Lightweight Solar Panel (12V Battery Charger)

This is, I hope, not just "Yet Another Home-Made Solar Panel".

My plan is to eventually install a considerable number of dismountable panels on a sailboat. For that, they have to be lightweight, efficient, relatively inexpensive, and relatively rugged.

For efficiency, I chose monocrystalline wafers and an MPPT charge controller. These wafers are brittle with a relatively large minimum bending radius, so I needed a rigid yet lightweight support. I'm using a plastic honeycomb sandwich with polyester GRP skins. The finished panel with 35 wafers weighs 1.5kg, not including the controller. For redundancy in the face of wafer breakage, I chose a wiring scheme where groups of wafers are wired in parallel and then groups are connected in series. In the prototype, which I completed before finding a controller, there are 7 groups of 5 wafers. Each wafer gives about 0.6V, so that I have a total of about 4V. Failure of a single wafer will not break the series circuit, so that the panel should function with one or more wafers completely cracked. However, I could not find a commercial boost controller with a minimum voltage less than 5V. Chips exist from e.g. Texas Instruments to extract power from a single wafer (or N in parallel), but I chose to build a second prototype panel to connect in series with the first (giving 8V total) rather than design and build a custom controller.

Some prior experience with GRP (glass-reinforced plastic, aka. fibreglass) is recommended. It's not exactly difficult, but you should know how to mix resin for different temperatures, how to wet cloth properly, and how to spot the gelling that indicates the resin is about to set up and you need to finish a section immediately. Also how to clean brushes (unless you discard them), clean yourself, and are prepared for some spills (i.e. do not use the dining-room table). If you don't have this experience, practice with some offcuts until you do.

This was what I used for the prototypes:

Materials (things that get used up)

• White plastic "egg crate" lighting panel

• Marine grade polyester resin with MKP hardener

• Clear polyester potting resin
• Glassfibre cloth
• 3" x3" solar cell kit from heartofthesun-solar on eBay (includes tabbing wire and flux pen)
• solder paste (as used for SMT components)
• Rosin-core solder
• spray release wax

• mixing pots (used food cans, plastic pots etc.)

• Acid brushes (very inexpensive, for resin)

• polyester twine
• Duct tape
• Polyethylene sheet (or garbage bags)
• Genasun 8A 12V lead-acid Boost MPPT Controller

Tools (things that survive construction)

• Coroplast plastic sheet

• 35W temperature-controlled soldering iron with shovel tip
• scissors (for cutting cloth)
• tinsnips or saw (for cutting lighting panel)
• vacuum pump (optional)
• copper tubing (optional)
• G-clamps
• wooden laths, plywood sheet, foam rubber or bubble-wrap sheet, weights
• sandpaper
• x-acto knife or similar
• "surform" rasp or similar
• Putty knife
• Wire cutters, needle-nosed pliers

I purchased a vacuum pump halfway through building the prototype as it seemed like a good idea. I have done many projects that involve gluing and clamping material, and find it hard to apply sufficient pressure evenly over a large area. G-clamps only have so big a throat, I only have so many car batteries etc. Vacuum bagging ought to be able to apply up to 100,000N (14psi) in theory, equivalent to a layer of car batteries about 5m high. I drilled some small holes in copper tube to make a wand and flattened one end, and have had some success making bags with polythene sheet and tuck-tape.

Originally, I was going to use the Coroplast sheet for the core of the sandwich. However, the GRP did not stick to it, and lighting panel grid was a similar weight and thicker and hence more rigid. I ended up using the coroplast for a layup base.

The lighting panel grid adheres better to GRP, but still not that well. In order to prevent the finished GRP skin from delaminating from the grid, I used polyester twine that I happened to have on hand. The twine is woven through the grid in a spiral pattern so that it bridges cells of the grid on each side, and when laid up with the GRP cloth, absorbs resin and bonds to the GRP.

Assembly steps:

1. Trim the panel grid material to the desired size - enough to take an array of NxN wafers, with a margin of some 2cm on all sides. Some of the margin is used for electrical connection, and some is for handling and just because it's hard to get the reinforced edges flat. The grid I bought had half-height edges, so I needed to remove the outer row on all 4 sides to get a clean full-height edge. I cut the grid with tinsnips and then cleaned it up with a rasp.
2. Roughen the surface of the grid with sandpaper
3. Thread several lines of twine through the grid in a spiral pattern (see first photo), keeping it tight and knotting it at the end of each line.
4. Cut one sheet of GRP cloth to fit the grid, leaving a small overlap of perhaps 1cm on all sides
5. Prepare the coroplast panel on a suitable table or plywood sheet, and spray the area that will take the panel with release agent (not essential, but it helps)
6. Mix (regular) resin according to directions
7. Paint one side of the grid with resin, including the exposed twine on that side.
8. Lay the cloth on the cloroplast, making sure it is square and flat
9. Remove the grid and carefully wet the cloth with resin using a brush without pulling the weave
10. Set the wet side of the grid onto the wet cloth and press firmly in place
11. Option 1: cover the grid with plywood sheet and weight it down. I have some old lead-acid batteries that make useful weights
12. Option 2: cover the grid with plywood sheet, insert the complete sandwich of lower plywood, cloroplast, GRP, grid and upper plywood into a plastic bag, seal it, and connect the vacuum pump
13. Allow the resin to set firmly
14. Disassemble the press system
15. Insert the putty knife between the cloroplast and now-rigid GRP skin, and gently separate the GRP from the
    

    cloroplast, bending it slightly if necessary (the cloroplast, that is).

