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RE: [EnergyPlus_Support] Light shelf surface temperature
Vassilis,
Thanks for filling in some more of the details for me. It does sound like a mysterious result to me too. The incident solar on the shelf, the absorption of solar energy, and its release to the zone should all be governed by fairly standard heat balance processes within EnergyPlus...really the same ones that occur with the floor surface or any other zone surface. I will have to take a look at your file to see what else might be going on. Could you please post your file to the Problem Submittals folder? Thanks.
P
-----Original Message-----
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx]On Behalf Of Vassilios GEROS
Sent: Wednesday, December 13, 2006 6:40 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Light shelf surface temperature
Dear Peter,
Thank you for your answer. I fully agree with you but in my own model that concerns a building that is located in Athens-Greece, I'm facing the same problems. Even though the model is realistic (ventilation, infiltration, A/C etc) I also observe high surface temperatures as in the example model. Also, something that makes me feel uncomfortable is the fact that when I reduce the size of the inside shelf, the surface temperature of the shelf is increased. For example for my own model under the Athens climatic conditions the peak surface temperature of the shelf is 86C when its "depth" is 0.3m, while the peak temperature is increased to 163C when the size is reduce to 0.1m. The solar absorptance of the shelf is 0.3 for both cases and the glazing of the window is double pane with clear glazings. Of course the effect of these thermal conditions is almost the elimination of the heating load.
Vassilis
----- Original Message -----
From: Ellis, Peter
To: EnergyPlus_Support@ <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com
Sent: Wednesday, December 13, 2006 6:47 AM
Subject: 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 inside sh
elf; 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@ <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com [mailto: EnergyPlus_Support@ <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com]On Behalf Of Vassilios GEROS
Sent: Tuesday, December 12, 2006 11:27 AM
To: EnergyPlus_Support@ <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com
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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