Jesse,
Running PVWatts for various tilt angles with an azimuth angle of 135°
(or 225°) for locations in Minneapolis, shows that the optimum tilt
angle for annual energy production is about 35°, that's almost the same
as the optimum tilt with an azimuth of 180°. To maximize December energy
production with the 135° azimuth, the optimum tilt angle is about 60°
and that gives 12% more energy in December than the 35° tilt would
(without considering how much better the steep tilt does at shedding
snow). The SE roof with the steeper pitch is certainly the better choice.
Kent Osterberg
Blue Mountain Solar, Inc.
www.bluemountainsolar.com
On 5/14/2013 8:23 PM, Jesse Dahl wrote:
Allan,
That is pretty much my thinking. They have 0 interest in stepping
foot roof and the SE give more tilt and easy ground access to clear
the array during winter.
I appreciate the advice.
Jesse
Sent from my iPhone
On May 14, 2013, at 10:15 PM, Allan Sindelar
<al...@positiveenergysolar.com <mailto:al...@positiveenergysolar.com>>
wrote:
Jesse,
Based only on the info you provided, I'd recommend the SE face. A low
slope is best for maximizing annual gain, which is what a
conventional grid-tie customer would want. But you are in essence
giving them a system that will function as GT now, but as an off grid
system if the grid goes down. Under that scenario, I would sacrifice
annual spin-the-meter-backward production in favor of maximizing
winter output, when days are shortest and loads are greatest. A
steeper array pitch will maximize available charge during winter,
while the shallow pitch will be great in summer but close to nothing
in winter. Given their stated objective, explain the tradeoff as
offering the best preparation for lack of grid.
I'd advise otherwise if they clearly want to seasonally adjust their
array if there is an outage, and you install adjustable-tilt racks,
and the owners are afraid of neither ladders, heights, or snow.
We just completed a 4 kW GTBB system for a customer with similar
objectives. We used 16 240W modules on two 8-module pole-top racks,
with the instruction to leave the tilt angle shallow as long as the
utility is present, but tilt to 45-55 degrees in winter during an
extended outage. (And a pair of Classic 200s to handle the 4-module
series strings.)
Allan
*Allan Sindelar*
_Allan@positiveenergysolar.com_ <mailto:al...@positiveenergysolar.com>
NABCEP Certified Photovoltaic Installer
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder and Chief Technology Officer
*Positive Energy, Inc.*
3209 Richards Lane (note new address)
Santa Fe, New Mexico 87507
*505 424-1112*
_www.positiveenergysolar.com_ <http://www.positiveenergysolar.com/>
*
*
On 5/14/2013 4:34 PM, Jesse Dahl wrote:
Hello,
I was asked to look at a site today for a homeowner looking for a grid-tie with
battery-back up. They are Doomsday Preppers for lack of a better term and want
certain loads if and when the grid goes down for good. Aside from parts selection,
they have two roof lines, one facing SE one facing SW. the SW is very low slope and
is only accessible with a ladder and climbing on the roof, the other is SE, much
steeper slope and accessible from the ground or deck. I mention the access because
of snow removal. We had lots of snow last year and I spent lots of time on my roof
removing 8" snow falls. Both have good solar access.
I'm inclined to chose the SE roof line, but I am interested in arguments for
the SW.
Thanks!
Jesse
Sent from my iPhone
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