PVGIS confusion.

rogerblack

I'm just above Edinburgh, on the sticky-out bit in Fife.

http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php - climate-SAF.

An optimum angle (42 degree) 1kW panel produces 949kWh/year - more or less as expected.

A similar panel northfacing produces 391, again more or less as expected - summer sun can go on for aaages with the sun in the north up here.

A 2-axis tracking system produces 1250.

949+391>1250.

Is there a something that I'm missing, or is it assuming massive amounts of indirect light?

legoman62

Have you seen the 2nd graph on the same PVGIS page?
Tried my system. Came out as yours and then I saw the second chart.
.....When you enter 2-axis;)

rogerblack

Yes, I was looking at the right chart, I don't think that's it.

Martyn1981

rogerblack wrote: »

Is there a something that I'm missing, or is it assuming massive amounts of indirect light?

I don't think you're missing anything, I think it's simply down to twice as much kWp and the benefits of indirect sunlight. I guess what you've spotted is a very British thing, where a fair chunk of our generation is indirect.

Imagine the two panels situation and the sun is due south. The north panel will generate close to zero in good weather, but as the weather gets worse, the south panel will drop down from 1kW(ish) and the north panel will head up, till they meet, maybe around 200W or 300W or so each.

My ESE system in the summer after 4pm (BST) can generate anything from zero to 800W depending on how much white is in the sky. Only happened once, but the sky was so perfectly clear that it actually shut down. But if a pleasant pale white, nearer 800W. Normally it would be around 300W to 400W.

Mart.

PS When considering north panels, roof pitch is critical, ideally 30deg or less. Obviously a flat roof can't really be considered north, but as you get to 20deg you can hit around 70% of south. M.

zeupater

rogerblack wrote: »

... An optimum angle (42 degree) 1kW panel produces 949kWh/year - more or less as expected.

A similar panel northfacing produces 391, again more or less as expected - summer sun can go on for aaages with the sun in the north up here.

A 2-axis tracking system produces 1250.

949+391>1250.

Is there a something that I'm missing, or is it assuming massive amounts of indirect light?

Hi

There is a certain amount of double accounting by adding the two figures together .... a simple way of thinking about is is that when a south facing system is in full sunlight, then that's all of the irradiation it can collect, so adding the simultaneous 'indirect' generation of a north facing system is flawed .... another example would be generation when the sun is directly to the west or east, this would be double accounting at double the rate generated ....

HTH
Z