Dissimilar panels in series

tkc100 · March 2010

Please view the attached file.
Is this possible?

If it is possible as you can see I'm not sure what some of the outcomes would be or if there are any draw backs from attempting a configuration such as this.

Thanks!

Windsun · March 2010

Re: Dissimilar panels in series

In series like that you are basically going to end up with the Imp of the smallest panel for the total string.

mikeo · March 2010

Re: Dissimilar panels in series

Is this possible?

I have never seen it done but I guess it would be possible to parallel 3 X 17 volt panels with a 200 watt panel in a series string. That would give you 8+ amps at 42 volts. per string.

BB. · March 2010

Re: Dissimilar panels in series

Like MikeO said, and I posted earlier in another threade--your only choice would be 3x of the 55 watt in parallel with 1 of the SB 3200 panels... But you do not have enough of the 55 Watt panels (as I recall). You would need 12 of the 55 watt in series with 4 of the 200 watt panels.

-Bill

tkc100 · March 2010

Re: Dissimilar panels in series

You right BB I had made reference to this in another thread and the only reason I started this thread was because I have found it confusing to address more than one subject in one thread.
Also I don't understand.
It's my understand that a PV module is a number of photocell in series. Any given cell produces somewhere around .5 volts. I will use round number here to make things simple.
A 12 volt module (panel) would have 24 cells. A 16 volt panel 32 and so on.
Why then is in not possible to configure 24 cells in series with and additional 32. I'm sure there must be a simple answer but it's not reaching me yet.
Why can't 2 panels be configured to preform as one.

BB. · March 2010

Re: Dissimilar panels in series

Generally, the panel with the higher current has a larger solar cell (more surface area)...

So while the voltage is the same, the higher current panel cell has (in this case) almost 3x the surface area, so can output ~3x the amount of current per cell.

Just to be clear (;)), the cell voltage is pretty much fixed--once you have enough sun to make any useful voltage (dim direct sun)--then any additional sun simply makes more current available for use by the load (charge controller, GT inverter, etc.)...

You can look at this data sheet for a crystalline silicon panel and see that Vmp for different levels of sunlight is about the same Vmp voltage for all of the different I*V/Power curves.

It sort of like flashlight batteries... I can have 2x AAA cells in a 3 volt Pen Light, or I can have 2x D Cells in a larger hand held industrial flashlight.

Both AAA and D Alkaline cells output ~1-1.5 volts--but the larger cell can output more current and still maintain that ~1-1.5 volts per cell.

In this case, roughly 3x of the 55 watt panels in parallel will output about the same amount of current as 1x 200 watt panel.

The surface area to current ratio is also dependent on cell manufacturing (some cells are more efficient than other cells)--And different physical cells (Mono Crystalline, Poly Crystalline, and Amorphous Silicon) have different conversion efficiencies too.

Mono Crystalline cells are slightly more efficient than Poly Crystalline. And Amorphous Silicon cells are about 1/2 as efficient as either of the Crystalline cells.

However, Mono Crystalline cost the most to make. And Amorphous Silicon cells tend to be much cheaper--So, you will find all three types widely used in solar various solar applications.

-Bill

mikeo · March 2010

Re: Dissimilar panels in series

I can have 2x AAA cells in a 3 volt Pen Light, or I can have 2x D Cells in a larger hand held industrial flashlight.

Another way to think of it is if you series a 2 volt 100 amp forklift battery cell with a D-cell flashlight battery for a 4 volt battery pack, the D cell will limit the current you can draw to what the D-cell is rated at. IHTH

tkc100 · March 2010

Re: Dissimilar panels in series

Thanks BB and MikeO
It is perfectly clear now;)
But I've got to tell you it wasn't last night. I pulled the cover up thinking about it. Somehow I pictured PV cells as all being the same. The comparison to batteries makes perfect sense.
So in a series configuration you would obtain the series voltage but are limited by the current carrying capacity of the smallest cell.
In MikeO example I got a feeling the smaller cell would not just limit the current but would more than likely react as a fuse.
More mass / surface area = more current.