16. Using a sharp knife, trim excess GRP from the edge of the panel. Using a rasp, smooth the edge of the GRP skin flush with the edge of the grid
17. Prepare another sheet of glass cloth, repeating steps 5-16 to create the GRP skin on the other side of the grid.
18. Prepare two more sheets of cloth
19. Roughen the two existing GRP skins with sandpaper.
20. Mix more resin.
21. Lay the cloth on one side of the grid, over the existing skin, and work resin into the cloth with the brush to build the skin up to 2 layers of cloth.
22. Allow the resin to set to at least the tacky stage, turn the grid over and repeat on the other side.
23. Trim the new edges with the knife and rasp

It is probably possible to combine all this into one operation, using a second sheet of cloroplast and laying up all four cloth layers in one go. I haven't tried.

As stated before, the GRP skin tends to delaminate from the grid material, so in addition to using twine to secure the middle of the panel, the edges are overlapped with a layer of GRP.

Steps

1. Cut strips of glass cloth, one for each of the 4 edges of the panel, allowing about a 1.5cm overlap on the top and bottom surfaces
2. Clamp the panel vertically with one edge exposed horizontally
3. Cut strips of polythene sheet (or garbage bag) just larger than the cloth strips
4. Spray a bit of release agent on the cloroplast, to take the cloth strips
5. Mix resin, and paint the exposed edge of the panel along with 1.5cm of the top and bottom panel surfaces
6. Paint a strip of cloth with resin, set it on the panel edge, and paint it some more
7. Cover the wet cloth with the plastic strip
8. Secure the strip with duct tape, pulling it down on both sides of the grid so that the cloth is firmly held against the edge of the grid (left to itself, glass cloth will pull away from sharp corners and set up with a void underneath)
9. Press the resulting edge assembly between two wooden laths and G-clamps; allow the resin to set
10. Repeat for the other 3 edges

I actually did two opposite edges at once, then the two remaining edges after trimming the GRP from the corners.

You should now have a lightweight rigid panel with a flat surface, ready to accept PV wafers.

Allow the resin to set hard, then sand the best face of the panel flat to within about 0.2mm flatness, to within 2cm of the edge. It should actually be that flat already, but there are sometimes resin drops or stray glass fibres that need to be reduced. It doesn't need to be glass-smooth, just roughened a bit, since there will be another layer of resin.

Wire up groups of wafers to fit across the panel, following the instructions in the solar panel kit. As I mentioned earlier, my prototype had 7 groups of 5 wafers which gave me only 4V total - not enough from one panel. My second prototype has 6 groups of 6 wafers, arranged in subgroups of 2 wafers. That gives about 10V, easily enough for the boost controller to work with. So each group is two wafers connected in parallel with tabbing wire, in series with two other pairs of wafers. The photos show the backside of the wafers, with one length of tabbing wire soldered across two wafers then dipping underneath to connect the front sides of the next two wafers. Leave a 2cm tail at each end of the wafer groups.

To precis the soldering instructions: First test the wafers in a jig, and set aside those with a low voltage. I marked the backside of each wafer with the voltage in pencil. Select pairs (or your desired subgroup size) of wafers with about the same nominal voltage to connect in parallel. On a flat surface, run the flux pen over the solder stripe on the wafers, then press the tabbing wire onto the wafer with the soldering iron. Apply a little bit of solder to the iron tip to act as a heat transfer agent. There should be enough solder pre-tinned onto the tabbing wire to reflow onto the wafer and make a good connection, but I added some solder paste as well. The paste comes in a syringe with a needle - squirt a tiny (0.5mm) spot of paste onto the solder stripes every 2cm.Then gently turn the wafers over, holding them by the attached tabbing wire, and connect the other side.The wafers are brittle and have a minimum bending radius of something like 1 metre - they can (maybe) be bent over an obstruction of at most 1mm high. So they can be bent over a tabbing wire, providing it doesn't have big blobs of solder on it. The instructions that came with my kit recommend practising with damaged or low-voltage wafers. If at any time during construction you break a wafer, you will have to replace it in the group without damaging the other wafers. Fortunately I have not had this happen, but I suspect it is not trivial - it may be easier to cut the tabbing wire and splice it than to unsolder it cleanly from a broken wafer.

Test each assembled group of wafers with a voltmeter to make sure that it is working properly before moving on.

