PVs - summer vs winter

[Russdl]

@SteamyTea  Got back to the computer and the PVGIS website gives a 'slope angle' for the easterly facing array of 28 degrees, so to my understanding that would be 28 degrees up from the horizontal, it doesn't make that clear, apart from the words 'slope angle' which to my mind implies that.

 

@Ed Davies If I've got that correct then I guess your assumption must be correct because it wouldn't get a great deal of direct sun from the low early morning sun at that angle.

[Ed Davies]

8 minutes ago, Russdl said:

… so to my understanding that would be 28 degrees up from the horizontal, it doesn't make that clear, …

 

Yep, pretty sure that's right. Apart from anything else, my playing with steep angles for winter generation earlier in this thread wouldn't make any sense if those angles were from the vertical.

[SteamyTea]

Though not clear, this is what PVGIS has to say , and show, about slope angle.

 

[H F]

In light of all of this, do installers take any of this into account, or do they just slap the panels on your roof using its existing pitch?

[scottishjohn]

25 minutes ago, Home Farm said:

In light of all of this, do installers take any of this into account, or do they just slap the panels on your roof using its existing pitch?

highly unlikely the extra cost of making another framework that is adjustable would ever pay back the extra cost 

but ground mounted could be different as you already have to make some sort of mounting  anyway.

If you look at commercial solar farms --i think you answer to this question is there 

not worth the extra complications

Edited November 19, 2019 by scottishjohn

[PeterW]

No other way really unless you have a ground mounted array. 

[Ed Davies]

For a new build, though, it can affect roof design.

[Russdl]

2 hours ago, Ed Davies said:

 

For a new build, though, it can affect roof design.

 

That’s why we went for 40 degrees as our roof pitch, plus it gave a tad more head height in the fully useable loft space. 

[richi]

18 hours ago, Home Farm said:

The penny dropped after I posted my last comment. kWh makes perfect sense. 

 

As a rather confused friend of mine once said, "It generates up to 4 kWh per hour." ?

[SteamyTea]

8 hours ago, Home Farm said:

In light of all of this, do installers take any of this into account

Yes as part of the MCS yield estimate.

Also, if modules are fitted on a flat roof, the the angles can be changed.

You get less module area ratio on flat roofs, do increasing yield is more important.

[H F]

Just checked our paperwork. Our roof inclination is 35 degrees. 

 

Since we’re most starved for solar in winter, would it have been a difficult exercise to make a frame that took us to 40 degrees? I assume from what’s been said here we would have increased our winter production a bit. 

[SteamyTea]

7 minutes ago, Home Farm said:

would it have been a difficult exercise to make a frame that took us to 40 degrees

Yes, mainly down to the wind loading and possibly snow loading.

Would look silly as well.

And would do nothing for the reduced hours of daylight.

Or the azimuth angles.

 

[joth]

Ours happens to already be 32° which is near as perfect for maximizing annual yield and I'm very happy with that. (Enerphit Plus rating is based on annual yield so that is convenient too)

 

 

[H F]

9 hours ago, SteamyTea said:

Yes, mainly down to the wind loading and possibly snow loading.

Would look silly as well.

And would do nothing for the reduced hours of daylight.

Or the azimuth angles.

 

Fair enough.

[H F]

8 hours ago, joth said:

Ours happens to already be 32° which is near as perfect for maximizing annual yield and I'm very happy with that. (Enerphit Plus rating is based on annual yield so that is convenient too)

 

 

Good to know. Thanks for sharing.

[joth]

1 hour ago, Home Farm said:

Good to know. Thanks for sharing.

To clarify, this is for my latitude (~51.8°) and orientation of my solar array (SW).

 

If you've not seen it, the PV estimator tool as an "optimize slope" option: https://re.jrc.ec.europa.eu/pvg_tools/en/tools.html#PVP

 

 

If you click to choose this option, PVGIS will calculate the slope of the PV modules that gives the highest energy output for the whole year. This assumes that the slope angle stays fixed for the entire year.

Edited November 21, 2019 by joth

[H F]

On 18/11/2019 at 17:56, Ed Davies said:

I think it's a combination of the angle of incidence and the longer path length of the sunlight through the atmosphere. Overall annual production from panels is reduced noticeably for more steeply mounted panels but winter production is severely clobbered by the shallow angles most panels are mounted at; they need to be mounted nearer vertically to optimize that.

 

For a point at the north end of the runways at RAF Syerston (between Nottingham and Newark - an arbitrary East Midlands point I happen to be sufficiently familiar with to pick as a point with a clear southern horizon) putting the panels at various angles PVGIS gives the following outputs:

 

35° -  Annual: 1020 hours, December: 35.8 hours.