BB. · March 2010

Re: Dissimilar panels in series

If you want to be even more confused... We are used to "batteries" that are voltage sources--pretty much, no matter what (reasonable) current you pull from your 12 volt storage battery, it outputs around 12 volts.

Solar cells are "current sources"... No matter what load you place on the cell/panel (between 0 volts and Vmp)--It will still output around Imp.

The output resistance of a voltage source is near zero ohms. The output resistance of a current source is very high.

-Bill

tkc100 · March 2010

Re: Dissimilar panels in series

Why is it you got to hit me with something like this just before I go to bed......
I pretty much understand the battery part and my experience would seem to agree but the solar cell current generator part.

I attached a file with the figures of a panel I recently tested perhaps you could use it as an example.

BB. · March 2010

Re: Dissimilar panels in series

Current Sources and Voltage Sources is a fundamental part of electrical theory classes... I cannot do it (or you) justice with a little bit of typing and hand waving here...

Wiki has some pretty good information--but it does get a bit technical a bit quickly...

Current Source
Norton Equivalent Circuits

From your point of view... Every time you see one of the basic Electrical Equations:

V=I*R
P=I*V
P=I^2 * R
P=V^2 / R

We are all used to looking at battery/AC circuits and assuming V (voltage) is a constant. To a degree, with solar arrays, we plug in I (current) as the constant and solve for the other term(s)...

Like any voltage source has real limits (a AA battery cannot start a car). A real voltage source has a maximum limit on voltage (Vmp/Voc).

We can simulate a "real battery" with an ideal battery and a Series Resistor.

The equivalent simulation for a solar panel would be a Current Source with a Parallel Resistor.

To get you thinking about something in "real life" that can be both a voltage source and a current source with the flip of some bits in a micro processor (and a whole lot of design work)...

Take the typical Off-Grid Inverter... It is a 120 VAC voltage source. With zero load or 100% load, the voltage is still 120 VAC and the output current goes from zero amps to 100% of rated current.

Now, flip some bits, and it becomes a Grid Tied / Utility Interactive Inverter that we use to power our home AC power and even feed power back to the Utility.

Our GT Inverter is now a current source... It regulates its output current based on the AC Voltage Wave Form (follows the Utility voltage) and outputs current based on how much solar (or DC battery power) it has available.

You could not put a Off-Grid inverter in parallel with an AC Utility feed... The O-G inverter would be "fighting" the AC utility trying to set the voltage, frequency, etc. This would be like putting a 12 volt car battery in parallel with a 9 Volt transistor radio battery and expecting any thing good to happen.

When the inverter is set to G-T mode--now it does not care what the voltage is (as long it is within the correct voltage and frequency range)--it just needs to satify the P=I*V equation--or--I=P/V where V is fixed by the utility and P is the available power from the solar / battery arrays.

The Xantrex XW Hybrid Inverter System (and others too)--make good use of the differences between voltage and current sources to address the needs at hand (Grid Tied current source or Off Grid voltage source inverter mode). Control theory mixed with Electronics is actually pretty cool once you have a handle on what is happening and the math behind it.

-Bill

lorelec · March 2010

Re: Dissimilar panels in series

The equivalent simulation for a solar panel would be a Current Source with a Parallel Resistor...

...and a parallel diode, to make things even more complicated.

tkc, if you plotted the amp/volt data from your pdf file you'd end up with the solar cell's characteristic I-V curve, which is nearly identical (mathematically) to that of a plain old diode. In fact, you can generate electricity (tiny amounts) with a glass-packaged diode and ambient light.

In simple terms, a voltage source maintains a steady voltage over a wide range of currents. A current source maintains a steady current over a wide range of voltages. It's easier to visualize a voltage source because we deal with them all the time (batteries, for example).

There is no such thing as a perfect voltage or current source, though. A perfect voltage source would maintain its voltage even with a short circuit (zero resistance for a load)...which means it would be able to supply an infinite amount of current. A perfect current source would maintain its current even with an open circuit (infinite resistance for a load)...which means it would be able to supply an infinite voltage.

Marc

Dissimilar panels in series

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