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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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> Thanks
> 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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