Update: the performance of wafers is seriously degraded by shading. If just one wafer in a series-connected string is completely shaded, the power generated by the panel will be significantly affected. In one test I did, a wafer normally gave about 500mV under indoor light, so that a string of 6 gave about 3V at 30mA. With one wafer 50% or 70% shaded, there was little difference, but at 90% shading the wafer voltage dropped to zero and at 100% shading the waver voltage was reversed to -1200mV while the current dropped to 12mA. This effect can be prevented by fitting bypass diodes, such as a 1N5822 Schottky diode, which in my test limited the reverse voltage to about -150mV and kept about the full 30mA current. I suggest adding bypass diodes to each group of parallel-connected wafers if partial shading is expected, e.g. in a marine application where spars may cast shadows across the panel.

1. Spray some release agent onto the cloroplast sheet
2. Lay the groups of wafers face-down on the cloroplast, carefully
3. Mix some clear potting resin
4. Paint resin onto the good side of the completed GRP sandwich panel
5. Paint the backside of the wafers with resin
6. Carefully lay each wafer group face-up on the panel. You can pick them up by the tabbing wire tails. You really don't want to break any at this stage. Arrange the groups in a zig-zag fashion so that when connected in series, the voltages all add up instead of subtract. I.e. the tail of one group connected to the backside of a wafer should be next to a tail connected to the topside of the wafer in the next group. Leave a 2cm margin all around to make connections. Press the wafers gently into the resin. Wipe up any large blobs that form.
7. Apply pressure while the resin sets:
8. Option 1: cover the panel with a polythene sheet, then a layer of foam rubber, then a sheet of plywood, then add weights.
9. Option 2: put the assembly in a plastic bag, insert the wand then connect a vacuum pump. The photo shows an actual plastic bag; it worked better with a thicker stiffer bag made with polythene sheet and tuck-tape. The one in the photo, I added the foam rubber and weights, too, as the bag leaked but if the vacuum is working properly it should not be necessary
10. Wait for the resin to set, then disassemble the press and carefully peel the polythene sheet away from the wafers. Again, you really don't want to break one.

Connect the wafer groups in series, and make the connection pads:

1. Check each group again with a voltmeter to make sure it is still working
2. Bend each tabbing wire tail 90 degrees with pliers, about 1cm from the wafer, so that it points towards the tail from the next wafer. Make a fold at 45 degrees as you would with a paper strip (try first with a paper strip if that's not clear)
3. Cut lengths of tabbing wire to join the tails together, with about a 4mm overlap. Solder them in place.
4. Cut 3cm lengths of wider bus wire to make the pads. Solder them to the tabbing wire tails at either end of the chain (e.g. bottom left in the photo).
5. You should now have the entire set of wafers connected in series in a zig-zag pattern. Re-check the voltage across each subgroup, and across the entire chain between the pads. If you made a mistake it's a bit late to fix it, except that if you just got one group the wrong way round you could run insulated wire across to connect it correctly
6. If the pads and wire connections won't lie flat, glue them down. I used UV-cured superglue. Alternatively, apply point pressure later with polythene sheet and clamps.
7. Cut some glass cloth to fit the entire panel, leaving perhaps a 5mm margin. This goes over everything, including the connection pads. Cut 4 small patches about 2cm x 4cm to reinforce the pads. I used a lighter weight of cloth for this.
8. Mix some clear potting resin
9. Lay up the cloth patches over the connection pads
10. Lay the cloth over the panel. Paint it with resin, making sure the wafers and cloth are wetted properly so that the final assembly is transparent and waterproof
11. Wait for the resin to set
12. Using a sharp knife, carefully cut away an 8mm x 4mm area of GRP in the centre of each connection pad, exposing the tinned bus wire
13. Identify the positive and negative terminals with a voltmeter and mark them, then solder appropriate coloured wire (red and black, usually) to the pads. I used something like 14 gauge marine grade multistrand wire that I had to hand, which is probably the best in any case.
14. Connect the charge controller, and a 12V lead-acid battery, and test it. The charger should show the battery charging if there is any daylight at all. The panel voltage will drop from the open-circuit value as the charger finds the optimal charging current.

Optionally, add reinforced GRP pads for mounting holes by building up cloth and resin on the margins of the panel. I have not decided how best to do this - perhaps anodized aluminium strips bolted along the edge. The prototype has unfinished holes with polyester cord threaded through them, as in the last photo. Although the panel weighs less than 2kg, that is not a long-term solution as the GRP will abrade the cord.

The finished panels are fairly stiff and robust, and should survive a moderate amount of abuse. I have not tried actually standing on one, or immersing it in water, or dropping a screwdriver on it, but they should be a lot more rugged than a glass panel. I am also not certain of the long-term stability of the potting resin - whether it will survive years of UV exposure without becoming slightly opaque, for instance. (I will, perhaps, make another small prototype to test to destruction, in which case I will update this instructable).

Andrew Daviel, May 2016