Re: [EnergyPlus_Support] Simple Steady State simulation

["Richard J. Liesen" <r-liesen@xxxxxxxx>]

At 11:08 AM 6/5/2002, colin harris wrote:
> The outdoor temperature is 24
> degrees and indoor was around 19.5 degrees. I did what you suggested by
> setting the absorptance to a low number and the inside temperature rose
> to very near 24 degrees. I am now wondering what is the Sky temperature
> phenomena, and how is this derived from the input data? 
>
> Thank you very much for your help. So far i have been very impressed with
> EnergyPlus ease of installation and its comprehensive documentation and
> now with the email group support.
>
> Best regards
>
> Colin.

Hello,
The sky temperature is calculated using the Sky Radiance Model in the
program.  At night the sky looks like a cold black-body surface
which buildings exchange with radiantly.  This is the answer to the
age old question of why puddles on the roof of a building can freeze when
the air temperature never drops below freezing.  A description from
the Engineering Manual is included below which you can reference in more
detail.

Thanks Richard Liesen

Sky Radiance Model

In EnergyPlus the calculation of diffuse solar radiation from the sky incident on an exterior surface takes into account the anisotropic radiance distribution of the sky. This calculation is done when the user has chosen the anisotropic sky radiance option in BUILDING input (SkyRadianceDistribution = 1). In this case the diffuse sky irradiance on a surface is given by

AnisoSkyMult(SurfNum) * DifSolarRad

where DifSolarRad is the diffuse solar irradiance from the sky on the ground and SurfNum is the number of the surface. 

AnisoSkyMult is determined by surface orientation and sky radiance distribution, and accounts for the effects of shading of sky diffuse radiation by shadowing surfaces such as overhangs. It does not account for reflection of sky diffuse radiation from shadowing surfaces.

The sky radiance distribution is based on an empirical model based on radiance measurements of real skies, as described in Perez et al., 1990. In this model the radiance of the sky is determined by three distributions that are superimposed (see Figure 16)

 (1) An isotropic distribution that covers the entire sky dome;

 (2) A circumsolar brightening centered at the position of the sun;

 (3) A horizon brightening.

The proportions of these distributions depend on the sky condition, which is characterized by two quantities, clearness factor and brightness factor, defined below, which are determined from sun position and solar quantities from the weather file.

The circumsolar brightening is assumed to be concentrated at a point source at the center of the sun although this region actually begins at the periphery of the solar disk and falls off in intensity with increasing angular distance from the periphery.

The horizon brightening is assumed to be a linear source at the horizon and to be independent of azimuth. In actuality, for clear skies, the horizon brightening is highest at the horizon and decreases in intensity away from the horizon. For overcast skies the horizon brightening has a negative value since for such skies the sky radiance increases rather than decreases away from the horizon.

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