The Effects of Clouds on a Solar Panel

Photovoltaic Cell

• Solar photovoltaic cells are built from semiconductor materials. Two differently prepared "flavors" of semiconductor material are layered, creating a junction region that acts as a diode---more easily allowing current to flow in one direction than another. When sunlight shines on the panel, it generates current that will flow in one direction through the junction. When the cell isn't illuminated, it presents a "reverse bias" with the opposite voltage of an illuminated cell. The problem is that no cell stands alone.

Within the Panel

• To produce useful amounts of current at a useful voltage, individual solar cells are put together in "strings." For mono-crystalline or poly-crystalline silicon photovoltaic cells, many cells are on a single wafer, then the wafers are strung together. If one cell is shaded, because the cells are strung together, the "bad" cell will suck current out of the good cells, reducing output of the other cells. This can be minimized with bypass diodes. Because of the manufacturing details, mono- and poly-crystalline silicon cells can't use as many bypass diodes as thin film solar panels. In a numerical simulation paper (see References), V. Quaschning and R. Hanitsch note that 2 percent shading of a module leads to a performance loss of 70 percent.

Within the Module

• To smooth out variations in power output and convert the DC output of solar panels into AC voltage used in most houses, the output from the panels is put into an inverter. Frequently, many panels are connected together in a module through a single "string inverter." If just one of those panels is shaded, it pulls current from other panels within the module, the same way that a bad cell can ruin the output from a whole panel. This problem can be addressed by using micro-inverters, which are smaller devices attached to each individual panel.

Cloudy Days

• The problem underlying the output disruption in shade is non-uniformity from cell to cell or panel to panel. When one element within a string performs badly, it pulls all the rest down. So what about cloudy days? As long as the output of the cells and panels is uniform, then the output will simply be reduced. To get better performance, as stated in the "Improved Photovoltaic Energy Output" paper, "It is well-known that 2-axis tracking, in which solar modules are pointed at the sun, improves the overall capture of solar energy by a given area of modules by 30--50% ... ."