumb is that if the two string voltages are matched within 5% and the strings are then put in parallel the loss of power will be less than 1%. Maybe only .5%?
Similarly if two panels have the same Imp within 5% they can be put in series without losing more than 1% of the output power.