PVs - summer vs winter

[H F]

On a comparable days, with blue skies and lots of light, what is the main reason why in summer the panels produce their full potential, while in November they do half? Is it the position of the sun in the sky and its consequent angle to the panels, or is there more to it?
 

In our case, we’d do 6kW in summer and only peaked at 3.5kW today.
 

Curious.

[Dreadnaught]

I would guess it is lower incidence angle combined with shorter days.

 

The panels being cooler will actually help the generation.

[MikeGrahamT21]

Angle is definitely a huge influencer. Length of day again a massive factor.

[SteamyTea]

3 minutes ago, Home Farm said:

why in summer the panels produce their full potential

Are they actually doing that, or just seem to.  Modules are rated at a particular light intensity i.e. 800 or 1000 W.m^-2. In summer, levels may be higher than that.

5 minutes ago, Home Farm said:

Is it the position of the sun in the sky and its consequent angle to the panels

Basically yes, and the sun has to travel through more atmosphere before it hits the module.

6 minutes ago, Home Farm said:

In our case, we’d do 6kW in summer and only peaked at 3.5kW today

Go and find your nearest WeatherUnderground weather station that has a PV monitor and look at the historic data.  It is measured in the horizontal, rather than the optimal angle.

Then go and look, and play about with PVGIS and see what changing the azimuth and altitude of modules would do to production.

Also, is the 6 kW and 3.5 kW really that, or kWh.

If kWh, then that is partly down to shorter hours of daylight.

[H F]

Sorry. kWh.

[H F]

Having said that, why is this a factor when trying to establish a comparison between two similar days in different seasons? Is it because the daylight time is less?

[SteamyTea]

5 minutes ago, Home Farm said:

Sorry. kWh.

Makes a big difference, so you are comparing probably 14 hours of productive daylight with about 8 hours today.

[SteamyTea]

2 minutes ago, Home Farm said:

Is it because the daylight time is less

This is why kWh can seem a bit confusing to a lot of people.

It is Power x Time, so even a lower power in the summer can produce a greater amount of energy.  Energy can be measured in kWh, though is really MJ (Mega joules).

There is a direct conversion between MJ and kWh, basically the time cancels out.  This is because a W is a J.s^-1 and an hour is 3600 seconds.

[H F]

5 minutes ago, SteamyTea said:

This is why kWh can seem a bit confusing to a lot of people.

It is Power x Time, so even a lower power in the summer can produce a greater amount of energy.  Energy can be measured in kWh, though is really MJ (Mega joules).

There is a direct conversion between MJ and kWh, basically the time cancels out.  This is because a W is a J.s^-1 and an hour is 3600 seconds.

The penny dropped after I posted my last comment. kWh makes perfect sense. At 3kWh I’d be able to run a 3kW appliance for an hour. 

[SteamyTea]

13 minutes ago, Home Farm said:

At 3kWh I’d be able to run a 3kW appliance for an hour. 

Yes, or 10 300 W loads for an hour, or a 30 W load, like the security light I have just fitted for my Mother, for 100 hours.

More interesting is how much water and air you can heat up.

Water takes 4.2 kJ.kg^-1.K^-1, air is easier to remember as it is 1 kJ.kg^-1.K^-1, but oddly, stone, concrete and brick are less at about 0.8 kJ.kg^-1.K^-1, pine is 1.5 kJ.kg^-1.K^-1.

Taking just water as an example, and 100 litres (or kg as it is the same) of water at 40°C and an incoming mains water temperature of 10°C

That would be:

 

4.2 [kJ.kg^-1.K^-1] x 100 [kg] x (40 - 10) [ΔK]

 

12,600 kJ

 

To convert from kJ to kWh multiply by 0.00027778

 

3.5 kWh

 

So today, your PV has given you a free bath.

 

(If you take the reciprocal of 0.00027778, you get 3,600, which is the number of seconds in an hour)

[Ed Davies]

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.

Edited November 18, 2019 by Ed Davies
Add PVGIS link

[SteamyTea]

14 minutes ago, Ed Davies said:

longer path length of the sunlight through the atmosphere

Here is an interesting challenge for you (I am a bit busy right now).

Does that change for an equal latitude in the Southern Hemisphere?  I am wondering how much difference the weather makes?

I could get posh and start talking about seasonal turbidity, but cloudiness is a better word.

The UK is particularly cloudy, but never looked at similar positions in the Southern Hemisphere (as it is mostly water), but maybe places in Russia and Canada that are at a similar latitude have very different figures.

