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Mix and Match Different PV Modules

 

Tigo enables system designers to mix different module types within the same string. For example, if a particular module needs replacing and the technology is now outdated or the manufacturer is no longer around you can replace with something different.  It can eliminate the need of keeping additional inventory for such replacements throughout the life of your system. 

Retrofitting a system with Tigo optimizers allows you the same 25% flexibility when choosing replacement modules.

 

How to Calculate Different PV Modules Mismatch

When matching modules within a string, current rating is the important thing to consider. You can mix and match modules in the same string as long as the current ratio remains bigger than 0.75, which is equivalent to:

1-(lower current module/higher current module)= % mismatch; must be less than or equal to 25%

Matching voltage is a bit more complex, as it affects the string to string mismatch. 2 scenarios:

  1. If you have one string, voltage variance has no limit other than maximum voltage allowed per string.
  2. If you have more than 1 string, the voltage variance allowed is limited by the ratio between the lowest voltage string to the highest voltage string. For example, see the following drawing of a system with 2 strings, 15 PV modules per string; total array of 30 PV modules. The system is equipped with 2 different module types: 27 modules with Voc=33.5V and 3 PV modules with Voc=43V. Possible configuration: One string with 15 modules of 33.5V (1st string voltage is 15*33.5V=502.5V), second string with the remaining modules (2nd string voltage is 43V*3+33.5V*12=531V). The voltage variance is 502.5V / 531V = 0.95 so 5% voltage variation, and therefore for this specific combination it works.

At the end, ensure that overall mismatch doesn't exceed 25%. Going back to the example, the 33.5V modules have Isc=6.4A and the 43V have Isc=8A. The overall mismatch would be current mismatch   (6.4A / 8A = 0.8) multiplied by voltage mismatch, so 0.8*0.95 = 0.76: 24% overall system mismatch.

 

 

Notice that this example also demonstrates that the power variation between the different modules has no restrictions. As long as current and voltage variations obey the 25% rule, you can mix and match modules with any power rating or technology available: Mono-crystalline, Multi-crystalline and even Thin-film.

 

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