How can I set Solar Absorptance for windows?

asked 2017-10-25 12:37:45 -0600

updated 2017-10-25 19:44:42 -0600

I am employing a CFD software to study site-specific weather boundary conditions. It takes into account incoming solar radiation as well as convection heat exchanges, providing surface temperatures through a global energy balance. I can, therefore, use EnergyPlus actuators on each wall to override the surface temperature.

The surface temperature provided by the CFD software already takes into account the radiative heat exchange with the sun, so I can't include it twice. I have to exclude the solar heat gain impact on my building model, modifying the building material properties. I can't just set the incoming radiation to 0 otherwise I would deny the "light gains" and the "heat gains" that come with the light filtered through the window glass.

I am setting the Solar Absorptance on the walls equal to 0 and I get the result I want. However, how can I set the same for a window? Basically, what I am trying to do is to exclude the radiant heat gain from the surface heat balances. I would like to exclude the solar heat gain on windows surfaces, without excluding the light solar contribution. Is there any way to do that, besides the Solar Absorptance coefficient?

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Are you trying to exclude the solar heat gain on just the window surfaces, but still keep the solar heat gains to a zone? Could you just pass all the solar gain to the zone? Understanding your use case in a bit more detail may help determine an approach.

mdahlhausen ( 2017-10-25 17:13:31 -0600 )edit

I am trying to exclude the solar heat gain on the whole zone. On walls, setting solar absorptance to 0 works fine enough. However I am trying to figure out what to set on the windows. Maybe the solar heat gain coefficient? I can't just set the incoming radiation to 0 otherwise I would deny the "light gains" and the "heat gains" that come with the light filtered through the window glass.

carlobianchi89 ( 2017-10-25 17:19:09 -0600 )edit

Visible light is part of solar heat gain. Some of your heat gains include your light gains.

mdahlhausen ( 2017-10-25 17:39:32 -0600 )edit

Could you explain your use case and the problem you are considering? That will help those on the forum better recommend a modeling approach.

mdahlhausen ( 2017-10-25 17:51:57 -0600 )edit

Yes, sorry, I modified the question and I added more details.

What I am talking about is basically the effect of visible light that filter through a transparent surface. Like in a greenhouse: The incoming solar radiation affects the heat balance on the greenhouse external surface, but the visible radiation filters through and heats up the greenhouse. I would like to include in my model this second effect, getting rid of the first one (surface heat balance). Any idea how to do that?

carlobianchi89 ( 2017-10-25 18:45:24 -0600 )edit

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answered 2017-10-26 10:12:03 -0600

MJWitte

9740 ●6 ●10 http://www.gard.com/

You say that the CFD model "takes into account incoming solar radiation as well as convection heat exchanges, providing surface temperatures through a global energy balance." As mentioned in one of the comments above, in EnergyPlus, "solar radiation" covers the entire solar spectral distribution, including visible light. If the solar radiation in the CFD model covers the full solar spectrum, then allowing visible light through the window would double-count that heat gain. Or are does the CFD model include just an outside surface heat balance on the window surface with no tramsission of solar into the zone?

In either case, if you want a window to transmit only visible light in EnergyPlus, then use WindowMaterial:Glazing with Optical Data Type = Spectral. In the MaterialProperty:GlazingSpectralData, you can set the properties differently based on wavelength. For wavelength ranges that you want to exclude, set the front reflectance to 1.0 and the trasmittance to 0.0.

If you want to remove a window from the interior radiant exchange, set the Back Side Infrared Hemispherical Emissivity to near-zero for the innermost pane.