Cloud cover and orientation

[DrSmile]

I read an interesting statement here

NASA GISS: Science Briefs: Cloud Climatology: Distribution and Character of Clouds

Specifically

"Over land there are more clouds, with higher reflectivity, in the afternoon."

If I am to take this at face value, should it not be possible to make more overall annual solar power with panels facing slightly East instead of true South? I'm wondering if anyone has any hard data on this as obviously there would be a point where the increased morning production would be countered by lower peak production. I wonder if something like 170 degrees might be optimal.

 

[JohnRatsey]

If you've been in the tropics than you have probably noticed the tendency for days to start sunny and turn progressively more cloudy. This probably skews the global average. However, in some other parts of the world the weather is more random. I live in old England where the weather is best described as changeable depending on what atmospheric disturbance is coming from where (the jet stream is a big factor). My panels are facing just east of south but that's because it's the orientation of the house and roof. It just happens to be close to optimum, not on account of the average cloud behaviour but various trees which obscure the sun during the winter months.

 

[DrSmile]

On an island like England I can see that being the case, but I'm here in New York City I would venture to bet that there is more cloud cover in the afternoons on average. I'm just curious if there is any data to back that up.

 

[getakey]

South is going to be better most of the time. It gets exposure most of the day. What about summer in NYC? Seems that many afternoons are sunny

 

[JohnRatsey]

You could use the output of existing PV arrays - see the discussion here about how to find users who are uploading their data to PVoutput. However, you'll have to look at a load of plots for individual days for several sites to develop an opinion about whether mornings are better than afternoons for solar generation.

 

[getakey]

Maybe check households in Hawaii.
Her in Calif, we rarely get any clouds in the summer. In the winter, its usually cloudy all day long with rain or patchy or completely sunny as it has been all Feb, but nothing extra in the afternoon. I have arrays on my east facing roof and arrays on my west facing roof. Used to be equal amount and my production curve was a bell curve virtually identical on both sides. I have since added more on the east side only because the west was maxed out due to the roof construction

 

[jrweiss98020]

Here around Seattle, we get morning fog on lots of otherwise-sunny days. I think that would go against the headline...

 

[Ampster]

I wonder if something like 170 degrees might be optimal.

The best answer would come from doing some simulations on PV Watts at various azimuth. PV Watts does use some general weather conditions but in some locations I have lived in on the coast of California, early morning clouds can vary significantly within 2 miles of different locations. I don't think PV Watts is that granular so you may have to add your own weather knowledge.

 

[DrSmile]

The best answer would come from doing some simulations on PV Watts at various azimuth. PV Watts does use some general weather conditions but in some locations I have lived in on the coast of California, early morning clouds can vary significantly within 2 miles of different locations. I don't think PV Watts is that granular so you may have to add your own weather knowledge.

Thanks for that info! I graphed the results from my location for a 13KW system, here is what I got:

Interesting and at the same time quite disappointing, I'm very surprised at the broadness of the curve, from 160-200 degrees the annual production estimates vary less than 1%. The peak is in fact slightly East at 178 degrees but with a difference of only 1KW from 180 degrees it is meaningless (less than 0.01% of total production). The East/West difference is slightly more pronounced at less optimal azimuths, but even there it is hardly significant (less than 1% at 110 vs 250 degrees). Even with panels facing true North the production estimate is over 11,000 KWh, or roughly 2/3 of South facing panels.

I'm not sure how much weather factors into the PV Watts calculations, like you said it does seem to take it into account somehow, but really I'm not sure if the East/West differences are in fact due to the weather info or some other factors like precession or nutation.

 

[Ampster]

What was the angle of tilt relative to the latitude of your entries in PV Watts? I imagine as you make it more horizontal it becomes flatter until totally horizontal there is really no azimuth.

 

[JPP]

Slightly off topic...in NoCA we deal with PG&E. They have shifted the peak time (on TOU plans) later in the day (to like 3-8 pm or later). So it becomes advantageous to consider mounting panels facing west to get more solar later in the day. FWIW when SolarCity (now Tesla) did our 2013 install of 64 panels (16kW) our south roof was not usable. We have 4 strings split between east and west. Works great.

Oh and now I have 2 Powerwalls, so this matters less as I can time shift and arbitrage....

