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This section covers the thinking that went into the mount design, and the construction and installation of the PV panel mounts.
Why we chose ground mounts in the first place is covered here...
The goals for the mount system were:
- Strong enough to withstand very strong winds. Even though this is normally a low wind area, we do occasionally get storms with high winds. I did not want my PV panels to end up in the next county. - A long life. The panels will probably last for 30 years, so it seems like the mounts should last just a long. - A simple, uncluttered look. - As cheap a system as meeting the above goals allowed.This is all highly subjective, and there are no doubt lots of good ways to get to a good mounting system, but this is what we did. We considered buying a metal racking system. Several of these are offered. We decided against this partly on cost grounds and partly because I like the look of a simple wood rack better than a forest of metal poles. But, the advantage of going with a metal rack would be that you get a pre-designed system that should go together easily and hold up well -- these are advantages worth considering. Either way, you have to dig and pour the concrete footings, which is the biggest job. I settled on the simple treated lumber support rack that is made using relatively heavy (4X4) members spaced fairly far apart. I thought that this looked cleaner and would be easier to build than a more elaborate trusswork of smaller members. The PV kit that I bought came with IronRidge aluminum rails to mount the PV panels to. This is a very nice system which does a good job of securing the panels and micro inverters. Standard IronRidge hardware that came with the kit was used to secure the PV panels to the rails, and to splice the rails together. There are other manufactures of similar systems.
My first thought was to make sure that the rack was level across the full width, but the ground is slopped enough that this would have raised one end of the rack about another foot of the ground. So, instead, I established a line that was 12 inches off the ground on the east end and the west end, and connected these with a straight line. So, while the rack looks level, it actually slopes down about 1 ft over the 33 ft length.
Its downhill toward the house, so the electrons have gravity on their side.
I considered building the mounts so that the panel tilt could be adjusted by time of year -- steeper in the winter for the lower sun and less steep in the summer for the higher sun. I ran PVWatts for tilts of 30 deg, 45 deg, and 60 degrees, and then picked the highest output months for each tilt. I compared the yearly output for multiple tilts to the output for a constant tilt all year of 45 degrees. The increase in yearly output was very modest. The extra complication of a variable tilt mount and the ongoing chore of changing the tilt several times a year did not seem worth it to me, so I settled on a fixed tilt of 45 degrees -- this allowed for a very simple and strong structure.
Again, this is all rather subjective, and there are probably lots of good ways to do it. The important thing (I think) is to make sure the mounts are strong enough to stand up to your winds, and that the type of construction you use has demonstrated a long life in your area.
In order to have a good platform to mount the aluminum rails on, it was necessary to make sure that all 6 of the 4X4 support frames were in good alignment with each other. The L brackets used to mount the aluminum rails to the supports have slotted holes that allow for about a half inch of adjustment, but that's it.
All six support frames need to be carefully aligned along the surface that
the rails will be attached to.
Once the dimensions of the support frames were established using the little shop mockup shown above, I cut and built all 6 frames.
Click on pictures for full size
All six frames ready to go. The scraps of wood
hold the angle right until the ends are in concrete.
The next step is laying out and digging the holes for the concrete footings. In all 12 footings are needed -- one for the back of each frame, and one for the front.
The back (vertical) frame member is embedded in the concrete about 20 inches. The front frame member (the angled one) is attached to the fitting via a galvanized U strap that is embedded in the concrete. As shown just below.
I used stakes and strings to get the footings in the right place.
In order to make it easier to get the frames lined up, we put a temporary support along the back edge of the frames as shown in this picture. Its important to get this frame in with the 2X6 at the right height and straight over the full 33 ft width -- each support frame is temporarily screwed to this 2X6 guide for alignment until the concrete sets. I think that without some kind of guide structure like the 2X6 one shown, it would be a nightmare to get all 6 frames lined up properly.
This picture shows the temporary 2X6 frame that supports the back of each of the frames until the concrete is poured and set.
Note the small block of wood screwed to the back face of the vertical frame member -- this allows the frame to be easily placed at exactly the right height.
The two end frames were installed with quite a bit of care to get all the heights and angles correct. The 45 degree bubble on the level gets the right angle and the block of wood on the back of the vertical member setting on top the 2X6 temporary support gets that frame height right. Screwing the vertical 4X4's to the 2X6 guide insures that all the frames are in a straight line.
Pour the concrete for the two end frames and let it set so that these frames are solid, and can be used as a guide to set the other frames.
The footing holes are about 12 inches in diameter, and about 30 inches deep. I have a post hole auger for my tractor, but this proved to be worthless in the hard, dry soil. I ended up running two trenches with the rental trencher along the line of the foottings. This gave me a 3 inch wide start on each of the footing holes. I expanded them out to around 12 inches by hand with a shovel, iron rod, and post hole digger. This worked well, but you do have to really pack in the dirt around the footings where the trench is after the concrete sets -- I used the heavy iron rod with a mushroom end for this.
click on pictures for full size
In some cases, I used the Sonotube cardboard forms, but mostly I just poured the concrete directly in the holes.
My neighbor lent me his small concrete mixer, and that save a lot of time -- thanks Don!
In all, 3200 lbs of dry premixed bags of concrete were used, with the back footing holes getting a little more than the front ones.
After the two end frames were in and solid, I stretched a string between the two end frames at the positions where the aluminum rails would be mounted, and started to mount the middle 4 frames. With the back 2X6 frame to guide and support the middle frames, this all went pretty fast, and the end result was that all the frames aligned well.
click on pictures for full size
All six frames in
and aligned with footings poured. The straw is to protect the
concrete from expected night time freezing conditions.
So, this system worked pretty well, and the mounts seem very solid, and well aligned. Cost was roughly: treated wood $100, concrete $130, other miscellaneous $50 = $280ish
Update: September 2013:
There have been some comments on potential life problems with mounts made from treated lumber, so here is a report on how things are going so far, types of wood treatments and expected lives, maintenance, and a potential design detail change in the mounts....
These are the dimensions of the mounts for my system. You will, of course, have to adjust them for your PV panels and situation.
|Click on pictures for full size|
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Gary November 20, 2009, February 15, 2011