Low system output during indian summer?

[hduz5]

Hi Guys,

Anyone else noticing systems not reaching very high power outputs during the last week of sunny weather? Any if so any idea why? I'm an installer, not a system designer, so maybe there is an obvious answer to this that I am missing? We commissioned 12 systems on thurs and fri last week and were surprised by the output figures we were seeing, none of the systems getting above 60% of peak designed output despite facing dead south, 15 degree tilt, and being in glorious sunshine.

I've come across another forum aimed more at PV users than installers, where several people in different parts of the country were questioning why their outputs were not getting above around 60% in the direct sunlight we have been experiencing. Is anybody else experiencing this?

Cheers

 

[SolarCity]

Just because it is hot, does not mean that the systems should be doing well. They'll do better later in the week when it is still sunny yet not as warm.

The 15 degrees tilt could be an issue. Although we've had glorious weather, the sun will not be peaking as high in the sky as it would have done mid-summer. The lower the sun is in the sky, the lower your output - especially so with a low tilt. It often surprises people how low this can be.

 

[JulianC]

All the above, plus the sun is further from the earth at this time of year than in high summer. What insolation figures were you getting when you commissioned the systems?

 

[SolarCity]

All the above, plus the sun is further from the earth at this time of year than in high summer.

:thinking2:I'm not sure that is right. The Earth is actually getting very slightly closer to the Sun as we get closer to winter. I think the Earth is closest to the Sun around early January and furthest away around mid July.

I don't think it has much of an impact on our climates.

 

[hduz5]

Interesting, just came across this on another forum discussing output over the last week, not going to pretend it makes complete sense to me. Does this make sense to you guys?

NOCT of typical panel is about 48 C; air temperature of 30 C and insolation of 1000 Wpsm gives a panel temperature of 67.5 C, or 42.5 C above the nominal operating temperature. Typical power output temperature coefficient of -0.5%/C gives a loss due to temperature of 21%. That reduces a 4kWp array to 3.16kWp.

 

[JulianC]

D'oh. You're quite right. What am I banging on about then? Axial tilt probably. Sun weaker in winter due to shallower incident angle with the earth. Which is what you said re: sun lower in the sky. I'll shut up in a bit....

 

[pjcomp]

And don't forget that we're getting about 25% less daylight. A couple of months ago my array was waking up at 5.30am, now it's more like 7-7.30am. Take that off both ends of the day and a possible 15 to 16 hours of light have gione down to 11 to 12. I've also noticed quite a lot of haze about, which is actually dust and so forth suspended in the air, which also interferes with the light quality.

PJ

 

[Graeme Harrold]

Working my way down on the thread and the answer I was going to give is just above. The air over the past week has been more humid, and there has been high level haze, all of which combined with shorter days, lower sun angle causing a greater distance of atmosphere to get through all takes its toll. Not to mention pretty still air reducing panel cooling......

 

[TJLinks]

You seem to of missed the fact that heat makes things expand and cold makes them contract.....Hot summer = long days, cold winter = short days....:sunny:

 

[topquark]

:thinking2:I'm not sure that is right. The Earth is actually getting very slightly closer to the Sun as we get closer to winter. I think the Earth is closest to the Sun around early January and furthest away around mid July.

I don't think it has much of an impact on our climates.

Yep, perihelion is around 3rd January and we are approx 5 million km closer to the sun then.

 

[Energetic]

Not Sanyo Panels and SMA inverters by any chance... i'm preoccupied...

 

[hduz5]

Sharpe panels and fronius inverters.

Errrr, just to confirm, I do realise that there are fewer daylight hours each day now than in midsummer

I'm talking about a reduction in peak output, not daily output.

Throughout the period we were commissioning there wasn't a cloud in the sky and the panels were all in direct sunlight. Yet despite what appeared to be optimum light conditions output never rose above 60% of potential peak. I take the point about the sun being lower in the sky. Not so sure about the temperature of the panels being too high, we were only at 24 degrees. In July temperatures were higher than that and we were seeing systems way above 60% peak output.

 

[edexlab]

We've had recently installed systems do the same 2 x 4 kw on customers house originally did'nt do as well as his farm building also 2x 4kw sanyo/4000tls in same area but this has now reversed even though the house arrays have been affected by high voltage issues currently being monitored by the DNO
I explained the shallower angle of the sun in relation to the arrays being a factor and demonstrated the difference between pitch angle using the irradiance meter also I noticed a few local farmers were still harvesting

 

[Scooby]

I suspect Graeme is on to something when he referred to the haze level. I had a 3.2kW system installed last Tuesday and on the following day - a clear scorcher - it recorded a peak output of just over 3kW - which amazed me. Since then, despite what initially seemed to be equally sunny days, it has 'struggled' to exceed a peak of 2.5kW. Looking at the sun (not literally, of course), I noticed that the sky wasn't as crisply blue as the day before, even tho' shadows were still pronounced. It's amazing what a bit of haze can do...

 

[davidanson]

It is just the time of year. While this is my first year with PV I have been monitoring the panel output - before the autumn equinox (c 21 Sept) we would peak around 3kW on our system, now it is peaking at 1.8kW and will continue to drop. Winter is colder because the sun is weaker, PV simply converts the sun's energy to electricity. There is less energy ergo less electricity. My monthly forecast averages are still in line (usually better) with what is expected.