Lifetime Fuel Economy: 39.49 mpg

Wednesday, August 5, 2009


I've been contemplating this mod for a while - about 6 months. I was waiting to figure out the alternatorless stuff so it would actually be coupled with improved fuel economy, but I'm confident that I'll get that figured out soon, and my free time is running out fast with the school year approaching. Enjoy.
For starters, I had to clean up the roof.
I got this $45 piece of plexiglass for $25 at Lowes thanks to the damaged chunk. I thought this was a terrific deal until later when I noticed it is about twice as thick as the pieces I used on the Kammback, making it stiffer and more likely to split... Argh. I think I'll have to re-do this with a different material after this winter - time will tell.Plegiglass, cut to the roof's shape:Removing the remaining pieces of protruding roof-rack materials that I didn't delete yet - they're the loose pieces atop the roof.The roof's ready - or not.
I decided to paint the top white for three reasons.
1. Extra insulation from the bottoms of the conductive panels; there were bolt-holes and some surface rust on the roof.
2. Keeping the roof cooler in the sun - like school busses and mini coopers - for me, for the panels (they work better when cool), and to slightly reduce the Earth's albedo. :)
3. The silicon caulk that I plan to use to seal the plexiglass against the roof is white, so matching the roof will improve the W.A.F. (Wife Acceptance Factor)
The sandpaper to roughen up the surface:
The roof, lightly sanded:Taped off and ready to paint:Primer painted on:White paint:Now for the fun part - Solar Cells!
I bought a bunch of tabbed solar cells on Ebay in March for a project my students were doing in class. The auction was for 108 cells, way more than they could afford, but I assured them I would use the extra.
They run $2.50 apiece and are rated at: 3.7A, 0.5V apiece (1.85W) under maximum sunlight. Each cell has 2 tabs attached to the top side and two rows of solder points on the back. To attach them in series, you lay one cell's tabs atop the next cell's back, and solder at the solder points. I made strings that were 14 cells long. I was planning on doing 15 cells, then putting 2 strings together for 30 cells * 0.5V per cell = 15V for a good input to the charge controller I haven't built yet. My roof was slightly too short, so I'll probably have to put more cells on the Kammback to get the voltage up enough. I ran electrical tape across the tabs and put a dab of silicon in the center of each cell. (One string - ready to install, other - just soldered)

Laying out the strings on the roof. Yes, I realize I have 5 strings, and I can only use 4 with my plan to get the voltage I desire, but I figure if I do this, I'm going all out and I may want to put panels elsewhere later. This was my downfall; if I had only used 4 strings, I could've used the size available of the thinner, more flexible plexiglass.
I used bricks to hold the setup down overnight. Later, when I was screwing it down, I used about 15 bricks to hold the plastic to the roof's slight curvature. It helped to leave the protective peel-off plastic on until the end. (just had to remember to remove the underside when I was ready to screw it on)
Finished product photos: The back of the roof - notice one string of cells has a bit more space behind it than the other. I scooted it forward and added my homemade desiccant. I was worried about condensation on the inside of the glass looking horrible, decreasing performance, or damaging cells. I wrapped up some of my daughter's dry rice cereal mix inside a paper towel, and voila! Moisture trap! The visible wiring:More top pics:

My wife actually kind of liked it - she said it was "nice". First thing she's ever liked on this car! Now: Performance data: (preliminary)
In the shade (only using 4 of the 5 strings): 14V, 1A (14W)
Pushed the car into the sun at 2:45PM: 14.5V, 5.4A (78W)
So the whole roof is worth about 100W for probably 4-5 hours in mid-day sun after all wiring is eventually done. Cool.


David said...

AWESOME! I, too, am impressed at how nice it looks.

By the way, the highways in northern kansas city are RIDICULOUS!...especially in traffic!

b_cole06091 said...

Good sourcing on the cells. With a bit of creative positioning, and more work tabbing, higher voltage strings might have been possible, but this is quite fine. White membrane roofing material would likely have been a good underlayment - cushioning, no need to paint, and the possibility of making a moveable/removable array. I'm wondering how the cells will tolerate snow/ice loading, i.e. if there will be cracking. Good approach, and even sweeter once the alternator drag is minimized or eliminated. There are many avenues to explore in the effort to eliminate the alternator. This is a good use of roof-mounted solar.

BeetleDozier said...

can you talk a little more about the results?