[RE-wrenches] Using the North Facing Roof
Jarmo.Venalainen at schneider-electric.com
Jarmo.Venalainen at schneider-electric.com
Fri Jul 31 15:52:49 PDT 2015
Hi:
I went over and looked at my calculations whereby I arrived at the simple
expression that the change in harvested solar energy is bounded by the sin
of the angle of tilt to the North.
As I was doing it however, it became clear that the reason this simple
result popped up, is simply because,
1. The effect of tilting an array North is exactly the same as if the
system was physically relocated farther North by that amount of degrees
latitude.
2. There is a linearly decreasing amount of annual insolation which is a
linear function of latitude.
Latitude versus Average Annual Insolation
30 degrees latitude has 8.7 kWh-m2
40 degrees latitude has 7.8 kWh-m2
50 degrees latitude has 6.7 kWh-m2
60 degrees latitude has 5.6 kWh-m2
3. The SIN function is very linear for small angles up to about 40 degrees
Angle versus sin
sin(10) = 0.17
sin(20) = 0.34
sin(30) = 0.5
sin(40) = 0.64
The sin expression describing the effect of north tilt is a bounding
function, whereby it bounds the maximum reduction in energy harvest as a
function of tilt. It is a bounding analysis as it does not take into
account the effect of atmospheric diffuse radiation which has the effect
of making the "tilt loss" less than it would be if the earth had no
atmosphere.
For example if an array was tilted north by 40 degrees in Vancouver, with
no atmosphere the modules would see no sunlight for 6 months of the year.
With an atmosphere, there is still a lot of light to be gathered.
Regardless, my intent with the exercise from the beginning was to find a
bound for the potential "loss effect" of North tilt so that I could
continue to advocate the maximum use of roof space even when that roof is
North facing.
JARMO
_____________________________________________________________________________________
Jarmo Venalainen | Schneider Electric | Xantrex Brand | CANADA |
Sales Application Engineer
Phone: +604-422-2528 | Tech Support: 800-670-0707 | Mobile:
+604-505-0291
Email: jarmo.venalainen at schneider-electric.com | Site: www.Xantrex.com
| Address: 3700 Gilmore Way, Burnaby, BC V5G4M1
*** Please consider the environment before printing this e-mail
From:
<billbrooks7 at sbcglobal.net>
To:
"'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>,
Date:
07/28/2015 12:20 PM
Subject:
Re: [RE-wrenches] Using the North Facing Roof
Sent by:
"RE-wrenches" <re-wrenches-bounces at lists.re-wrenches.org>
Jarmo,
Unfortunately, simple is wrong in this case—and detrimental to the PV
industry that needs all the roof real estate it can find.
Bill.
Bill Brooks, PE
Principal
Brooks Engineering
From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On
Behalf Of Jarmo.Venalainen at schneider-electric.com
Sent: Tuesday, July 28, 2015 10:43 AM
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Subject: Re: [RE-wrenches] Using the North Facing Roof
Hi:
Granted that the description is very simple, but that is the intent.
The essence of it is that the "loss" for small variations in angle of
incidence is approximately bounded by, (less than), the sin of the angle
between the orientations of two panels/arrays in question.
10 degrees ---> minus 17%
20 degrees ---> minus 33%
30 degrees ---> minus 50%
If you go through the detailed math and take into account atmospheric
effects, especially when the sun is near the horizons, temperature,
location, weather, etc., the result will vary, but will not be worse than
the sin of the angle.
I'll draw out better picture with more detail for Vancouver. We're at a
fairly high latitude, so overall array orientation is a more sensitive
factor than farther south.
