Effect of shading on solar panels in series

[mperkel]

I'm not sure I agree. The way I see it if you have bypass diodes and if you have one panel in the sun you should be harvesting the power from that one panel. if you had an inverter that worked from 25V to 600V then you would have that. With the ABB inverter at a setting of 120 VSTART it takes 5 panels in the sun (@27.5V) to get the inverter to work normally. With less that 5 panels in the sun the inverter is wasting power that could be harvested.

Imagine this. Supposed I had 2 inverters - an ABB inverter and a cheap Chinese 1KW inverter - and I had them on a switch so that when the voltage dropped below 120V (due to shading) it switched to the cheap Chinese inverter, The Chinese inverter would produce a lot more power than the AAB. Thus watts are being left on the table. If the ABB inverter could do what the cheap inverter does in low voltage due to shading then it would be perfect.

I think the flaw is that there inverters are watching the voltage to do dawn/dusk detection and that voltage drop due to some panels being shaded is completely different.

[sensij]

I'm not sure I agree. The way I see it if you have bypass diodes and if you have one panel in the sun you should be harvesting the power from that one panel. if you had an inverter that worked from 25V to 600V then you would have that. With the ABB inverter at a setting of 120 VSTART it takes 5 panels in the sun (@27.5V) to get the inverter to work normally. With less that 5 panels in the sun the inverter is wasting power that could be harvested.

Imagine this. Supposed I had 2 inverters - an ABB inverter and a cheap Chinese 1KW inverter - and I had them on a switch so that when the voltage dropped below 120V (due to shading) it switched to the cheap Chinese inverter, The Chinese inverter would produce a lot more power than the AAB. Thus watts are being left on the table. If the ABB inverter could do what the cheap inverter does in low voltage due to shading then it would be perfect.

I think the flaw is that there inverters are watching the voltage to do dawn/dusk detection and that voltage drop due to some panels being shaded is completely different.

I don't understand the argument you are making. There is a large amount of research into how to make MPPT effective for partial shade conditions, not just sunrise and sunset. Google "mppt shading" for a taste. If your panels had been wired as a single string, or the shaded panels split up between the two strings, then turn on voltage wouldn't be a problem, and it achieves exactly the same goal you are describing. Seems like a easier fix than redesigning the inverter, no?

The SB3000-US-12 has a start voltage of 228 V that can drift down to 200 V before it turns off. The transformerless model SB3000TL-US-22 has a turn on voltage of 150 V and can drift down to 125 V before turn off. The push to go transformerless may be influenced by the greater design flexibility, but really, I would trust the inverter manufacturer to know what the market needs and to be designing their inverters to get as close to that as they can while hitting their profitability goals. If low turn on voltage was easy, it would probably already be in.

[bcroe]

In order to take advantage of getting the most amount of power from a partially shaded environment the ideal inverter would be capable of dealing with a very large range of input voltages. An inverter that could work from say 25 to 400 volts would be ideal.

Is there any inverters that have that kind of range? If not - why not?

Only in recent times, have practical power supplies capable of a 2 to 1 input voltage range
become common (120 or 240 VAC, etc). You are asking for a 16:1 input voltage (and current)
range. You might be able to design a supply capable of that input voltage range, at some
current level. But that would give in effect a power capability variability of 16:1, depending
on the available voltage. Trying to ramp up the current capability by 16 would cause a huge
increase in cost. In effect it is just a unit 16 times as large as needed at 400V, just so it can
work at 25V. Given the losses squared as the current in conductors, it might be a lot more
than 16.

Having done that, a solar app needs to maintain a good efficiency even when available power
is much reduced. Doing that with an operating power range of 50:1 or so is going to be a
real challenge. But if all this is achieved, the design will be FAR too large and expensive.

To be competitive, it would be much better to consider the % variation voltage range of PV
panels, multiplied by SOME variability in the number of cells that might be placed in a string,
at voltage compatible with efficient construction at the power level. If my array were
lowered from 400V to 25V, I would need 256 times as much copper to get the power to the
inverter (400' away).

Industry has already managed to cover substantial input voltage range, a lot closer to 2 than
16. The size, cost, and efficiency are excellent, large variations in power are handled
competitively by using different model inverters.

If shading is costing a lot of power, a series string can't be optimized. There are micro
inverters, one per panel, to cover that. What is available won't eliminate the need for some
design work. And you'll never achieve 100% utilization. good luck, Bruce Roe