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Re: [EnergyPlus_Support] wall and roof color
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
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Grad Student
Building Systems lab
Univ Of Illinois Urbana Champaign
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Univ Of Illinois Urbana Champaign
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