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Consideration of External Radiation

asked 2021-06-19 22:43:09 -0600

Keigo gravatar image

updated 2021-06-22 05:32:25 -0600

I'm trying to understand the basic calculation mechanics of EnergyPlus, and now I'm reading the I/O Reference and EngineeringReference on how external radiation (both short-wave and long-wave) is taken into account.

Please take a look at the following tables that I summarized. These tables with some relevant input fields just show my understanding.

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My questions are:

  1. For external longwave radiation, are View Factors to the ground, the sky, the air and the surrounding surface automatically caluculated in EnergyPlus? I refered to EngineeringReference, but I can't find input fields of those view factors in IDF files. SurfaceProperty:SurroundingSurfaces has input fields of Sky View Factor and Ground View Factor, but they are optional (not mandatory).
  2. For external longwave radiation, is radiation exchange between one exterior surface of the building and another exterior surfaces of the building itself considered in EnergyPlus? For example, in my summary table, does the roof with high surface temperature affect the surface temperature of the nearby wall? If so, where should I refer to in EngineeringReference or I/O Reference?
  3. For external longwave radiation, can we change the emissivity of the ground and the neighbouring buildings? EngineeringReference says that the emissivity is 0.9.
  4. Is there anything incorrect with the above tables? I'd appreciate it if anyone could point it out.

Answers to not all but just one of the questions would be appreciated.


I read I/O Reference and EngineeringReference again, and the answers to my question 1 should be

View Factors to surrounding surface: Need to specify. View Factors to ground, sky and air: Yes, but very roughly calculated only by the angle of the surface. image description

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I guess that the above very simple calculation is used because the calculation of view factors is generally quite complicated and the effect of longwave radiation on heat balance is not so significant. On the other hand, view factors are calculated in detail in shading of solar beam because it affects much on the heat balance.

By the way, I have another 2 questions.

5.For external longwave radiation, why is "View factor of surrounding surface to exterior surface" used when there is a surrounding surface? To me, "View factor of exterior surface to surrounding surface" is reasonable, and I/O Reference and EngineeringReference look incorrect.

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6.If Surrounding Surface 1 View Factor is specified in SurfaceProperty:SurroundingSurfaces, is it the same value for all external surfaces of the building including roofs and walls that are completely hidden from the surrounding surface? I think it's realy strange. But I/O reference says it's "constant". SurfaceProperty:SurroundingSurfaces doesn't have input fields to specify targeted building external walls.

Please refer to my supporting sketches as below.

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Is it for the case where the surrounding surface 1 is also placed on all four sides of the building?

llfan_123 gravatar image llfan_123  ( 2021-07-28 19:42:30 -0600 )edit

No, there is only one "Surrounding Surface 1" on one side in my sketches. It is the case that n=1 in the equation (3.66).

Keigo gravatar image Keigo  ( 2021-07-28 20:17:03 -0600 )edit

2 Answers

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answered 2021-06-25 02:00:38 -0600

Keigo gravatar image

updated 2021-06-25 02:15:34 -0600

It is unlikely that I will get further answers, so I just updated my summary tables at this time. They may be updated further in the future.

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The Answers at this time are as follows:

  1. View Factors to ground, sky and air are roughly auto-calculated based on the tilt angle of the building exterior surface. If we want to consider longwave radiation from surrounding building surfaces, we need to input View Factors related to the surrounding surfaces manually, but the View Factors are not "from the building exterior surface to the surrounding surface" but "from the surrounding surface to the building exterior surface". It's weird. It only seems to me that EngineeringReference is incorrect.
  2. Longwave radiation from exterior surfaces of the building itself is probably Not considered. It's just because I couldn't find any mention about that in EngineeringReference. I'm not 100% sure about that.
  3. We can't change the emissivity of ground and surrounding building surfaces, but I think we don't need to do so because EnergyPlus doesn't seem to consider it. The equation of q"LWR (Exterior surface longwave radiation fux[W/m2]) used in EnergyPlus only have the emissivitiy of the building exterior surface. (Note that in general, radiation exchange between two surfaces is affected by the temperature and emissivity of both surfaces.) image description
  4. Longwage radiation from surrounding buildings is hardly considered by default because the surrounding surface temperature is the same as air temperature, which means that the surrounding surfaces are almost ignored.
  5. I mentioned in Answer 1.
  6. Surrounding Surface 1 View Factor in SurfaceProperty:SurroundingSurfaces seems to be the same value for all exterior surfaces of the building including roofs and walls that are completely hidden from the surrounding surface. This is another very weird thing.

