Monday, November 3, 2008

Solar electric

I made some progress on my solar electric system - I built a rack to hold the panels.  I have the same modules as KP and the Skipper have out on the Island:  BP Solarex MST-43.  They had gotten their rack from Power Fab and gave me the part number for the rack to match these panels, but when I asked Power Fab for a price, they ignored that, asked for all the module specs, and came back with a quote of $2770.  For a rack! This was more than twice what I paid for the solar panels!  Clearly this was custom, hand-made-by-union-Ph.D.-plumbers pricing.  I was so disgusted I didn't even call them back but I thought about saying something like, you did hear me say rack, not crack or rock, right?  Mr. Universe joked that it would be cheaper to buy a Cadillac and bolt the panels onto to it - it would be long enough to hold all seven, and heavy enough not to blow away.

So for one-fortieth of that price I built my own rack, out of wood and deck screws because that's what I know.  This was a solid three-day project.

I designed the basic cross section in Sketchup, for a 60 degree angle.  The instructions for the modules recommended latitude-plus-fifteen-degrees tilt for this location.  They also recommended using the middle set of mounting holes (on 2-foot centers) rather than the holes at the corners.   The back leg of the rack is also at a 60-degree angle.  This suggests an equilateral triangle shape which I thought would be pleasing.
I worked out the 3-d details and added extra bracing as I went along.  It's made in two eight-foot sections and the center module spans over the joint.  
The basic strategy was to screw horizontal crossbars tucked inside the rails on the back of each module, and then to screw those crossbars to horizontal rails on the rack.  I figured this would be pretty forgiving of inaccurate construction and wavy wood.

Pocketing the face of rails for the screw heads.
I left a good inch between modules.  This helps it look okay even though the panels are not laser-parallel, and gave me space to clamp.
Because of wavy rails I had to shim here and there.  
It's ballasted with 4x4 timbers and concrete blocks.  I don't know if this is enough.  I positioned it as close to the house as I could in hopes of reducing the wind load.  There's no gutter on the front yet, the drip line is right above the concrete blocks.

I would have preferred to paint or stain it but I settled for caulking all the glue joints.  Now it's ready for some grounding & wiring.  

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