Optimum Voltages vs Location

Re: Optimum Voltages vs Location

aarif wrote: »

Ok, I've searched this question over a number of forums however I can't seem to find the right info that I'm searching for. I already know about the optimum angles associated with latitude, however is there a graph or series of data that correlate optimum PANEL voltage to geographic region?

Solar panels are "current sources" (more or less). Once the panels have enough (weak) sun to output any useful amount of power, they will operate at ~Vmp (voltage maximum power). If you try to draw more current than the panel can output, the panels cannot output any more current and the output voltage will collapse (or become near Vbatt-charging + any voltage drops). And the output current is basically equal to the energy of the sun hitting the panel. 100% Imp rated output is pretty much noon-time sun on a clear day.

Currently I have strings of panels, each 300W @ 36v put together to create a bank of 600W@72V. This is going into a 48VDC system. So my question becomes, is there a voltage wastage of me being at 36v. Would I see better results at 32 or 34 volts?

There are two (major) types of solar charge controllers. One is a PWM (pulse width modulation). The charge controller simply turns a "switch" (transistor) on and off to control the "average current" to the battery bank. If the bank needs more charging current, the switch is "closed" more than it is open (i.e., pulse width modulation). And if the battery is near full, the switch is off more than it is on.

The other is a MPPT charge controller--Maximum Power Point Tracking. More or less, it is switching power supply that behaves a bit like a variable AC transformer for DC power. The charge controller can take "high voltage" and "low current" and efficiently down converter to high current/low voltage needed to recharge the battery bank (power = V*I; and Vmp-array*Imp-array=Vbatt-charging*Ibatt-charging).

More or less, your Vmp-array of 72 volts is the correct operating voltage for connecting from a solar array to a battery bank with a PWM charge controller. Yes, there is a little loss of energy because PWM cannot "match" or down convert the differences in voltage, but Vmp-array is usually less because "hot" solar cells have lower Vmp. And this works out to be pretty efficient.

An MPPT charge controller can take the "extra high" voltage from a "sub freezing" array and "down convert" that extra voltage (Vmp-cold*Imp for the array) and give you 10-15% or so extra power for your battery bank.

However, the main reason for MPPT (in my humble opinion) is because you can run Vmp-array at 100 VDC (or higher for some controllers) and use smaller/lighter copper wire from the array to the charge controller/battery bank shed. (less copper used, less costs. And/or the ability to put the solar array in full sun while the battery shed is in poor sun, etc.).

I also do understand the correlation of I to V so I do know I'd be getting more current at lower voltages, and I also understand that my charge time would be less as a lower voltage panel will start charging later in the day as compared to a higher voltage panel. But my question remains to ask that if anyone knows of a "sweet spot" or some sot of white paper between voltage, current, and geographic location?

One other thing a MPPT charge controller can do for you--In very hot climates, the Vmp-array even if ~72 volts at STC (Standard Test Conditions), the array voltage can fall to Vbatt-charging/Vbatt-equalization voltages--So on hot summer days and with cool battery banks (cool batteries need higher charging voltages and cold batteries last longer), you may not be able to fully/quickly charge or equalize a battery bank with a "matched" array Vmp.

Because the MPPT charge controller can use a higher Vmp-array, you do not hit temperature limitations of the solar array in very hot climates.

In general, for arrays with less than ~400 watts, PWM controllers are usually "good enough" for a typical system (unless you have a long cable run from array to charge controller).

And for over ~800 watt arrays, it is usually better to choose a MPPT charge controller (less wiring costs, but more expensive charge controller).

One other issue that MPPT controllers are very good for--In the US/Europe, most of the "cost effective" larger solar panels are not designed for ~17.5 volt MPPT--But they have "random" Vmp ratings that do not match well with 12/24/48 volt battery banks and PWM charge controllers. So, you are left with getting more expensive $$$/Watt solar panels (typically 140 watt or less) and a "cheap" PWM controller. Or purchasing "cheap" >140 Watt solar panels with "non-standard" Vmp voltages and an expensive MPPT charge controller.

Usually, you have to do a couple different "paper designs" to see what will best meet your needs.

Install site: Southern Pakistan. lat: ~24 degrees

And here you are stuck in a difficult position... US/European charge controllers are not easy/cheap for you to get (they are not "cheap" in the US either for good quality controllers).

And many "MPPT" charge controllers from China/Asia are not MPPT charge controllers. They are just PWM controllers with "MPPT" stamped on the label.

Here is a PWM type charge controller (was properly labeled--this is a failed unit). Note there are no large inductors visible:



The charge controller on the right has a small inductor on the lower left near the green connectors. This is not really a full blown MPPT charge controller (as I recall):

Attachment not found.


Usually, a very quick check is to see if there is a large "inductor" (typically a circular black/grey ferrite core hand wrapped with copper wire) inside the controller. If there are no "large" inductors (must be capable of carrying the rated current/power of the controller), then it is almost for sure not an MPPT type charge controller.

From a Yahoo Groups "open source" MPPT controller design project. A torrid:



-Bill

Re: Optimum Voltages vs Location

You now have a choice of running Vmp-array at ~100 Vmp-array (or higher, depending on the exact controller and low temperatures in your area). That would allow you use smaller diameter copper wire/send your power longer distances because of the higher array voltage.

-Bill