[RE-wrenches] Using the North Facing Roof
billbrooks7 at sbcglobal.net
billbrooks7 at sbcglobal.net
Fri Jul 31 16:57:09 PDT 2015
Jarmo,
Your intent was laudable, but simple trigonometry just flat out fails with the complexity of solar geometry. PVWatts is so easy to use that anyone, without any knowledge of trigonometry, can use it with far more accurate results. Take advantage of nice, free software that your tax dollar paid for many years ago.
Your harvest calculations don’t take into account about 20 factors that impact solar energy on a PV array. You are only looking at instantaneous sunlight at noon on a south-facing surface. There is almost nothing that we can learn from that simple of an analysis which is why we turned to simulations over 25 years ago. I have all my complex geometry sun position equations from the solar class I took in 1985, but to calculate energy from those equations requires hourly weather data and a computer—no other method will work. PVWatts does this.
Use PVWatts and continue to advocate the installation of PV modules wherever they make economic sense—north, south, east, or west.
Bill.
From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jarmo.Venalainen at schneider-electric.com
Sent: Friday, July 31, 2015 3:53 PM
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Subject: Re: [RE-wrenches] Using the North Facing Roof
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: <mailto:jarmo.venalainen at schneider-electric.com> jarmo.venalainen at schneider-electric.com | Site: <http://www.xantrex.com/> www.Xantrex.com | Address: 3700 Gilmore Way, Burnaby, BC V5G4M1
*** Please consider the environment before printing this e-mail
From:
<billbrooks7 at sbcglobal.net <mailto:billbrooks7 at sbcglobal.net> >
To:
"'RE-wrenches'" <re-wrenches at lists.re-wrenches.org <mailto: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 <mailto: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> mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jarmo.Venalainen at schneider-electric.com <mailto:Jarmo.Venalainen at schneider-electric.com>
Sent: Tuesday, July 28, 2015 10:43 AM
To: RE-wrenches <re-wrenches at lists.re-wrenches.org <mailto: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: <mailto:jarmo.venalainen at schneider-electric.com> jarmo.venalainen at schneider-electric.com | Site: <http://www.xantrex.com/> www.Xantrex.com | Address: 3700 Gilmore Way, Burnaby, BC V5G4M1
*** Please consider the environment before printing this e-mail
From:
Brian Mehalic < <mailto:brian at solarenergy.org> brian at solarenergy.org>
To:
RE-wrenches < <mailto:re-wrenches at lists.re-wrenches.org> 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" < <mailto:re-wrenches-bounces at lists.re-wrenches.org> 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 <http://www.solarenergy.org/>
On Tue, Jul 28, 2015 at 10:24 AM, <billbrooks7 at sbcglobal.net <mailto: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: <mailto:re-wrenches-bounces at lists.re-wrenches.org> re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of <mailto:Jarmo.Venalainen at schneider-electric.com> Jarmo.Venalainen at schneider-electric.com
Sent: Tuesday, July 28, 2015 8:04 AM
To: RE-wrenches < <mailto:re-wrenches at lists.re-wrenches.org> 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: <tel:%2B604-422-2528> +604-422-2528 | Tech Support: <tel:800-670-0707> 800-670-0707 | Mobile: <tel:%2B604-505-0291> +604-505-0291
Email: <mailto:jarmo.venalainen at schneider-electric.com> jarmo.venalainen at schneider-electric.com | Site: <http://www.xantrex.com/> www.Xantrex.com | Address: 3700 Gilmore Way, Burnaby, BC V5G4M1
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From:
"Peter Parrish" < <mailto:peter.parrish at calsolareng.com> peter.parrish at calsolareng.com>
To:
"'RE-wrenches'" < <mailto:re-wrenches at lists.re-wrenches.org> 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" < <mailto:re-wrenches-bounces at lists.re-wrenches.org> 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
<tel:%28323%29%20839-6108> (323) 839-6108
<mailto:petertor at pobox.com> petertor at pobox.com
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