太阳阴影对光伏系统发电的影响

Shading. When an individual cell within a module or an individual module within an array is shaded, its output will be reduced—and this will
typically reduce module or array power to a degree much greater than simply the proportion of the module or array area that's shaded. This is
because PV modules are composed of numerous individual solar cells connected in series to provide the desired module voltage. Often, a
module will contain several series "strings" of cells that are wired together in parallel to the module terminals. The current output of any given
cell string is limited to the current output of the least productive cell in the string. Similarly, an array is typically composed of at least one
string of modules that are wired in series to provide a DC voltage compatible with the system's inverter. The current output of a series string
of modules, and thus the power that it can deliver, is limited to the current output of the least productive module in the string. So if part or all
of one module in the string is shaded, the power output of the remaining modules in the string will also be reduced.
It's difficult to predict the actual amount of reduction because it depends not only on the pattern of shading, which in itself can be quite
complex, but it also depends on the array layout. For example, if an array is composed of four series strings of modules connected in parallel,
and each string has at least one module that is partially or completely shaded for part of the day, the impact will be greater than if all of the
shaded modules are from one string and the other three strings remain completely unshaded all day. In the former case, the output of all four
strings will be reduced, and the array voltage may drop below the inverter's operating window, shutting the system down entirely. In the
latter case, the three unshaded strings will perform normally, keeping the array voltage up and allowing the system to deliver at least 75
percent of what an equivalent unshaded array would produce (Figure 1).

The impact of shading on the power output of a photovoltaic array depends both on the portion of the array that is shaded and the
electrical layout of the modules. In this array, if the four modules in each column were wired together in series and those four columns
were wired in parallel, then the output of all four series strings would be reduced because each string contains one shaded module. If,
however, the array were designed so that modules in each row are connected in series and the rows are connected in parallel to the
inverter, the output of the top three rows would be unaffected by shading—and more power is delivered to the inverter.
The ideal situation is of course to select a location where the array will remain completely unshaded all day throughout the year. But this is
often not possible because trees, neighboring buildings, or other objects will block sunlight at least part of the year. Measurement tools and
software from companies such as Solmetric (http://www.dyesuntech.com/_d274493277.htm ) are available to assess the degree of shading a proposed array will
experience throughout the year. Some utility rebate programs mandate the use of such tools, and some require that the proposed array have
a minimum number of unshaded hours per day (often six) throughout the year.