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Re: [EnergyPlus_Support] shape factors

Some clarification on radiant exchange.

View Factors in EnergyPlus

EnergyPlus uses an area weighted approximation to calculate "view 
factors" between surfaces within a thermal zone.  Each surface sums 
up the total area that it can see among the other surfaces.  The 
approximate view factor from this surface to each other surface is 
then the area of the receiving surface over the sum of areas that is 
visible to the sending surface.  

In order to account in some limited way for the fact that certain 
surfaces will not see each other, several assumptions have been built 
into this simple view factor approximation.  First, a surface cannot 
see itself.  Second, surfaces with approximately the same azimuth 
(facing direction) and tilt ("same" being within one degree) will not 
see each other.  This means that a window will not see the wall that 
it is placed on, for example.  Third, floors cannot see each other.  
Fourth, if the surface is a floor, ceiling, roof, or internal mass, 
the rule for the same azimuth and tilt eliminating radiant exchange 
between surface is waived when the receiving surface is floor, roof, 
ceiling, or internal mass as long as both surfaces are not floors.   

Note that this does not take into account that surfaces may be 
"around the corner" from each other and in reality not see each other 
at all.  Rooms are assumed to be convex rather than concave in this 
simple method.  

On 7 Oct 2002, at 8:24, Linda Lawrie wrote:

> At 07:02 AM 10/7/2002, Adriana Angelotti wrote:
> >does anybody know if in the radiation exchange between internal
> >surfaces in a zone EnergyPlus calculates shape factors? If yes, are
> >shape factors available as output variables?
> No, it does not calculate shape factors.
> The program bases view factors on an area weighting and then uses
> Hottel's method to perform the "exact" radiant exchange for gray,
> diffuse surfaces. At some point, we might put in some sort of view
> factor calculation or the ability for the user to specify view
> factors, but that is currently not in the program plan (through
> Version 1.1 -- release Spring 2003).
> Linda
> EnergyPlus WebSite: http://www.energyplus.gov
> Owner: EnergyPlus_Support list
> Member: EnergyPlus Development Team

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