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Re: [TRNSYS-users] Type 34, again ...



Christophe,
I have seen the same phenomenon; essentially, Type34 makes an isotropic sky assumption when it calculates the diffuse radiation on the receiver so you have to use that same assumption in your weather data / radiation processor. The isotropic sky model gives a somewhat conservative estimate of solar radiation than does Perez. That means that your building heating loads will be higher, your cooling loads lower, and the effects of shading devices will be less important. Both models (isotropic and Perez) are certainly well within the uncertainty of other modeling assumptions.
Kind regards,
 David


At 10:50 4/19/2007, Enertech wrote:
Dear TRNSYS Users,

When I want to simulate a building, the biggest difficulty is to take into
account the good insolation. For a big building with a lot of balconies,
it's impossible to be accurate in calculation of shading masks, without
using hundreds of type 34. So we have to simplify the phenomena.

I find a new problem (for me) with Type 34 : I simulate a building without
any overhang and wingwall and I find a certain quantity of Qheat = Qheat1
(for example). I want to test wingwall, so I add a type34. At first, I test
a type 34 with all overhang and wingwall lengths equal to 0. The problem is
that I don't find the same quantity of Qheat (Qheat2) for my building.
Qheat1 is bigger than Qheat2.

To refind the same Qheat in the 2 cases, I have to change the Sky Model for
diffuse radiation in the type 109 Wheather data, from the Perez Model to
the Isotropic sky model.

So, what should I think about that ? What is the impact of the Isotropic
sky model on the accuracy of a building simulation ? Is there a way to keep
the Perez Model and to refind the same Qheat in the 2 cases (with and
without type34).

Thanks for your help.
Christophe.


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