[RE-wrenches] Using the North Facing Roof

Tue Jul 28 13:31:42 PDT 2015

Bill and Wrenches,

I have been battling this issue for a while. Our sales folks are up against
fierce competition and want to design a system that has the best return on
investment. So, they will choose a reverse tilt system  over a North facing
array unless there is a strong deterrent to going with reverse tilt. This
deterrent could come in the form of a company policy “no reverse tilts” or
by adding significant costs to reverse tilt systems. We’ve tried some of
both and there are still cases where reverse tilts prevail – but we do them
very seldom and only with careful consideration.

Let’s say we have a 5 kW grid tied system (off grid is a whole other ball
of wax) and we’re analyzing our options between flush mount North 18
degrees (4:12 pitch) and reverse tilt to South at 5 degrees. The North
facing array will produce about 24% less than the South facing reverse tilt
system assuming no differences in shade for our Bay Area location or 5126
kWh/yr for North versus 6758 kWh/yr for South. At $0.20/w for electricity
this amounts to $326/yr of extra savings for the South facing reverse tilt
system. We might charge about an extra $1500 to install the reverse tilt
system to account for the engineering cost to check the structure and
 extra racking material and labor. This means that the reverse tilt system
pays off in less than 5 years. The numbers would be more in favor of the
reverse tilt if the North facing pitch was steeper.

I guess all I’m saying is that the pure numbers still support reverse tilts
in some cases. Despite that, my gut is that we should avoid them for the
same reasons that Bill lists below.

Best,

August

*From:* RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] *On
Behalf Of *billbrooks7 at sbcglobal.net
*Sent:* Tuesday, July 28, 2015 11:16 AM
*To:* 'RE-wrenches'
*Subject:* Re: [RE-wrenches] Using the North Facing Roof

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>
*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

_______________________________________________

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