Solar panels on flat roof: Does the angle of the solar panel really matters ?

[Paul Coetzee]

Hi,

I am living in Pretoria. I have a flat roof and my solar panels are flat on my roof. 

1) At what decrees are solar panels suppose the be installed?

2) What increase in sufficiency can I expect should I increase me solar panel decrees?

3) I have 20 300w solar panels.  Due to wind issues, how should I structure my panels? One structure or 5 structures ?

4) Do I need to anchor these structures? If yes, how can I do it, without causing waterleaks?

 

[DeepBass9]

About 35 degrees is optimal for Pretoria.

I can give an actual reading of the difference as I have one array at 35 degrees and one flat (actually slightly south), so right now (10:30am ) I am getting 59% of rated capacity from the 35 degree one, and 41% of rated capacity from the flat one.

I'll take another reading at local noon, which is 12:20 here.

[___]

What number of degrees shall we decree, folks? :-P

In the past getting it right was important because PV modules were expensive. Nowadays, it is cheap enough that other factors (aesthetics) are sometimes overriding factors, so if the roof you want to put them on is at 20 degrees... then use 20 degrees :-)

I have my own panels at 35 degrees. It's higher than it should be, but it optimises it somewhat for winter. The downside is that they have a lot of wind resistance when mounted like that. I'm thinking of moving the next frame down a tad.

[DeepBass9]

So at 12:20, 77% of rated capacity for 35 degree array, 45% for flat array. Quite a difference! The flat array will start producing later and finish earlier so to total production will be even less.

[OomD]

2 hours ago, DeepBass9 said:

So at 12:20, 77% of rated capacity for 35 degree array, 45% for flat array. Quite a difference! The flat array will start producing later and finish earlier so to total production will be even less.

But those figures will also be influenced by the azimuth.

[DeepBass9]

Azimuth is true north

[pilotfish]

9 hours ago, Paul Coetzee said:

1) At what decrees are solar panels suppose the be installed?

At RSA latitudes you can use [your latitude + 10'] as a good rule of thumb for best year round performance. So in Pta 35' about right.

 

9 hours ago, Paul Coetzee said:

2) What increase in sufficiency can I expect should I increase me solar panel decrees?

Depends when you measure - mid summer your panels will be fantastic, mid winter your panels will be pathetic. For best AVERAGE year round performance use 35'. The thing is in winter you will struggle with your panels laying flat unless you have MUCH more panels than you actually need (like double).

 

9 hours ago, Paul Coetzee said:

3) I have 20 300w solar panels.  Due to wind issues, how should I structure my panels? One structure or 5 structures ?

I recommend that you have your panels only 1 layer high on your flat roof [eg 5 rows of 4 panels wide], but remember to use spacing of at least 1.5x the height of your panels. So if 1st row is 500mm high after 35' tilt then 750mm space before next row - otherwise you will get one row shading the next row in winter.

9 hours ago, Paul Coetzee said:

Do I need to anchor these structures? If yes, how can I do it, without causing waterleaks?

Yes - the structure design would be up to you - you could anchor with sand bags if really worried about leaks, but most would use bolts and a waterproofing solution.

[Johandup]

Can an expert please do the sums regarding sun tracking.

My logic tells me that you would need fewer panels to get the requires kw input if the system can track the sun over a two axis system.

This will make up the cost required for the tracking components.

Surely a Raspberry pi computer can be made to do this.

[DeepBass9]

Panels are so cheap there is no point seems to be the consensus opinion. The additional cost and complexity of a tracker would be more than adding a few extra panels to get the same effect.

[Mark]

The other argument against trackers (given the cheap prices of panels) is to just have more panels in 2 strings facing sunrise and sunset (or NE and NW).

[pilotfish]

5 hours ago, Johandup said:

Can an expert please do the sums regarding sun tracking.

This has been done before by people much smarter than me;

	Fixed	Adj. 2 seasons	Adj. 4 seasons	2-axis tracker
% of optimum	71.1%	75.2%	75.7%	100%

The table above shows that there is hardly any value at all in adjusting panel tilt each season over a fixed tilt array, however there is a very real advantage to a 2axis tracker over fixed array.

The above numbers don't really tell the whole story - for a fixed array to theoretically match the annual output of a 2axis array you would need to add 40% panel capacity, but the larger fixed array would still give low output in the morning and evening with a huge output in middle day, whereas the 2axis array would give a much smoother output throughout the day.

This smoother output would deliver significant benefits in real life over the simplistic result of math on a piece of paper showing equal annual output. To have your output pick up quickly in the morning and extend PV output late into the afternoon would have significant benefits in an off grid situation (and most other situations as well).

 

 

[pilotfish]

Having said that^^^...

You would need a BIG area in practice and probably only practical in a rural setting. To slap 10 or 20 panels onto a suburban roof is doable in most instances, but filling your 1/4 acre plot with a big ground mounted 2D array (because your neighbors wont be overjoyed to see this mounted on your flat roof) is not going to leave much space to park the bakkie.

If you are on a farm then a 2axis array is doable so long as total cost is not more that fixed array x1.5. Also remember that your maintenance will be a bit more than the occasional spray down with the garden hose.

[Johandup]

Thanks for the insight @plonkster

I fail to see the logic of custom installing solar panels to suit all types of roofs. This is like demanding roads to be build to suit all transport. We expect cars to be able to cope with all types of roads - although my Pajero really battles with corrugations :-)

I can see that we will get standardised solar installations (much like bathrooms today) to put onto standardised roofs. 

