String Configuration for Different Surface Angles

[HRTKD]

My trailer is a toy hauler and it has a roof with two different angles. In the image below, the "A" panels are on the horizontal plane. The "B" and "C" panels are on a plane that is angled down towards the front of the trailer. The "A" and "B" panels are for sure, the "C" panels are optional. The "C" panels put me 30% over-paneled, but still within the max volts/amps of the Victron 100/50.

I'm not sure if the roof angle makes a difference in how I set up the strings. If it does, what should the string configuration be?
3s2p with panels A1, B1 and C1 on one series string and A2, B2, C2 on the other string?
2s3p with panels A1 and A2 on one series string and then B1/B2 and C1/C2 on the other series strings.

There is a roof vent between panels A1 and A2, so there is no room to place another panel between them. The "B" and "C" panels may have to slide forward due to the air conditioning unit in the middle of the roof (to avoid shade from it).

 

[svetz]

Great Question!

SAM could tell you based on the angles. To the MPPT, panels at different tilt angles on a string would be like partial shade on a string. It might not make much difference, but if you want to get every last erg I suspect each tilt angle would need to be on it's own MPPT input channel (or MPPT).

 

[HRTKD]

Great Question!

SAM could tell you based on the angles. It might not make much difference, but if you want to get every last erg I suspect each angle would neet to be on it's own MPPT input channel (or MPPT).

After I posted I started to think about perhaps putting the different angles/strings on different MPPT controllers. I also started thinking that limiting my configuration to 200 watt panels may not be ideal. Back to the (CAD) drawing board. This is going to increase my cost.

 

[svetz]

Let us know what you find! Really curious how this goes!

 

[HRTKD]

SAM is perhaps overkill for this. I used the PVWATTS calculator instead. I used my home (80130) as the location. Close enough.

With a 0° tilt, with the trailer facing south on a north/south line, a 1K panel system produces 1,378 kWh/Year.
I switched the tilt to 8°, keeping all the other variables the same and the output increased to 1,491 kWh/Year

No surprise that the tilt improves production. However, the trailer is never facing the same way twice. Sometimes I can pick the direction it faces, sometimes I can't. PVWATTS doesn't answer the question of what happens when two panels with different tilts are on the same string. I'm poking around in the SAM application now...

 

[svetz]

...SAM is perhaps overkill for this. I used the PVWATTS calculator instead....

PVWatts is from SAM, you can just run it from SAM (or the web link ;-).

 

[HRTKD]

Wow, there is a lot that goes into SAM. I muddled my way through entering two strings, one at 0° tilt and the second at 8° tilt, using a Trina Solar panel (TSM-300DD05a.05II) since it was in their database and it seemed to match up with a panel that is currently available. The database of modules in SAM is from Dec 2019. This is not the same panel I used in my prior post. The application also requires the selection of an inverter. I picked one pretty much at random since I'm not concerned about the inverter.

With both string at 0° tilt the output is 1,222 kWh/year.
With one string at 0° tilt and the other at 8° tilt the output is 1,259 kWh/year.

There isn't a way to setup a string with panels at different tilts. The model assumes that all panels on a string are at the same tilt. Consequently, I don't see a way to calculate the output if a string is made up of panels A1, B1 and B2.

I used a south facing orientation (180°). That was the default. Just for giggles, I chose an east facing orientation (90°) and the output numbers didn't change.

It looks like the "loss" for an array with some panels tilted and some not tilted is 3%. I'm backing into that number simply based on the simulated kWh output. 3% gives me a good indicator of the scope of the "loss" due to different angles. At 3% I'm not going to worry about it.

I think the only way to get a better handle on this is to get the system installed and switch the wiring around, using different panel arrangements on a string. Or, I'll just over panel and call it a day.

The SAM help manual has this to say about mismatches:

Subarray Mismatch Losses For a system with more than one subarray, SAM can estimate losses due to maximum power point mismatches between subarrays, but only with the CEC module submodel (either with database parameters or user-defined inputs, see Section 9.5). For the Sandia module model and simple efficiency module model, SAM does not estimate subarray mismatch losses as explained in Section 10.1. ​

​

The subarray mismatch loss algorithm (Dobos 2012b) is intended to model situations that may occur in residential rooftop systems when the array is installed on different parts of the roof with different orientations and connected to a single inverter. The group of modules on each roof surface is a subarray, and may have its own set of parameters defining its tilt and azimuth angles, tracking, shading factors, and DC losses. ​

​

In larger systems, modules are typically oriented uniformly across the entire array. SAM’s subarray mismatch algorithm is not suitable for such systems. SAM does not calculate mismatch losses due to maximum power point losses between individual modules in the array or in each subarray. ​

​

The algorithm determines the system’s maximum power point in a given hourly time step by running the CEC module model over a range of maximum power point voltage values to find the voltage that results in the complete array’s highest maximum power given each of its subarray’s effective incident irradiance, module performance and temperature parameters. ​

 

[svetz]

...There isn't a way to setup a string with panels at different tilts.

4 strings with 4 different tilts and Azimuths....did you check the checkboxes to add subarrays?

 

[svetz]

...The SAM help manual has this to say about mismatches:...

If you're using different PV modules in different strings, you can model them as two separate systems.


If a panel isn't in their database, you can select "CEC Performance Model with User Entered Specifications" on the Module tab.

 

[HRTKD]

4 strings with 4 different tilts and Azimuths....did you check the checkboxes to add subarrays?
View attachment 19393

Yes, I entered two different "subarrays", using different angles for each. In my write-up I did call out that I was using two different strings to come up with the numbers. I was hoping that it would tell me what the net result would be of putting panels at different angles on the same "subarray" (what we call a string).

I've seen some videos where panels of different specifications (volts/amps/watts) were put in the same string. That would be a nice calculator app to see! But figuring out the effect of different angled panels on the same string may simply be too complex. Or, it's just considered a really dumb thing to do.