Average sun-hours [RE-wrenches]

[Joel Davidson]

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I, too, find it hard to believe that winter PV production for a location in
the continental U.S. is greater than summer production. Even in Los Angeles
where were get "June gloom," with less solar input in June than in May, we
still get more total sun hours in the summer 6 months than in the winter 6
months. Where are the exceptions?

----- Original Message ----- 
From: "Kurt Nelson" <sunwise at cheqnet.net>
To: <RE-wrenches at topica.com>
Sent: Thursday, August 05, 2004 8:04 AM
Subject: RE: Average sun-hours [RE-wrenches]




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Thanks Tom,

I would imagine MPPT CC's are changing the formula in some instances as
well.  That said, I've got to wonder why a program such as NREL's
PVWatts program doesn't reflect the true solar conditions you are
experiencing and why maps designed to indicate sun-hours are not
representative of this as well.

Here in the upper Midwest we do experience a very cloudy period in
November and December which some times sticks around for the better part
of the winter.  In such winters you can design around published winter
hours and still put a lot of hours on a gen-set.

I am just trying to come up with a good standard for off-grid homes on
which to base system sizes that may be receiving State funding (we
provide incentives in WI for off-grid) and am hoping to find a balance
between funding systems that are charge regulating by noon much of the
year versus systems that run gas generators too often to provide power
for the "solar" occupants.  I'm thinking most years that's closer to
"average" sun-hours.

Kurt Nelson

-----Original Message-----
From: Tom Elliot, Guffey Energy Works [mailto:telliot at wagonmaker.com]
Sent: Thursday, August 05, 2004 4:24 PM
To: RE-wrenches at topica.com
Subject: Re: Average sun-hours [RE-wrenches]



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OK, this is where my pet peeve comes in.  How about starting this
discussion
with the realization (and clear statement) that there are fairly large
areas
of the country where winter sun produces more power than summer sun.  I
get
my best power output on the shortest days of the year here in Colorado
and
far less output, because of bad horizontal sun angles at sunrise and
sunset
and common mid-day clouds, during the summer months.  Someone who
designed
for real world sun conditions here, and throughout most of the Rocky
Mountain states and the Southwest, would be starting from a false
premise by
assuming summer is more productive.  I understand completely that there
are
very large populations for whom the winter/summer dichotomy is as you
assume
it to be but we do not do the world of solar any favors by reinforcing
the
misconception of winter=lowpower/summer=highpower meme as a basic truth
of
solar design.

Please be accurate enough to put in the appropriate modifiers in such a
statement to identify that it is not a universal truth.

Sorry, but the whole country does not reside on the two coasts...

Tom

____________________________________

Greetings all,

Can I request a thought or two on using annual average sun-hours versus
winter sun-hours for designing off-grid PV systems?

While it would be really nice to not design a system around any
generator run-time, is that practical when one looks at the fact that
the resulting system is so oversized for the most of the solar year?
BTW, I am not saying there would be no generator or generator run-time
in either case, but am interested in it from a system design
perspective.

Thanks in advance.

Kurt Nelson
SOLutions

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