HB UTCI Calculation at night (No Solar Exposure) with and without shade

hi @charlie.brooker,

Thanks a lot for digging into this with me! I will go through everything that you sent and see if I can get a better understanding on what happens inside the UTCI component calculations.

However, I have a quick question. You mentioned that the temperature of the exposed surfaces may be high and carry late into the evening. However, I thought that with grasshopper (unlike ENVI-met for example), calculations for each hour are not built on the previous hour, and it is done for each instant based on the numbers obtained from the epw file. am I wrong about this? If so, this can be a good explanation to the results obtained.

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Hi @regwan,

No worries, getting stuck into these problems helps me learn more about the tools.

The Honeybee UTCI Map is driven by a couple processes. There’s the energy model side with energy plus which is used to simulate surface temperatures. The simulation carried out by energy plus is a dynamic simulation that will account for the previous hours condition. Honeybee does this for surfaces only, whereas I believe ENVI-met also does this for air temperature, humidity, and other variables (my understanding is that in reality the variation in those factors are fairly minor in external environments where air can move freely). The other part is the Radiance based simulation which accounts for solar exposure of the person, and that’s just an instantaneous effect with no time dependency (the solar effect on surfaces is calculated by E+).

If you use Ladybug instead of Honeybee then it will just give the instant results based on the epw inputs.

Hi @charlie.brooker ,

Thanks for the explanation. I did not know that. It helps a lot. And it possibly explain the results. I will further dig into this and will post any updates I find.

Thanks a lot

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@charlie.brooker it is just as simple as that. You’re correct.
@regwan, I’ve used the legacy version before in a study in Cairo. Not sure if this holds true for LB1.5, Point MRT mainly depends on the surface temperature from EnergyPlus, and during the night, it is the sole constituent of the solar adjusted MRT (Sky temp is not accounted for). Regwan, you’re correct, the calculation is done on an hour-by-hour basis in Honeybee; however, all the heat transfer processes (conduction, convection, radiation, etc.) hold still in EnergyPlus. So, as Charlie suggested, do check the surface temperature, and I am pretty sure it is going to be the main driver. With shades, surfaces are not radiated, so their temperatures stay lower, and so do their LW share in the point MRT, and vice versa in the non-shaded mode. Hope that helps!

All the best,

@YASSER_IBRAHIM
Thank you Yasser for your reply. I am so surprised to see your name, because I was just looking at some of your published papers!
|A parametric optimisation study of urban geometry design to assess outdoor thermal comfort| and |Improvement of the Ladybug-tools microclimate workflow: A verification study|.

I was going to reference your work in a paper I am working on. Because my study is on Cairo, I found your latest work very beneficial. Especially the validation for the use of Ladybug Tools.

Thank you for your help on the above issue, it sure helps a lot.

Regarding the MRT calculation at night, how come it is only based on the surface temperatures? This shall be a bit different than reality then. Any additions on how to fix that? Because I thought that a low sky view factor at night should have a negative effect on the thermal comfort.

Thanks

Hi @regwan

Glad you found the papers useful. Forgot to mention, the sky temperature is already accounted for in the PointMRT component, by assuming that sky and air temperature (running weighted mean) are equal, which is fairly accurate, especially in cloudy conditions.
https://doi.org/10.1016/j.enbuild.2018.11.037
During the day, a new component of sky temperature (based on EPW sky radiation - same as in EnergyPlus) is added. This usually comes with a value of air temperature [minus] some 11 C (ISO 13790). This value is then multiplied by the SVF which - in the case of indoor conditions as in the legacy version - only comes from the window aperture, thus will always be minimal. This is why it is neglected at night, given that prevailing outdoor temperature is already accounted for in the PointMRT. If this value is added at night for outdoor conditions, sky temperature will be overestimated (considered twice with large SVF). I’m pretty sure @chris has taken care of that in the new LB1.5.
About the negative effect on thermal comfort, this is a two-fold aspect - radiative and convective. Radiative transfer to the atmosphere is already considered in E+. The reduction in surface temperature due to shading during the day is fairly higher than the attenuation of the longwave diffusion.
http://dx.doi.org/10.1016/j.renene.2012.12.040
Note that you’re getting UTCI which is mainly shaped by MRT rather than air temperature. You may be correct here that air temperature should be higher under covered canyons; however, the convective (turbulent) motions -the second aspect- are not considered in LB, neither in E+. This, however, could not improve thermal comfort significantly - only a matter of 1-2 C; sometimes, it may deteriorate cooling rates due to convective heating.

Hope that helps!
All the best,

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Thank you so much @YASSER_IBRAHIM for explaining all this to me. I appreciate it.

I have been trying to dig deep into how calculations are made beyond the components used, to try to understand more. It has been a bit of a challenge.

I wonder is there a source that identifies changes that has been made between the ladybug versions? Especially in the way calculations are made.

For example, in older versions there used to be a wind component that would adjust the wind for the urban areas, now it is included in the HB UTCI component, and basically divides by a factor (1.5) to calculate it at ground level. But I have only known this when I asked Chris on another post.