>> In a cloudy area, mirrors can be used (carefully) to 
>> increase the
>> concentration of sunlight while not exceeding the ratings 
>> on the panels.

> I'm under the impression the limiting factor is 
> temperature rather than
> anything else, IE you could put 4x the sunlight onto your 
> cells with
> mirrors if you water cool them back below around 60C. 
> (which wont use
> that much energy really)

"Concentrator" cells are commonly run at many suns. Cooling 
is a major issue. A coating that stands the energy flux also 
helps.

Standard panel output drops quite noticeably with 
temperature. I found that I could get 10% plus more output 
from a real world panel by running a thin film of cooling 
water over the top surface while operating. Properly 
designed I decided the flow rate could be quite acceptable 
if water was not scarce. Also by "washing a panel to cool it 
down and then watching the output you clearly see the 
change. And it's in all good spec sheets but not noticed by 
many people. Note that performance specs are given at 25C or 
20C and never at 50C :-).

Substantial rear fin / air cooling of a panel may help 
substantially.

A variable mirror arrangement could improve panel output 
very substantially at off peak sun periods - either cloud 
cover or end of days. With no cooling I could get 
polycrystalline small panels to 150% to 200% output with 1 
or 2 low cost mirrors each equal to the panel area. Gain is 
as much about getting the light spread evenly as about 
level. A PV cell's on resistance drops dramatically under 
illumination. Shadow a single cell and the whole panels 
output drops to a small percentage of total. eg a tree 
branch shadow across the edge of a panel reduce output to 
say 5%-10% of expected. Forward diodes across each cell help 
prevent this. Similarly, when adding mirrors, if the 
illumination does not cover all cells then the gains will be 
much lower than expected or even almost nonexistent. In 
fact, if you add heat you may reduce total output.

My tests show that pointing a panel at a blue sky on a sunny 
day but without sun input is vastly superior to pointing at 
even bright sunlit items. I haven't tried this, but 
concentrator mirroring a bright sky with a large low grade 
mirror may be useful as the sky extends semi-infinitely as 
opposed to the sun which is of finite size.

While panels which do not day track usually have minimal 
losses and even lack of season tracking is not too too bad, 
mirrors will probably require tracking for direct sun 
concentration. Energy needs for this need not be overly 
large by arrangements  can get cumbersome. For very small 
panels a controlled mirror concentrator should be able to be 
implemented at low cost.



        Russell McMahon

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