Best Solar Charge Controller? And Series vs Parallel Wiring

[E350]

OK guys I would like to follow this rabbit hole to the end, so if you have any suggestions please continue to help.

But some of this ground is beginning to look familiar. In fact, I think I may see some of my footprints from when I first designed (mis-designed) my system in 2005.

Scalff77: You said "The voltage of a panel will drop in higher temp, so as the voltage goes down there is little room for a MPPT controller to squeak and extra power out."

So, does voltage go down as temps go up? Or does amperage go down as temps go up which is what I thought Blue Sky's sheet meant.

Edit: Ok, I found this statement:

"An unwanted side-effect of the encapsulation of solar cells into a PV module is that the encapsulation alters the heat flow into and out of the PV module, thereby increasing the operating temperature of the PV module. These increases in temperature have a major impact on the PV module by reducing its voltage, thereby lowering the output power."

from:
http://www.pveducation.org/pvcdrom/modu ... emperature

So at first blush it seems to me that Scalff77 is right that as temperature goes up voltage goes down and Blue Sky's sheet is wrong to say that as temperature goes down current (i.e., amperage) goes up. Wouldn't the inverse of Scalff77's statement be true? I.e., as temperature goes down, voltage (not current) goes up?

To say I am "confused" is truly an understatement.

What a I missing?

And then there's this goofy guy:



Measuring solar cell efficiency in Antarctica. Solar cells love cold sunny environments. (Photo Antony Schinckel)

[daveb]

Originally Posted by E350

So, does voltage go down as temps go up?

Seems to be the word and also in very cold temps and/or low light and why 2-100w panels are better than 1-200 for the most part when using a MPPT controller. In mild weather using a single panel, a PWM controller works about the same.

[E350]

Hmmm... I wish I could quote daveb for something.

I think I will give Alex and BS another chance tomorrow. Maybe the kid knows his stuff, he just didn't show his homework.

[daveb]

Well tell us what they say cause I'm far from an expert. I was just going by what I remember when my system was installed and back then MPPT was just coming to the market. I would think Greg will know.

[E350]

More Foster's Premium Ale (the Green Can) inspired research:

"A MPPT Solar Controller tracks the maximum power point, which is going to be different from the STC (Standard Test Conditions) rating under almost all situations. Under very cold conditions a 120 watt panel is actually capable of putting over 130+ watts because the power output goes up as panel temperature goes down – but if you don’t have some way of tracking that power point, you are going to lose it. On the other hand under very hot conditions, the power drops – you lose power as the temperature goes up. That is why you get less gain in summer.

MPPT’s are most effective under these conditions:

Winter, and/or cloudy or hazy days – when the extra power is needed the most.

Cold weather – solar panels work better at cold temperatures, but without a MPPT you are losing most of that. Cold weather is most likely in winter – the time when sun hours are low and you need the power to recharge batteries the most.
Low battery charge – the lower the state of charge in your battery, the more current a MPPT puts into them – another time when the extra power is needed the most. You can have both of these conditions at the same time.
Long wire runs – If you are charging a 12 volt battery, and your panels are 100 feet away, the voltage drop and power loss can be considerable unless you use very large wire. That can be very expensive. But if you have four 12 volt panels wired in series for 48 volts, the power loss is much less, and the controller will convert that high voltage to 12 volts at the battery. That also means that if you have a high voltage panel setup feeding the controller, you can use much smaller wire."

from:

http://blog.solars-panel.com/?p=23

Ok so the above works for my Winter application. Use a MPPT controller right? But wire in parallel or series for my four particular 17.6V panels? Or should I wire each pair in series then combine the two pair in parallel? Or should I wire each pair in parallel and then combine the two pair in series?

(Those are questions by the way.)

But what controller and wiring is optimized for 100F+ Summer in the Sacto. valley?

[daveb]

I don't know but I've been told if you put your panels in series you get a higher voltages that the MPPT controller can work with so in hot weather it will be more efficient even though the panel output is lower. How high in voltage you can go is beyond me. In a home array they go way high but the system is designed for that.

[E350]

daveb: You are correct. Also IIRC I wired my entire array of four panels in series, which is susceptible to shading as discussed below:

"Re: Camper Van System - charge controller options?