[scottishjohn]

really you want the angle the same as  your  latitude with solar thermal  for summer,

and even steeper for winter due to low angle of sun --hanging vertical almost

54was my best summer angle 

 

[joe90]

For all the above reasons I quite fancy a ground array where I can alter the angle to suit the season.

[H F]

38 minutes ago, joe90 said:

For all the above reasons I quite fancy a ground array where I can alter the angle to suit the season.

Yeah. That makes sense.

[H F]

2 hours ago, SteamyTea said:

Yes, or 10 300 W loads for an hour, or a 30 W load, like the security light I have just fitted for my Mother, for 100 hours.

More interesting is how much water and air you can heat up.

Water takes 4.2 kJ.kg^-1.K^-1, air is easier to remember as it is 1 kJ.kg^-1.K^-1, but oddly, stone, concrete and brick are less at about 0.8 kJ.kg^-1.K^-1, pine is 1.5 kJ.kg^-1.K^-1.

Taking just water as an example, and 100 litres (or kg as it is the same) of water at 40°C and an incoming mains water temperature of 10°C

That would be:

 

4.2 [kJ.kg^-1.K^-1] x 100 [kg] x (40 - 10) [ΔK]

 

12,600 kJ

 

To convert from kJ to kWh multiply by 0.00027778

 

3.5 kWh

 

So today, your PV has given you a free bath.

 

(If you take the reciprocal of 0.00027778, you get 3,600, which is the number of seconds in an hour)

What’s your background SteamyTea?

[H F]

2 hours ago, 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.

I think ours we mounted at 30 degrees. 

[SteamyTea]

13 minutes ago, Home Farm said:

What’s your background SteamyTea?

Engineering and lecturing.

And catering and dropping out of the rat race at a decent age.

I could have invented the term FIRE, but I was too busy dropping out of mainstream work.

[Russdl]

I guess all the above is based on a southerly facing array? My (non connected array) faces just west of south, 210 degrees, and is at 40 degrees from the horizontal. In the (distant) future I plan some easterly facing panels (120 degrees). 

 

I’d read somewhere that 40 degrees was the best compromise for the southerly facing array (hence the 40 degree roof pitch) and that for an easterly facing array, 30 degrees was the optimum. I was quite surprised that 30 degrees up from the horizontal was the ‘optimum’ for the easterly facing panels, I was expecting more like 60 degrees. I could of course have misinterpreted the information and it should be 30 degrees from the vertical (thus 60 degrees from the horizontal) but I’m sure that’s the information I gleaned. 

 

Have I got that completely wrong?

Edited November 18, 2019 by Russdl
Pour spilling, agen!

[SteamyTea]

1 minute ago, Russdl said:

Have I got that completely wrong?

Possibly.

Though the majority of the generation will be in summer, so may make less difference than you think.

Go and play about on the PVGIS site and see what the numbers show.

 

[Russdl]

@SteamyTea that’s probably the site I used to get that 30 degree angle, too far from a computer to check, but I’ll revisit it when I get the opportunity. 

[A_L]

13 minutes ago, Russdl said:

I was quite surprised that 30 degrees up from the horizontal was the ‘optimum’ for the easterly facing panels, I was expecting more like 60 degrees

 

30°, particularly in summer, allows southerly (noon) sun to come 'over the top' while 60° hides the PV surface until later in the afternoon

[SteamyTea]

Also the modules may be getting the optimal amount of sunlight to reach peak efficiency.

Too much light just turns into thermal energy, and you don't want that.

[Ed Davies]

18 minutes ago, Russdl said:

I guess all the above is based on a southerly facing array?

 

Yes. And being off directly south hammers you more in the winter, too, as when the sun is above the horizon it's to the south. In the summer, if the array is, say, east facing you gain a bit in the morning to compensate for the loss in the evening but in the winter you just lose.

 

20 minutes ago, Russdl said:

I’d read somewhere that 40 degrees was the best compromise for the southerly facing array (hence the 40 degree roof pitch) and that for an easterly facing array, 30 degrees was the optimum.

 

This depends on what you're optimising for. If it's total year-round energy harvesting then those sound plausible numbers. However, spare electricity in the summer is less valuable than any you can get in the winter so the results might be different if you're trying to find the optimum energy value.

[Ed Davies]

26 minutes ago, Russdl said:

I was quite surprised that 30 degrees up from the horizontal was the ‘optimum’ for the easterly facing panels, I was expecting more like 60 degrees. I could of course have misinterpreted the information and it should be 30 degrees from the vertical (thus 60 degrees from the horizontal) but I’m sure that’s the information I gleaned. 

 

That sounds reasonable as the panels will then be making more from indirect light scattered from clouds or just the blue sky above through the rest of the day.

Edited November 18, 2019 by Ed Davies