"Real" Sun Hours [RE-wrenches]

[Bill Brooks]

Carl and Travis,

Most insolation data provided by folks like NREL and others is in units
referred to as "peak sun hours". This is kWh/m2/day. Values are often
published for monthly or yearly averages as in the NREL "Red Book". Goto
http://rredc.nrel.gov/solar/pubs/redbook/ and look at this. You can also
order a copy online.

Measurements are made with pyranometers and the insolation recorded is
listed in units of peak sun hours. This is an average daily energy delivered
by the sun (averaged over 30 years in the case of the "Red Book"), not an
instantaneous irradiance measurement.

I hope this helps.

Bill.


> -----Original Message-----
> From: Carl Emerson [mailto:freepower at freepower.co.nz]
> Sent: Tuesday, September 25, 2001 2:10 PM
> To: RE-wrenches at topica.com
> Subject: Re: "Real" Sun Hours [RE-wrenches]
>
>
> Travis,
>
> I'll stick my neck out on this one...
>
> Real sun hours is very complicated to measure and requires very
> sophisticated
> recorders that can integrate the varying insolation levels during
> the day and
> come out with an index for 1000 watts/SqM.
> Very early recorders used a magnifying glass to burn a track in a
> paper roll and
> could therefore only record a threshold.
> Using a C40 might indicate a threshold and you could relate this
> to a current
> reading and an insolation level, but this will not be able to account for
> integrating the intensity variations.
> Personally I have gone away from using 'sun hours' in my design
> calculations and
> use the irradiance data. I find this much more useful and accurate partly
> because even today the sun hour recordings in our country are
> useful only to
> tourists who might want to calculate the gallons of sun cream
> required for their
> holiday...
>
> "Travis Creswell, Ozark Solar" wrote:
>
> > My second question of the day.
> >
> > Noticing the high module temps I am seeing even on cool days I
> am wondering
> > about the accuracy of Sun Hour numbers.
> >
> > I use the DVM on my C40 to make a crude calculation of the
> daily sun hours I
> > am getting.  Of course, it's accuracy is questionable and
> commonly I have
> > found the C40 DVM's that I've installed to read amps about 10%
> low.  Voltage
> > is usually right on.  Mine is no different.  Simply, I divide
> the rated amps
> > of my array into the amp hours for that day and that tells me
> how Sun Hours
> > I got.   Currently I have only part of my array installed so I
> only have 7.1
> > amps (Four Astro power 120's at 48 volts).
> >
> > But what I just realized about this number is that it factors
> everything in,
> > including a de-rate for heat and actual insolation.  Are these things
> > already factored in to the commonly published Sun Hours numbers?
> >
> > Travis Creswell
> > Ozark Solar
> >
> > - - - -
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> --
> Regards,
>
>
> Carl Emerson
> Manager
> FREEPOWER LTD.  "Harnessing Natural Energy"
> P.O. Box 10-1207 N.S.M.C. Auckland, NEW ZEALAND.
> Ph. 09 4275206 fax 09 4275208  cell 021-633999  web www.freepower.co.nz
>
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