Usually one does not use an MPPT type controller in an application like this. However, if it's in the budget the Kid would work well as you need <30 Amps to charge 220 Amp hours of batteries.

You would not want to put more than two panels in series, though. For one thing it increases the problems with incidental shadows knocking out the whole array and for another there is a reduction in controller efficiency the higher array Vmp is in respect to system Voltage. With four panels configured for two parallel strings of two in series you'd be at about the best (and no fuses required on only two parallel connections).

If you do need to exceed the 30 Amp rating the Kid can be paralleled with another, and for the most part there is no difficulty in running more than one controller on a battery bank."

This is quoted from a thread on the N. AZ Wind and Sun forum. They are talking about the Midnite Solar the "Kid" model MPPT controller, which is American made:

http://www.midnitesolar.com/productPhot ... ame=Charge Controllers - KID&productCat_ID=43&sortOrder=1&act=p

I think I will email or call Midnite as well as Blue Sky today.

[E350]

For those interested in further development of this topic see:

http://midniteforum.com/index.php?topic ... 2#msg19682

I will post a writeup of the results here.

[Scalf77]

I hate when I do a long detailed post, hit send and find out I was logged out. Should have copied it.

Anyway I will try to repeat it. The specs on your panel were Vmp of 17.6 and a Imp of 4,5 or 17.6 X 4.5 = 79.2 or 80 watts. These numbers are based on Standard Test Conditions (STC) that are not real world but they work for comparison sake. The temperature of the panel can have a pretty big effect on the voltage output of the panel, it will drop with higher temps and temps on top of the van can get pretty high. This is why you want some airflow under the panel. While current output is also effected by temp, the real driver is caused by solar irradiance.
We can simplify things because we are using the same panel for the both MPPT and PWM controllers. With a PWM controller we would take that 80 watt panel and again for simplicity we use a 13 volt battery charge set point.. So the best power we can do with a PWM controller is Imp of 4.5 amps X our 13 volt set point. This gives us an output 58.5 watts, so really your 80 watt is putting out only 60 watts . As temperature goes up the Vmp will go down, but the PWM controller is just regulating the output voltage down to the set point. If the Vmp went down to 15 volts it would not affect the output of a PWM controller.

A MPPT controller is designed to use the extra voltage headroom. A simplest way to look at this is we would get the same 58.5 watts out of panel, but we would also look to gain power left , we have Vmp 17.6 minus a the 13 volt set point and get 4.6 volts * 4.5 Amps or 20 watts to gain back. Again for simplicity sake let’s say we can get 90% efficiency out of this conversion. This would give us 72 watts or 5.5 amp of output at the set point of 13 volts. So in this case we gain 1 amp of output over the PWM controller. If we now look at the same 15 volt Vmp do to temperature we get ((15 * 4,5) *90%) we get a current of 4.67 amps at 13 volts. So after taking into account for temperature we really only see a 0.17 amp increase with our MPPT controller.

Now if you are using a grid tie panel with a higher Vmp, it would become a lot easier to see the gain that a MPPT controller was designed for. You can wire two of your panels in series and get a higher Vmp, this would allow you to take advantage of a MPPT controller. The problem with wiring panels in series is that shading affect , specifically if one panel is partially shaded it can have affect on the output of the other panel that is in series with it.

I think it would make sense with your panels to go with a PWM controller.

Anyway I see from your post over at midniteforum that you are generally getting idea on the difference between the two types of controllers. Outside of cost I do not see a negative of going to MPPT. Grape has some pretty good prices on mono-crystalline panels 160 watt output. They have a Vmp of 18.5 volts making it better for MPPT , they are 58 " x 26". Mono-crystalline has less voltage drop due to temperature than Poly-crystalline.

Morningstar makes a couple of comparable controllers a Pro Star 30 and a step up would be the Tristar 45. The tristar brings with it more configurability for your battery charging, and also supports voltage sense line for managing the set point to your battery.. The longer the run from your controller to battery this become more important. Be aware that there pricing usually does not include a monitor (that is usually extra)

-greg

[Scalf77]

E350,
I just glanced at the specs and manual for the KID, that looks like a pretty nice controller.

-greg