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

[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]

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]

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.

[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.

[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?

[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.

[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.

[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.