Radiation

filipe.madureira · Posts: 10

Dear Andy:

I'm studing the effect of the presence of neighbour buildings in the radiation gains of my case study building.
To do that, I built a very simple model with a small building with a 90% glazed north facade. I defined also a 0% glazed area on the other facades. In front of this building, I created a second one, very tall and which south facade is in front of the north facade of the first one.
I' ve studied the reflection effect varing the distance between the two buildings. I made simulations for separation distances of 5, 10, 15 and 20m. I also made a simulation unchecking the 'Include all buildings in shading calcs' option on the Simulation model Tab. The simulations were carried out in a Winter day (1 december) and in a Summer day
(1 July).
I' m sending to you charts illustrating the results and the 20 m distance between buildings DB model.
The winter results are in agreement with the expectations. The closer the buildings are, the greater the solar gains of the smaller building.
Unexpectedely, the same does't happen in the Summer. As the buildings get closer, the solar gains get reduced. At the beggining, we thought that it could be related with the settings of the bigger building especially the outside surface temperature, but now we know that EnergyPlus models Radiation gains taking in to account diffuse solar and diffuse sky solar reflection radiation and beam-to-beam reflection. Other curious fact is that the effect of the presence of the neigbour building is more pronounced in the Summer.

Could you help us with this?
Best Regards

Filipe Madureira

Andy Tindale · Posts: 1205 Location: Stroud

Dear Filipe,

I have had a look at your model and checked the DesignBuilder IDF output of shading surfaces and made a some modifications which will improve the accuracy of the amount of glazing on shading surfaces. However I don't think these changes will affect the results you are getting. I would make the following observations about the results you have obtain:

1. The results for winter are for a clear sunny day with a predominance of direct normal solar radiation, whereas the results for summer show a lot of diffuse light. The EnergyPlus reflection specular algorithms for Windows only treat direct radiation. In other words Windows do not reflect diffusely in EnergyPlus.

2. The glazing on your adjacent building is only 10% of the total facade and is not highly reflective, so the main effect of the tall building is diffuse reflection of solar radiation.

3. When the tall shading building is close to your actual building, it reflects solar radiation received on its south face, but it also blocks part of the sky. I suspect that the reason you are seeing reduced radiation in the summer is that:

a) the shading building is blocking the sky and because you have chosen a cloudy summer day it is quite bright in the northern part of the sky.

b) the solar radiation incident on the shading building is less in the summer because the solar incident angle is smaller. In the winter sun is much closer to being normally incident to the south facade of the shading building.

Having looked at the results in quite a lot of detail I don't think there is a problem, but I would recommend downloading the new version of DesignBuilder (1.1.0.006) which has improved glazing output on shading surfaces when it is ready, use a sunny summer day and recheck your calculations.

One other thing I noticed in the course of my investigation was that wall facades (solar reflectance values are 0.2-0.3) can be far more reflective than standard glazing (reflectance values of 0.1-0.2). This was not what I expected.

Please let me know how you get on.

Andy

Login