JARMO
_____________________________________________________________________________________
Jarmo Venalainen | Schneider Electric | Xantrex Brand | CANADA |
Sales Application Engineer
Phone: +604-422-2528 | Tech Support: 800-670-0707 | Mobile:
+604-505-0291
Email: jarmo.venalainen at schneider-electric.com | Site: www.Xantrex.com
| Address: 3700 Gilmore Way, Burnaby, BC V5G4M1
*** Please consider the environment before printing this e-mail
From:
Brian Mehalic <brian at solarenergy.org>
To:
RE-wrenches <re-wrenches at lists.re-wrenches.org>,
Date:
07/28/2015 09:48 AM
Subject:
Re: [RE-wrenches] Using the North Facing Roof
Sent by:
"RE-wrenches" <re-wrenches-bounces at lists.re-wrenches.org>
The analysis of 50% of south facing production is too simplistic; running
some modeling shows that, depending on the latitude, the difference can be
much smaller, approaching 25% less for the north facing. I think this
layout could become more common especially on low slope commercial roofs,
where the north facing module would occupy space that was already unused
due to interrow shading. Of course the closer to the equator the less
difference between production of the north and south arrays...and you
better be careful when stringing them in series so as not to mix N and S
facing..plus filling in all those gaps between rows could make servicing
the array a bit problematic!
Cheers,
Brian Mehalic
NABCEP Certified Solar PV Installation Professional™ R031508-59
PV Curriculum Developer and Instructor
Solar Energy International
http://www.solarenergy.org
On Tue, Jul 28, 2015 at 10:24 AM, <billbrooks7 at sbcglobal.net> wrote:
Jarmo,
The sun’s geometry is not nearly that simple. To understand the impact of
north-facing arrays, you have to perform a simulation. PV:WATTS does this
just fine and it is easy to show that a 18-degreed North-facing tilt
produces 75% of a perfect 30-degree south-facing array. Far more than your
assumption of 50%.
To compare 15-degrees South to 15-degrees North, the numbers are slightly
better at 77%. We are going to see a lot of north-facing arrays once
people understand that low tilt angles are very forgiving on North slopes.
Steep slopes are a totally different story and you have to run the
numbers….
Bill.
From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On
Behalf Of Jarmo.Venalainen at schneider-electric.com
Sent: Tuesday, July 28, 2015 8:04 AM
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Subject: Re: [RE-wrenches] Using the North Facing Roof
I did a slide on the effect of North facing modules. For even a fairly
aggressive rotation North as shown, the effect is "only" a 50% reduction.
The questions of whether or not to do it, are,
- is the mounting structure simpler, lower cost
- security against wind
- can I put a larger array on the roof (typically yes, if you make back
to back pyramid shaped structures)
- overall, what is the cost versus benefit
JARMO
_____________________________________________________________________________________
Jarmo Venalainen | Schneider Electric | Xantrex Brand | CANADA |
Sales Application Engineer
Phone: +604-422-2528 | Tech Support: 800-670-0707 | Mobile:
+604-505-0291
Email: jarmo.venalainen at schneider-electric.com | Site: www.Xantrex.com
| Address: 3700 Gilmore Way, Burnaby, BC V5G4M1
*** Please consider the environment before printing this e-mail
From:
"Peter Parrish" <peter.parrish at calsolareng.com>
To:
"'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>,
Date:
07/28/2015 12:22 AM
Subject:
[RE-wrenches] Using the North Facing Roof
Sent by:
"RE-wrenches" <re-wrenches-bounces at lists.re-wrenches.org>
I recently read a short piece that caught me up short, and I quote:
“The fast dropping cost of solar, while a huge boon to the adoption of
solar PV, has counter-intuitively altered design parameters. No longer is
the north-facing roof considered unusable because limited application in
less-than optimal orientations can still show a positive net benefit.
Arrays are thus designed now with elements or sub-arrays in these
locations, increasing overall kW installation while reducing the energy
production per capacity installed. This might have been anticipated based
on sheer economic analysis from a users perspective, but so long has solar
been expensive that these less optimal orientations were never seriously
considered.”
I doubt that the individual who wrote this piece came to these conclusions
him/herself. Does anyone know of a recent article that argued this
perspective? Is this an emerging design practice? If so, I’d like to know
more about it.
- Peter
Peter T. Parrish, Ph.D.
President, SolarGnosis
1107 Fair Oaks Ave., Suite 351
South Pasadena, CA 91030
(323) 839-6108
petertor at pobox.com
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