Re: [EnergyPlus_Support] Some issues about non-convex surfaces

One reason which I can think of is -- In case of FullExterior, all the surface will get equal amount of diffuse radiation (after the floor absorbs the beam radiation), i.e. they will have "SOME" share of radiation that came inside. But in case of FullInteriorAndExterior, since it calculates the amount of beam radiation falling on each surface in the zone, and there could be some surfaces (in non-convex zone) which are not able to "see" (in terms of view factor) the exterior window, hence those surfaces will not have "ANY" share of incoming radiation. That is why we get warnings saying e.g. "There are 14 surfaces which are casting surfaces and are non-convex. Shadowing values may be inaccurate." But this is my hypothesis, someone should correct me if needed.

PS: Farhang have you checked Engineering reference on this?

Vaibhav Jain

www.vaibhavjain.co

On 11 September 2012 19:49, Javed Iqbal <eee.javed@xxxxxxxxx> wrote:

Hi,

I agree with jason and came across similar problem sometime back. it will cause inaccurate solar radiation distribution calculation in the space. See the discussion on page 16 of the I/O guide (pg. 60 of 1932 in the PDF). The impact is severe if the shading surface in larger in size.

Here is a very good example http://www.youtube.com/watch?v=GzJSHQHTHy4 Even if you don't use OpenStudio, you'll get the idea of what needs to be done.

Hope this helps!

On Tue, Sep 11, 2012 at 6:32 AM, Jason Quinn <jason.e.quinn@xxxxxxxxx> wrote:

I actually haven't researched why the non-convex surfaces are bad but I have seen how to fix them (at least with OpenStudio). Essentially you break your surface into smaller segments that are all convex. In OpenStudio all you do is draw additional lines in Sketchup to segment the surfaces to squares or triangles and the warning goes away.

Not sure if this helps as your question 4. below seems to imply that you knew this. The folks from NREL (who are not E+ experts but do a lot of modeling and wrote the OpenStudio interface) told me this fixes this issue. If they are correct than the answer to question 4, is Yes if you break it up it should work and the results should be good.

On Tue, Sep 11, 2012 at 9:23 AM, Farhang <farhang.tahmasebi@xxxxxxxxx> wrote:

Some issues about non-convex surfaces:

1. I have a non-convex roof in my model and I get a warning about shadowing calculation of this surface. I change the "Polygon Clipping Algorithm" to SutherlandHodgman. But again I get this warning. Why?

2. I don't get this warning for the floor. Doesn't the floor play any role in shadow calculation (as a receiving surface), even in "FullInteriorAndExterior" solar distribution?

3. This zone is L-shaped. I change the solar distribution to "FullInteriorAndExterior", but I get no warning or errors, while in the reference it's been clearly stated that this solar distribution mode should not be used with non-convex zones.

4. If in an L-shaped zone we break down the roof and floor to convex surfaces, can we rely on the "FullInteriorAndExterior" solar distribution simulation?

--

Javed Iqbal, LEED AP, CEA

Energy Analyst