add content to cross-string mismatch

Author: mikofski

Mismatch_from_cross-string_shade.md

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+Effects from shade are complicated, but can be summarized in two orthogonal categories:
+
+1. shade across all modules parallel to strings
+2. shade across few modules perpendicular to strings
+
+## shade parallel to rows
+Row-to-row shade in fixed-tilt systems, typically in winter, is an example of
+shade across all modules that is parallel to strings. When I originally wrote
+about [PV electrical mismatch]]({filename}PV-electrical-mismatch.md), I analyzed
+this type of shade using [PVMismatch](https://sunpower.github.io/PVMismatch/)
+to simulate shade across the bottom row of a single string of 10 modules in a
+10 string system. The conclusion of that post was that the string performed as
+well as the most shaded cell, so even though only the bottom cells were shaded,
+the modules in the string lost most of their power. I shaded the bottom cells
+80% to simulate only diffuse light, and the string lost roughly 80% of output.
+The other 9 strings operated at full capacity, so the system only lost 8%. The
+[NIST ground mount array](https://www.nist.gov/el/energy-and-environment-division-73200/heat-transfer-alternative-energy-systems/photovoltaic-1)
+is an example of a system that will have row-to-row shade in winter that will
+cause most of the strings to lose almost all of their output even when only
+the bottom row of cells is shaded.
+
+![NIST Google](./images/nist-ground-array.png)
+
+This topic is also covered in 
+
+## shade not parallel to rows
+However, that post also contained a picture of a rooftop with non-uniform shade
+that was not consistent across each module of the string. The shade cast from
+the roofline cut diagonally across the modules in the string, which was wrapped
+in two rows to fit.
+
+![non uniform shade on a roof](./images/20150923_170418.jpg)
+
+I didn't analyze the shade from this system in that post, so it raises the
+question whether the rule of thumb I recommended would still apply?
+
+## shade perpendicular to rows
+To simplify the question, the rest of this post analyzes a PV system with a
+shade obstacle like a wind turbine tower or a telephone pole that casts shade
+perpendicular to the rows.
+
+>TL;DR: When shade cuts perpendicular to strings, cells go into reverse bias,
+bypass diodes activate in the shaded submodules, and the other modules operate
+at higher voltage to match the voltage of unshaded parallel strings.
+
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