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Re: [EnergyPlus_Support] wall and roof color




Hello,

regarding to this subject, I have one doubt:

Does E+ consider same value for Thermal Emittance than
for Thermal Absortance?

I say this, because in the idf editor, the fields you
find for a material:regular or material:regular-r
objects are the following (for radiative properties):

MATERIAL:REGULAR-R,
    ,          		       !- Name
    ,             	       !- Roughness
    ,            	 !- Thermal Resistance {m2-K/W}
    0.9,                     !- Absorptance:Thermal
    0.7,                     !- Absorptance:Solar
    0.7;                     !- Absorptance:Visible

If this is not a mistake, how could I set the value
for emittance and emissivity of an opaque material?

For glass materials there is a field for the IR
Emissivity, but I cannot find how to introduce it for
an opaque wall material.

Thanks,

Carmen.

--- Rahul Chillar <chillarrahul@xxxxxxxxx> escribió: 
---------------------------------
hello
Regarding your first question, Color does have an
effect but COLOR can be varied by changing the
Emittance , absorptance of the material 
For Example: If you want your walls to be black then
  0.9000000    ,   !- Thermal Emittance
  0.9500000    ,   !- Solar Absorptance
 
These values are function of color  of material.  
another example:
if you walls are white then
  0.9000000    ,   !- Thermal Emittance
  0.2600000    ,   !- Solar Absorptance
the values vary and you could look up them from any
heat transfer book. The values i have written are at
30Deg C.
 
Again if you wall is made of say asphalt then the E+
material values will have the correct values of these
fields to take into account the effect of color of
asphalt.
 
 So i can is that the software E+ is taking care of
it, however feel free to cross check the values from
any data book. The values vary with temperature,
roughness which will be very difficult to quantize. 
 
Regarding 2: a possbile reason for deviation from what
you expect could be that in the materials you
mentioned CONCRETE - 3/4 IN MORTAR, CEMENT SA 3 / 4
IN, A1 - 1 IN STUCCO 

These values will very with color or your concrete/
material which may be a little darker, or lighter
basically vary from place to place so you may feel
these are not satisfing your requirements. Then again
the Roughness of the materials are different .

 

Hope this helps

Thanks

Rahul Chillar

Building Systems lab

Graduate Research Assistant

Univ of Illinois Urbana Champaign


teklay Weldeabzgi <teklay1979@xxxxxxxxx> wrote:

hello ever body i need some help on these questions
1. how does E+ considers the effect of wall and roof
color in cooling load. i know that color has a
considerable effect on cooling load of a building but
i can't understand how to take this effect in E+.

2. i have some doubt with material properties given in
the dataset of E+. especially on the thermal
emittance, solar Absorptance, and Solar Absorptance.
for example if u see the following three materials i
think they sould have the same property but it differs
greatly. 

     MATERIAL:Regular,CONCRETE - 3/4 IN MORTAR,  !-
Material Name
Rough,  !- Roughness
  1.0000000E-02,  !- Thickness {m}
  0.7200000    ,   !- Conductivity {w/m-K}
   1858.140    ,   !- Density {kg/m3}
   830.0000    ,   !- Specific Heat {J/kg-K}
  0.9000000    ,   !- Thermal Emittance
  0.2000000    ,   !- Solar Absorptance
  0.2000000    ;   !- Visible Absorptance

  MATERIAL:Regular,PLASTER - CEMENT SA 3 / 4 IN,  !-
Material Name
Smooth,  !- Roughness
  9.9999998E-03,  !- Thickness {m}
  0.7200000    ,   !- Conductivity {w/m-K}
   1858.140    ,   !- Density {kg/m3}
   830.0000    ,   !- Specific Heat {J/kg-K}
  0.9000000    ,   !- Thermal Emittance
  0.7800000    ,   !- Solar Absorptance
  0.7800000    ;   !- Visible Absorptance

MATERIAL:Regular,A1 - 1 IN STUCCO,  !- Material Name
Smooth,  !- Roughness
  2.0000000E-02,  !- Thickness {m}
  0.6900000    ,   !- Conductivity {w/m-K}
   1858.140    ,   !- Density {kg/m3}
   830.0000    ,   !- Specific Heat {J/kg-K}
  0.9000000    ,   !- Thermal Emittance
  0.9200000    ,   !- Solar Absorptance
  0.9200000    ;   !- Visible Absorptance

  MATERIAL:Regular,CONCRETE - 3 / 4 IN STUCCO,  !-    
                                   Material Name
VeryRough,  !- Roughness
  9.9999998E-03,  !- Thickness {m}
  0.7100000    ,   !- Conductivity {w/m-K}
   1874.160    ,   !- Density {kg/m3}
   830.0000    ,   !- Specific Heat {J/kg-K}
  0.9000000    ,   !- Thermal Emittance
  0.7200000    ,   !- Solar Absorptance
  0.7200000    ;   !- Visible Absorptance

why is this happen ?  if these properties are not the
exact value can u help me how to get the exact value.
you can imagin that these properties varies the
cooling load greatly.

tahnks
tek



            
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Thanks 
Rahul Chillar 
Grad Student 
Building Systems lab
Univ Of Illinois Urbana Champaign

            
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