75° -  Annual: 889 hours, December: 45.8 hours.

90° -  Annual: 752 hours, December: 44.4 hours.

 

One of the reasons people say that PV falls off a cliff in winter is that they only look at the output for relatively shallowly mounted panels. For that 75° case the month with the maximum output is April with 96.8 hours production so not much more than twice the December production.

 

Actually, for 75° the worst month for production is January at 43.7 hours. December is the worst month for 35° mounted panels. I guess that's to do with the mix of direct sunlight and indirect sunlight scattered from clouds.

Is there an optimal angle for panels in the uk they get the most out of winter months?

[SteamyTea]

It all depends if you want to increase the probability of maximum power (the W) at a particular time, or maximum yield (the kWh) over the winter months.

All you need to do is some fairly basic (but boring) trigonometry.

Start by getting the azimuth and altitude angles of the sun. 

Edited December 11, 2019 by SteamyTea

[H F]

Thanks ST. I’ve also gone on PVGIS and pulled data for my postcode. Not sure what I’m looking for in terms of relevance to my array. 

[H F]

24 minutes ago, SteamyTea said:

It all depends if you want to increase the probability of maximum power (the W) at a particular time, or maximum yield (the kWh) over the winter months.

All you need to do is some fairly basic (but boring) trigonometry.

Start by getting the azimuth and altitude angles of the sun. 

Is there a way of reflecting light back towards the panels during dark, short months.

 

i assume we’re not generating much power because the sun is a light lower so the angle of the panels isn’t great at this time of year, but much better when the sun is high in the sky.

Edited December 11, 2019 by Home Farm

[SteamyTea]

When the sun is at a low altitude, it is passing though more of the atmosphere, that reduces the power.  Not a lot you can do about that.

 

When modules were expensive, I looked into, and did a few experiments with movable mirrors, was still cheaper to just add an extra module.

 

If you have a ground mounted system, you could increase the angle, but you run the risk of getting shading.

alternatively, you could lay the modules flat to the ground, that can increase yield, but not power much.  Does collect more dirt on them.

 

 

[H F]

Wow. I’m impressed you did a mirror experiment. Interesting.

 

From all that’s been said, I guess we just have to live with our 35 degree system that matches the pitch of our roof.

[Ed Davies]

54 minutes ago, SteamyTea said:

All you need to do is some fairly basic (but boring) trigonometry.

 

It's more complicated than that. As you say the low angle of the sun causes it to travel through a lot more atmosphere so the light is absorbed and scattered more. One effect of the increased scattering, though, is that proportionally more of the light comes from the global radiation across the whole sky.

 

58 minutes ago, SteamyTea said:

It all depends if you want to increase the probability of maximum power (the W) at a particular time, or maximum yield (the kWh) over the winter months.

 

Exactly. PVGIS said I should put my panels pretty much vertically to maximize yield in January but I've actually designed my house so the roof they're on is at 60° on the theory that that will only slightly reduce the yield on the brightest days but will significantly increase the amount of scattered light received on the hazier days. Having more even output from the panels would be better in an off-grid situation but not so useful on grid.

 

I held a panel at various angles with a meter on the current range short circuiting it on a hazy day in February, could just see where the Sun was through the cloud, and found the angle which gave the most current. That was 60°.

[joth]

I love the enthusiasm here, but it  does start to feel like a case of "if it was that easy, everyone would already be doing it." ?    Besides anything else, to get meaningful energy transferred you would need a lot of reflection. This can cause all sorts of new problems from dazzling airline pilots to starting forest fires, and who knows what else. Not to mention, very expensive to purchase and install and complex to control that many movable mirrors.

That said, at commercial scale it seems it can work - to melt salt for energy storage rather than directly onto PV panels, as the focal point of the mirrors creates temperatures way higher than a PV panel would like.  They're quite stunning to look at.  I drove past one in the Mojave desert in 2016, which was first time I heard of the idea -- seeing the beam of intense ionizing light going into the sky over the horizon in the middle of the day, right on the Nevada border, we naturally assumed  aliens were landing. (OK not actually ionizing light, but in the shimmer of the desert it looked like the air was melting as the dazzling beams cut through it. It was painful to look at the focal point)

 

[Ed Davies]

It doesn't need to be that complicated. E.g., when I was thinking of nearer vertical panels I did consider putting a pond immediately south of the house to pick up a bit of reflection. Not sure how much difference that would make but it'd provide some extra light, I'm sure.