PV north facing in US [RE-wrenches]

[Bill Brooks]

Tom,

I agree with your previous post about the north side of a 7:12 pitch (30
degrees) being in the shade during the winter. However you are mistaken with
the information below.

I live in Vacaville and we get very little Tule fog since we are on the
extreme edge of the Big Valley. Tule fog causes scatter such that it almost
does not matter where your collectors point in the winter (north is almost
as good as south--both are poor but that are not zero).

Where are you getting your information? The sources of data for Tom Hoff's
Clean Power Estimator and PVFCHART are backed up by good correlations to
field performance. Where do you get the less than 50% number? --sounds like
a suburban myth to me.

People should not put systems in the shade on the south roof. My example was
merely to stimulate the mind to find a location that is closest to south
with the least amount of shade. For low-pitch roofs, that could be the north
side as crazy as that sounds. The whole purpose is maximizing performance.
Site performance is a function of two variables--orientation and shading.
These factors need to be calculated separately and multiplied together to
determine the overall effect.

I was hoping someone would question my results. If you can produce good data
that contradicts this analysis, I am more than happy to change my mind.

Bill.


-----Original Message-----
From: Tom Lane, Energy Conservation Services
[mailto:solar8 at ehostingbiz.com]
Sent: Wednesday, July 23, 2003 12:08 PM
To: RE-wrenches at topica.com
Subject: Re: PV north facing in US [RE-wrenches]


This is ridiculous a 4:12 pitch roof in Vacaville, CA will not produce
50% of the solar gained from on a year round basis if you had clear
skies every day.  The "Tule fog" in the Vacaville area results in almost
0 gain no matter how you face solar panels during the winter.  Lurkers on
this wrenches site will pick up the wrong impression.

You have to take the microclimates into account if
microclimates are not taken into account serious misinformation can get
spread to people who do not understand the local situation.  In no cases,
should people be encouraged to face solar panels north on a north facing
roof above 30 degrees latitude with a 4:12 or steeper pitch.

Gator Tom


----- Original Message -----
From: "Bill Brooks" <billb at endecon.com>
To: <RE-wrenches at topica.com>
Sent: Tuesday, July 22, 2003 6:41 PM
Subject: RE: PV north facing in US [RE-wrenches]


Bill and Joel,

My SPAM filter caught this because it said north facing PV in the US (not
really). However, Joel asks a good question and there is very good answer.

Case in point. A homeowner has a south-facing house with a 4:12 roof pitch
(18 degrees). The detailed version of Clean-Power Estimator works for a
North facing array. In Vacaville (Northern California) the north roof face
will produce 75% of the south face on an annual basis. Now consider a short
and fat tree on the south side that provides lovely shade to the house and
south roof, but leaves the north roof unshaded. The Pathfinder shows that
the tree limits performance by 40% on an annual basis.

Question: Which location will produce more energy?
Answer: North face (and we're not in New Zealand!)

Of course system performance could be improved by going flat or tilted on
tilt legs to the south, but the owner may think that is ugly (and they would
be right).

As long as you and the customer know the impact of the decision, go for it.

Bill.


-----Original Message-----
From: Bill Roush [mailto:solarguide at everestkc.net]
Sent: Tuesday, July 22, 2003 1:50 PM
To: RE-wrenches at topica.com
Subject: RE: PV north facing in US [RE-wrenches]


Why North facing? I will never speak ill of a client, they are so rare
in these parts, so my lips are sealed.

Best,
Bill Roush
Solar Electric Systems/KC, Inc.




-----Original Message-----
From: Joel Davidson [mailto:joeldavidson at earthlink.net]
Sent: Monday, July 21, 2003 9:17 PM
To: RE-wrenches at topica.com
Subject: Re: PV north facing in US [RE-wrenches]

Bill,
You can input north-facing azimuths in PVWATTS
http://rredc.nrel.gov/solar/codes_algs/PVWATTS/version1/text_only.html
I've heard about north-facing arrays in Hawaii because the islands are
so
close to the equator and I saw a grid-connected 25 kW array in India
that is
manually tilted north and south depending on the season. But PV on a
north-facing roof in the U.S.? Why?
Best,
Joel Davidson

Bill Roush wrote:

> I am not finding the references I see periodically to de-rating for a
> north facing roof in the US. Any links for me?
>
> Best,
> Bill Roush
>
>
>
>
>

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