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Re: [EnergyPlus_Support] wall and roof color



This is correct, emittance=absorptance for a given wavelength region (solar, 
visible, IR).  One small correction, the relationship between properties is

(absorptance+reflectance+transmittance)=1.0  

Some handbooks state reflectance rather than absorptance.  For opaque 
materials, transmittance is zero, so absorptance = 1.0 - reflectance 

Mike


On 30 Jun 2004, at 8:27, Rahul Chillar wrote:

> hello
> by defination Emittance is equal to Absorptance.
> 
> For finding indiviual exact values of material you would need to refer to a heat
> transfer data book. You would need to know your exact roughness of material etc.
> 
> IR is the thermal part of radiation, and since the sum of all the Similar type
> of interactions ( absorptance+ emittance+ reflectance+transmittance)=1.0 ,
> Energy plus takes care of the value you want to enter  by its internal
> alogrithims.   
> 
> REMEMBER i dont mean to say ( Absorptance:Thermal+ Absorptance:Solar
> +Absorptance:Visible ) =1    , this is not true. 
> 
> For glass we need IR specifically because we need to know how much thermal part
> of the visible radiation is entering the building ( how much of sunlight is
> coming in to heat up the space) , through the GLASS pane. I think this field
> would not apply to opaque structure.    
> 
> 
> THanks
> Rahul Chillar
> Graduate Research Assistant
> Building Systems Lab
> Univ of Illinois Urbana Champaign.
> 
> 
> carmen herrero prada <herreroprada@xxxxxxxx> wrote:
> 
> 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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> Rahul Chillar 
> Grad Student 
> Building Systems lab
> Univ Of Illinois Urbana Champaign
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> Thanks 
> Rahul Chillar 
> Grad Student 
> Building Systems lab
> Univ Of Illinois Urbana Champaign
> 
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