Hopefully still in my lifetime...,.

[___]

1 hour ago, Johandup said:

although my Pajero really battles with corrugations

You should watch some of Andrew St Pierre White's videos, he tells the whole story of how his experience with a Pajero, the review he wrote and the resulting response from Pajero owners eventually cost him his job :-)

[pilotfish]

13 hours ago, Johandup said:

I can see that we will get standardised solar installations (much like bathrooms today) to put onto standardised roofs. 

Unlike you I hope that I am dead long before that eventuality - the thought of my solar system and roof design being prescribed by some government lackey and forced on me against my will fills me with horror, almost as bad as Nersa removing regulating my right to employ the suns rays to produce my own power 

[pilotfish]

Just now, plonkster said:

the review he wrote and the resulting response from Pajero owners eventually cost him his job :-)

And fair enough too - I have watched the video and it is clear that he has no idea how the Pajero drive system works (which is amazingly capable, I know because I have one) and then uses his privileged position in the media to lambasts and destroy the reputation of a great product, because he was embarrassed at looking like an idiot in that group when he portrays himself as the master of 4 wheel drive. 

[pilotfish]

Obviously having a bad day, I think I need a whisky

[Johandup]

8 minutes ago, plonkster said:

You should watch some of Andrew St Pierre White's videos, he tells the whole story of how his experience with a Pajero, the review he wrote and the resulting response from Pajero owners eventually cost him his job :-)

Refuse to fire myself lol

[___]

2 hours ago, pilotfish said:

no idea how the Pajero drive system works

He did criticise more than just the capabilities in sand :-) Only reason I really thought of him was because @Johandup mentioned that he too finds the drive quality on some road surfaces a little wanting (if I may paraphrase). And to be fair, there are other perfectly capable vehicles (such as the Defender) who are equally unkind to its passengers when driving long distances. And to be even more fair, it remains somewhat of a personal preference thing, one is often willing to forgive a multitude of sins for reasons others would not understand.

[Gabriël]

On 2018/05/09 at 10:40 AM, plonkster said:

I'm thinking of moving the next frame down a tad.

better you move them than the wind

so much for my decree

[pilotfish]

8 hours ago, plonkster said:

He did criticise more than just the capabilities in sand :-)

He did yes - after making a fool of himself in the sand because he was no doubt too arrogant to spend the 30s required to find out how the system works, he went about defending his fragile ego by attacking everything about the vehicle with bulging eyes, pulsing veins and spittle spray.

Some of the points were valid and some were silly nitpicking, but all delivered with such outrage that anyone would swear that marketing a Pajero was akin to a criminal offense. As an experienced vehicle journalist he could have made light of his stupid mistake in the sand and accepted responsibility, and then gone on calmly to highlight both

good and bad points of the vehicle.

8 hours ago, plonkster said:

Only reason I really thought of him was because @Johandup mentioned that he too finds the drive quality on some road surfaces a little wanting

Which would be a valid criticism of the Gen 4 Pajero (mine is Gen 3 and doesn't suffer this malady), there is a private fix available on the [4x4 Community Forum] and [Pajero Owners Club of SA] - which I suggest @Johandup look into, apparently transforms the vehicle when used on corrugated roads.

9 hours ago, plonkster said:

And to be fair, there are other perfectly capable vehicles (such as the Defender) who are equally unkind to its passengers when driving long distances.

90% of Landrovers made are still on the road, the other 10% eventually made it home

OK - back to PV stuff now!

[Johandup]

38 minutes ago, pilotfish said:

Which would be a valid criticism of the Gen 4 Pajero (mine is Gen 3 and doesn't suffer this malady), there is a private fix available on the [4x4 Community Forum] and [Pajero Owners Club of SA] - which I suggest @Johandup look into, apparently transforms the vehicle when used on corrugated roads.

Done the dashfix - a cure (not successful on the Kgalagadi paths - can’t call them roads) and not addressing the cause which is the front suspension.

only a problem when driving slooowly on corrugations - the best 4x4 in my opkinion.....

This forum is more lively and comtributing than the Pajero one ;-/

Back to solar....

[Johandup]

20 hours ago, Mark said:

The other argument against trackers (given the cheap prices of panels) is to just have more panels in 2 strings facing sunrise and sunset (or NE and NW).

Cheap is a relative concept but not when it is your own money.

I will need 15 panels to give me 4kw (debatable of course) so 30 panels is way out of my budget.

This discussion will stop when solar roof tiles is affordable and the whole roof is covered in it.

[pilotfish]

1 minute ago, Johandup said:

not successful on the Kgalagadi paths

No - those "roads" on the RSA side can loosen your teeth!

If you haven't already then try going in the Bots side next time, that is a good test of of any vehicles sand capabilities and the paths that we took were much more pleasant twee-spoor tracks that the bone jarring corrugated roads on RSA side - very interesting Lion interactions as well, stick to roof-top tent unless you are happy to wake up starring into a Lions eyes from 18" at 2am!

[pilotfish]

8 minutes ago, Johandup said:

This discussion will stop when solar roof tiles is affordable and the whole roof is covered in it.

I now understand the "standardised roof" input better. I agree that if these were affordable it would be a wonderful solution.