Questions about radiation results

Discussions about the Version 4 concerning the core model simulations
BGovehovitch
Posts: 27
Joined: Mon Mar 12, 2018 9:30 am

Questions about radiation results

Postby BGovehovitch » Thu Feb 06, 2020 8:58 am

Dear ENVI-met team,

I'm trying to get a better understanding of the radiation model of ENVI-met. But I get strange results.

I use a simple cube (edges of 15m) as a case study.
simple_cube.PNG


1) My first question is about the shortwave radiation received on the roof. Here is the radiation that I used as the input.
radiation_input.PNG

My understanding at this point is that the value "SW dir max" is the shortwave DNI and the value "SW dir hor" is the shortwave DHI. If I am right, I should get {DHI + diffuse} on the roof. But it seems that I get {DNI + diffuse}. Is it the expected result? What do I miss?
radiation_roof.PNG


2) I carried out 3 different simulations with the same profile of irradiation and a constant air temperature. It has no physical meaning because the temperature was set to 0, 25 and 50°C. But the goal was to make sure that the irradiation is independant from the air temperature. Unfortunately, it does not seem to be the case. Indeed, for the same irradiation level, I have different results.
For a temperature of 0°C:
radiation_roof_temp_0.PNG

For a temperature of 25°C:
radiation_roof_temp_25.PNG

For a temperature of 50°C:
radiation_roof_temp_50.PNG

Everything is equal for the 3 simulations but the air temperature. But I don't understand how the air temperature can influence the irradiation level.

3) I tried to understand how the global irradiance on façades is evaluated. I used the formula G = DNI*cos(angle)+Gdiff. But, as you can see, the result is not exactly equal to the result of the simulation.
diffuse_difference.PNG

The difference is low but it makes me think that I am doing something wrong. I am pretty confident on the evaluation of the direct irradiance, therefore I am wondering about the evaluation of the diffuse component of the solar radiation.

I thank you in advance for your reply.
Best regards,
Benjamin

helge
Posts: 107
Joined: Mon May 15, 2017 12:27 pm

Re: Questions about radiation results

Postby helge » Thu Feb 06, 2020 10:31 am

Dear Benjamin,

what exactly are your boundary conditions? Are you using full forcing? Some of our radiation outputs are relative to a normal surface (always perpendicular to the incoming direct shortwave radiation).

Best
Helge

BGovehovitch
Posts: 27
Joined: Mon Mar 12, 2018 9:30 am

Re: Questions about radiation results

Postby BGovehovitch » Thu Feb 06, 2020 1:46 pm

Dear Helge,

I used either the beginner or the intermediate option with the default options as the boundary conditions.

I am particularly interested in the incoming radiation on buildings' façades, therefore I am not looking to the results in the radiation folder but the variable "Wall: Shortwave radiation received at facade". Thus, the incoming radiation on the roof should be horizontal, and not normal.

Benjamin

Tim
Posts: 1089
Joined: Mon May 15, 2017 3:46 pm

Re: Questions about radiation results

Postby Tim » Thu Feb 06, 2020 3:02 pm

Hi,

I do not know if that model area works fine (very little borders at the sides and at the top). However, could you upload the files here=?

Best regards,
Tim

BGovehovitch
Posts: 27
Joined: Mon Mar 12, 2018 9:30 am

Re: Questions about radiation results

Postby BGovehovitch » Thu Feb 06, 2020 4:38 pm

Hi Tim,

I know the area is very small. But it should not affect the radiative part. Should it?
Here are the four projects that I used as case studies.

Thank you for your help.

Benjamin
Attachments
Block_summer_tests.zip
(50.51 KiB) Downloaded 1 time

Tim
Posts: 1089
Joined: Mon May 15, 2017 3:46 pm

Re: Questions about radiation results

Postby Tim » Fri Feb 07, 2020 9:18 am

Hi Benjamin,

Your shortwave radiation values differ because of the diffuse portion. It is reduced due to a high water vapour pressure in the atmosphere. That is caused by your increased air temperature values whereas the relative humidity was set the same for all simulation. The absolute amount of water is thus much higher in the hot simulations causing the diffuse radiation to be reduced.

