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

[Bill Brooks]

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Jeremy,

I'm a little confused--I saw lots of trig flying around and some good
advise, but I'm not so sure the concept has been properly relayed. The
vertical distance from the top of the first row to the bottom of the second
row is the concern. If you don't want to do a lot of trig and worry about
which way the cells are facing on the module and so on, three times the
vertical distance is best for 38deg N up into the mid 40degN latitudes.
Twice the vertical height is okay in lower latitudes (30-33degN), but it is
a real compromise at 38degN.

I'm not saying it's wrong or evil, it is just not good practice. If the room
is available, always go 3 times the height in all three directions (east,
south, and west). Anything less will begin to produce shading that will
eventually become very significant. I can see a lot of folks starting to use
2 x vertical height as a standard row spacing, and that is not what we
should be preaching. If you can't fit the extra space, then you need to
explain to the customer that the row spacing chosen is a compromise between
available space and inter-row shading. It is nearly always better to reduce
tilt angle to achieve the 3:1 spacing than it is to push down to the 2:1
spacing.

Bill.


-----Original Message-----
From: Jeremy Rodriguez [mailto:allsolar at ris.net] 
Sent: Monday, January 10, 2005 6:14 PM
To: RE-wrenches at topica.com
Subject: Re: Mutiple Array Spacing at 38deg N Latitude [RE-wrenches]


 
Dean,
Thanks a buch!
I actually came up with 38" spacing today.
I got that by taking the total height at the rear of the first array and
doubled it. I guess that will work if you say use 36.4

Jeremy
----- Original Message -----
From: "Dean T. Newberry" <deant at dcn.org>
To: <RE-wrenches at topica.com>
Sent: Monday, January 10, 2005 11:41 AM
Subject: Re: Mutiple Array Spacing at 38deg N Latitude [RE-wrenches]

HI Jeremy,
I went to http://www.susdesign.com/sunangle/ to get the sun altitude angle
at 10:00 AM for your location  22.41degrees This site has the trig
functions. http://www.univie.ac.at/future.media/moe/galerie/wfun/wfun.html

You want leg b and know  leg a and angle (a).
b = cot(a) * a,  cotangent is not a windows calculator function, so... cot
(a) = 1 / tan(a) therefore b = 1 / tan(a) *  leg a, simplify b = leg a / tan
(a) in your case leg a = 15" the solution then is: b = 15 / tan 22.4 b = 15
/ .412 b = 36.4"

Your row spacing should be at least 36.4" to avoid inter-row shading in
December between 10AM and 2PM.

The rule of thumb derived for solar thermal collectors, is bascially correct
for the noon elevation angle, but that is not when you are generating power.
The tendancy of PV modules to dramatically reduce output under only a little
shading requires different assumptions.

cul  deant

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