Re: [EnergyPlus_Support] Re: Solar distribution in a double skin facade against simple skin results

[ppeixeiro@xxxxxxx]

Hello Aymeric

Sorry for the late reply.

"the radiated part in the SHGC value is small compared to the directly
transmitted one."
Thats true dependent on the optical properties of your window. so if you
have a high transmitance and low abortance, its only natural that the
direct radiation into the room is the big part of the SHGC.



"In the case of the double skin I do find solar gains that are almost only
half than those of the simple skin case"

The solar radiation that is directly transmitted into the main zone by
having the DSF as a single zone should be pretty close to a window defined
in the main zone with the same glazing configuration (provided that you're
using FullExteriorAndInterior). I think this should be your main basis for
comparison between single window and a DSF.

If you're using the SimpleGlazingSystem to compare SHGC values, i
recommend changing to Material:Glazing (you can determine equivalent one
glazing transmission/reflection of a multi-glazed window by using the
equations on Eng. Reference page 207); also dont use shading devices while
running these tests to keep results more simple.



"E+ reaches some kind of limit in handling properly the solar distribution"
Thats a possibility. I don't recall seeing validation for this situation,
and it might just be the case that the optical behaviour of a DSF is
distorted by the fact that the software doenst well the "link" of the
radiation transmition between the glazings (through a thermal zone) as
good as it should.

(i'll reply seperatly to your other post)





> Hi Pedro,
>
> Nice to continue exchanging about this project with you!
> What you say I get it but I had the pre conceived idea
> that the radiated part in the SHGC value is small compared
> to the directly transmitted one. When you look at the the
> absorption and transmission coefficient it does look like that.
> Anyway, even if that is not true and my simulation results
> tend to confirm that I have another problem here:
>
> In the case of the double skin I do find solar gains that are almost only
> half than those of the simple skin case, proportion that I found
> irrealistic, but again it may be my preconveiced idea. Do you really think
> the cavity airflow does remove that much of the heat brought by the sun,
> assuming of course it is correctly ventilated?
>
> Then the second issue is that, even if I have that difference on the solar
> gains I don't see a lot of difference on my sensible cooling load...And
> that bothers me a lot.
>
> Now I'm afraid that due to the complex geometry of this project, E+
> reaches some kind of limit in handling properly the solar distribution but
> that's true I was first afraid of this because of the difference I
> observed between simple and double skin.
> Any thought on that?
>
> Aymeric
>
>
> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Pedro Peixeiro <ppeixeiro@...>
> wrote:
>>
>> Hi Aymeric
>>
>> SHGC is the amount of solar radiation that enters a room both directly
>> transmitted through the facade and the fraction of radiation that its
>> absorbed by the facade and then transmitted into the room by both
>> convection and radiation heat transfer. In the pressence of a DSF, there
>> is a certain amount of heat transfer occurring inside the cavity (heat
>> removed by the air flow). This amount is not stationary, because it
>> depends on wind incidence, wind velocity, temperature difference between
>> cavity air and outside air, and surface temperature of the panes.
>> Because of this un-steady state heat flow, its not possible to establish
>> a convectional SHGC value for a DSF. Also, in reality, the heat flow
>> also varies with the height of cavity (temperature inside the cavity
>> increases with height) and so will heat transfer through the inside
>> pane. So it basically means we would get a difference SHGC value in
>> function of height, which complicates even more a definition of a single
>> SHGC value for a facade.
>>
>> Pedro
>>
>> On 01-04-2011 11:43, aymericnovel wrote:
>> >
>> > Dear all,
>> >
>> > I am studying a double skin facade design. I want to compare with a
>> > design using a simple skin with windows that have a SHGC of 0.25
>> > because it is the assembly SHGC I have to reach according to the local
>> > chinese standard. See the architect have asked me to justify the cost
>> > of using the external skin...good question when you think about it!
>> >
>> > So I have chosen inner skin and outer skin glazings so that it is
>> > equivalent first with a simple approximation of SHGCin x SHGCout =
>> > 0.25 . I know it is too simple but still it should not be too
>> > different in the zone inside the building.
>> >
>> > I am having trouble understanding the following fact:
>> >
>> > Having a combined SHGC of the inner and outer skin very close to the
>> > simple skin I try to compare with, I do not obtain at all the same
>> > amount of sun in the zone inside the building. Why? Normally the sum
>> > of the direct and diffuse solar transmitted through the internal
>> > windows should be close to the one through the external window in the
>> > case of the simple skin no?
>> >
>> > I have the feeling I am totally missing something basic here but it's
>> > late and I don't see it.
>> >
>> > Your help is more than appreciated!
>> >
>> > Thanks
>> > Aymeric
>> >
>> >
>>
>
>
>



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