exterior blind unexpected results

asked 2019-12-14 14:19:36 -0500

Determinant gravatar image

I ran parametrics on exterior blinds, checking only the solar gains through the windows. I've varied the azimuth, reflectance, tilt and spacing of the blinds and compared it to a no-slat baseline to get the savings.

Some results follow the trends I expect: towards the south there is more savings, lower reflectance has more savings, but others are unexpected. As the depth of the slats decreases, there is less savings which makes sense, but at a certain point, a somewhat short slat (e.g. depth:spacing = 1:4) has higher solar gains than a no-slat baseline.

My only guess is that maybe at the hours when the sun would normally be at a very indirect angle to the windows, the no-slat baseline case benefits from the glancing angle which spreads the sun's energy over a larger area (i.e. it is less intense), but maybe with slats somehow the redirection of sunlight makes the angle more direct and therefore results in more solar gain. I find this "redirection logic" difficult to believe for two reasons:

  1. I think sunlight bounces off the slats diffusely which would result in a generally low intensity.
  2. According to this logic, as tilt angle increases, sunlight would be bounced more directly at the windows and solar gains would be higher, but results show the solar gains to be lower as tilt angle increases (which is as expected without this "redirection logic").

Does anyone have any similar experiences or thoughts on what might be happening?

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