Dragonfly - influence of albedo and SHGC on Temperature


#1

Hello,
I’m usign Grasshopper Dragonfly plugin to simulate the urban heat island in a Sao Paulo (Brazil) Urban District.
I’m setting buildings envelopes by using the component “edit typology envelope”.
In this component I have to parametrize window SHGC, wall and roof albedo.
Now, I’m confusing about the influence of these parameters on the simulated Air Temperature, Mean Radiant Temperature and Surface Temperature of outdoor spaces.

My doubts are:

-if the value of window SHGC is increased (higher trasmission), the buildings surfaces temperatures increase too while the external temperature decrease due to the subtracted heat, right?

  • if the albedo of walls and roofs is increased (higher reflection), the building surfaces temperatures decrease while the external air temperature increase due to the reflected radiation dumped into outddor spaces, right?

Thank you in advance.

Best,

Martina


#2

@matrame ,

In honesty, both of those parameters have a relatively small effect on urban heat island and so, if you are off in your estimate of these parameters, your simulation will not be off by much.

Increasing wall albedo typically means that lower-angle sun gets reflected out of the urban canyon more easily and so it should result in an slight overall decrease in surface and air temperature in the canyon. The impact of SHGC entirely depends on what the building fact_heat_2_canon is. If all of the heat rejection of the building’s cooling system is not going to the canyon (ie. to the roof), then a higher SHGC can end up decreasing the air temperature in the canyon, since heat that would otherwise go to the canyon goes through the building windows and then gets rejected by the cooling system to the roof. If the fact_heat_2_canon is not zero, there is a chance that the heat the building takes in through the windows just gets rejected back to the canyon, thereby heating it up further than it would have been otherwise.


#3

Hello Chris, thank you very much for your answer.

So, if albedo and shgc do not influence the urban heat island, which parameters really affect the urban heat island effect? I’m testing the influence of traffic parameter, fract canyon,albedo, shgc, under the component DF City…any other idea?

And also, where can I set fact_heat_2_canon? I’m not using this parameter until now…

Really thanks,
matrame.