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DIY Solar Panel Design

Lets get a couple terms out of the way; just for clarity. Their is a hierarchy to a solar system, and the names are often misunderstood. It all starts with a solar cell, which is a the small, individual 'windows' you see when looking at a panel. Panels are a specific number of these cells assembled together physically and electrically in the correct number to create the desired system voltage. Arrays are a collection of these panels assembled into a system.

To start my system design, the first thing I had to decide was what system voltage I wanted. Common practice is 18v panels for 12v systems, and 36v panels for 24v systems. This overhead is necessary to charge batteries and continue providing sufficient voltage at times of reduced lighting. I decided on a 12v system because of my intentions to use 12v devices directly. 12v panels are also arguably easier to build; because of smaller size and the fact that one bad connection can disable an entire panel. Commonly, a 12v/18v panel will be constructed with 36 cells. I chose to use 39 for a little additional overhead and because the physical arrangement of 3 rows of 13 fit my enclosure plans well.

Cell connection diagramYou may have surmised from the cell counts, but all these cells get connected in a series circuit. That means the positive of one cell is connected to the negative of the next and so on. This results in adding of the voltages of the cells. The total current will be equal to a single cell's current in this arrangement. It also means they will function a lot like christmas lights; one broken connection opens the circuit and shuts all of them down.

Finding Cells

So, where do we get some cells? The web, of course. There are many sources on the web; I personally have had good luck with eBay. I started out with small cut cells for experimentation; and they work well but it takes a lot of them to make any meaningful power. As I gained some experience making these small panels, I moved up to 3x6 cells - and actually found them to make a lot more sense. They are not as fragile, and a single panel generates a fair amount of power. I'd suggest a similar path for anyone starting out; though I'd recommend moving to the larger cells after just a couple practice panels.

Soldering (aka Tabbing)

There is much on the web about soldering these things; I don't find it all that difficult to do. To keep costs of the project at a minimum, I made the first one with regular wire, rosin core solder, etc. I'd recommend skipping that step - its not easy to get it to stick, takes a long time, and leaves a lot of high spots which will keep the cells away from your glass covering. I would strongly recommend buying actual tabbing wire and a flux pen.

Choosing the right soldering iron is important. I started out with my 25W iron that I use for PC boards. That wasn't enough; as you move along the length of the cell, it cools off too much. I've found 40W works very well; but be cautious to not move too slowly - overheating will cause the white screen-printed contact points to separate from the cell. It also helps to use/make a tip with a flat wedge shape rather than a point. Apply the flux(with a flux pen) to the white surface of the cell, lay an appropriate length of tabbing wire on top of it, and run your iron across it. Move slow enough to melt the solder, but not too slow.

Bypass diodes

Yes, I have to complicate the simple series circuit just a little. As stated above, a single bad connection can interrupt the whole panel. Unfortunately, that happens easier than you might think; because a shaded cell will act as an open circuit too. Thus a shadow, a stray leaf, or a bird dropping is all it will take to cause problems. So, bypassing diodes are added to allow bypassing sections of the panel. These will allow some power to be produced, and protect the shaded cells from absorbing the power of the rest of the panel.

Not that difficult to understand, but there is a couple of tradeoffs to consider. More diodes will allow more panel to remain effective if a small number of cells are inoperative. On the other hand, more diodes will cause stacking of the junction voltage if a larger section is inoperative. Obviously, more diodes also adds to the cost.

Because I am using 3 rows of 13 cells, I chose to bypass 6 cells and 7 cells at a time. This is a bit more bypassing than is conventional, but it seems to serve me well. It helps that I found an entire roll of diodes(1250) for a great price; so cost isn't an issue in my case. I consider small inoperative areas to be more likely, so I did not have as much concern with stacked junction voltages as I did with loosing large chunks of voltage from bypassing a lot of cells with a single diode.

Selecting proper diodes for this purpose is not too difficult. You will need a diode that can handle the forward current rating of the cells you are using, and a reverse voltage equal to the sum of the voltages of however many cells you decide to bypass at a time. In addition to that, finding the lowest junction (VF) voltage is advantageous for efficiency.

Efficiency

This is probably the area where I learned the most from experiment and thought; rather than research. As you research the subject of solar power, you will find that it is always about efficiency; be it cell composition, panel construction, wiring, etc. Of course, high efficiency is a great thing but I believe its a bit over-rated. Squeezing a few extra mA/h out of a panel can often cost far more than simply building an additional panel. Of course, you'll have to make those decisions for your situation .. but consider cost of materials(you may have a less costly source of an efficiency improving product), available space(you may or may not have room for the extra panel), etc.

In my opinion, commercial research efforts would be better spent finding cells that don't require such elaborate and expensive encapsulation techniques than the current more power per square inch approach. Cost of system is still the primary limiting factor to deployment of solar energy, and as you get into the project you will find that the enclosure is far more costly than the cells are.

Next : Construction Techniques
(Yep, this needs a whole page of its own - coming soon.)