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Re: [EnergyPlus_Support] Re: Modifying a weather file to reflect "real" weather



James,

>  
> Since sky cover is reported in many weather data sets, what if I just 
> gave the actual % sky cover to E+ in a custom data set?  As mentioned 
> in my first post, my (perhaps simplistic) assumption is that the sun 
> does the same thing every day, but the clouds vary a lot.  How can we 
> represent that factor simply to E+?  If I did that, is there some 
> other factor that renders it meaningless?
>

Well, yes, your assumption is simplistic. The clear-sky irradiance (or 
above the clouds) does vary from one day to the other due to changes in 
aerosols, water vapor, ground albedo and ozone (in that approximate 
decreasing order of importance).

> Since the sun has a big impact on many building designs, I want to 
> reflect it's impact on HVAC and overall energy use.  I'm trying to get 
> a "reasonable and defensible" approach, not "precise to several 
> decimal places".
>

The best radiative models can predict the irradiance components within 
only 5% if all atmospheric variables are perfectly known locally, which 
is essentially never the case in practice. So in the real world, errors 
of 15-50% (on a hourly basis) are typical, especially under variable 
cloudy conditions. We are still quite far from being precise to several 
decimal places...

Chris

>
>     ------------------------------------------------------------------------
>     *From:* EnergyPlus_Support@xxxxxxxxxxxxxxx
>     [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] *On Behalf Of *Chris
>     *Sent:* Tuesday, June 02, 2009 6:36 PM
>     *To:* EnergyPlus_Support@xxxxxxxxxxxxxxx
>     *Subject:* Re: [EnergyPlus_Support] Re: Modifying a weather file
>     to reflect "real" weather
>
>     I know at least 100 papers that have been devoted to this
>     question, most
>     of them proposing a simple empirical formula to obtain either the IR
>     (aka thermal or longwave) irradiance, or equivalently the sky
>     temperature or the sky emittance. Most of them, however, are for
>     clear
>     skies only (when the sky temperature is significantly lower than
>     the the
>     air temperature). Still. some offer models for overcast and
>     partly-cloudy conditions.
>
>     Chris
>
>     > Adding to the discussion, the weather file needs solar radiation
>     data which could be estimated based on the total horizontal solar
>     radiation which could be "cheaply" recorded, but eplus also
>     requires the horizontal IR radiation, for which I have yet to find
>     a method to estimate it using normally reported weather data (like
>     temp, humidity, wind speed and direction, rain).
>     >
>     >
>     >
>

-- 
Chris A. Gueymard, PhD
Solar Consulting Services
P.O. Box 392
Colebrook, NH 03576, USA
Tel. (603) 237-8550
Fax  (603) 237-5314
http://www.SolarConsultingServices.com



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