Mixed Array on One Contrller Vs. Two Controllers

tvengineer · December 2009

I have asked a few questions here before and gotten good answers.. So some of you may remember that I have 2 x 200 watt panels wired for 12v to a Morninstar Prostar 30 charge controller.

I have recently acquired 4 additional 100 watt panels (effectively doubling my solar array)

From the answers to my last question here.. I have decided to go to a 24volt system now.. (to make wiring easier, fewer parallel batteries..etc)
Well that, and a really good deal I got on a 24v Exeltech 600watt inverter :-)

Now for my new question...
Switching to 24volts means that my Prostar 30 could potentially handle up to 700watts of solar.. (not quite enough)

My original thought was to buy another Prostar 30 for the new panels..
but now that I think about it..
I could get a Morningstar TriStar 45 controller for about the same price as another Prostar 30, and could sell my Prostar on Ebay to make up for some of the cost.

So. the question..
Is it ok to run a mixed array (2 x 200w + 4 x 100w) into one charge controller even if they panels have slightly different maximum power voltages etc.....

or is it better to separate them into two controllers???

I'm talking mostly efficiency as the wiring and cost of one new charge controller are basically the same.

thanks
Louis

BB. · December 2009

Re: Mixed Array on One Contrller Vs. Two Controllers

Realistically, you should be able to match panel Vmp's to within 10% for parallel connections. For parallel connections match Imp's to within 10%.

If the Vmp/Imp matches go beyond >10% differences (i.e, Vmp=32 volts for panel A and 35 volts for panel B, etc.), the MPPT controllers could "maximize" the wrong peak power and you simply are not using the panels to their maximum ability (say you have a 17 volt panel and a 34 volt panel--whichever Vmp/Imp the MPPT tries to maximize, you will be loosing power somewhere).

If you have a PWM controller, then as long as Vmp>Vbatt-charging+controller-drop, then Vmp matching is not really important (Panel Vmp are >~30 VDC) as a PWM controller cannot gain any extra power from a Vmp=40 VDC panel on a 24 volt battery bank (hope that makes sense).

A 200 watt Vmp=40 volts @ 5amp on a PWM controller will have exactly the same performance as a 150 watt Vmp=30vdc @ 5 amp panel when charging a 24 VDC battery bank.

-Bill

tvengineer · December 2009

Re: Mixed Array on One Contrller Vs. Two Controllers

I looked up the specs on my panels..

the 200 watt ones are 18.4 vmp 22.8 voc so two in series would be. 36.8vmp 45.6 voc

the 100s 17.4mp 21.4oc two in seris 34.8mp 42.4oc

So what you are saying is that since they are within 10 percent of each other I would be OK using them parallel into ONE PWM charge controller?

Only IDENTICAL panels will be in series.. so the currents will be the same

Louis

BB. · December 2009

Re: Mixed Array on One Contrller Vs. Two Controllers

Actually, since you have have a PWM controller, as long as both sets of panels meet your minimum voltage requirement to charge the battery bank (plus voltage drop from controller and wiring)--there is nothing gained or lost by having a higher Vmp panel string in parallel with the other string.

PWM just uses the available currents from the two (or more) strings once Vmin is met.

The only time you would possibly lose is if you had an MPPT type charge controller with mixed Vmp solar panels in parallel. MPPT type CC use the equation P=I*V ... So if V is depressed by mixing "high" and "low" panels together, you end up with with V held down by the "low voltage" panel, and then lose power.

-Bill

tvengineer · December 2009

Re: Mixed Array on One Contrller Vs. Two Controllers

Actually... I think my current Prostar 30 amp PWM controller will be fine...

Originally I did the math figuring 800 watts / 24 volts = 33.75 amps... too much for my 30 amp charge controller..

but if I look at the ISC current for my panels.. the total short circuit current is around 25 amps.. meaning the prostar 30 amp controller will work after all..

I would like to move up one day to MPPT controllers.. but they are just too expensive now...

I will wire the arrays separately down from the roof.. so that in the future it will be easy to switch to separate MPPT controllers if the time comes.l.l

thanks for the help

Louis

BB. · December 2009

Re: Mixed Array on One Contrller Vs. Two Controllers

Louis,

This is an example of how MPPT could help:

tvengineer wrote: »

the 200 watt ones are 18.4 vmp 22.8 voc so two in series would be. 36.8vmp 45.6 voc

the 100s 17.4mp 21.4oc two in seris 34.8mp 42.4oc

18.4v * 10.87 amps = 200 Watts
17.4V * 5.75 amps = 100 watts
==========================
Roughly 22.3 amps at Vmp=~17.4*2=34.8 volts

22.28*17.4v*2 series panels = 775 watts avaiable power

The rough equation for a MPPT controller on how panel Vmp*Imp turns into Vbatt*Ibatt:

Vmp*Imp*0.96 eff = Vbatt*Ibatt (during bulk charging)

So, the current to the battery (assuming 29 volt charging):

96% eff * 775 watts / 29 volts battery charging = 25.66 amps (less controller losses of a few percent)

The improvement (on cooler days) with an MPPT vs PWM controller:

25.66 amps / 22.28 amps = 1.15 or ~15% improvement during cool, sunny days with a MPPT vs PWM controller

Probably not worth it for you at this time--unless you are going to replace the controller for other reasons (long wire runs from panels, very hot summer weather causing Vmp to fall below Vbatt-charge, etc.).