Best regards,
Tim

BGovehovitch
Posts: 27
Joined: Mon Mar 12, 2018 9:30 am

Re: Questions about radiation results

Postby BGovehovitch » Fri Feb 07, 2020 11:45 am

Hi Tim,

Thank you very much for the answer. It's much clearer for me.

But what about my first question? Why do I get {DNI + diffuse} on the roof instead of {DHI + diffuse}?

Benjamin

Tim
Posts: 1089
Joined: Mon May 15, 2017 3:46 pm

Re: Questions about radiation results

Postby Tim » Fri Feb 07, 2020 12:51 pm

Hi Benjamin,

You get the horizontally received shortwave radiation since in general you want to see how much radiation is actually received by that facade element. In order to so, exposition and inclination are considered for recalculating the values according to lambert.

Best regards,
Tim

BGovehovitch
Posts: 27
Joined: Mon Mar 12, 2018 9:30 am

Re: Questions about radiation results

Postby BGovehovitch » Fri Feb 07, 2020 3:32 pm

Hi Tim,

I'am not sure to understand your answer correctly. Perhaps, my question is not clear enough. I'll try to make it clear.

My principal focus is on the variable "Wall: Shortwave radiation received at facade" in the buildings folder, which is, if I am right, the sum of direct and diffuse shortwave radiation. Theoretically, the level of radiation on building's façade can be calculated with flux(façade) = DNI*cos(angle)+diffuse, with angle, the angle between the sun rays and the façade. Similarly, DHI = DNI*cos(angle).

When I use this equation on the vertical façades, I get roughly the same result as the one given by ENVI-met. But, the case of the roof is different. I should get the horizontally received radiation but, when I compare the results with the theoretical ones, ENVI-met seems to provide with the normal received radiation.

Here are all the comparisons between theoretical and simulation results. The curves match for the vertical façades but not for the roof.

======================
INPUT
radiation_input.PNG


======================
WORKS FINE
radiation_north.PNG
radiation_south.PNG
radiation_east.PNG
radiation_west.PNG


======================
NOT AS EXPECTED
radiation_roof.PNG


Best regards,
Benjamin

Tim
Posts: 1089
Joined: Mon May 15, 2017 3:46 pm

Re: Questions about radiation results

Postby Tim » Mon Feb 10, 2020 11:41 am

Hi Benjamin,

Now we understood your question and indeed found a bug there. DNI was used instead of DHI. Thank you very much for the hint! We need to have a look at the changes since the last release and see if it is possible to make a bugfix release soon.

Best regards,
Tim

BGovehovitch
Posts: 27
Joined: Mon Mar 12, 2018 9:30 am

Re: Questions about radiation results

Postby BGovehovitch » Mon Feb 10, 2020 12:39 pm

Hi Tim,

Thank you for your reply. I'm looking forward to your fix.

I have one more question. Is the bug affecting only roofs or all kinds of horizontal surfaces, like ground surface?

Best,
Benjamin

Tim
Posts: 1089
Joined: Mon May 15, 2017 3:46 pm

Re: Questions about radiation results

Postby Tim » Mon Feb 10, 2020 1:11 pm

Hi Benjamin,

No, only the roof.

Best regards,
Tim

Tim
Posts: 1089
Joined: Mon May 15, 2017 3:46 pm

Re: Questions about radiation results

Postby Tim » Fri Feb 14, 2020 4:20 pm

Hi Benjamin,

An updated version including that bugfix is available online.

Best regards,
Tim

BGovehovitch
Posts: 27
Joined: Mon Mar 12, 2018 9:30 am

Re: Questions about radiation results

Postby BGovehovitch » Thu Feb 20, 2020 9:51 am

Hi Tim,

Thank you for the info. Now everthing looks right, I will be able to rerun my simulations.

Thank you for your support.

Benjamin


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