Unexpected internal temperature in 4 walls building using EnergyPlus

energyplus

asked 2019-05-24 04:51:21 -0500

Marco
11 ●2

updated 2019-05-27 15:29:08 -0500

__AmirRoth__

4436 ●5 ●17 http://bleedinggreenna...

Hi all,

I am doing some researches about the R-C representation of the building thermal system and I would like to create a model using Energy Plus to check if my model can be used every day. I decided to start in the easiest way: I created a building (idf file: https://pastebin.com/YMnYajix) with four walls, a roof and a floor. To simplify the calculations, I decided to use very low conductivity values for the roof and the floor and no internal loads. Furthermore, I decided to carry on simulations by means of a custom .epw file where all the values are fixed (e.g., a row of .epw: “2019,1,1,1,0, ,23,5,0,101000,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0 “) and the ground temperature is always equal to 23 °C like the external air temperature. No Sun is present as well as wind.

The problem arises when I perform simulation: at the end of it, the internal temperature is equal to 19.24 °C even if I expect to have 23 °C. What am I missing?

Using my RC model, this means that there is a heat flux of 15 W even if no internal loads are present. What could the cause be? Is there any variable that I should monitor?

Thanks!

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answered 2019-05-24 10:06:25 -0500

Aaron Boranian

13830 ●76 ●24 http://bigladdersoftware.com/

I would recommend doing a test simulation of just January 1 (set in RunPeriod object) to see what initial values are for the following output variables:

Output:Variable,*,Site Ground Temperature,Timestep;
Output:Variable,*,Zone Air Temperature,Timestep;
Output:Variable,*,Surface Inside Face Temperature,Timestep; 
Output:Variable,*,Surface Outside Face Temperature,Timestep;

In your IDF, it looks like you are already reporting Zone Air Temperature but have commented out the other three. For this test, I would also recommend using 15-minute timesteps instead of 1-minute timesteps that the IDF is currently set up for (for testing, timesteps that short are overkill and not necessary).

EnergyPlus performs a warmup process to determine appropriate initial values for temperatures at the beginning of the run period simulation. Surface and air temperatures are set to 23C at the beginning of this warmup process, but EnergyPlus might be converging on different values. You can see this in the output variables from the very first timestep of your run period simulation, which appears in the output CSV file. If temperature values are different between surfaces and the zone air, then heat transfer will occur between them and cause them to change temperatures as a result.

You could also include various output variables for weather data to see their values in the output CSV and confirm that you created your custom EPW file correctly. In your IDF, it looks like you are already reporting Outdoor Air Drybulb and Outdoor Air Relative Humidity. I would recommend adding other variables that could impact surface or zone air temperatures (Wind Speed, Solar Radiation, etc.).

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Thank you! I checked the variables and they correspond to the EPW values. The outdoor air dry temperature is equal to 23 °C, wind speed is equal to zero as well as solar radiations. All the external and internal walls temperatures are equivalent among them and are equal to about 19.2 °C. The floor has an outside temperature of 23 °C (expected) and an inside temperature of 19.2 °C. The roof has an external temperature of 11.36 °C: I think this is the problem, I must taking into account the dispersion of the roof even if I use a very low conductivity.
Thank you and have a nice weekend!

Marco ( 2019-05-24 10:57:11 -0500 )edit

Do you know what variable I can use to monitor the amount of dispersed heat by means of the roof?

Thanks for you patience

Marco ( 2019-05-24 10:59:17 -0500 )edit

In your IDF, the roof surface uses the "Tetto" construction, which is just the "Roof Insulation Mia" layer. This layer has a very low conductivity (50,000x smaller than normal insulation), which might be causing this issue. Some useful output variables to check surface heat transfer are: