Intentionally shading modules

[heynow999]

I know this sounds wierd but let me explain.

I have a buddy who has an old feed-in-tarrif contract here in Ontario, Canada. He can install up to 12kw according to the terms of his contract and recieve an amazing 80 cents per kw/hr produced. Unforunately he has a small roof. His house is a townhouse about 100 years old. He has about 15ft x 45ft of flat roofspace, maybe enough room for for 3.5 kw if properly spaced. A good angle for this latitude is between 25 and 30 degrees.

The idea is to really cram the modules on the roof so that in the few weeks around dec 21st they will shade each other. For an angle I think about 15 degrees would be good to still keep them self cleaning with rain and snow should still slide off. I could fit in an extra row for an extra 1.75 kw. In this area in dec we only have about 1.5 hours of sunlight in winter where we have over 5 hours in the summer. That is the rational for doing this. We will use micro-inverters.

Any thoughts?

[Cariboocoot]

I see what you're saying; the power production in Winter will be so poor anyway that the amplified loss due to angle shading isn't going to make much difference, whereas in Summer the additional panels allowed by the tight spacing will boost production significantly. Makes perfect sense, but have you work out an approximate $ cost/return? Some place on the forum there's some links to various design tools to examine just this situation.

Only suggestion I have is to go for monocrystaline panels, as they take up the least space per Watt and therefore would minimize the effect.

[niel]

not too sure if you mean 15 degrees from horizontal or vertical. in any case be sure the cramming of the pvs does not create a bad angle for good summer time production. it may actually produce more spaced out a bit to allow a better summer angle.

[inetdog]

I see what you're saying; the power production in Winter will be so poor anyway that the amplified loss due to angle shading isn't going to make much difference, whereas in Summer the additional panels allowed by the tight spacing will boost production significantly. Makes perfect sense, but have you work out an approximate $ cost/return? Some place on the forum there's some links to various design tools to examine just this situation.

Only suggestion I have is to go for monocrystaline panels, as they take up the least space per Watt and therefore would minimize the effect.

Without knowing more about the exact layout within each panel, the amount of overlap, and the orientation you will be putting them in, it will be hard to say whether the partial shade will still give you any output from the panels at all. If you shade just one entire cell from each bypass-diode-group in the panel, your output from that panel will drop to zero.

If it still works out to your advantage even if you calculate for zero output for part of the Winter, then you don't have to worry about it until you look for the best way to mount them.

Can you cantilever panels past the edges of the roof? Or would the neighbors complain?

[heynow999]

The proposed slope would be 15 degrees from horizontal.

We would mount the modules in landscape so they should still produce power even if one row of cells are shaded on the long side

Peter

[westbranch]

putting them in portrait should get the max winter output from the first row, which would still produce its max with ~ 2x the number of panels, but then can you get in all the rows. Maybe the back one elevated a bit in portrait?

hth

[ggunn]

I know this sounds wierd but let me explain.

I have a buddy who has an old feed-in-tarrif contract here in Ontario, Canada. He can install up to 12kw according to the terms of his contract and recieve an amazing 80 cents per kw/hr produced. Unforunately he has a small roof. His house is a townhouse about 100 years old. He has about 15ft x 45ft of flat roofspace, maybe enough room for for 3.5 kw if properly spaced. A good angle for this latitude is between 25 and 30 degrees.

The idea is to really cram the modules on the roof so that in the few weeks around dec 21st they will shade each other. For an angle I think about 15 degrees would be good to still keep them self cleaning with rain and snow should still slide off. I could fit in an extra row for an extra 1.75 kw. In this area in dec we only have about 1.5 hours of sunlight in winter where we have over 5 hours in the summer. That is the rational for doing this. We will use micro-inverters.

Any thoughts?

Do you mean that in the winter they will shade each other all day long or that they will only do it in the morning and evening? If they shade each other all day long in the winter they will shade each other for part of the day all year long. You need to look at the window, not just the sun's elevation at solar noon.

[Cariboocoot]

I don't know where the 15 degree angle came from. According to Macslab the optimum Summer angle for daily kW hour output is 25.5 The Winter angle is 63.0
Now if you set up a little diagram with the panels at 25.5 degrees and the sun angle straight on to the panels it will cast a shadow line behind the each row of panels at an angle. Where this hits the horizontal is the minimum distance between rows. Otherwise like ggunn said you'll be shadowing panels year-round.

45 feet of length is good. Ought to handle eight panels of about 200 Watts each in a landscape row. So 1600 Watts per row. How many such rows can you fit in 15 feet? Probably five maximum. Got to do some trigonometry to figure it exactly, and I'm not up to it. But even at 3 rows you'd have over 4kW. That should max out the 12kW hour daily allowance (4800 Watts of panel @ 77% efficiency * 4 hours good sun [would be longer in Summer] = 14.784 kW hours). Even 6 x 3 200 Watts would yield over 12kW hour with 5 hours sun.

Do yourself some scale models and run some PV Watts simulations.