[RE-wrenches] Bi-facial optimum site

[Zeke Yewdall]

We've been using bifacial modules for ground mounts in Colorado this last
year and they seem to be doing very well.  No hard data with a side by side
comparison with a regular module, but it seems that they generate better.
We use snow as our reflection surface.  I've also heard anecdotally that
they are good in cloudy conditions in the pacific northwest where you
aren't collecting much direct beam radiation, but again, I don't curently
have side by side monitoring data.  PVwatts claims a 5 to 10% increase in
most climates, and less sensitivity to exact orientation, which could also
help with off grid sites.  It also seems like the dark backside heats up a
little faster on grounmounts and melts snow and ice off just a little
faster than regular modules.

As far as backside shading goes. The back side behaves just like the front
side from small hard shadows (like from racking).  A significant drop in
production.  But diffuse light, which is almost always what the back side
is receiving, should be less affected by racking shading -- but I still
think it would make sense to try to reduce racking shading of the back side.

The biggest thing that concerns me about bifacial modules is that most of
them are not tempered glass.  Two 2mm layers of heat strengthened glass is
supposed to be stiffer than one layer of 3.2mm tempered glass, but still
less impact resistance if you are in any potential hail area in the 1" to
2" range (higher than 2" is liable to break even the tempered glass modules
anyway).  Other than that, I don't see any downsides, as they are usually
cheaper per watt than regular modules too.  A lot of people were using them
on flush roof mounts because of the cheaper price, though you get zero
benefit from the bifacial aspect there.

Z

-- 
Zeke Yewdall
PV Engineer
NABCEP #031508-89
zeke at darkforestsolar.com
303-523-3592
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