Some questions about longwave radiation remain. They may not have much impact on energy simulations, but I just wanted to know how EnergyPlus takes them into account.

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answered 2021-06-21 13:04:41 -0600

sashadf1 gravatar image

updated 2021-06-21 13:59:27 -0600

I looked through my IDFs for residential buildings, and for all of my BuildingSurface:Detailed and FenestrationSurface:Detailed objects, the field View Factor to Ground was blank (specified with a ","). I believe that this means E+ autocalculates it. It appears for those surface objects that specifying View Factors to anything other than the ground is optional.

In I/O reference, it says for Field: Sky View Factor, "the sky view factor used in solar radiation calculation of the surface would be overwritten with the value of the fraction if defined here." If left blank, E+ autocalculates Sky View Factor, but it can be manually defined in the SurfaceProperty:SurroundingSurfaces object.

Similarly, for the Ground View Factor field, if left blank E+ autocalculates it according to certain rules (see link), but it can be overwritten/manually defined in SurfaceProperty:SurroundingSurfaces object.

It appears that Field: Surrounding Surface 1 View Factor, which defines surface view factor for neighboring buildings or overhangs, is not autocalculated by E+ and must be user inputted. I could be wrong here. It can definitely be specified, so make that correction in the bottom right cell of your table.

Note: you can define multiple surrounding surfaces (overhangs + neighboring buildings) for a particular external surface if you wish.

"The last three fields are extensible to define multiple surrounding surface name, temperature and view factor sets."

I suppose this bit of documentation helps you out with sky, ground, and surrounding surface view factors but not air view factor.

In your table, in the column "blank," for row "and the sky", I would replace "Sky temp: Auto-calculated" with "Sky temp: Global", and make the similar correction for column "input" (i.e. Sky temp: Global, but can be specified).

Leave out the question mark b.c. the docs clearly show that you can override/specify your own Sky Temp to be used with the field Sky Temperature Schedule Name. I would make similar "Global" replacement corrections for ground temp. E+ says that if ground temp is left blank, than the global ground temperature would be used.

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Thank you for your reply. Is your answer summarized as follows?

  1. VF to ground and sky: Auto-calculated. VF to air: don't know. VF to surrounding surface: Need to spacify.
  2. don't know
  3. don't know
Keigo gravatar image Keigo  ( 2021-06-21 22:28:22 -0600 )edit

One more question. What does "Global" mean? Does weather data (epw file) have global sky temperature and global ground temperature?

Keigo gravatar image Keigo  ( 2021-06-21 22:37:12 -0600 )edit

Yes. My answer is summarized as such. I do not know what Global means, but I don't think it means "Global" as in "the world." B.c. there are both "Global" sky temps and ground temps, I infer that it probably means "Global" from a "Global variable" programming standpoint, or that the temp is constant spatially.

sashadf1 gravatar image sashadf1  ( 2021-06-22 12:16:05 -0600 )edit

I don't believe that the view factor is the same for all surrounding surfaces. If you look at "The last three fields are extensible to define multiple surrounding surface name, temperature and view factor sets" statement, I believe that you can define a seperate view factor for each surface (1 through n), through Field: Surrounding Surface (1...N) View Factor. It would not make any sense for all surrounding surfaces to have the same view factor to that one external surface, since they are at different geometries relative to that external surface.

sashadf1 gravatar image sashadf1  ( 2021-06-22 12:24:39 -0600 )edit

Thanks. One thing I would like to mention is that please do not confuse "surrounding surface" with "building external surface".

What we can define in SurfaceProperty:SurroundingSurfaces is a View Factor of each "surrounding surface". The building has so many "external surfaces", but only one constant View Factor can be defined for one surrounding surface. Therefore, the View Factors of one surrounding surface to all building external surfaces should be the same. I think it's ridiculous, but as far as I read I/O Reference and Engineering Reference, it seems to be like that...

Keigo gravatar image Keigo  ( 2021-06-22 13:23:58 -0600 )edit

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Asked: 2021-06-19 22:43:09 -0600

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Last updated: Jun 25 '21