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re: solar uneconomical??
29 mar 2001
"dlehmicke"  wrote:

> please explain how shadows degrade an array.

douglas otis  replied:  

>blocking diodes are not a complete solution to this problem of shadows...
>if not producing a voltage, the leakage of the energy produced by
>neighboring cells tear at the imperfections...

colorful... i can imagine a gradual degradation that gets worse over time. 
local heating makes more imperfections, which makes more heating, but i've
never heard of that happening in a switching power supply with 100,000
"shadings per second" :-)

another explanation from rich komp's 1995 _practical photovoltaics_:

shunt diodes

a cell can be permanently damaged if a large reverse voltage is applied
to the electrodes. a solar cell is an electrical rectifier which means
that it passes current in one direction only. in the dark, the silicon
solar cell acts just like any other silicon diode rectifier. however,
in the manufacture of solar cells, no real attempt is made to build in
an ability to withstand a large reverse bias since the cell is not usually
used in this way. there is one circumstance under which this reverse voltage
condition can occur: when one cell is shaded while the rest of the cells
in a series string are in sunlight. when this happens, the current through
the string immediately stops and the sum of all the open-circuit voltages
of all the other cells shows up across the shaded cell. if the cell cannot 
withstand this voltage, it will break down electrically and begin to conduct.
the resistance heating effect of the current can make a cell hot enough
to melt the solder connections and destroy the fingers.

sooner or later, such shading will occur: for example, a leaf falls on
one cell or a tree casts a shadow on a module. since it is impossible
to prevent such occurrences, it is necessary to take precautions so that
shading will not destroy a cell. with 33-cell modules, reverse bias
breakdown is rarely a problem, as virtually all cells will take 30 volts
or more in reverse bias. but if higher voltage strings are needed, shunt
diodes should be installed. .. some manufacturers have constructed
experimental cells with the shunt diode built into the cell structure and
some amorphous silicon modules have shunt diodes built into each long row.
with these cells and modules, no external shunt diode is necessary.


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