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Shading on solar panels and how to defeat it without breaking the bank

Think Christmas trees lights. When one light failed, the whole string of lights failed because all the lights were connected in series. Now decent Xmas lights are wired in parallel but solar panels continue to have to be series connected. Shade on a solar panel causes resistance. Resistance in a series electrical circuit not only causes heat at the point of resistance, but it also forces the current down for ALL the solar panels in that circuit to match the worst performing panel. The solution for defeating shade is to make sure that shade is either entirely avoided using effective panel placement, or else to use power optimisers.

Whether it's Huawei, Tigo, SolarEdge or Enphase...all optimisers do exactly the same job, in the same way, and cost about the same too.
$80 each + $10 for new installations....a bit more for retro fits where panels have to be lifted up, disconnected, and then the optimiser fitted.

 

Huawei's new FusionHome hybrid inverter has DC power optimisers that can be selectively deployed. If, for instance, two panels might get some late afternoon shade then just put optimisers on them. In conjunction with Huawei's smart safety box, you can also view the output of all your optimised panels and see that they are really working !!

If you already have an inverter and shade problems then Tigo are a great retro fit solution.

 

SolarEdge and Enphase require an optimiser on EVERY panel with Enphase also putting more hardware inside each box to convert DC panel power to AC. (More to go wrong in a very inconvenient place in our opinion). Either way, both of these products, while excellent become relatively expensive because of their requirment to put an optimiser on every panel, shaded or not.

 

 

How they work.

Whilst it would be useful to use some analogy of a river or a garden hose to explain it, unfortunately it's really only Year 11 science that can properly do it.

A typical 275W solar panel produces 25 volts pushing 8 amps of current when the sun is shining strongly on it. 25 x 8 = 200 Watts of power. That's the first thing to know...your 275W solar panel, almost never makes anything close to it's ideal rated factory maximum of 275W.

 

At 8am that same panel will probably be producing the same 25 volts (voltage stays very constant all day long), but the current might only be 0.5A. So, Power being Voltage x Current, would be just 12.5 Watts. The current is low because the sun is weak.
Shade inhibits the sun getting to the solar cells and so the current flow is reduced.

 

Optimisers monitor the current flowing through the circuit and if their panel's current drops (eg shade hits it), then the optimiser decreases their panel voltage which in turn makes the current go back up to match the current of the other panels.

 

In this way, whilst the optimised shaded panel now has a lower voltage and therefore less power being produced, the voltage and current of all the other panels is unchanged so the combined power for ALL of the panels is only very slightly reduced by the one under-performing panel.

 

Now for something just as clever as that

Conventional wisdom says that all the panels on a string MUST face the same way. The reason is exactly the same as for the shade issue. If some of the string are on the North East roof, and some are on the North West roof, then, at different times of day, one group will be making less current than the other because they aren't getting the full force of the sun at the same time.

Here is an example of two groups of panels (arrays to use their proper name) , some on North West fitted with optimisers, some on North East, but both on a single string.

In the morning and through most of the day, the optimised North West panels will adjust their voltages to make sure the current of the strongly performing panels on the North East is maintained, and in the later afternoon, when THEY are making more current than the North East panels, they will increase their voltage so that their current drops to match the North East panels. In the end, the total power produced throughout the day is the same as if the panels were on separate strings.

Selective deployment of power optimisers is very useful sometimes when we are designing a system with maximum output in mind but a small or tricky roof area to work with.

 

 

 

 

 

 


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