-Bill

niel · December 2009

Re: Mixed Array on One Contrller Vs. Two Controllers

"the 200 watt ones are 18.4 vmp 22.8 voc so two in series would be. 36.8vmp 45.6 voc

the 100s 17.4mp 21.4oc two in seris 34.8mp 42.4oc"

the current does not double in a series arrangement, only the voltage.

a0128958 · December 2009

Re: Mixed Array on One Contrller Vs. Two Controllers

BB. wrote: »

...
18.4v * 10.87 amps = 200 Watts
17.4V * 5.75 amps = 100 watts
==========================
Roughly 22.3 amps at Vmp=~17.4*2=34.8 volts

22.28*17.4v*2 series panels = 775 watts avaiable power
...

Bill, how did you arrive at 22.28 amps at Vmp?

And why is Vmp = 17.4 volts for two panels connected in parall with different Vmp?

Many thanks!

Best regards,

Bill

BB. · December 2009

Re: Mixed Array on One Contrller Vs. Two Controllers

Bill,

The one time I don't show my work because I think I am boring people--I get asked. :roll:;)

The Original Poster did not give the Imp of the panels--so I just guessed that 200 watts / Vmp would give us:

the 200 watt ones are 18.4 vmp 22.8 voc so two in series would be. 36.8vmp

200 watts / 18.4v = 10.87 amps

Since the two 200 watt panels are in series, we get:

400 watts at 36.8 Vamp and 10.87 Imp

For the 100 Watt panels (cut from two different posts):

4 x 100w & the 100s 17.4mp 21.4oc two in series 34.8Vmp

100 watts / 17.4V = 5.75 amps

These 100 watt panels are two in series, then the two panel strings are put in parallel for:

400 watts at 2*17.4V * 2*5.75 amps = Vmp 34.8 volts and Imp 11.5 amps

I chose the Vmp of the lower voltage panel string (the 100 watts units) because that would give us the worst Vmp--a MPPT Charge Controller might bring Vmp up a bit (and give more power), but this is a worst case amount.

For the 800 watt array (2*200 watt panels + 4x100 watt panels arranged in Series/Parallel connection for Vmp=~34.8-36.8 VDC)) we get an Imp total of:

Imp-array=10.87amps + 11.5 amps = 22.4 amps
Vmp-array=34.8 volts (minimum of the two Vmps of the two different panels)

Paste in the rest of my post's math so that it is all in one place (fix round-off error to 22.4 amps instead of 22.3 from earlier post):

So, the composite array of the 800 watt array works out to:

22.4 Imp * 34.8 Vmp = Pmp 780 watts available power (of array)

The rough equation for a MPPT controller on how solar panel array Vmp*Imp turns into Vbatt*Ibatt for charging the battery bank (assuming 96% MPPT charge controller efficiency):

Vmp*Imp*0.96 eff = Vbatt*Ibatt (during bulk charging)

So, the current to the battery (assuming 29 volt charging and our 800 watt composite solar array) for a MPPT Type Charge Controller would be:

0.96 eff * 780 watts / 29 volts battery charging = 25.8 amps into battery bank

The improvement (on cooler days) with an MPPT vs PWM controller assuming PWM controller passes through 100% of the panel current (probably 99% of current to run the little bit of electronics of the PWM controller):

25.8 amps / 22.4 amps = 1.15 or ~15% improvement during cool, sunny days with a MPPT vs PWM controller

Hopefully the above makes more sense... Since I did not have all of the specifications for the panels, there was a little extra math required to pull out the Imp of the panels. Also since the panels were in series/parallel configuration with two different types of panels, I sort of punted on trying to write a post showing all my work because I was not sure I could to it clearly.

People could pick a Vmp that was the average of the two different Vmp's of the two different types of panels... I was just going for (hopefully) worst case numbers (being conservative).

-Bill "the other" B.

a0128958 · December 2009

Re: Mixed Array on One Contrller Vs. Two Controllers

BB. wrote: »

... These 100 watt panels are two in series, then the two panel strings are put in parallel for:

... For the 800 watt array (21*200 watt panels + 4x100 watt panels arranged in Series/Parallel connection ...

... (fix round-off error to 22.4 amps instead of 22.3 from earlier post)

Bill, this is exceptionally appreciated. You spent a lot of time putting this together - thanks!

I didn't earlier comprehend that it's a series-parallel configuration for the 100 W panels.

I think you meant to write (above) "2*200 watt panels..."

The comment on rounding helped to complete the picture on Imp.

Many thanks!

Best regards,

Bill

BB. · December 2009

Re: Mixed Array on One Contrller Vs. Two Controllers

Thank you Bill, I fixed the typo.

And you are very welcome! I am happy to help.

-Bill

Mixed Array on One Contrller Vs. Two Controllers

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