why do we need Diode ?

[Mike90250]

..... I've heard that some manufacturers actually void their warranty if the panels have been installed in shade, as they are not sure about the long term degradation caused by shading on solar panels with bypass diodes installed.

Actually they are VERY sure about the damage caused by shade. Count each day as a thermal cycle. 365 cycles in a year. 7300 cycles in 20 years. Add 2 daily shade events, and you now get 21,900 cycles in 20 years. Pooof. that's not counting the stress caused by the diode hot spot.

You are close on your diode wattage calc, but forgot HOW to dissipate the heat to keep the diode junction temp below it's kill threshold. You hjave to use the Vf at Imax, which is closer to .8V (even for schottky diodes) Oversize the diodes enough, and then you get the Vf down to .5V, but you still have several watts of heat to get rid of.

[jg6871]

Mike,

Thanks for looking at the diode watt calculation.

Interesting point you make regarding 21,900 cycles over a 20 year lifespan. Surely the panel lifespan is not only a function of number of cycles but also the magnitude of each cycle (i.e. how high is the temperature difference) and the rate of change of each cycle (i.e. how quickly the cell itself becomes hot). Hence if some cells are going through a thermal cycle every day (on top of the standard temperature cycle as experienced by an unshaded panel) and this thermal cycle is hitting +80 deg c at some points/cells at a very fast rate, the lifetime of the panel is reduced. As you said, heat is not good in electrical circuits. This would explain the logic behind some manufacturers voiding their warranty if shading is apparent over the modules. What are your thoughts?