[RE-wrenches] Using the North Facing Roof

Wed Jul 29 07:56:53 PDT 2015

I want to give back to this forum that has been so valuable to me, so here
it is... a tool for you to quickly compile data for multiple compass
orientations for a given pitch...

http://floridasolardesigngroup.com/pva/do-pvwatts-wrenches.php

This PVWATTS derived tool will give you 36 data points for solar energy
production with every azimuth from 0º to 350º at 10º increments for a given
location and pitch. You can enter your desired system size to model its
output and include your desired PVWATTS derate (system loss) factor. It
will download a .CSV file each time you run the tool. You can run it for a
variety of tilt angles based on your needs to compile data for your own
study.

There are limits on use, but if everyone does not run out and use it at the
same time it should be fine.

Once you get your data into a spreadsheet you can easily slice and dice it
any way you want and make some great visualizations.

[image: Inline image 1]

Enjoy!

Jason Szumlanski

On Tue, Jul 28, 2015 at 7:01 PM, Allan Sindelar <allan at sindelarsolar.com>
wrote:

>  I am paying close attention to this thread, but for different reasons. I
> have designed and will install next month an off grid system for a
> high-elevation research hut. At 14,242' I believe this will be the highest
> elevation off grid system in the continental US (Alaska too?), at the
> Summit Hut atop White Mountain, east of the Sierra crest in eastern
> California (wmrc.edu/facilities/bar/summit.html). For me it's sort of a
> post-retirement summer working adventure. A nearly identical system will
> serve an older observatory at 12,700' in the same area. The Summit Hut will
> get a roof array where winds of 190 mph have been measured, I'm told. The
> roof has about a 5º north-facing tilt.
>
> I'm old-school too, thinking in terms of a 25º tilt to south. But other
> than powering an internet repeater all year, the system is only used during
> the summer months, mid-June to mid-October, when the snow has melted and
> it's accessible by a long jeep trail. So in that respect it's similar to a
> flat- or north-facing array in a grid-tied system, where only summer gain
> matters much and a southern tilt matters less that I want to believe.
> Jason's chart is pretty useful here, suggesting that I should pay more
> attention to handling wind loads with a low angle than maximizing summer
> gain.
> Allan
>
>  *Allan Sindelar*
>  <allan at sindelarsolar.com>allan at sindelarsolar.com
> NABCEP Certified PV Installation Professional
> NABCEP Certified Technical Sales Professional
> New Mexico EE98J Journeyman Electrician
> Founder (Retired), Positive Energy, Inc.
> *505 780-2738 <505%20780-2738> cell*
>
>
>  On 7/28/2015 11:15 AM, billbrooks7 at sbcglobal.net wrote:
>
>  Larry and Peter,
>
>
>
> You are too old-school to think outside the box. It’s not about direct
> sunlight—it’s all about kWh/m^2/day and those numbers don’t lie. Your
> analysis is not correct and this is why simple analyses will always give
> you a wrong answer.
>
>
>
> North-facing arrays have been financially attractive for years, but many
> have not done it due to taboos or bad analysis. Reverse-tilt arrays often
> look horrible and should be avoided particularly on the street-side of a
> house. Also, the structural impacts of tilted arrays on residential
> rooftops are not well-understood so wind-loading calculations are complex
> at best.
>
>
>
> We have been using east and west facing roofs for your years so what’s the
> big deal about north? I put together the one of the first tables of
> orientation version performance way back in 2001 for the California Energy
> Commission to combat the misconceptions that PV arrays had to be mounted at
> 45-degrees facing South (the prevailing misconception at the time). I
> didn’t print the North facing numbers because the concept would have blown
> people’s minds at the time—they weren’t ready for the truth.
>
>
>
> 30-degrees facing south is optimal in most latitudes from 20-degrees to
> 50-degrees. (perfect in most locations)
>
> 4:12 pitch (18-degrees) facing south is 97% of perfect.
>
> 4:12 pitch east or west is 88% of perfect.
>
> Flat is 89% of perfect.
>
> 4:12 pitch facing north is 75% of perfect.
>
>
>
> The truth shall set you FREE.
>
>
>
> Bill.
>
>
>
>
>
> *From:* RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org
> <re-wrenches-bounces at lists.re-wrenches.org>] *On Behalf Of *Starlight
> Solar Power Systems
> *Sent:* Tuesday, July 28, 2015 9:41 AM
> *To:* RE-wrenches <re-wrenches at lists.re-wrenches.org>
> <re-wrenches at lists.re-wrenches.org>
> *Subject:* Re: [RE-wrenches] Using the North Facing Roof
>
>
>
> In Yuma, AZ, north facing modules will have direct sunlight for small part
> of the year. In the picture, look at the yellow area above the East-West
> line. Thats direct sunlight from the north. The green top line in the
> picture shows summer solstice showing sunlight from sunrise to about 0930
> and from 1530 to sunset. The energy harvested during those hours will be
> tiny compared to the peak sun hours on the south side. The angle of
> incidence will also reduce the total power generated during those hours.
>
>
>
> The thin brown middle line is the equinox. By then, there is no direct
> sunlight on the north side. I can not see any benefit in AZ even at todays
> low prices. Now, if I were building in Quetzaltenango, Guatemala, that
> would be a different story. But then again, I would have to clean off the
> volcanic ash each morning.
>
>
>
> Larry Crutcher
> Starlight Solar Power Systems
>
>
>
>  chart came from http://www.gaisma.com/en/
>
>
>
>
>
> On Jul 27, 2015, at 11:21 PM, Peter Parrish <peter.parrish at calsolareng.com>
> wrote:
>
>
>
> 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>petertor at pobox.com
>
>
>
> _______________________________________________
>
>
>
>
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