How Shade Hurts
A solar panel is a series-connected string of cells, so, as in the familiar "weakest-link" analogy, the current generated by the panel will be limited to the current generated by the weakest cell. This fact explains why shading just a few cells of your panel can reduce the output current to just the minuscule amount produced by the shaded cells. Small or difuse shadows, like that from shrouds or guy wires, will not have a mortal effect on solar panels because they cannot shade fully shade individual cells. There are a few subtleties in the cell current-voltage curves, like reverse breakdown, that can sometimes work in your favor, but the basic effect remains.
Note that shade-tolerant doesn't mean shade-proof. A shade-tolerant panel performs exactly the same as a conventional panel under full shade. The only gains come when the panel is partially shaded.
Two Roads to Shade-Tolerance...
We've just seen that a shaded cell can kill the power output for an entire panel, so manufacturers have come up with two strategies. First, prevent the cells from being fully shaded, or second, provide a path for the panel current around the shaded cell.
Most cells are rectangular, so it's easy for them to become completely shaded. Some amorphous panels are built with very long narrow cells that each run the length of the panel. With this geometry, it's less likely for an individual cell to be shaded, so the power reduction is closer to (but still worse) than proportional to the fraction of area shaded. Note that these amorphous panels suffer a big hit in efficiency compared to more common poly- or mono-crystalline panels.
The other option for making a panel shade-tolerant is to connect a bypass diode across each cell. This way, when the cell is shaded, the current from the panel can travel through the diode. Instead of contributing 0.4-0.5V, each cell and diode combo now drop 0.2-0.7V (depending on the diode type), but the current can still get through. However, if more than, say, a fifth of the panel is shaded, the power will fall off rapidly because the panel maximum power voltage will fall below the battery voltage.
...No, Make that Four Roads
As soon as your panels are mounted, they aren't panels anymore, they are part of a system. So, it's important to make sure that every component of your system fully works towards your goal of producing power. Considering the installation on a system level, there are two more approaches that can result in a system that works well in shady conditions: system decentrallization, and Maximum Power Point Tracking.
System decentrallization, for lack of a better term, takes the bypass-diode approach up a level. Instead of a system with one large panel, consider a system built with several smaller panels. Now, if one of the panels becomes shaded, the others can still operate at full power, and we have just built a shade-tolerant system.
And the fourth road to building a shade-tolerant system is the use of a Maximum Power Point Tracking Charge Controller like our GV-series units. When a panel is partially shaded, the maximum power voltage goes up, and the gains available with MPPT increase. We quote typical power gains of 10-30% with MPPT, but in shady conditions, we've seen gains of 50% and more.
Be aware that MPPT charge controllers are not all created equal. As of 2006, there are several other companies making small MPPT controllers. The most popular of our competitors estimates the maximum power point (MPP) using the panel open-circuit voltage, while Genasun's controller actively finds the MPP. Now, when a panel is partially shaded, the open-circuit voltage goes down, but the MPP goes up, so our competitor's units will move the panel operating voltage in exactly the wrong direction, throwing away power! Genasun's units will behave correctly and give you the greatest gains, since they find the the true maximum power point.
And Some Numbers to Back it all Up
To find out the benefit of shade-tolerant panels on a sailboat, let's engage in some estimation. S/V Genasun has one of our 110W panels, not shade tolerant, mounted on a tripod just behind the stern. Since the sails are so darn big, this is probably similar to a panel mounted on deck near the stern of a "normal" sailboat. The panel is roughly 4' by 1.8', long axis across the boat.
Now, imagine the sails in some typical configuration, and mentally swing the sun 360 degrees around the boat. Our task is to figure out for how many of those degrees the panel is actually partially shaded. Fortunately, this needn't be as hard as it sounds.
The panel usually can't "see" the foresail, since it is generally behind the mainsail, so we'll only consider the mainsail. As far as the panel is concerned, the mainsail has two edges, the luff (at the mast), and the leech. The mast is roughly 13 feet forward of the panel. Thus, the panel will be partially shaded by the front edge of the mainsail over roughly 4'/13'*57 degrees/radian=18 degrees of solar azimuth. I've used the small-angle linear approximation here. Now, the leech of the mainsail probably averages 5 feet from the panel, but the apparent width of the panel may only be 3 feet when seen from the angle of the leech. The panel will be partially shaded over roughly atan(3'/5')=31 degrees (asin, or the radian approximation are arguably just as good for the accuracy we need here).
We'll be generous to the shade-tolerant panels and assume that only one edge falls on the panel at a time. Thus, the panel will be partially shaded for about 18+31=49 degrees of the solar azimuthal circle. Let's also generously assume that the conventional panel produces no power when partially shaded, while the shade-tolerant panel captures all the power possible (and thus averages 50% power). Let's estimate that the panel above is fully shaded for 11 degrees of azimuth. Thus, the conventional panel will produce full power over 360-49-11=300 degrees, while the shade-tolerant panel will also produce half power for an additional 49 degrees of azimuth, or equivalently, 325 degrees of full power. So, the gains resulting from a shade tolerant-panel are only 25/300=8.3%! And remember, this analysis was generous to the shade-tolerant panel. You'd be much better off upgrading to a charge controller with MPPT than a shade-tolerant panel. And if you had gone to an amorphous shade-tolerant panel with 2/3 the efficiency of a crystalline non-shade-tolerant panel, you'd actually be losing 25% for equally sized panels.
"So, you still want a shade-tolerant panel, huh?"
It's a solar panel, why do you want to mount it in the shade? Really though, if you're looking for shade tolerance, first ask yourself, "Is there really, without a doubt, positutely, cross-my-heart, no better mounting location for my panel?" Sometimes, the answer really will be no. In that case, do some estimation like that above to see how much a shade-tolerant panel will help you, look at prices, and decide whether shade-tolerance is worth the increased cost. At the same time, consider the techniques mentioned above for making your system shade-tolerant. Follow these guidelines, and you'll be well on your way to a cost-effective solar installation with good performance.
-Alex 11/2006