Solar Array Tilt

Re: Solar Array Tilt

mike90045 wrote: »

I opted for more of a winter optimized tilt, because summers have longer days.

Mike (or others): for a traditional fixed mount that you
"set and forget" after initial installation:

I don't understand the rationale for doing the winter tilt.
Won't that dilute your summer harvest, when you have
the most potential production? Yes, it would increase
winter harvest, but increasing the larger summer amount
to start with (including at the higher summer electricity rate)
would capture more dollar value it would seem to me.

I note that the referenced Tilt Paper also advises
doing a winter mount.

--

I am debating making a change to my own recently
installed array. My roof is aligned 30 degrees west
of true south at Lat 37.5 degrees. We placed one
facet of five panels at this azimuth tilted at 20 degrees
(same as roof line). We placed the other facet of eight
panels on my north roof, and tilted them up 30 degrees
from the roof, for an effective 10 degree tilt. [See
photos below.]

[EDIT...roof angle is 20 degrees, corrected from earlier
post which had 30 degrees; 5-panel set is 20 degree
tilt angle, 8-panel set is 10 degrees tilt angle.]

Because the two facets are physically close to each
other, have the same azimuth, and are fairly close in tilt
angle, I figured that they would perform about equally.
Actually, going in we thought the 5-panel set should be
slightly better than the 8-panel set. Neither expectation
came out exactly correct.

Starting about 2:30 pm in the afternoon, the power
generation on the 5-panel starts to drift downward
from the 8-panel set. The divergence reaches 20 watts
per panel around 5 pm, then both sets converge around
6 pm and uniformly diminish to zero as the sun sets.
This is a 10% reduction over a 3-4 hour period, or
an effective 3% loss on a daily basis.

The change I am considering would be to the 5-panel facet,
and could be one, or both, of the following:

1. Lift the bottom edge of the facet by, say, 6", to
have the facet be more perpendicular to midday
sun. This reduces the tilt angle by 5 degrees.
2. Lift the left (west) edge by, say, 6" to have the facet
be more southernly facing.

I wouldn't want to go more than 6" for either of the
mods, to keep the facet's appearance somewhat normal.

My contractor advises against the changes. First he thinks
that the 5-panel facet, being installed closer to the
roof than the 8-panel set, is suffering from heat effects.
Second, when he ran PV-Watts, it shows that flattening
actually reduces the annual production.

Lastly, if I have the contractor do the work, it will likely
cost at least $200. If you assumed the potential gain
from one or both tilt angle mods is a best case 10%, the
gain over 25 years is $200 at current electricity rates,
i.e., no net dollar gain. That being said, perhaps I could
do the work myself, in which case, the only cost would be
new rail mounting parts.

Net net, the economics are marginal, but if I am going
to do it, I might as well do it now. Any suggestions?

Re: Solar Array Tilt

Summer, batteries are full by noon. Winter, still needs daily genset time, so winter angle is much more important, as in summer, the clear skies and long days do pretty well. I'm thinking of possibly adding a small tracker array for winter, but that will be a few more years in the future.

Re: Solar Array Tilt

Niel, thanks for your comments! I sense that your overall
strategy can be interpreted as follows: summer light is
so long and strong, that no extra help is needed; with
short duration, weak winter light, you should do everything
you can to maximize energy capture.

niel wrote: »

i don't think i'd have mounted the pvs quite the way you did. the ones in the back, or north facing roof, are apt to be shadowed at some point by the peak of the roof. now for that set of pvs i would increase the tilt on them as they are nearly flat...

The potential roof peak shadowing has not been a problem, as
the majority of the panels have been clearing the roof peak
by 8:30 am. Even if we moved them a couple feet closer to
the roof peak, there is a line of trees across the street that
blocks the direct sunlight until 7:45 am.

At only 10 degrees, these are pretty close to being flat,
but the contractor advised against a steeper tilt to
avoid them becoming "sails" during high wind conditions.
The rail manufacturer ran a wind load analysis on the config
to confirm that the 10 degree tilt was safe.

The combination of tilt and distance from the roof peak
was influenced as well by my requirement that the north-
roof-mounted PV panels not be visible from the front of
the house. We also had to contend with the existing
roof vents, though we could have moved them if push
came to shove.

niel wrote: »

as to those 30 degree off south westerly facing pvs if you raise the front pvs for a better angle then the whole back row will need raised up at least that much to stay out of the shadow of the front pvs, but if you can do it even some then you will benefit some from it, especially in the winter.
hope i'm not confusing you with this as we are looking not for peak power, but overall power production. pv angles of latitude +10 to +15 degrees offers the best overall imho.

Just by way of clarification, when I described changing
the tilt of the 5-panel facet, I meant changing the entire
facet's tilt. In other words, think of it as one solid plane
that you are lifting from the bottom (or left side). Thus,
there would be no shadowing of one panel by another. To
implement this type of "facet tilt," I'd need to have
increasingly taller legs for the rails as they go down the
roof line.

I am a bit confused as to whether you would recommend
either or both of the tilt mods. Lifting from the bottom
is an attempt to make this facet's performance closer to
the other facet's. Lifting from the left side is meant to
get the panel oriented more true south and increase
harvest efficiency. I think either change would be
beneficial and would not detract from the esthetics.
I am tempted to experiment with one panel to see if
changing either the up/down or left/right tilt would be
worth the work to change the whole facet.

Re: Solar Array Tilt

stephen,
it doesn't matter if you are off grid or grid tied as max overall production will be the same. he may have a problem anyway with his indicating shading that may occur.

john,
ok i understand the front pvs as a whole you wish to reorient. go for it and try at least your latitude (37.5 degrees) and try to get it more pointed south. if you can place the other pvs on the north facing roof so that they are tilted similarly towards the southerly direction with at least the same latitude angle it should help your production.
can you try to verify outputs now and after adjustment? you want to get an idea of the improvement you'll get.

Re: Solar Array Tilt

stephendv wrote: »

It will likely be different, because off grid users would orient and size their PV for the worst solar months, not the best months. So assuming a static array, off grid users will likely have theirs at a much steeper angle than grid tie users to maximise winter production.

Most definitely! The winter solstice drives most all design! If you ever watch a dual axis track (anytime of year) you will learn quite a bit about the sun!