Often it is unavoidable that buildings, aerials or roof dormers cast their shadows onto the surface of a Solar PV Panel, however shading can be quite a problem, because, in simple terms in most instances, the panels are wired in series, like the batteries in a torch.
If one battery in a torch is completely dead, then it can effectively stop the torch working, so it is (or was!) with Solar PV Panels. The general rule was that if one panel in a string of panels was affected by shadow, then this would pull the output of the whole string down, and indeed with cheaper panels and inverters, this is still true.
Another situation were similar problems occur is when an installation needs to be split either east / west or south east / south west, where the panels will give different light inputs at different times of the day. (East South East getting the strong morning light and West / South West getting the strongest light in the afternoon).
Well two companies have worked hard to solve this, I’ll deal with the second solution in part 2 of this article.
The first solution:
Uses a combination of two advances,
1) By-pass diodes in the PV Panels themselves.
2) An intelligent inverter
By-pass diodes, do exactly that, if a part of a panel is not performing, due to say shading, then the panel’s built in electronics by-pass a part of the panel, this gives a slightly reduced output from the panel, however it now doesn’t work as a ‘drain’ on the output. Without by-pass diodes, that part of the panel in shade, effectively works in reverse and can heat up quite dramatically, test results even show these parts of the panels acting as light emitters. In this case then the output is reduced as opposed to blocked.
Intelligent inverter, by-pass diodes are only part of the solution though. Inverters convert the DC electricity generated by the Solar PV Panels into useable AC electricity. Obviously we want them to convert as much as possible as efficiently as possible, they do this by tracking and balancing the DC voltage and current from the panels (called Maximum Power Point or MPP tracking), the best inverters do this more efficiently, and often have multiple MPP’s installed to take best advantage of configurations with shading or a split east / west system.
One particular inverter manufacturer, SMA has taken their 20 year knowledge of inverter manufacture and design, and come up with an MPP tracking system that reacts dynamically to the changes caused by shading and lighting conditions, and optimises itself, giving a higher output yield, and so optimising your return on investment. In SMA inverters, this job is done by the operation management system OptiTrac.
When individual modules of a string are shaded, the power curve changes significantly, the string of panels now shows several operating points of different quality, which cannot all be found and used by a conventional MPP tracker. In this case, the current output of the PV plant might be considerably lower than it actually should be in terms of shadowing.
SMA’s unique solution, OptiTrac Global Peak allows the inverter to use the energy offered by the PV modules almost completely – in any circumstance.
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