This post was originally published in 2012. The tips and techniques explained may be outdated.
What happens if I have an east-facing PV array and a west-facing PV array connected to the same string inverter with one MPP tracker?
This is a fairly common question that SMA Technical Support receives from customers and installers. There is only one answer that is correct 100% of the time: Do the design!
Running a computer simulation like Sunny Design or doing manual calculations is the only way to precisely know what the optimum system size and expected yield will be.
One thing we can say for sure is that connecting east and west-facing PV arrays to a string inverter with one MPP tracker will not break the inverter or void the warranty (assuming the install and design were done per SMA requirements). In most cases the inverter will operate and produce power.
How much power?
That is the where we have to take a closer look at the system behavior.
SMA Solar Technology AG has published a study involving “Polystring Operation” which is defined as two or more strings at different orientations on one MPP tracker.
There are, of course, many possibilities for strings in different orientations. In this post I will focus on the scenario of a direct east-facing string and direct west-facing string connected to a single MPP tracking inverter.
The first thing to note is that the power produced by the PV array is directly proportional to the amount of sunlight it is exposed to. As the sun rises and moves across the sky, the power production on a south-facing array will produce a traditional bell curve graph. However, on a PV array facing due east, there is significantly more sunlight in the morning than in the afternoon.
Likewise, for a due west array, there will be much more sunlight in the afternoon. So, these strings are producing their peak power at different times. You essentially have two smaller bell curves, one for each string, instead of one larger combined curve. The east-facing string’s power curve peaks in the morning and the west peaks in the afternoon, with some overlap in the middle.
What does this mean?
According to the SMA study, “With non-uniformly aligned module a surface, less inverter power is required.”
Let’s say you have a 6 kW PV system, two strings of 3 kW in parallel, one facing east and one facing west. The peak power of the two strings is not going to happen at the same time, so the inverter is not going to see a 6 kW DC input. This means the system can be operated with a smaller inverter, which reduces the overall cost of the system.
How much smaller of an inverter?
That’s where the design is necessary; the exact orientation and inclination angles are the most important factors.
With arrays that are not facing due east or west and are facing some degree of southeast or southwest, the power curves for the two strings will overlap to a higher degree. The inclination angle of the array is also a factor. The steeper the angle, the sharper the rise and fall of the power curve, which affects the degree of overlap between the curves. These factors must be taken in to account when sizing the inverter.
Wait, won’t the two string voltages be different if they are facing opposite directions?
No! Luckily unbalanced string voltage is a completely separate issue. As the SMA study says, “The currents from the PV modules may vary significantly in the substrings over the course of the day. However, the voltage at MPP is practically identical.” This is because it only takes a small amount of light for the modules to achieve their rated DC Voltage.
Overall, it is not an optimum system design to have strings at significantly different orientations. However, there are instances where different orientations are the only option or the desired option. The results of the SMA study shows that you can operate a string inverter with a single MPP tracker with strings at different orientations. One test that was done in the study compares a single MPP tracking inverter with a dual MPP tracking inverter, both with due east and due west facing strings and the same site conditions. The results were only a 0.25% yield loss for the single MPP inverter.
Sunny Design Web Now Available
SMA has released an online version of Sunny Design with sizing systems using different string orientations, and Polystring Operation is a new feature! Sunny Design Web offers the same great features as the desktop software in an easy-to-use web format.
Visit SMA America’s YouTube playlist to view a short video introduction on Sunny Design Web.