Off-ideal Array Tilt Output [RE-wrenches]

[Dan Rice]

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Bill and all,
    While I don't have a vast amount of field experience, I have been
puzzled in the past while measuring voltage drop in array home run
conductors. On a very cold, sunny Utah winter day, I saw a much lower than
expected voltage drop for the amperage being produced. While consulting
Table 8, Chapter 9, I finally took a look at the notes at the bottom of the
table, and saw the temperature correction formula. Wire resistance in Table
8 is for conductors at 75 C. It was probably 10 C on that particular
morning. The correction for this lower ambient temperature results in wire
resistance being 21% lower than the table value. I measured still lower wire
resistance than this calculated value.
    This temperature correction factor could negatively affect wire
resistance in instances where wiring and raceways are exposed to direct
sunlight, but for long, buried home runs, this temperature correction can
result in 10% lower wire resistance in cool soil. Not that I'd use this
correction in design calculations, but it may be a positive contributing
factor in some (especially low voltage) installations.
    I guess that my main point is, are folks actually measuring voltage
differences across conductors as they assess the contributions of wire
losses, or are they using table data? Note 1 in Table 8 seems worth
considering...

Dan Rice
Abundant Sun, LLC
Escalante, UT

----- Original Message ----- 
From: "Bill Brooks" <billb at endecon.com>
To: <RE-wrenches at topica.com>
Cc: "Tim Townsend" <ttownsend at firstsolar.com>
Sent: Tuesday, April 06, 2004 11:51 AM
Subject: RE: Off-ideal Array Tilt Output [RE-wrenches]


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William,

You brought up a very important issue. We get sloppy about calculating
system losses. The more accurate we get with estimates, the more we know
about where the power is going. My experience is that we generally
underestimate wire losses in low voltage systems and over estimate them in
higher voltage systems. That being said, we almost always underestimate
system losses in general.

Bill.

-----Original Message-----
From: William Korthof [mailto:wkorthof at earthlink.net]
Sent: Tuesday, April 06, 2004 9:31 AM
To: RE-wrenches at topica.com
Subject: RE: Off-ideal Array Tilt Output [RE-wrenches]
>
>William,
>
>The overall average wire losses should be about 75-80% of peak power
because
>for a typical system in the U.S. (except Alaska) 50% of energy is generated
>above 800 W/m^2 and 50% of energy is generated below 800 W/m^2.
>
>Bill.

Thanks, I stand corrected.

/wk

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