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 Post subject: Extraterr in SIT FilePostPosted: Mon Aug 27, 2018 5:22 pm

Joined: Fri Aug 21, 2015 3:21 pm
Posts: 16
Hi folks,

I've had a Client come back to me asking what the 'Extraterr' in the PVsyst SIT file is and where I got it.

Since they asked I've checked my other SIT files and sure enough they all have:

GlobalH=<13 entries>
DiffuseH=<13 entries>
TAmb=<13 entries>
Extraterr=<13 entries>

I thought this might be "extra-terrestrial" radiation - i.e. estimated irradiation before it passes through the atmosphere, but in some cases it diverges rapidly from the GHI readings which I wouldn't expect if this was the case.

I can easily explain the other three as these are fairly obvious but I'm stumped with the last one - basically I need to know:

a) Does this parameter impact a standard PV simulation (or is it used for concentrating/other solar simulations?>
b) Roughly how is it calculated? I don't need a technical description but an outline would be appreciated, i.e. "This is x, it is calculated using y model" - I can then begin to formulate a response to my Client.

Thanks all,

H

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 Post subject: Re: Extraterr in SIT FilePostPosted: Tue Aug 28, 2018 11:17 pm

Joined: Mon Apr 16, 2012 7:29 pm
Posts: 1680
This is indeed the extra-terrestial irradiance, i.e the irradiance outside of the atmosphere.

This is equal to the solar constant (1367 W/m2), which is indeed not a constant, but a yearly average. The extraterrestrial irradiance varies by about +/-3% around this value due to the earth's trajectory ellipticity. This "normal" extraterrestrial irradiance has to be multiplied by the sin(HSol) for representing the Horizontal global.

The extraterrestrial value is used for the evaluation of the clearness index Kt, involved in the Transposition and Diffuse models.

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 Post subject: Re: Extraterr in SIT FilePostPosted: Thu Aug 30, 2018 1:25 pm

Joined: Fri Aug 21, 2015 3:21 pm
Posts: 16
Wonderful, thank you for the detailed response - that should clear things up quite nicely.

Best,

H.

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