[RE-wrenches] Fwd: An interesting article

[Jason Szumlanski]

There are two issues that make the article overly simplistic I think.

   - As the author admits, the calculations only work for arrays with due
   south orientation and states that "Solar panels should always face true
   south."
   - The article ignores weather/temperature, which is one reason the
   statement above is not completely accurate.

For example, where I live the summer afternoons are cloudy and very hot
just about every day. Winter days are crystal clear with little haze. For
fixed arrays the annual optimum array orientation is about latitude minus
three degrees and 170 degrees from true north. The benefit you get here
from adjusting tilt twice a year is less than other areas may be for
weather reasons. In fact, you could possibly gain more by keeping the same
annual optimum tilt and adjusting the orientation twice a year (in a pole
mount scenario). In the summer the best orientation is somewhere east of
170 and west of 170 in the winter (presumably close to 180).

Granted, I’m now at 25 degrees north latitude, so the article is on the
edge of being accurate in my situation, but I know for a fact that local
weather patterns make the “due south” recommendation inaccurate in many
places, and the benefits of tilt changes can’t be optimized without taking
weather into account.


Jason


On Mon, Feb 27, 2017 at 2:10 PM, Sindelar Solar <Allan at sindelarsolar.com>
wrote:

> Wrenches,
>
> One of my clients sent me this article on seasonal array tilt angles:
> http://www.solarpaneltilt.com/#2season
>
> It challenges some of the long-held numbers, although not substantially,
> and I found it worth a review.
>
> I have long advocated adjustment twice a year, staying at latitude tilt
> for nine months and latitude +15 for winter, presuming that for most homes
> a latitude tilt will be adequate over the summer months without a second
> pair of adjustments. I have long figured that the fewer required
> maintenance operations the more they'll actually get done. After reviewing
> this article I replied to the client who sent it to me, whose home is at
> 7,000' at 35.5 ° latitude:
>
> Bill,
> Thanks for this. I'll assume that you're welcoming my response, so in
> short...
>
> For me as an advisor principally to off grid homeowners, I'll err on the
> side of practicality, simplicity and ease of remembrance over accuracy.
> That's why, for example, I recommend two angle adjustments/year over four.
> Also, as Landau himself states, "A difference of a few degrees will make
> very little difference in the energy you gather". This information was of
> greater consequence when PV was far more expensive than it is today.
>
> My original and purposely simplistic recommendation was to adjust to
> latitude +15 for winter and back to latitude for the rest of the year. I
> gave an approximate date around November 10th, (midway between the fall
> equinox and the winter solstice) to increase the tilt, with a return to
> latitude around February 10th (midway between the winter solstice and the
> spring equinox). Extrapolating using his calculations for 4X/year, I would
> adjust to winter angle earlier (his target is October 7th), such as "during
> October" and returning later (his target is March 5th), such as "late
> February or early March".  Using Landau's figures for 35°, that suggests
> 32.0 for spring/summer/fall and 55.2 for winter, adding half a degree or so
> for your location, with an unknown adjustment for the higher elevation.
>
> Given your DP&W pole-top rack with its 10-degree adjustment increments,
> this means only that we have no need to drill the additional hole in the
> adjustment bar at 50 degrees. The supplied holes at 35 and 55 are best.
> You'll want only to broaden the duration of your winter tilt angle on both
> ends.
>
> Good exercise! I appreciate your sending this along, as I have always
> worked from longtime collective-wisdom guidelines, having never seen these
> calculations laid out with such accuracy.
>
> I'll be interested in any comments from other Wrenches who take the time
> to review the link.
> Allan
>
> --
>
> *Allan Sindelar*
> 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*
>
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