of the problem solving for my site has to do with the fact that none of
the roofs are south facing. As you can see, I had to build on to the
south end of my workshop to mount the solar collectors that I use to
heat the building. So I originally envisioned mounting the panels to a
2 axis tracking array. But that proved to be problematic, partly
because its size would make it a giant eyesore in the garden. I made a
composite image (at left) and decided that this would
By using a tracking array the modules always face directly toward the sun
increasing the performance by over 30% per module. But my friendly local solar dealer, Naoto
Solar Market, advised me against that
because the motors don't hold up well under heavy wind and snow load with a very
large array. Also
the number of panels I would need could not fit on the largest
tracking system available, so I would need 2 trackers at a cost of over
$8000 each. Even if I had room in the yard the money would be
better spent on panels!
Just to get a sense of scale
I built a model of my workshop in
SketchUp and then a simple scale model
of a tracking array. Wow, that thing is huge! And as shown
it would only meet about 2/3rds of my needs! This was my
first attempt at using
SketchUp and I could not figure out how to get
the sun positioned correctly - so it is shining from the north in these
images! The pain of learning new software - at least it's free!
my next thought was to install the panels on the roof, but since the
ridge line is oriented to magnetic north-south I felt it would be best
to try and tilt the panels up and mount them on both sides of the roof.
I got the sun position right and was able to move the virtual sun through the
daylight hours and seasons to evaluate the shading issues and to
optimally space the panels. This is a really cool feature of
SketchUp and I
learned a lot about the shading issues arising from the stove pipe. By
moving the sliders around I could watch the shadows move across the
model and see when there were shadows on the solar panels at different
times of day and year (see movie below).
While this design could work, it would have a lot of trade-offs such as
wind loading, not to
mention it makes my workshop look like a giant Space Bug!
Not shown in this model is the tree-lined road on the east side of the
building (near the car port) that would shade the east panels for much of the morning.
Notice that I refined the model with architectural details and modeled all of my
solar collectors. It took a few hours to learn the software, but
it's worth it to see a nicely rendered preview.
So then based on advice from Naoto (my
solar dealer) I decided to look at covering the entire west facing slope with
panels. This faces about 18 degrees south of due (solar) west, so
it is acceptable. Using the
PV Watts calculator I learned that 30 panels would achieve the
desired power rating and it seems that this is the best option for my
site. The panels have no shading from trees or other obstructions at
all. (We later decided to install only 21 panels to start with, based
on fiscal realities).
The movie below was taken from the SketchUp screen, by saving "scenes"
you can create transitions from one scene to the next. Here I do a
fly-around from noon to dawn on a June day, then show the sun arc from sunrise to sunset in June so you can see the
shadows changing and the solar exposure hours of the panels. If
you pause the movie and grab the timeline slider (at top) you can move
through time back and forth to watch the blue slider changing the time
of day as the movie plays.
The roofing shingles are curling up at the corners, indicating that they
are due for replacement. The building is nearly 20 years old and
the shingles were not high quality. Since it would be more
expensive to re-roof the building after installing panels I called some
local roofers for estimates. They confirmed that it would be wise
to re-shingle the roof now rather than wait since it is nearing the end
of its lifespan. The lowest bidder ($2800) offered to install
architectural shingles at no extra cost, and he will install them right
over the existing shingles. Architectural shingles are thicker and
more durable and easier for the roofer to install. I also chose a
light gray because lighter colors last longer and reduce the heat
buildup in the summer. For more information look at the
Cool Roof Rating Council site.
March 2013 update. As I have been adding panels, I have been
updating my Google SketchUp model to pre-visualize shading issues
before I install them. The image at left was created before I installed
the two panels tilted out on the South facing wall above my solar
collectors. I was able to learn that the new panels would not
adversely shade my solar collectors, particularly since I drain those
during the summer months.
January 2014 update:
A study released in November 2013 shows that west facing solar
arrays (in the northern hemisphere) are better at cutting peak electric
demand on the grid because demand peak falls later in the day than solar
peak in warm climates. This validated my decision to use the
available roof space.