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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorrevor
    • CommentTimeOct 1st 2021
    I recently got to know of his phenomenon by studying what my inverter was doing as it was dropping out one of the strings of my solar panels. This effect is caused by sudden increase in irradiance due to reflection of the passing cloud focusing more sunlight to the array. The inverter to protect itself, senses this and controls the current by dropping out the string. My inverter has uneven MPPT i.e. the 6 Kw is divided into a 4 kw and 2 kw strings it is the 2Kw (MPPT2) that drops out or reduces in output and does it for just a couple of seconds but if the current is high drops out for 10 seconds. I was wondering if others with solar PV systems has knowledge of this effect and how does their inverter manage it. How good the inverter is at doing this job can dramatically affect the efficiency. I am led to believe that different manufacturers do it differently. I am curious as to how they do it. It is possible you have not noticed the effect and put down the decreasing output with passing cloud as just that. I only noticed when I started watching the strings current on the display. Seems to be a lot of papers written on the topic and is well known effect in PV circles but I have only just learnt of it.
    • CommentTimeOct 1st 2021
    Posted By: revorI have only just learnt of it.
    Well, as of reading your post, I've only just learned of it too. Fascinating. Seems like it might be another case where microinverters can offer some benefit?
    • CommentAuthorSteveZ
    • CommentTimeOct 2nd 2021
    Micro-inverters and the Solar Edge system do this sort of thing well. They both work on individual panel outputs, so the string effect is not present.

    My ideal weather (for PV at least) day is bright sunshine in the morning, with a cloud bank moving in from the East in the afternoon - it makes a tremendous difference to the daily power figures on my mainly E-facing array. I have seen 2kW produced from the reflected sunshine on a cloudy afternoon, when the relevant roof should be in shadow.
    • CommentAuthorJeff B
    • CommentTimeOct 2nd 2021
    Our PV system has microinverters but I had never thought about this cloud reflection effect. Perhaps I should mount an array of mirrors on my garden fence so as to reflect the afternoon sun, when most of my S-E facing PV panels are coming into the shade!
    • CommentAuthorEd Davies
    • CommentTimeOct 2nd 2021
    Bizarrely, I first heard about this cloud-edge effect years ago (late 1990s, early 2000s?) in a non-PV context. Somebody at a gliding competition was talking about some insolation measurements they'd been making as a little private research project and was puzzled about spikes in the output just as clouds came or went. Somebody else explained the reflections and said it was known to affect PV.

    Another consideration is that when the panels have been shaded by cloud for a while they'll be a bit cooler so their initial generation when the clouds move away will be a little higher until they warm up.

    I don't understand why revor's inverter is dropping out under these conditions, though. Normally inverters are designed to handle the case of panels producing more power than they're capable or configured to handle, up to a point. They should just throttle the output to whatever their limit is by running the panels off their maximum power point.

    I'm not sure what grid-tied inverters do but I gather MPPT charge controllers (for batteries) normally do this by increasing the current and so dropping well below the maximum-power-point voltage. This commonly means that the limit on the amount of panels which can be connected is that the total short-circuit current of the panels should not exceed the nominal current of the controller.

    I don't see what microinverters have to do with all of this as the reflection effect won't differ significantly between the panels.
    • CommentTimeOct 2nd 2021
    Posted By: Ed DaviesI don't see what microinverters have to do with all of this as the reflection effect won't differ significantly between the panels.
    The cloud cover and any related effects affect individual panels, and so with microinverters only individual panels are affected. With strings of panels on an inverter, the whole string is affected by any effects on just one panel, resulting in a greater disturbance to the output. Or at least that's what I understand.
    • CommentAuthorEd Davies
    • CommentTimeOct 2nd 2021
    Posted By: djhThe cloud cover and any related effects affect individual panels…
    But, unless the array is much more spread out than a typical domestic installation, the panels will all be affected essentially the same way. Clouds just aren't that sharp edged and area for reflections will be even more spread out. It's nearer objects casting shadows that microinverters help with.
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