Mixing PV modules

NZRR

Hi. Sorry if this question has been posed before. I am looking for advice on mixing PV modules from different manufacturers that have different electrical specifications. As an example, a customer has 4 Isofoton I-150 modules connected to the same charge controller as 8 ARCO 35W modules. The Vmp and Imp of these modules are different by a factor of 2, and I am looking to replace the ARCO modules. Any advice as to what to look for in modules to allow me to not lose power from the new set of modules? This is for an off-grid situation, so all the power we can make is critical. Thanks.

BB.

Re: Mixing PV modules

In general, for series connections match Imp within 10% of each other.

And parallel connections, match Vmp within 10%...

The power curves are fairly flat over Vmp +/- 5% (10% span). If you go outside that range (one panel >>10% higher or lower specification--then you may lose the power contributed by one or more strings of panels--or in the case of MPPT controllers, they may not find the "true maximum power" but just a local power peak (tracking locks onto one string's worth of energy instead of 1+1 strings' worth of energy--if my explanation makes sense).

If the Vmp of one panel is 34 volts, and the Vmp of two other panels are 17.5 volts--Put the twox17.5=35 volts in series. Then add that in parallel with the other Vmp=35v panel -- and so on.

PWM controllers, for best efficiency should have Vmp~Vbatt+a few volts for controller drop and wiring drop... But PWM "don't care if you have a 17Vmp and and a 32Vmp panel in parallel--you waste power with the 32Vmp connected panel--but you still gets it current (power into the battery is 14.4 volts * X.X amps--so you lose about 1/2 the 32Vmp output due to voltage miss-match with PWM).

For example rough minimum PWM and MPPT solar panel Vmp requirements:

• Vmp=17-18 volts for "12 volt bank"
• Vmp=34-36 volts for "24 volt bank" etc...

MPPT type charge controllers can take a wide range of voltages. From the Vmp recommended for a PWM system to Vmp~100 volts and still work efficiently (the higher end MPPT controllers have Vpanel-max of 150 VDC or so).

Also remember that Hot panels have reduced Vmp output... So a 14.4 volt battery may will not get full charging current from a Vmp=15volt panel on anything but a very cool day.

All About Charge Controllers
Read this page about power tracking controllers

There does become an issue where all of the "issues" of trying to mix and match solar panels, adding series protection fuses, running old panels not designed for higher voltage MPPT system, lots of small panels and wiring vs a few large panels--may reach a point of frustration.

-Bill