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Dear Fernando and TRNSYS Users,<br>
<br>
<i>(b) If the upstream component is type68, <br>
<br>
Sky diffuse on window = Shaded sky diffuse on window without
overhang<br>
* (1-Fws)<br>
<br>
In both cases (a) and (b) I suppose that the sky diffuse radiation
input<br>
is always the input number 4 of type34. The label in the proforma is<br>
somewhat error prone (it says “sky diffuse on the horizontal”).<br>
</i> <br>
I think you’re right !! There’s an error in the TYPE 34 proforma :
input(4) has to be the diffuse radiation <u>on surface</u> and not the
horizontal diffuse radiation. You can check it in the TYPE34 source
code.<br><br>
I’m a little bit angry to discover more and more errors in proforma
definitions. How can we trust in TRNSYS simulation results if there is
these kinds of error in models ?<br>
<br>
I’m working on complex building projects with a lot of far-away masks
(TYPE68) and a lot of overhangs, with very short notice (of course!!).
I’ve no time to check that all inputs defined in proforma are the ones
needed by source codes…<br>
<br>
I’m sorry to be so hard on this subject but modelling a building is not
an easy job and I need to trust in models I’m using otherwise it only
remains for me to say to my boss that I can’t do this job anymore,
there’s too much errors in the software.<br>
<br>
Salutation,<br>
C. Plantier<br><br>
,At 11:38 16/08/2005, Fernando Domínguez Muñoz wrote:<br>
<blockquote type=cite class=cite cite="">Dear TRNSYS users,<br><br>
I have some doubts about TYPES 34 (overhang and wingwall shading) and
68<br>
(shading by external object) that I would like to discuss with you.<br>
These problems are related to the diffuse radiation
calculations.<br><br>
In order to model a vertical window with an overhang and a wall in
front<br>
(of the window), I tried to use types 16, 68 and 34 connected in<br>
cascade. The first parameter of type34 changes the way in which the
view<br>
factors are calculated, being offered the following two choices:<br><br>
(a) If the upstream component is type16, there is no problem:<br><br>
Sky diffuse on window = Sky diffuse on horizontal *
(0.5 Fws)<br><br>
Where Fws = view factor between window and sky (takes account of the<br>
overhang and the windwalls)<br><br>
(b) If the upstream component is type68, <br><br>
Sky diffuse on window = Shaded sky diffuse on window without
overhang<br>
* (1-Fws)<br><br>
In both cases (a) and (b) I suppose that the sky diffuse radiation
input<br>
is always the input number 4 of type34. The label in the proforma is<br>
somewhat error prone (it says “sky diffuse on the horizontal”).<br><br>
In case (b), locations behind the window were already excluded (by<br>
type16), so the first term is 1. I do not understand the second
term,<br>
because the overhang is obstructing part of the sky over the window,
so<br>
the involved radiation should be that coming from the vault of
heaven,<br><br>
(b*) Sky diffuse on window = 1*Shaded sky diffuse on window
without<br>
overhang - Sky diffuse on horizontal*Fws<br><br>
In fact, suppose that type68 is used without obstacles defined (all<br>
obstruction height angles equal to zero). In this case<br>
shaded_view/full_view = 1, so<br><br>
Shaded sky diffuse on window without overhang=0.5* Sky diffuse
on <br>
horizontal <br><br>
The results of expressions (a) and (b) are inconsistent:<br><br>
(a) Sky diffuse on window = 0.5* Sky diffuse on horizontal Fws*
Sky<br>
diffuse on horizontal<br><br>
(b) Sky diffuse on window = 0.5* Sky diffuse on horizontal
0.5*Fws*<br>
Sky diffuse on horizontal<br><br>
A different and more difficult situation is when the overhang or a<br>
wingwall (or part of it) sees the external obstacle (that is, there
are<br>
two obstacles in series). In this case, part of the shaded radiation
is<br>
obstructed. So as to solve this problem, I would try to compose the<br>
geometry of the problem by combining several masks.<br><br>
Other questions are:<br><br>
Q1) The geometrical dimensions of the window can be comparable to
those<br>
of the obstacle in some cases. In this situation strict view factor<br>
algebra should be used, but if we use type68 it is not clear where
to<br>
place the measurement reference for the obstruction angles, because<br>
these angles are different for different points on the window. The
best<br>
place seems to be the centre of the window.<br><br>
Q2) When type68 is used in conjunction with type1 (flat solar<br>
collector), I think that the description given in the proforma is
error<br>
prone, because the total radiation given by type68 does not include
the<br>
ground reflected radiation, so it should not be directly connected
with<br>
the total radiation input of the solar collector type. This term can
be<br>
difficult to calculate when the obstacle is close to the collector
(in<br>
this case the radiation reflected by the obstacle can be
significant),<br>
but shall be included anyway.<br><br>
I hope this helps.<br><br>
Fernando Domínguez Muñoz<br>
University of Málaga (Spain)<br>
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