Balancing photo-voltaic system?

[misterroy]

A friend of a friend is putting in a loft extension, with new dormer windows. The new window will cast a shadow on part his neighbour's photo-voltaic array. The neighbour seems to know his stuff, but part of what he is saying sounds strange.

As best as I can remember, the panel nearest the dormer will drop in efficiency, by say 20%. This bit I understand, due to the shade. (The neighbour has built a model and used a torch to help his estimates).

However, according to the neighbour, this is not the end of his losses. Because one panel is at 80% efficiency, the others have to drop their efficiencies too.

The neighbour is taking his loss on the chin and not hassling, but how can shade on one panel effect the efficiency of every panel? ta

[Bill Taylor]

The current through the shaded cell will be reduced by the shade. As all the cells are in series the entire system suffers from reduced output.

There are 2 common solutions; use micro-inverters or optimisers for each panel.

[Cynic]

Some systems have more than one array running in parallel

[rmlaws54]

Another relevant question is whether the PV owner actually has a right to light for his panels. Did he buy an easement when he set up the panels? I am guessing not. In which case I dont' see why it is the dormer-builder's problem.

To cite an obvious example: you can't prevent a building being built simply by setting up an array of panels that will be shaded by it.

[Martin Brown]

It limits the maximum current flow through the weakest link in the chain - the panels are in series so one in shade limits them all.

[Bill Wright]

Why should it?

Bill

[tabbypurr]

The other thing to note is that shading 20% of a panel drops its output by much more than 20%, for the same reason.

NT

[Brian Gaff]

Well there is something in the back of my mind about the way the panels work together and when the system switches from supply to grid etc. If the whole is measures by output of everything then in effect there will be a drop in efficiency as the system will not look at them individually but as a whole.

It might of course be that the panel in question could be relocated to preserve the status quo, it will much depend on orientation and when shadows are cast. I don't think I'd be so charitable about losing what small power I'm making to such an extent, I'd suggest compensation might be needed. Brian

[Martin Brown]

The current is produced by light creating free electrons. In the shade the silicon is almost an insulator. The maximum current that the thing produces is limited by the panel that can provide the smallest current.

Without photoelectrons to move the rest just heat up a bit more and make the voltage across the weakest panel higher. The panel in shade behaves like a resistor preventing a higher current from flowing.

It isn't unlike the situation where you put a mixture of used and unused batteries in a torch. The weakest one limits performance greatly.

[The Natural Philosopher]

Good question aqnd one that had been botrehring me

...and other sources imply that by and large PV cells are current sources and thats why series connecting them is a Bad Idea.

OTOH series connecting them makes inverters more efficient and means less wires flying around the roof, so its a Good Idea.

Really PV are just pants anyway so who cares?

[misterroy]

Thank you all.