Part One: Irradiance Losses
Obstructed Panel Losses: Shade
Obstructed Panel Losses
Shade Losses
In his book, Renewable Energy and Efficient Electric Power Systems, Stanford University’s Gil Masters demonstrates how shading just 1/36 cells in a small solar module can reduce power output by over 75%.
To conceptualize why shading results in such severe losses, we’ll use the analogy of water flowing in pipes. The flow rate of water through a pipe is constant, much like the current through a cell string. But when a pipe is clogged, it restricts the flow of water through the entire pipe. Similarly, when a solar cell is shaded, the current through the entire string is reduced.
Essentially, the shaded cell is the weakest link, reducing all the remaining cells' power availability — unless the panels have bypass diodes (which most do, see the section below for more detail). This is significant because every cell in the cell string has to operate at the current set by the shaded cell, preventing the unshaded cells from operating at maximum power.
See page 6 under "Irradiance Mismatch Losses" for more ways to mitigate shaded panels.
Obstructed Panel Losses
How to Reduce Shading Losses
Bypass diodes (which, in practice, every panel has 1-3 of) are devices within a module that allow the current to “skip over” the shaded regions. By utilizing bypass diodes, the higher current of the unshaded cell strings can flow around the shaded cell string. This comes at the expense of losing the output of the cells that are skipped over.
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