Roof design for solar PV

ed110220

Martyn1981 wrote: »

Hiya Ed. This is still going round in my head, and I'm having second thoughts. My concern about mixing S&N panels, was that with high summer sun on such a shallow pitch, the north roof would generate at the same time as the south, so going large could mean capping.

But, even with the sun shining over the ridge onto the N panels, I doubt you'd hit 3.68kW assuming S panels at 80% and N panels at 50%.

Also, if I'm even close on the extra costs of those higher efficiency panels then you'd have around £1k extra to spend on regular panels, allowing 5kWp in total - 3kWp South + 2kWp North.

I've lost track of where you are, but for my house in Cardiff, using a 22d roof pitch and SSW (+10d) and NNE (-170d) I get the following:

SSW only 4kWp * 943kWh = 3,772kWh pa

SSW, 3kWp * 943kWh = 2,829kWh pa +
NNE, 2kWp * 639kWh = 1,278kWh pa = 4,107kWh

FiT would be 10% less, which equates to 4,107 * 0.9 = 3,696 (comparable?), whilst export and leccy savings aren't reduced.

The big question is, would there be any/much capping during peak generation, and I suspect not (I was wrong in earlier post).

Actually, there is a second big question, can the roof structure support the extra PV.

Assuming I'm right and that capping would be none to minimal, then you'd also have other options, such as 3kWp on the north, leave the soil pipe alone and shift an extra panel to the north, use 260Wp to 275Wp panels for an additional boost etc etc etc.

Mart.

Hi Mart,

I took this photo of the the panels on a house on our road, identical apart from the new roof covering of Decra or Metrotile (or something similar, basically coated steel with an imitation tile effect, though I would prefer something more contemporary, but functionally it's all the same thing). They have 9 panels on the SW side and 7 on the NE (their house is aligned slightly differently to ours). Looking at it, it looks as though they could have fitted an extra panel on each row, or two rows of 6 if the vent was removed or lowered.

I'd remove or reposition the vent anyway as the way it is boxed-in in the bathroom makes the handbasin position awkward and also the fewer roof penetrations the better in my opinion, especially when dealing with sheet materials. This leak problem we have all started with the flashing failing around the point where the flue passes through the top of the chimney box, in getting up there to fix it the asbestos got cracked, it would probably have been fine for quite a few more years if it hadn't been damaged.

Ed

Fruit_and_Nut_Case

ed110220 wrote: »

I took this photo...

I'd be tempted to ask the owner of that house if there was any specific reason for choosing that arrangement (5+4) of panels. I've found that most people are only too pleased to discuss their set-up.

mac2008

ed110220 wrote: »

Hi Mart,

I took this photo of the the panels on a house on our road, identical apart from the new roof covering of Decra or Metrotile (or something similar, basically coated steel with an imitation tile effect, though I would prefer something more contemporary, but functionally it's all the same thing). They have 9 panels on the SW side and 7 on the NE (their house is aligned slightly differently to ours). Looking at it, it looks as though they could have fitted an extra panel on each row, or two rows of 6 if the vent was removed or lowered.

I'd remove or reposition the vent anyway as the way it is boxed-in in the bathroom makes the handbasin position awkward and also the fewer roof penetrations the better in my opinion, especially when dealing with sheet materials. This leak problem we have all started with the flashing failing around the point where the flue passes through the top of the chimney box, in getting up there to fix it the asbestos got cracked, it would probably have been fine for quite a few more years if it hadn't been damaged.

Ed

ah, now I know why WestonDave bothered mentioning a specific location in his post.

Anyway, my comment was going to be, if the existing structure can't support regular tiles, solar panels weigh 15-20kg each and if mounted on rails off the roof covering, the wind loading can be significant.

I'd hope this is not a problem as you point out identical houses have had panels, but I'd question any potential installer carefully about the calcs they will do to demonstrate it won't be a problem, otherwise I could see insurers being a bit funny about it.

Also regarding fixings, you might find some suggest a through the tile fixing which is obviously harder to seal compared to the conventional type which slot up between tiles. Can the covering you suggest use conventional fixings? If coated steel 'tiles' I doubt it?

I don't know about 'in roof' systems I'm afraid, but I think Southern Solar have done some. They did my install and I was happy with them.

Definitely worth knocking on the door of the existing house to ask them about fixings and what structural calcs were done.

Martyn1981

ed110220 wrote: »

Hi Mart,

Looking at it, it looks as though they could have fitted an extra panel on each row, or two rows of 6 if the vent was removed or lowered.

Ed

Hiya Ed. I'd agree, using MkI eyeball it certainly looks like 6 panels would fit across, and 2 high. So with vent removed I reckon you have lots of options:
12 regular panels on south,
12 regular high efficiency panels on south,
16 small high efficiency panels on south,
16 to 20 regular panels spread over north and south,
20 to 24 regular panels spread over north and south (depending on capping or DNO permission).

As suggested, worth knocking on the door for info, and ask how they are finding the mix of panels. Fingers crossed they have some generation estimates/targets, (which you can check against PVGIS) and perhaps some generation figures, that you could use to verify the PVGIS findings you get.

Mart.

ed110220

I think this in Shirehampton, Bristol, clinches it (12 panels in a 6 + 6 portrait arrangement on a British Iron and Steel Federation house). In this case the panels are on the front of the house, the side without the chimney but in our case it is the back of the house that faces generally south.

Google street view, Portway, Shirehampton

Their neighbours have the same arrangement on the back (funny how solar is often found in little clusters), vent removed or lowered and a smaller chimney box over the flue pipe.

Google street view, Ermine Way, Shirehampton

Edit: looking round there are four more houses solar panels nearby, all in the same arrangement:

Catherine St, Shirehampton

Watling Way, Shirehampton

Watling Way, Shirehampton

These people in Gloucester have managed to cram 14 on, and onto the original asbestos too (looks rather cramped and as though it might take off!).