Lowest DAYTIME temp records [RE-wrenches]

bbassett at rockisland.com bbassett at rockisland.com
Fri Feb 3 10:32:44 PST 2006


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I was in Olympia, WA when we set our all time record low temp (-6 F), 
and was outside at sunrise. The temperature did not rise more than a 
couple of degrees after the sun was up and shining, so I'd tend to 
assume lowest record temp could happen while there is sun on the array. 
It might not happen often that the array would be exposed and tilted 
right to get full voltage at that time, but it could happen.

I usually calculate the actual array voltage at design temperature and 
then make the voltage correction factor fit what I find. My engineering 
submittals always include that calulation.  This does make certain 
(larger well known) inverters impossible to use in colder climates, they 
just do not have a wide enough voltage range.

Brad
AEE Solar



joeldavidson at earthlink.net wrote:
> 
> Hello Allan,
> 
> I did a project for SMUD over 10 years ago that calculated over 600 Voc
> based on historical climate data. SMUD accepted the calculation and the
> project. None of the 240 inverters failed due to high Voc.
> 
> Data recorded by inverters can be used to void warranties and can also 
> be
> shared with system designers so that inverters and systems can be 
> improved.
> 
> Joel Davidson
> 
> Original Message:
> -----------------
> From: Allan Sindelar allan at positiveenergysolar.com
> Date: Thu, 2 Feb 2006 19:50:24 -0700
> To: RE-wrenches at topica.com
> Subject: Re: Lowest DAYTIME temp records [RE-wrenches]
> 
> 
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> 
> Joel,
> I think Bill Brooks has the right idea. You can get record cold ambient
> temps, or very close to them, when the sun first hits the array. Voltage 
> is
> a hard limit, and most (all?) GT inverters record it internally, so an
> overvoltage failure wouldn't be warranted. One overvoltage condition 
> could
> fry an inverter. It's open circuit voltage that matters, before the 
> inverter
> latches and reduces the voltage to MPPT, so current can be zero; that 
> is, it
> wouldn't take any current for the voltage to be excessively high.
> Anyway, my two cents worth.  Genuine conservatism, from someone who 
> doesn't
> identify as a conservative...
> Allan at PosE
> 
> ----- Original Message ----- 
> From: "Joel Davidson" <joeldavidson at earthlink.net>
> 
> > Should PV systems be designed using nighttime temperatures when it is
> > obvious that voltage is not present in the dark? Should climate data 
> > from
> a
> > weather station be used because it is available even if the system
> location
> > has a different microclimate? Does this seem reasonable? SMA's string
> sizing
> > tool has the right idea. They ask you to select the coldest record low
> > ambient temperature when sunlight will be on the array.
> >
> >
> > www.weather.com only lists record high and low temperatures. NEC 
> > 690.7(A)
> > specifies the lowest expected ambient temperature which implies sunlight
> on
> > the array to cause voltage. Except in rare situations record low
> > temperature occurs at night or before sunrise when no voltage is 
> > present.
> >
> > Does anyone know a source for the lowest expected ambient temperature 
> > when
> > sunlight will be on the solar array?
> >
> > Joel Davidson
> >
> >
> >
> 
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