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Re: [EnergyPlus_Support] Light shelf surface temperature
Dear Peter,
Thank you for your reply. It is true that the behaviour of the model shows that the inside shelf receives the same amount of solar radiation regardless its size. Your approach to solve the problem was also one of the solutions I was thinking, but what I'm trying in this project is the minimization of energy consumption (heating, cooling and lighting) based on the optimization of the dimensions of light-shelf, the thermal and optical properties of the window and optionally the size of the window. Therefore it is important to have one model that is capable to perform all the calculations at once. My opinion is that the definition of the inside shelf affects dramatically the solar radiation distribution in the room (maybe there is a bug in the algorithms). When I use a darker glazing (see the uploaded file) the surface temperature of the shelf is decreased importantly. Anyway, thank you again for your help.
Vassilis
----- Original Message -----
From: Ellis, Peter
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Sent: Tuesday, December 19, 2006 9:09 PM
Subject: RE: [EnergyPlus_Support] Light shelf surface temperature
Sorry, Vassilis, I do not have a final answer for you yet on this problem. However, I can tell you that one of the modeling simplifications for the inside shelf is that all beam solar that comes through the associated window is assumed to strike the shelf. No solar can pass the end of the shelf regardless of the beam incident angle or the position and shape of the shelf. This is documented in the Engineering Reference description of the model. Of course, this is not entirely realistic because some beam will miss the shelf at low angles. This explains why a shallow shelf can get much hotter than a deeper shelf: the same amount of solar gain is absorbed by a smaller mass. The consequence is that this model will tend to work better when the shelf depth is greater (or much greater?) than the window height.
What I have not been able to figure out is if the overall heat balance on the shelf is incorrect or not. I'm not sure if the extra solar absorbed is enough to account for the high temperatures or if there is some other problem. It still seems high to my intuition. Right now I am having trouble getting some of the window solar variables to report a value, so I do not know the answer yet. I will post when I have it figured out.
Depending on what you are trying to do, there might be an alternate way for you to continue with your analysis in the meantime. You can do two simulations: one for daylighting and one for energy. First, set up your model for daylighting using the DAYLIGHTING DEVICE:SHELF object, just the same way you've been using it. Check the illuminance set points until you are satisfied that the daylighting design is what you want. Ignore the temperatures and energy numbers for now. Now, delete or comment out the DAYLIGHTING DEVICE:SHELF object and run your model again. These results should be very close to the correct temperatures and energy numbers. The downside is that if you are doing automatic dimming of the electric lights (which you probably are), you will not see the benefit of the light shelf as a device for diffusing light farther into the zone, in which case this approach might not be very useful to you. Unfortunately, that's the best that I have for the moment.
P
-----Original Message-----
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx]On Behalf Of Vassilios GEROS
Sent: Thursday, December 14, 2006 1:47 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Light shelf surface temperature
Dear Peter,
I have uploaded the file in the Problem Submittals folder under the name Cell_LightShelf.zip. Thank you.
Vassilis
----- Original Message -----
From: Ellis, Peter
To: EnergyPlus_Support@ <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com
Sent: Thursday, December 14, 2006 1:16 AM
Subject: 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@ <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com [mailto: EnergyPlus_Support@ <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com]On Behalf Of Vassilios GEROS
Sent: Wednesday, December 13, 2006 6:40 AM
To: EnergyPlus_Support@ <mailto:EnergyPlus_Support%40yahoogroups.com> yahoogroups.com
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
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@ <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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