Wiring a large array to minimise the effect of shading?

[DiploStrat]

Ralph Hiesey has a discussion of PWM vs. MPPT under question C.1 here: http://www.bogartengineering.com/support/faq/

He makes a strong case for PWM when you use "12v" panels - that is the common panels rated at around 18v. He also makes a case for MPPT when you have higher voltage panels, e.g. when you put panels in series as you might do on a house roof or where your panels are physically a long way from your controller/battery.

I am still undecided. The lower cost of a PWM controller appeal and I have certainly seen my MPPT controller get upside down, that is, produce less current than it takes in on very sunny days.

 

[dwh]

Different array but same question, 3x Renogy RNG-100D panels (100w, vmp 18.9V, imp 5.29A) and a 12v battery bank: with a PWM controller in series would be wasting efficiency due to a drastic voltage mismatch (54v vs 14.4v), right?

Yup.

But you'd not have to worry much about voltage drop due to shading, whereas in parallel you'd have ~18v and it wouldn't take much shade to drop that too low to charge, but your amperage would be stronger all the time, right?

Yup.

If I've understood that correctly, PWM benefits most from being parallel and MPPT benefits most from being in series?

Not exactly.

PWM benefits most from reducing the voltage mismatch as much as possible. Ideally, to take a battery to 14.8v, you would want a PV with a Vmp of 15v. Of course, that's a bit too close for the real world. It might have a 15 Vmp under STC, but when the PV gets hot and/or the sun isn't shining dead on the panel, and the Vmp goes down some, it would be below 14.8v . So building the module for a Vmp of 18v gives some overhead for when the Vmp goes down a bit.

MPPT doesn't care. Series or parallel it still does its job of squeezing the most watts out of the solar.

*Shade tolerance* is what benefits the most from being in series.

What if I found room/$ for 4 panels, would 2 paralleled strings of 2 panels in series with one another be better?

No, not with 18vmp panels charging a 12v battery through a PWM controller. You would be kicking up the array voltage to 36v and losing even more efficiency from the 36v -> 12v mismatch.

But with an MPPT controller (and anything over 200w should have MPPT IMHO) you would harvest more watts, and rigged series/parallel would also increase shade tolerance.

 

[rayra]

so how about an array of smaller discreet panels, each wired directly as in a star topology. Wouldn't that give a quasi- 'fault tolerance' whereby a shadowed panel or subcell structure doesn't drag down the whole array?

/and the whole thing makes me lean further towards having a suitcase-style array / clamshell that mounts in the open flat position on the roof normally, that can be detached and erected on the ground in the full sun when encamped. And such a setup could be arranged to allow releasing one half and tilting it to the proper sun angle to maximize energy capture / production when its on the vehicle. Or even better yet, make a hinge pin arrangement that lets you separate the halves and set angle on both, on roof or ground

Just a little work and design / fabrication skills can create a great deal more flexibility - and even security from theft - in a vehicle-borne/-mounted solar array.

 

[dwh]

so how about an array of smaller discreet panels, each wired directly as in a star topology. Wouldn't that give a quasi- 'fault tolerance' whereby a shadowed panel or subcell structure doesn't drag down the whole array?

Hmm. Well, that's just parallel then isn't it?

But more smaller panels ends up having more total bypass diodes, so partial shading would have less impact on total array output.

 

[grizzlyj]

Hi folks

I was going to delete this the morning after posting because I just thought I was being daft, it will just have to be two strings to produce 24v into each controller.

But I am very glad I didn't, and thank you all for the replies

The panels are 12v 100W, and I have two controllers. I will look at maybe another pair of small ones depending on fitout space and cost.

I need to understand this lot first though

 

[dwh]

So rig three panels in series to make 6 series strings, then three strings paralleled into each controller.

Provided the controllers are MPPT that is

 

[Rando]

I would be a little careful with the description of the downsides of MPPT controllers from a company that only sells PWM controllers. They are right that there are some limited circumstances where a PWM may out perform an MPPT controller, but the vast majority of the time an MPPT controller wins. Secondly their cost differential argument is kind of weak as you can buy a higher quality MPPT charge controller of the same rating for less than what their PWM set up will cost you.

Ralph Hiesey has a discussion of PWM vs. MPPT under question C.1 here: http://www.bogartengineering.com/support/faq/

He makes a strong case for PWM when you use "12v" panels - that is the common panels rated at around 18v. He also makes a case for MPPT when you have higher voltage panels, e.g. when you put panels in series as you might do on a house roof or where your panels are physically a long way from your controller/battery.

I am still undecided. The lower cost of a PWM controller appeal and I have certainly seen my MPPT controller get upside down, that is, produce less current than it takes in on very sunny days.

 

[LeishaShannon]

They are right that there are some limited circumstances where a PWM may out perform an MPPT controller, but the vast majority of the time an MPPT controller wins.

Fortunately this is pretty easy to work out if you have the panel specs and your average charging voltage. Here's one i prepared earlier :


This based on a "standard 12v" panel charging a lithium battery.
RED = PWM losses higher than MPPT
GREEN = MPPT losses higher than PWM
BLACK = panel voltage dropped below battery voltage so no more charging


As you can see PWM wastes a -lot- of power.

 

[dwh]

Just fired up one of my netbooks and Firefox still had a tab opened to this thread.

So grizz, how'd you end up rigging it?