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EnergyPlus-calculated design sensible cooling load extremely small

asked 2017-04-27 12:53:56 -0500

W. Chiang gravatar image

updated 2017-04-27 13:03:54 -0500

Hello everyone,

I am trying to simulate the design cooling load of a simple conditioned building with no ventilation but with cooling and heating set point. However the "design sensible cooling load" I've got is only about 20W/㎡, which is too small compared to the results I calculated with CLTD method and by TRNSYS(about 140W/㎡, a normal value by my experience).

I examined output variables and had found all values under "surface inside/outside face convection heat gain rate" to be negative, meaning that the building loses heat to its surroundings all the time. That is apparently abnormal since it is also shown in output variables spreadsheet that surface inside temperature have always been lower than surface outside temperature for all the hours of design day.

Below are my IDF file (Google Drive shareable link, sorry for the inconvenience since I don't have enough points to be able to upload file directly): https://drive.google.com/file/d/0B_io...

Here's a screenshot of the abnormal "surface inside/outside face convection heat gain rate": https://drive.google.com/open?id=0B_i...

Deeply appreciate it if anybody could come to my aid.

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answered 2017-04-27 13:52:52 -0500

I would investigate the ground temperatures used in your model. See the description in the InputOutputReference in section Site:GroundTemperature:BuildingSurface. The eio file will report the ground temperatures used in the simulation, although I am not sure if they are reported when the Site:GroundTemperature:BuildingSurface object is not included in the input.

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I have considered this problem before, but the problem I encountered has something to do with surface convection, while heat exchange between surface and the ground is in the form of conduction. Also, by default, ground temperatures are set to 18 degrees C(64.4 degrees F). Since heat exchange between the ground and building surfaces is in the form of conduction, heat loss to the ground shouldn't influence surface heat gain so much. In summer design days, surface heat gain from air and solar radiation should be way more than heat loss to the ground.

W. Chiang gravatar image W. Chiang  ( 2017-04-27 20:56:56 -0500 )edit

I tried to change ground temperature to be the same as my cooling set point temperature, but the problem remains, still losing heat by convection.

W. Chiang gravatar image W. Chiang  ( 2017-04-29 09:05:29 -0500 )edit

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Asked: 2017-04-27 12:53:03 -0500

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Last updated: Apr 27 '17