[RE-wrenches] White Paper on on the affects of a "snow, melt, refreeze process" (no pun intended)

Bill Hoffer sunengser at gmail.com
Thu Aug 5 19:34:40 PDT 2010


Lee


I worked on a job on Washington pass in Washington state about 12 years ago,
near the border of Canada in the North Cascades wilderness area.  Design
snow load was specified 200 #/sq ft as I recall (far greater than the forces
on a module at 120 mph wind load) and the Cascades are known for wet heavy
snow storms.  Mt Baker Ski area (very close as the crow flys to the pass)
had the record recorded snow fall of over 1,140 inches total for the 1998-99
snowfall season.  The storm that took out the array was over 200" in one
storm, they had to shut down to dig the lifts out, riding the lifts
afterwards the snow was over the towers and well above the chair lifts!  It
was a low pitch metal roof (in snow country a big no no).  I ran main beams
North and south and then connected Solarex rails going East to West.  The
rails were 4 inches above the roof, leaving the modules approximately 7"
above the roof.  The melt freeze melt freeze process you mentioned built up
under the modules until it frost upheaved and broke every module shattering
the glass from behind. The mounting system actually held the frames down!
It was a 3Kw system that had to be replaced (some how covered by
warranty???).  Now every winter they take the array down until the spring
(was not being used during the winter anyways).

So I have experienced one rare time the situation you are asking about, but
it was a severe weather situation and an uncommonly low sloped roof for the
environment.  I have never heard of it ever happening to anyone else, but if
I had to design for this location again I would do it very differently!  A
rare occurrence, probably outside of the 100 year storm statistic,  but it
did happen and it was my first job as a solar engineer!

That's my story and I am sticking to it!

Bill


On Aug 5, 2010 12:47 PM, "Lee Bristol" <leebristol at standardsolar.com> wrote:

Wrenches,

Has anybody come across an analysis of the potential damage to a flat
roof from snow on solar modules melting to form an ice layer on the
roof and then forming subsequent layers as more snow melts or slides
down the modules?  If the sheet gets thick enough then the next bit of
water could form ice layer under edge of panel and lift it up.  Seems
very unlikely to me but the prospect has questions....and prospect
questions must be answered.

Thanks!

Lee

--
Lee Bristol
NABCEP Certified Solar Designer/Installer

Chief Technology Officer
Standard Solar, Inc.
1355 Piccard Drive
Rockville, MD 20850
(301) 944-5105
(240) 479-1510 (c)
www.standardsolar.com
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