Thursday, December 24, 2009

Our solar installation is nearly complete, and with that in mind, I have been looking into some of the more subtle, yet quite important, issues
of photovoltaic installations. Foremost among these is the problem of shade.
Shade can have an impact that is surprisingly disproportional to its apparent coverage. This is because of the construction of a sol
ar panel. A given panel generally consists of dozens of cells connected in series. If one of those cells is shaded, it may not only not contribute to the output of the panel, it may detract from it by becoming a resistive element. The impact of one non-contributing cell of the dozens in a panel can be as much as 50% of the panel output (see these links:,,, and there are lots of others).
There are a number of ways solar panel manufacturers try to limit the effects of shade. Bypass diodes are one of the most common, but it is surprisingly difficult to find much information on how these are employed. (Here is a link to a simplistic explanation.) For example, our CS6p-200p panels have bypass diodes, but none of the documentation states how many are in the panel. These function by passing current when a cell becomes resistant, but when the cell is operating the diode is reverse biased, and therefore resistant. There are also higher-tech solutions, such as SolarMagic®, from
National Semiconductor, which acts as a voltage regulator. It has two benefits: first, to regulate the panel voltage level so that it continues to contribute to the overall output of the installation, and second, to report on the panel performance, which helps to identify when a panel is under-performing.
Shade is a problem for us in the morning, which is when Puerto Rico receives the most reliable sun (afternoons tend to get convection-generated clouds). There is a ridgeline just east of us that has many tall trees. We have trimmed some of these and will have to trim more. By mid-morning, the sun is above the trees.
Finally, we just found out that the law will not allow us to turn on the system until we complete two final steps. These are certifications, one by
the design engineer and the other by the power company. The first is simple and should be done soon (with a slight Christmas delay). The second is hard to predict, as is everything having to do with government agencies. However, if the power company does not respond within 10 days of notification, we have the right to turn on the system ourselves.
This website, in Australia, has some useful tools if you are interested in the benefits of solar installation.
The following are a selection of photographs of our experience.
The pictures are:
1. The SMA Sunny Boy 4000US Inverter
2. The panels from below, showing the mounting
3. The view from the ridge east of our house.

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