Solar Panel Guide Discussion

zeupater

1961Nick wrote: »

For a 15 minute 0.86kW timeslice, generation has to be a continuous 3440kWh...ie the maximum.

0.86kW timeslices occur very frequently & over a full range of ambient temperature....probably 2C - 25C. That would appear to discount temperature as being the limiting factor. Trust me, it's the inverter & 3440kW is the maximum that reaches the meter - ever!

Whatever the effects of temperature are, they mainly occur somewhere between 4kW & 3.44kW.

Hi

With 4kWp of panels and a 3.6/3.7kW inverter, a 3.4kW (even if it 3440!) maximum generation over an extended period will be due to temperature, not inverter capping, unless the inverter is otherwise limited to 3.4kW.

The panel will, in it's published specifications, have a (standard test) rated power at an irradiation value of 1000W/sqm with a panel surface temperature of 25C, this is referred to as 'Pmax' - there will also be a panel temperature coefficient, which will be something like -0.440%/degree (but usually taken as being a reduction of ~0.5%/C) - this describes the reduction in panel output as it warms-up in direct sunlight ... the higher the level of generation, the more irradiation you're getting, the warmer the panels get, the higher the temperature coefficient losses ... a higher ambient temperature will serve to raise panel surface temperatures by reducing cooling (that's one reason for reduction in generation on hot summer days), with a strong breeze across the panels helping by increasing the convective cooling effect ...

HTH
Z

1961Nick

zeupater wrote: »

Hi

With 4kWp of panels and a 3.6/3.7kW inverter, a 3.4kW (even if it 3440!) maximum generation over an extended period will be due to temperature, not inverter capping, unless the inverter is otherwise limited to 3.4kW.

The panel will, in it's published specifications, have a (standard test) rated power at an irradiation value of 1000W/sqm with a panel surface temperature of 25C, this is referred to as 'Pmax' - there will also be a panel temperature coefficient, which will be something like -0.440%/degree (but usually taken as being a reduction of ~0.5%/C) - this describes the reduction in panel output as it warms-up in direct sunlight ... the higher the level of generation, the more irradiation you're getting, the warmer the panels get, the higher the temperature coefficient losses ... a higher ambient temperature will serve to raise panel surface temperatures by reducing cooling (that's one reason for reduction in generation on hot summer days), with a strong breeze across the panels helping by increasing the convective cooling effect ...

HTH
Z

How do you explain the 3440kW maximum occuring throughout the hottest summer days & also at 2C?

Bombarding me with science doesn't change the fact that the 3440kW maximum is experienced across the full temperature spectrum.

nigelpm

FWIW, I'm with Nick on this one.

My PV system is capped at c. 3.6Kw and shows an identical distribution curve to the one Nick's posted on a completely clear day - whether that be in June or April.

zeupater

1961Nick wrote: »

How do you explain the 3440kW maximum occuring throughout the hottest summer days & also at 2C?

Bombarding me with science doesn't change the fact that the 3440kW maximum is experienced across the full temperature spectrum.

Hi

:wall: ..... it's nothing to do with 'Bombarding me with science', if you have 15 minute data records it's simply what happens within a 15 minute period that counts ... take an example of a partially-cloudy day where the panels (4kWp) have been in the shade for a while and the panels are close to ambient (say 20C) and are generating (say) 500W ... suddenly the cloud clears and with cloud edge effect (temporary increase in brightness/sqm due to the addition of reflection/refraction from clouds) the irradiation on the panels hits 1100W/sqm - in theory the panels will deliver ~10% over DC rating (1100/1000) due to irradiation with an additional ~2%(0.44*5) from being cooler than 25C .... you'll get an initial high DC generation of 4488W, which with inverter losses (say 5%) would result in an AC potential of value of ~4.26kW ... however, this is where the inverter's control software algorithm comes into play and caps the generation. If you're looking at real-time generation on the inverter you might be lucky to see this peak for a couple of seconds ...

... Now for some pretty simple 'bombarding with science' ... clouds are simply condensed water ... clouds can seemingly 'bubble-up' from nowhere and disperse just as quickly, however, the amount of 'vapour' in the atmosphere doesn't change that quickly, just the dew-point, temperature and pressure at altitude .... when there's relatively humid air in 'clear' sky conditions the sky will be 'light' blue and when there's little humidity it'll be a rich, much darker blue (usually cold/frosty day) ... the lighter blue the colour the more solar irradiation is reflected to space, the darker, the better for your panels ... <boring General Science over>

... So now we've got panels generating 4488W with cloud edge effect, but that soon drops as the sky clears and the cloud edge moves away. There's still vapour in the air, but now above dew-point, so with no cloud edge reflection & refraction, the panel irradiation drops to (say) 950W/sqm, the panel is still at 20C, so generation drops to 3876W DC ... around 3680W AC ... at this point, if the inverter is 3680W limited, there's no capping, it simply generates at maximum ....