 

[DrSmile]

What was the angle of tilt relative to the latitude of your entries in PV Watts? I imagine as you make it more horizontal it becomes flatter until totally horizontal there is really no azimuth.

20 degrees, which is my roof angle.

 

[aesculus]

Oh and now I have 2 Powerwalls, so this matters less as I can time shift and arbitrage....

Yes. With Powerwalls this becomes moot but without them, facing west might not net you the most power but the most money as utilities start to push peak periods out further and further.

See this thread:

Why are we limited to selling solar energy in real-time only?

 

[Dan123]

I read an interesting statement here

NASA GISS: Science Briefs: Cloud Climatology: Distribution and Character of Clouds

Specifically

"Over land there are more clouds, with higher reflectivity, in the afternoon."

If I am to take this at face value, should it not be possible to make more overall annual solar power with panels facing slightly East instead of true South? I'm wondering if anyone has any hard data on this as obviously there would be a point where the increased morning production would be countered by lower peak production. I wonder if something like 170 degrees might be optimal.

I think you are talking about 1-2% difference. In most cases, people don't have a choice and have to put it on their rooftop. Another reason why east is slightly better, is because temperature is lower in the AM.

On other hand, in cases with TOU electric rates, afternoon power is more valuable than morning power.

 

[charlesj]

Thanks for that info! I graphed the results from my location for a 13KW system, here is what I got:

View attachment 514179

Interesting and at the same time quite disappointing, I'm very surprised at the broadness of the curve, from 160-200 degrees the annual production estimates vary less than 1%. The peak is in fact slightly East at 178 degrees but with a difference of only 1KW from 180 degrees it is meaningless (less than 0.01% of total production). The East/West difference is slightly more pronounced at less optimal azimuths, but even there it is hardly significant (less than 1% at 110 vs 250 degrees). Even with panels facing true North the production estimate is over 11,000 KWh, or roughly 2/3 of South facing panels.

I'm not sure how much weather factors into the PV Watts calculations, like you said it does seem to take it into account somehow, but really I'm not sure if the East/West differences are in fact due to the weather info or some other factors like precession or nutation.

My buddy doesn't want to expose panels to the front of the house where it would be visible and the company I recommended for he said place it on the north side. I was surprised to see the small difference, small for him, not me, by placing it north.

Come to think of this, I wonder if that calculator web site takes into account of also having a negative incline degree as well as north orientation.

 

[Dan123]

My buddy doesn't want to expose panels to the front of the house where it would be visible and the company I recommended for he said place it on the north side. I was surprised to see the small difference, small for him, not me, by placing it north.

Come to think of this, I wonder if that calculator web site takes into account of also having a negative incline degree as well as north orientation.

North orientation should be a pretty significant difference. It depends on the pitch of the roof. But even a 3/12 pitch should result in at least 30% decline of annual production.

For example, in Miami, on a 15% roof slope, 10kw of panels facing south will produce 15500 kwh, and the same 10kw facing north will produce 12700 kwh. If the slope is 20%, then north will produce 12000 kwh.

This difference will be more pronounced in the winter. In the summer, north facing panels might produce the same amount of energy. But in the winter, they will probably produce less than 50% of the southern panels.

 

[charlesj]

North orientation should be a pretty significant difference. It depends on the pitch of the roof. But even a 3/12 pitch should result in at least 30% decline of annual production.

For example, in Miami, on a 15% roof slope, 10kw of panels facing south will produce 15500 kwh, and the same 10kw facing north will produce 12700 kwh. If the slope is 20%, then north will produce 12000 kwh.

This difference will be more pronounced in the winter. In the summer, north facing panels might produce the same amount of energy. But in the winter, they will probably produce less than 50% of the southern panels.

Yep, winter is the killer as it already has shorter days and lower sun.

I was just wondering about that roof pitch if the calculator web site know it will be a negative angle on top of orientation, a double wammy.

But my buddy would be ok with that as for some reason aesthetics is more important.

 

[Dan123]

Yep, winter is the killer as it already has shorter days and lower sun.

I was just wondering about that roof pitch if the calculator web site know it will be a negative angle on top of orientation, a double wammy.

But my buddy would be ok with that as for some reason aesthetics is more important.

PvWatts does know that the pitch is negative.

Aesthetics can be improved by all black panels.

 

[charlesj]

PvWatts does know that the pitch is negative.

Aesthetics can be improved by all black panels.

Thanks.
He won't listen