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RE: [EnergyPlus_Support] Light shelf surface temperature

 
Vassilis,
 
The problem is that the input file "DaylightingDeviceShelf.idf" was designed to be the simplest example of how to use the DAYLIGHTING DEVICE:SHELF object.  The input file, however, does not necessarily represent a realistic building model.  If you check the zone temperatures, they are also very high, peaking at 41 C.  The reason for all the hot temperatures (and cold ones in the winter) is that these zones are not conditioned, i.e., they do not have any heating or cooling systems.  Nor do they have any mechanism for exchanging air with the outdoors--no infiltration or ventilation.  Therefore, because of the windows you have a lot of solar gain in the summer with no way to remove the heat (although there is no insulation in the walls and the windows are single pane, so that helps a little).  All of these things were left out for the sake of simplicity of the example.  
 
The surface temperature of the inside shelf is naturally going to be even hotter than the zone because the direct beam of the sun strikes the surface during much of the day.  (By the way, this probably does reflect the real behaviour of light shelves.)  I would expect that the inside shelf and the floor will be the hottest surfaces in the zone, which then give off their heat to the zone air by convection and to other surfaces by radiation.  One thing that the light shelf model WILL NOT capture is the "fin effect" of an outside shelf.  For example, if you have a light shelf constructed from a single slab of material that forms the inside part and the outside part of the same shelf, there will be some conduction losses that will cool down the inside shelf, assuming that it is hotter than the outside.  So you might find that the simulated inside shelf is slightly hotter than reality.  If that's a problem, you can decrease the Solar Absorptance of the finished surface material of the ins

ide shelf; it won't absorb as much solar and won't heat up as much.  
 
If you are interested in simulating the energy performance of a real light shelf, I recommend that you study the example file to learn how to use the DAYLIGHTING DEVICE:SHELF object.  Then, create your own model of a realistic building or zone to which you can add the DAYLIGHTING DEVICE:SHELF.  You should get good results that way.  
 
P
 
 
-----Original Message-----
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx]On Behalf Of Vassilios GEROS
Sent: Tuesday, December 12, 2006 11:27 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] Light shelf surface temperature



Dear all,
I'm interesting to simulate the energy performance of a light shelf and I'm facing the following problem: the surface temperature of internal part of the shelf is very high. For example under the climatic conditions of San Francisco and when I'm using the provided example "DaylightingDeviceShelf.idf" the surface temperature of the shelf reaches 68C. This fact also influences the cooling (it is increased) and the heating (it is decreased) loads importantly due to the existence of a heat source into the space. 
Do you have any ideas on how to solve this problem?

regards,

Vassilis Geros

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