Hot September

Not said a lot here for quite a while.  It has been an excellent summer in many ways.  With so much water in the ground after the long wet winter, things have grown very well – and there’s been quite a lot of rain, very nearly as much as we had two years ago – which was a summer everyone thought terrible.  But this time most of the rain fell at night and the days were warm and sunny.

But this isn’t about gardening.  We have noticed an interesting and puzzling phenomenon here with the Solar PV.  And I, at least, am not getting any answers.  The solar PV consists of two identical strings each of nine panels, mounted side by side on the same, south-facing roof.  After a year, I noticed that the instantaneous power figures displayed by the inverter differed by about 10%, whereas they had been almost identical when the system was first installed (as one would expect).  I mentioned this to the installer – i.e. I complained!  By earlier this year, which is 2 years after the panels were first installed and about 18 months after we moved into the house, this discrepancy had widened to about 17%.  Now it is nearly 19%.

There have been visits by both the installer and the panel manufacturer. The two DC inputs to the inverter have been changed over – the difference changes to the other channel, so it would not seem to be the inverter.  Indeed a second inverter has been connected – and the figures remain different, so everyone now agrees it is not the inverter.

Repeated measurements on the DC output from the two arrays (strings) have also been made.  Open circuit voltage measurements have been made on both strings – and are reported to be closely similar.  Even more puzzling, short-circuit current measurements have also been made on both strings – and again are reported to be closely similar. Now, if cells within a panel had failed short-circuit, then you would expect the voltage to be down a very little bit – unless a lot of cells had failed, in which case the voltage would be noticeably lower.  But as the failed cells would not be generating power, the current would also be lower.  But it isn’t.  If the cells had failed open-circuit then, because an array has 3 diodes, you would expect to lose 1/3 of that array’s power – but the loss is not that large.  It’s only when the array is called upon to deliver power that the difference shows.  This does not behave in the manner I expect from what I was taught at school or university.

Needless to say, no-one wants to put up scaffolding and remove the panels since it would appear to be difficult to find the defective panel – if, indeed it is a defective panel. Oh yes, each panel is identified and its initial performance is known – and they are all initially similar to  better than 2%.

Any ideas, anyone?

On a quite separate point, the experts point out that south and west facing rooms can suffer from over-heating in autumn and, to a lesser extent, spring.  This is because the sun is noticeably lower in the sky and the shading measures (see picture elsewhere – we have a large overhang to the south) are therefore less effective.  Well, it’s true!  One room, which has windows to both south and west has got noticeably hotter during sunny days this last month than it did at the height of summer.  It’s not a problem – you open a window.  But from the thermometer in the room it has been getting 2 or 3C hotter this last month than in the fine weather of June/July.

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