As east-west arrays become increasingly common, they bring an important additional requirement: The modules on the east and west orientations need to be wired to different inverter MPP zones, or different inverters entirely. This can slow down electrical installation and, more importantly, create opportunities for installation mistakes. These mistakes—specifically, when east- and west-facing modules are wired in the same series circuit—can drop the array’s energy yield by 8 to 10%.
Why east-west wiring is hard to get right (and even harder to diagnose)
In east-west arrays, series circuits need to skip a row at the end of each run as adjacent rows are facing opposite directions. Skipping rows becomes doubly challenging when the rooftops are irregular (see the example above for a simple illustration of this).
Incorrect wiring can be difficult to detect in commissioning. Open-circuit voltage checks will not show potential problems at all, as the string Voc will be identical whether the stringing was done correctly or not.
How bad are the losses?
Mismatch losses from an incorrectly strung east-west array at a 10° tilt are roughly 8% over the course of the year. It is important to note that mismatch losses will vary based on the time of day and time of year. In the example below, while the annual mismatch losses are 8%, the mismatch ranges from 1% or less from 11 a.m. to 1 p.m. (when the sun is overhead), to 15 to 25% in the morning and afternoon.
What can be done?
There are a few steps installers can take to avoid these problems:
- Design for installability. Use design blocks (such as “Frames” in HelioScope) to reduce the complexity of the installation by reducing the number of irregular stringing runs.
- Take extra care during installation. Extra planning and thoughtfulness on-site can eliminate (or dramatically reduce) the odds of running into a mis-wiring problem. This might also include color-coding the conductors for the two module faces or even wiring up one set of modules before installing the second set of modules.
- Upgrade commissioning. Because a failed string might only be 5% off from a healthy string (depending on the time of day), extra care should be taken when tracing the string IV curves. And because any string could be at risk of being mis-wired, it may make sense to trace each source circuit, rather than a sample.
- Run IV curve traces before 10 a.m. or after 2 p.m. Running IV traces in the morning or evening when the sun is significantly stronger on one part of the array versus the other is the only way to spot errors in wiring. Otherwise, there won’t be any mismatch at high noon, even in an incorrectly wired system.
East-west racking can significantly improve the economics of rooftop solar in space-constrained applications. But as with any new technology, it’s important to apply it correctly.
This installation tip was provided by Paul Grana, co-founder, Folsom Labs