[RE-wrenches] Using the North Facing Roof

[billbrooks7 at sbcglobal.net]

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] 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 <mailto: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 <mailto:petertor at pobox.com> 

 

_______________________________________________



 



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