Mutiple Array Spacing at 38deg N Latitude [RE-wrenches]

[Jeff Clearwater]

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Hey Jeremy,

Twice row height is okay but not necessarily 
great.  You really should do the trig so you know 
how much shading you are getting and translate 
that to energy lost and see how that affects the 
economics of  your system.  For residential 
systems it's not a big deal to lose some near 
Winter Solstice.  When you get into larger arrays 
you really want to know exactly where you stand. 
You basically are deciding what time in the 
morning (or afternoon) on a winter solstice day 
do you want to have no shading (or how close to 
Winter Solstice  you are willing to have total 
shading for tighter arrays if you feel it's okay 
to lose that much)

The basic formula for row spacing for flat ground 
(assuming an true EW row orientation) is:

S=H/Tan(ø)

Where S= Row Spacing
H = Row Height
ø = Minimum Sun Altitude Angle below which you don't want shading

Winter Sun Angle (Altitude)= 90-(Latitude + 23.5º) (at noon)
Summer Sun Angle (Altitude) = 90-(Latitude - 23.5º) (at noon)

(So at 38º latitude you have a winter solstice noon altitude of 28.5º)

So you decide how early or late on winter 
solstice you don't want any shading (or if you 
don't have the luxury of that much space how many 
days each side of solstice you'll allow shading) 
-  so you might look up your sun angle on 9 am on 
winter solstice to have practically zero shading 
from the array year round - depends on your 
application and on existing (trees or such) 
shading)

If you use an angle of 15-20º for ø at your 
latitude you'll have very little total energy 
lost but you'll get a row spacing more than twice 
the height. .  You can go for a higher angle like 
22-25º for ø if the space needs indicate to. 
It's all a matter of how much you want to lose. 
Whatever angle you choose you can then look up 
what days and times you are going to have shading 
in the winter - then figure the energy you are 
throwing away and see if it's worth it 
economically vs. the available roof space.

That formula is true only for flat and south facing arrays.

For a truly south facing pitched roof you simply 
draw it out and add (or subtract depending on how 
you reference it) the roof pitch angle to your 
trig.

It can get complicated if the array is not on a 
truly south facing roof or if you have compound 
tilts (like when mounting the array facing south 
on a west facing roof pitch)

For azimuths other than 0º with a tilted roof you 
have to do 3 dimensional trig to be exact.  You 
can get pretty close though by solving for the 
extremes using regular trig.

If you don't trust your math, then  you can 
define the shading extremes at your site with an 
inclinomter and a siting device sited from the 
highest point of your most south row.  I always 
check my trig by using this method before I bolt 
down the second row.

I hope that helps!

Jeff Clearwater
Village Power Design

>
>Wrenches,
>Can someone confirm spacing of one array to another to be object height x 2.
>I know it depends on your latitude. The site it at 38N
>I have studied some pages sent to me by DP&W but I still have a question.
>Is the object height the height from the roof?
>I have figured out that the highest point of the 
>modules will be 15".  If I use a factor of 2, it 
>seems to be quite a distance to the next mount.
>Sorry, I normally do pole mounts and this will 
>actually be my first grid connected pv system to 
>install here in Colorado.
>
>Thanks in advance,
>Jeremy
>All Solar
>CO, USA
>
>
>
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--
~~~~~~~~~~~~~
Jeff Clearwater
Village Power Design Associates
Sustainable Energy & Water Solutions for Home & Village
http://www.villagepower.com
gosolar at villagepower.com

530-470-9166
877-SOLARVillage
877-765-2784
425 Nimrod St.
Nevada City, CA 95959
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