seasonal tilt gain [RE-wrenches]

[David Palumbo, Independent Power & Light]

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<<Another thing though that we have seen is that you can get up to around
10%
more from snow reflectance on bright clean snow.>>


I have been designing and installing pv racks (top of pole, hanging wall,
roof top etc.) that tilt up to 85 to 90 degrees for winter. Tilt up mid to
late November and back to 45 degrees around the end of March. Primarily
because of the consistent snowfall Northern Vermont receives (140" annual
average at 1,400' elevation and more at higher elevations) and the cold
temperatures here. Obvious big advantage is that with vertical modules you
never have to brush snow off. We also pick up the gain of the white snow
reflectance as the ground is generally (95% of the time) covered in snow
November to April.
I have never tried to measure what the % gain because of reflectance
actually is because I am tilting to vertical (or close) because we don't
want to have to brush the snow off.

It is cold enough here that snow does not shed off on its own, even with
modules at 75 degrees. I personally witnessed a particular array with 3" to
5" of snow cover for four to five weeks at a time in the late eighties,
early nineties, until the homeowner hired me to redesign the rooftop rack
for vertical winter adjustment. At 75 degree tilt the snow would not totally
shed off until the temperature warmed up to, at least, 25F. So during the
coldest times, daytime highs from -5F to 10F, even with clear skies and
strong solar insolation, this array would not produce any power. I have seen
the same issue many times on other Arrays in cold snow country.

Dave

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