Finally got around to mounting our large PV panel to the roof, and the routing of the wires from the PV panel to the Charge controller. All the details below.
See the main Electrical Page for more on the full electrical installation.
We decided to use one large PV panel (325 watts) rather than two or three smaller ones. Its a bit more awkward to mount the big panel on the roof, but it seems like it makes for a cleaner and simpler installation with fewer holes in the roof to use the single panel, and no need for roof top wiring of multiple panels together.
See the main electrical page for the details on how to pick a large PV panel and a compatible charge controller.
Lining Things Up
I decided to keep the PV panel low to the roof along the idea that this would minimize aero drag due to the PV panel. There will be a hit for this on the power output of the panel as it will get less ventilation, run hotter, and be a bit less efficient.
Wanted the PV panel to clear the center of roof by only 1/4 inch. The picture above shows determining how much the brackets have to extend below the PV panel frame on the edges to get the 1/4 inch clearance at the center — about 3/4 inch.
After reading a number of descriptions on PV panel mounting and looking at some of the commercial mountings available, we decided to go with some simple, homemade brackets.
The brackets are made from some left over aluminum channel from the old Boeing Surplus before they closed it. I cut it down the middle to make two sets of 90 degree mounting brackets (see pics). These are likely heavier duty than required, but they work fine. I made the brackets on the long side (4 inches) in that I wanted the PV panel to be held in place primarily by adhesive and the longer brackets provide more bond area.
I installed the brackets on the PV panel in the shop. Each bracket is screwed to the PV panel frame by two #14 (1/4 inch) stainless steel sheet metal screws. The frame of the PV panel is a box section, and the screws go through both walls of the box section.
Mounting the PV Panel
The brackets were mounted to the PV panel frame in the shop and before placing the PV panel on the roof.
The pilot holes through the brackets are about 1/4 inch (to just clear the full diameter of the screws), and the pilot holes through the PV frame are about 3/16 to provide good holding power for the screws. It would be a bit more desirable to use 1/4 inch bolts and nuts with lock washers, but once installed, it would be nearly impossible to remove the bolts with the PV panel so close to the roof.
The panel is big, awkward, and breakable making it a challenge get onto the roof of the van. I ended up using the tractor bucket with extensions on it to get the panel in place, but two people would not have much trouble getting it up working from ladders. I tied pieces of towel to the mounting brackets so that they would not scratch the van paint.
In attaching the PV panel to the roof, I had originally decided to use only adhesive. I read about a build in AU in which SikaFlex adhesive sealant only had been used to hold the PV panel to the roof and it had held up fine to a rough trip around AU. But, in the end I chickened out and decided to use one #14 (1/4 inch) stainless steel screw in each bracket along with the SikaFlex. While I expect that the SikaFlex will be doing all the work, the stainless screw provides some backup, and it also acts a clamp to compress the bracket into the SikaFlex. So, four 1/4 inch holes in the roof won’t be so bad to seal up if for some reason the PV panel has to go.
Update: Here is an build in which the solar panels are held in place with 3M VHB tape and then sealed over with Dicor sealant. They say that AM Solar has been using this method for years and has not had any problems.
I’ve made some small test brackets glued with SikaFlex and also with a polyurethane adhesive that I plan to pull apart in a few weeks and measure the failure force. I’m waiting a long time because the adhesive in from the edges cures very slowly — its a water vapor cure, and it just takes a long time to cure when gluing materials that are not permeable to water vapor. A two component adhesive that cures when the components are mixed would really be better for this job. I’ll report on how the test goes.
After getting the PV panel in place, I sanded the bottom of the bracket lightly, and then cleaned paint and bracket with Acetone.
I then applied the SikaFlex under the bracket. Note that the bottom of bracket surface is not parallel to the roof metal, so the thickness of the SikaFlex is greater on the outside of the bracket than on the inside. This means that the adhesive used should be able to accommodate this variation in thickness of the glue line (or you could bend the brackets to get a parallel fit).
I sprayed a very fine mist from the water bottle in the area just before tightening the screw down on the sealant — the hope being that this would help the cure go a little faster. The sealant cures around the edges in a day or so, but I expect that the cure in the center will take quite a while.
The #14 stainless steel screw is then tightened to bring the bracket into close contact with the van roof. Then smooth out the SikaFlex that oozes out to seal around the bracket.
I do plan to add some beads of the SikaFlex at the points near the center of the PV panel where the panel is 1/4 inch from van roof. The idea is just to keep the panel from vibrating or striking the van roof.
Running the Wires
I was able to run the wires through the backup camera housing and into the van — so, avoided another hole in the roof.
The PV panel had standard MC4 connectors on it. I bought a 50 ft long MC4 extension cable on Amazon and cut it in half to make the two wires that come down from the PV panel to the charge controller. The two weather proof MC4 connections that connect the PV panel wires to the MC4 extension cable wires are the only outside connections. And, the MC4 extension cord halves were long enough to reach the charge controller, so no splices inside the van either.
The MC4 wire is #10. At the full 315 watts (very optimistic), the PV panel output is about 36 volts, so the current through the #10 wire is about (315 watts/36 volts) = 8.75 amps. The voltage drop for the 25 ft of #10 wire is 0.44 volts, or 1.2% — this seems fine. This is another advantage of the larger, higher voltage PV panel compared to multiple 12 volt panels — smaller gauge wire can be used to connect between the panels to the charge controller. One not advantage is that the charge controller for this kind of panel is more expensive — see the electrical page for charge controller details.
I was able to run the wire through the backup camera housing, and then into the van using the same opening as the camera wires go through, then along the space above the back doors over to the wall, then down the middle of the sheet metal vertical post at the van corner, then forward through the bed platform to the charge controller. A little fishing, but pretty easy.
I used an electrical fitting from Home Depot that has a rubber gasket to seal the hole where it enters the camera housing. The other end was a fairly snug fit for the two #10 wires and was sealed with the SikaFlex sealant. I also added a bead of silicone caulk along the top of the camera housing where it joins the fan — it did not seem all that secure and I thought the silicone would reinforce it some, but would also be relatively easy to remove if need be.
The PV panel hooked up to charge controller — PV suppling 186 watts and 14.2 amps charging current. The charge controller is maintaining a charging voltage of 13.2 volts.
I would like to work out some sort of aero treatment for the PV panel that reduces the drag increase that it causes. Perhaps a fairing at the front that brings the air smoothly up to the top surface of the panel. Also thinking that an about 6 inch long sheet of aluminum might be mounted across the back of the PV panel with vortex generators mounted on it — this might help to reduce the separation at the back of the van?
Any ideas on this are very welcome.
August 7, 2015
Comments, Questions, Suggestions?