Mixing Monofacial and Bifacial panels in the same string...SS says don't.

[dhy4buva]

Has anyone tried mixing monofacial and bifacial panels in the same string? As a newb, I'm wondering if I've badly under-logic-ed this one. All I've seen online / on the forums about mixing panels is that 1) It's generally not advised 2) If you must do it, make sure the operating voltages and currents are close -- in my reading, mostly out of concern for efficiency, not necessarily major problems.

Background: I'm needing to add PV power to my system (EG4 6000ex) and the closest spec match to my existing 6s2p array of Aptos DNA-120-MF26-370W appears to be the Boviet 370W Bifacial BVM66100-370-HC-BF. The Aptos panels of the identical item number that I can purchase today are of a different spec for some reason and more expensive than the Boviets. So I bought the Boviets.

	Aptos DNA-120-MF26-370W​	Boviet 370W Bifacial BVM66100-370-HC-BF​
Maximum Power Current (Imp)​	10.76 A​	10.96 A​
Maximum Power Voltage (Vmp)​	34.40 V​	33.81 V​
Short Circuit Current (Isc)​	11.29 A​	11.54 A​
Open Circuit Voltage (Voc)​	41.90 V​	40.56 V​
Bifacial Power Gain 10%​	N/A​	407 W​
Bifacial Power Gain 20%​	N/A​	444 W​

I was just on the phone with Signature Solar tech support to get some last minute details cleared up before wiring everything up this morning and got an unexpected statement from the tech saying that they don't recommend mixing monofacial and bifacial panels. Not sure why I didn't just know this caveat (went back and googled around to see if I just missed this detail, found nothing), but anyhow, I did not know this. The tech said that the resistance of the bifacial panels would be higher than the monofacial panels and therefore the current would backflow into the bifacials...? Doesn't really make sense to me, but I'm not an electrical engineer. Seems to me that the resistance of dissimilar makes of panels would likely always be different in some way--but no one mentions this as a problem that I've seen. Can anyone corroborate or defy the logic the tech gave?

At any rate, now I'm reticent to try this config at all. My questions now are:

1. Would mounting the additional bifacials with minimal rear-light access (at the cost of bifacial gain), remedy this resistance phenomenon?
2. Why didn't I think of this problem before?
3. Can anyone else think of a way to get these panels integrated into the system without running a separate charge controller?

Thanks, all.

Danny

 

[Solar Guppy]

Should not make any difference, worst case is you don't get the marginal benefit of reflected light on the bifacial, but unless the panels are at a high tilt and in snow, bifacial is more of a marketing thing.

 

[dhy4buva]

Should not make any difference, worst case is you don't get the marginal benefit of reflected light on the bifacial, but unless the panels are at a high tilt and in snow, bifacial is more of a marketing thing.

This was my thought as well and why I was so surprised when he brought up resistance as a problem.

 

[wdwtx2.0]

bifacial is more of a marketing thing.

This guy would disagree.

 

[Solar Guppy]

This guy would disagree.

As someone whom made a living running array tests, there are flaws in the mythology used in the video.

Comparing factory new to some unknown aged array is the most obvious ( easily 3-5% just there ), keeping the back clear ( moved the micro inverter to prevent shading ), time of year when there is sun morning, evening coming from behind, not swapping location on the array ( end located panels tend to run cooler ) or monitoring the panel temperatures.

I'm not saying bifacial never outperform, but that on average, it's not worth a premium of perc-half cells.

Back to the topic, you can run them just fine in series, the IV curve is fairly flat on solar panels on the VMP knee, so panels that can produce more current, can also produce more with a higher voltage and lower current. If viewed each voltage point in a series string, the bifacial panels will run a tad higher in voltage, still providing whatever gain they might provide.

 

[dhy4buva]

Back to the topic, you can run them just fine in series, the IV curve is fairly flat on solar panels on the VMP knee, so panels that can produce more current, can also produce more with a higher voltage and lower current. If viewed each voltage point in a series string, the bifacial panels will run a tad higher in voltage, still providing whatever gain they might provide.

Very interesting and great to know. Thanks a lot! So just to confirm, SS’ notion of the bifacials back feeding into themselves is…fiction, right?

 

[Solar Guppy]

Very interesting and great to know. Thanks a lot! So just to confirm, SS’ notion of the bifacials back feeding into themselves is…fiction, right?

I can't really comment on thoughts from others as I don't know there expertise. My background includes design and testing of solar products with Xantrex and Morningstar ( past ), patents in mppt algorithms and published reports. I can and do make mistakes as well, so no throwing stones from me.

 

[dhy4buva]

I can't really comment on thoughts from others as I don't know there expertise. My background includes design and testing of solar products with Xantrex and Morningstar ( past ), patents in mppt algorithms and published reports. I can and do make mistakes as well, so no throwing stones from me.

Fair enough. I appreciate your insights and will give wiring the bifacials in series with the others a crack.

 

[dhy4buva]

So I wired in 3 of the bifacials into each of my 6s mono facial strings. Produced what seemed to be correct additional power during the day. Only weird thing is, now that it’s night, Solar App is showing 138v from PV (0 watts). The history shows it fluctuating between 0V and ~140V, all with 0 watts.

 

[MattiFin]

So I wired in 3 of the bifacials into each of my 6s mono facial strings. Produced what seemed to be correct additional power during the day. Only weird thing is, now that it’s night, Solar App is showing 138v from PV (0 watts). The history shows it fluctuating between 0V and ~140V, all with 0 watts.View attachment 161970

Solar panel wont need much light to generate almost nominal voltage. Current is just minuscule and thus you wont get any power.

There could be a huge variation in nearly dark performance between different solar panel models and types as it depends on internal "shunt resistance" or internal leakage currents. Panel would need at least enough light to generate more than the internal leakage currents.

Typically you won't find any specification for ultra low light performance as it usually makes zero difference if one 1kWp panel produces 0.01W and another produces 100 times more at 1W.