[RE-wrenches] Using the North Facing Roof

Jason Szumlanski jason at floridasolardesigngroup.com
Thu Jul 30 10:16:09 PDT 2015


I figured while I'm at it I might as well release something else that you
can actually print out and present to your clients or use for your own
visualization purposes. This is a very similar tool that will output a web
page with a radar chart (like a spider web representing the compass) and a
data table down to the secondary intercardinal directions. Hover over the
points to see the value.

http://floridasolardesigngroup.com/pva/do-site-analysis.php

Run it at two different tilt angles and check out the differences.
Eventually I plan to update this to compare two (or more) tilt angles on
the same radar chart. For now, this is all you get. :)


[image: Inline image 3]



Once again, enjoy!

Jason



On Wed, Jul 29, 2015 at 10:56 AM, Jason Szumlanski <
jason at floridasolardesigngroup.com> wrote:

> 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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