.... as time passes the panels rise in temperature - within a minute or so they reach 25C, 3600W(3680-(0.44%*5)) and a couple of minutes later reach (NOCT, say) ~45C and generate 3280W AC (3600-(0.44%*20)) ....

.... of course, all of this has happened within relatively small time, so with 15 minute time-sliced data, the recorded generation would be somewhere between, 3280W and the inverter cap of 3680W ... taking the mid-point, it would be 3480W and taking the initial period at 500W into account, probably much less ...

... Right then, real world ... panels have manufacturing tolerances, usually +ve rated, so to move from the above example resulting in 3280W stable generation over 15minutes to 3440W represents +4.9%, well within the combination of a 5% tolerance band on panels, variations in inverter/cable losses, meter accuracy and anything else ...

It's all an example, but I do hope that in the event that you're still not convinced, it helps others understand the process which would limit generation on their own systems ....

HTH
Z


(... unless, of course, you're 16A limited and always receive 215V, (16Ax215V=3440W) in which case the inverter should be continually shutting down due to grid connection being out of standard tolerance (230V +10%/-6%) ..... or the 'pulse' monitor is set to a voltage with isn't typical for your supply ... )

zeupater

nigelpm wrote: »

FWIW, I'm with Nick on this one.

My PV system is capped at c. 3.6Kw and shows an identical distribution curve to the one Nick's posted on a completely clear day - whether that be in June or April.

Hi

?? Curve or values, and importantly, would that change if there was no cap ?? ... We're not inverter-capped to 3.6kW and would show almost the same results on a bell-curve day, although on a different orientation distribution ... it's simply what silicon panels do and why undersized inverters are so-often specified ...

HTH
Z

1961Nick

Very educational Z, but....

3440kW is the maximum real time reading on my energy monitor as well as the 15 minute slices, regardless of the temperature.

If temperature was the limiting factor rather than the inveter, then the plateau would vary with the temperature curve throughout the day & from day to day.

I saw 3440kW this morning with an ambient of 8C & this afternoon at 15C.

Try to imagine a 4kWp with a 2kW inverter & that's the kind of data I'm seeing. There's not one single piece of data suggesting that temperature is a factor on this systems maximum output.

Martyn1981

Hiya Z. I understand what you are saying, and whether right or wrong, it certainly makes sense over a 15min slice, but Nick did say earlier that he's seeing a constant 3,440W on his monitor which is a meter pulse type.

Now, I've got two of those, and whilst there may be some small error factor, they do seem extremely accurate. But here's the rub, the more you generate, the faster the 1Wh pulses and as I understand it, the monitor works out generation by measuring the time between pulses. So at 3,440W, that's extremely close to 1 per second (3,600W), so if the monitor is displaying 3,440W then that would suggest an actual realtime generation, rather than a 15 min averaged generation. If that doesn't go higher, then it suggests capping.

I admit I'm guessing, but that output figure does sound odd given the inverter is a 3.7kW model. I've tried to look up the specs, but can't find it. The 3.6kW model specs say that it is capped at 3.68kW. Now the issues could be one of meter error, misinterpretation etc etc, but I wonder if there's a tiny chance that the inverter could have been set up wrong by the installer - bit of a long shot!

[Edit - Oops, took too long replying, so what Nick says. Got distracted by new MSE format (Yuck!). M.]

Mart.

orrery

1961Nick wrote: »

3440kW is the maximum real time reading on my energy monitor as well as the 15 minute slices, regardless of the temperature.

You are likely to see maximum output from the panels at peak sun, on a cloudy day - in the gap between the clouds. i.e. direct sun, plus additional scatter from the clouds.

On a hot sunny day, I see just over 3.2kW from my 4kWp system, but throw in clouds and it maxes out in the gaps at over 4.1kW.

1961Nick

I'm going to go into the loft tomorrow & see what the inverter says when the geo monitor is maxed at 3440kW. If there's no sun tomorrow blame me!

I'll also check the settings to see if I can find the 16a cap that Z was suggesting.

Interesting!

EricMears

Martyn1981 wrote: »

[Edit - Oops, took too long replying, so what Nick says. Got distracted by new MSE format (Yuck!). M.]

Mart.

I noticed another 'new' format a couple of hours ago (and second the Yuck! ) but it seems to have vanished again :j
I strongly suspect that whenever they're having a quiet day at MSE Towers, some idiot decides to mess about with things. Have they never heard of the old saying : "If it ain't broke don't fix it " ?